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Transmission_I_03_200909 SDH Basics
Course Objectives:
To master the SDH frame structure and
multiplexing/demultiplexing procedure
To master the overhead bytes and common alarms
detected by the overhead byes
To master the NE type in SDH network and their features
To master the features of different SDH network topology
To master different protection principles of the
self-healing network
To master the transmission performance
Reference:
Unitrans ZXSM Series SDH Equipment Training Manual
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Contents
1 SDH Overview ............................................................................................................................................ 1
1.1 SDH Concept .................................................................................................................................... 1
1.2 SDH Generation Background ........................................................................................................... 1
1.3 Limitations of PDH ........................................................................................................................... 2
1.3.1 Interfaces ................................................................................................................................ 2
1.3.2 Multiplexing Method ............................................................................................................. 3
1.3.3 Operation and Maintenance ................................................................................................... 4
1.3.4 No Unified NMS Interface ..................................................................................................... 5
1.4 Advantages of SDH ........................................................................................................................... 5
1.4.1 Interfaces ................................................................................................................................ 5
1.4.2 Multiplexing method .............................................................................................................. 6
1.4.3 Operation and Maintenance ................................................................................................... 7
1.4.4 Compatibility ......................................................................................................................... 7
1.5 Limitations of SDH ........................................................................................................................... 8
1.5.1 Low Utilization Ratio of Frequency Band ............................................................................. 8
1.5.2 Complicated Pointer Justification Mechanism ....................................................................... 8
1.5.3 Impact of Much Use of Software on System Security ........................................................... 9
2 SDH Frame Structure and Multiplexing ................................................................................................ 11
2.1 SDH frame structure ....................................................................................................................... 11
2.1.1 Payload ................................................................................................................................. 13
2.1.2 Section Overhead (SOH) ...................................................................................................... 13
2.1.3 Administrative Unit Pointer (AU-PTR) ............................................................................... 14
2.2 Structure and Process of SDH Multiplexing ................................................................................... 14
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2.3 Concepts of Mapping, Aligning, and Multiplexing ......................................................................... 16
2.3.1 Mapping ................................................................................................................................ 16
2.3.2 Aligning ................................................................................................................................ 16
2.3.3 Multiplexing ......................................................................................................................... 16
3 SDH Overhead and Pointer ..................................................................................................................... 18
3.1 SDH Overhead ................................................................................................................................. 18
3.1.1 Concept of Overhead ............................................................................................................ 18
3.1.2 Section Overhead Bytes ........................................................................................................ 18
3.1.3 STM-N Section Overhead .................................................................................................... 22
3.1.4 Path Overhead ....................................................................................................................... 24
3.2 SDH Pointers ................................................................................................................................... 28
4 Logic Structure of SDH Equipment ........................................................................................................ 31
4.1 Common Network Elements in SDH Network ................................................................................ 31
4.1.1 TM — Terminal Multiplexer ................................................................................................ 31
4.1.2 ADM — Add/Drop Multiplexer ........................................................................................... 32
4.1.3 REG — Regenerator ............................................................................................................. 33
4.1.4 DXC — Digital Cross-Connect Equipment .......................................................................... 33
4.2 Logical Functional Blocks of SDH Equipment ............................................................................... 35
5 Topology and Protection of SDH Network ............................................................................................. 39
5.1 Significance of Network Protection ................................................................................................. 39
5.2 Basic SDH Network Topologies ...................................................................................................... 39
5.3 Concept and Classification of Self-healing...................................................................................... 41
5.3.1 Overview............................................................................................................................... 41
5.3.2 Self-healing Concept............................................................................................................. 42
5.3.3 Self-healing Classification .................................................................................................... 43
5.4 Chain Network Protection ............................................................................................................... 44
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5.4.1 Overview .............................................................................................................................. 44
5.4.2 Basic Chain Network Protection Types ................................................................................ 45
5.5 Self-healing Ring Protection ........................................................................................................... 46
5.5.1 Self-healing Ring Classification........................................................................................... 46
5.5.2 Two-fiber Unidirectional Path Protection Ring .................................................................... 47
5.5.3 Two-fiber Bidirectional Path Protection Ring, ..................................................................... 50
5.5.4 Two-Fiber Bidirectional MS Protection Ring ...................................................................... 51
5.5.5 Four-fiber Bidirectional MS Protection Ring ....................................................................... 53
5.5.6 Comparison of Common Self-healing Rings ....................................................................... 56
5.6 Dual Node Interconnection (DNI) Protection ................................................................................. 57
5.6.1 Terminologies ....................................................................................................................... 57
5.6.2 DNI Principle ....................................................................................................................... 59
5.6.3 Application Instance ............................................................................................................. 60
5.7 Error Connection and Error Squelch ............................................................................................... 63
5.7.1 Error Connection .................................................................................................................. 63
5.7.2 Error Squelch of Error Connection ...................................................................................... 63
5.8 Logical Subnet Protection ............................................................................................................... 64
5.8.1 Overview .............................................................................................................................. 64
5.8.2 Basic Principles .................................................................................................................... 65
5.8.3 Categorization ...................................................................................................................... 66
5.8.4 Application Instance ............................................................................................................. 66
5.9 Topology and Features of Complicated Network ............................................................................ 72
5.9.1 T Network ............................................................................................................................ 72
5.9.2 Ring-chain Network ............................................................................................................. 73
5.9.3 Tributary Cross-Over of Ring Subnets ................................................................................. 74
5.9.4 Tangent Rings ....................................................................................................................... 74
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5.9.5 Intersected Rings .................................................................................................................. 75
5.9.6 Hinge Network ...................................................................................................................... 76
5.10 Overall Architecture of SDH Network .......................................................................................... 76
6 Timing and Synchronization .................................................................................................................... 81
6.1 Synchronization Modes ................................................................................................................... 81
6.1.1 Pseudo Synchronization ........................................................................................................ 81
6.1.2 Master/Slave Synchronization .............................................................................................. 82
6.2 Working Modes of Sub-Clock in Master/Slave Synchronous Network........................................... 83
6.2.1 Normal Working Mode - Track and Lock the Upper Level Clock ....................................... 84
6.2.2 Hold-on Mode ....................................................................................................................... 84
6.2.3 Free Run Mode – Free Oscillation Mode ............................................................................. 84
6.3 Network Synchronization Requirements of SDH ............................................................................ 84
6.4 Clock Source Types of SDH NE ...................................................................................................... 85
6.5 Selection Principle of Clock in SDH Network ................................................................................ 86
6.5.1 Synchronization Principle of SDH Network ......................................................................... 87
6.5.2 Instance ................................................................................................................................. 88
7 Optical Interfaces ..................................................................................................................................... 93
7.1 Optical Interface Types .................................................................................................................... 93
7.2 Optical Interface Parameters ............................................................................................................ 94
7.2.1 Optical Line Code Pattern ..................................................................................................... 95
7.2.2 S Point Specifications-Specifications of Optical Transmitter ............................................... 95
7.2.3 R Point Specifications-Specifications of Optical Receiver ................................................... 96
8 Transmission Performance ...................................................................................................................... 99
8.1 Bit Error Characteristics .................................................................................................................. 99
8.1.1 Generation and Distribution of Bit Error .............................................................................. 99
8.1.2 Measurement of Bit Error Performance .............................................................................. 100
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8.1.3 Bit Error Specifications Related to Digital Section ............................................................ 101
8.1.4 Measures to Reduce Bit Error ............................................................................................ 102
8.2 Availability Parameters ................................................................................................................. 102
8.3 Jitter/Wander Performance ............................................................................................................ 103
8.3.1 Generation Principles of Jitter/Wander .............................................................................. 103
8.3.2 Jitter Performance Specifications ....................................................................................... 104
8.3.3 Measures to Reduce Jitter .................................................................................................. 106
8.3.4 Notes .................................................................................................................................. 106
9 Test ........................................................................................................................................................... 109
9.1 SDH Test Method .......................................................................................................................... 109
9.2 SDH Tested Items ......................................................................................................................... 109
10 Introduction to Network Management ............................................................................................... 111
10.1 TMN Fundamentals .....................................................................................................................111
10.1.1 TMN Management Frame .................................................................................................111
10.1.2 Physical Structure of TMN ............................................................................................... 112
10.1.3 TMN Interfaces ................................................................................................................ 113
10.1.4 TMN Layers Division ...................................................................................................... 114
10.2 SDH Management Network (SMN) ............................................................................................ 114
10.2.1 SMN and TMN ................................................................................................................ 114
10.2.2 SDH Management Interfaces ........................................................................................... 115
10.3 SDH Management Functions ...................................................................................................... 116
10.4 OSI Model and ECC Protocol Stack ........................................................................................... 116
10.4.1 OSI Concept ..................................................................................................................... 116
10.4.2 ECC Protocol Stack Description ...................................................................................... 117
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1 SDH Overview
Key points
Limitations of PDH
Advantages of SDH
Limitations of SDH
1.1 SDH Concept
Synchronous Digital Hierarchy or SDH is an international standard for wide band
transmission hierarchy, which defines the transmission rate, frame structure,
multiplexing mode, and optical interface specifications of digital signal transmission.
1.2 SDH Generation Background
The high developing information society nowadays requires the ability of
communication networks to provide various telecommunication services. The amount
of information transmitted, switched, and processed by telecommunications network
keeps increasing requiring modern communication networks to develop towards
digitalization, integration, intelligentization and personalization.
Transmission system is an important part of communication networks. The quality of
transmission system makes a direct effect on the development of communication
network. Lots of countries are developing information highway by constructing optical
transmission network with bigger capacity. The optical transmission network based on
SDH/WDM is the basic physical platform of the information highway. The
transmission network should have universal unified interface specifications, so that
every user in the world can communicate conveniently anytime and anywhere.
The multiplexing method used by PDH cannot satisfy the transmission requirements of
bigger capacity. The regional specifications of PDH make network interconnections
difficult, and restrict the transmission network development to higher rates.
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1.3 Limitations of PDH
Limitations of traditional PDH mainly lie in the following aspects.
1.3.1 Interfaces
1. PDH only has regional specifications for electrical interfaces, and no universal
standard.
The current PDH system has three different signal rate standards: European,
North American, and Japanese systems. They have different electrical interface
rate levels, signal frame structures, and multiplexing methods, which makes
international inter-working very difficult and does not adapt to the development
of current communication industry. The electrical interface rate levels of the
three systems are shown in Fig. 1.2-1.
565Mbit/s
139Mbit/s
34Mbit/s
8Mbit/s
2Mbit/s
1.6Gbit/s
400Mbit/s
100Mbit/s
6.3Mbit/s
1.5Mbit/s
274Mbit/s
45Mbit/s
6.3Mbit/s
× 4 × 4
× 4
× 4× 4
× 4
× 4
× 4
× 6
× 7
× 3
European
system
Japanese
system
North American
system
× 5
32Mbit/s
Fig. 1.2-1 PDH rate levels
2. PDH does not have universal unified standards for optical interfaces.
Different manufacturers use their own line code pattern to monitor the
transmission performance of optical lines. The typical instance is the mBnB
code, where mB is the information code, and nB is the redundant code. The
function of redundant code is to monitor the transmission performance of
optical lines, which makes the signal rate at the same level of optical interface
greater than the standard signal rate at electrical interface. However, it adds
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requirement for the transmission bandwidth of optical channels. Meanwhile,
different manufacturers add different redundant codes to the information codes
when coding line signals, resulting in different code patterns and rates for the
same optical interfaces at the same rate level from different manufacturers, and
the equipments from different manufacturers are incompatible with each other.
Thus the hybrid networking by equipments from multiple manufacturers is
restricted in the transmission network and the cost of network construction and
operation is increased, which brings difficulty to networking application,
network management and interconnection.
1.3.2 Multiplexing Method
In PDH system, only PCM equipment adopts synchronous multiplexing method to
multiplex 64 kbit/s signals to a basic group rate; while all other groups adopts the
―Plesiochronous Multiplexing‖ method. Because the signals at all levels of PDH rates
are asynchronous, positive signal rate justification is required to adapt and
accommodate the rate difference of tributary signals at various levels.
Since PDH uses asynchronous multiplexing method, when low-speed signals are
multiplexed into high-speed signals, their locations in the frame structures of the
high-speed signals do not have a regular pattern. In other words, low-speed signals
cannot be located easily in high-speed signals, resulting in low-speed signals being
unable to be directly dropped from or added to high-speed signals. For example, 2
Mbit/s signals cannot be directly added to or dropped from 140 Mbit/s signals, which
causes two problems:
1. The low-speed signals have to be dropped from or added to high-speed signals
level by level. For example, to drop/add 2 Mbit/s signals from/to 140 Mbit/s
signals, we need to follow the procedure shown in Fig. 1.2-2.
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2Mbit/s (electrical signal)
140/34(Mbit/s)
34/8(Mbit/s) 8/34(Mbit/s)
34/140(Mbit/s)
8/2(Mbit/s) 2/8(Mbit/s)
PDHOptical
signal
Optical/
electrical
Dem
ultip
lex
Dem
ultip
lex
Dem
ultip
lex
Multip
lex
Multip
lex
Multip
lex
Electrical
/Optical
Fig. 1.2-2 Drop/add 2Mbit/s signals from/to 140Mbit/s signals
The figure shows that adding/dropping 2 Mbit/s signals to/from 140 Mbit/s
signals use lots of ―back-to-back‖ equipment. 2 Mbit/s signals are dropped
from 140 Mbit/s signals by three levels of demultiplexing equipment, and then
2 Mbit/s signals are added to 140 Mbit/s signals by three levels of multiplexing
equipment. One 140 Mbit/s signal can be demultiplexed to sixty-four 2 Mbit/s
signals. Even if only one 2 Mbit/s signal needs to be dropped from the 140
Mbit/s signal, a full set of three-level multiplexing/demultiplexing equipments
are required. This not only increases the equipment volume, cost, and power
consumption but also reduces the equipment reliability.
2. To drop/add 2 Mbit/s signals from/to 140 Mbit/s signals, we need to follow the
process of multiplexing/demultiplexing level by level, which can damage
signals and cause deterioration in transmission performance. For large-capacity
long-distance transmission, such defect is intolerable.
1.3.3 Operation and Maintenance
The PDH signal frame structure has very few overhead bytes for Operation,
Administration and Maintenance (OAM). This is why we need to add redundant codes
to monitor line performance when coding optical line signals. And this is unfavorable
for hierarchical management, performance monitoring, real-time service scheduling,
and control of transmission bandwidth, alarm analysis, and troubleshooting of the
transmission network.
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1.3.4 No Unified NMS Interface
PDH has no network management function, and no unified NMS (Network
Management System) interface, which is unfavorable to build unified TMN
(Telecommunication Management Network).
PDH transmission hierarchy becomes more and more unsuitable for transmission
network development because of the above defects. Therefore, Bell Communication
Institute of the U.S. first introduced the Synchronous Optical Network (SONET)
hierarchy which consists of a full set of leveled standard digital transmission structure.
CCITT accepted SONET in 1988, and renamed it Synchronous Digital Hierarchy
(SDH), making it the general technical hierarchy applicable not only to optical fiber
transmission but also to microwave and satellite transmission.
1.4 Advantages of SDH
The inherent disadvantages of PDH pave the way for the steady development of SDH
as a brand new generation of transmission hierarchy.
The main objective of SDH is to construct a digital communication network from the
aspect of uniform national telecommunications network and international
interconnection. Take the case of Integrated Services Digital Network (ISDN)
especially Broadband ISDN (B-ISDN), SDH plays an important role since SDH based
network is a highly uniform, standardized, intelligent network. It adopts a universally
uniform interfaces to make equipment from different manufacturers compatible,
manage and operate all networks efficiently and coordinately, implement flexible
networking and service scheduling, implement network self healing, improve the
utilization ratio of network resource, and saves expenses for equipment operation and
maintenance.
We will describe the advantages of SDH from the following aspects: interfaces,
multiplexing method, operation and maintenance, and compatibility.
1.4.1 Interfaces
1. Electrical interfaces
Interface standardization is the key point in determining equipment
interconnection from different manufacturers. SDH made uniform standards for
Network Node Interface (NNI). The standards include rate levels of digital
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signal, frame structure, multiplexing method, line interface, and monitoring
management making it easy for interconnection of SDH equipment from
different manufacturers, i.e. equipment of different manufacturers can be
installed on one transmission line, which presents transverse compatibility.
SDH has a set of standard hierarchy of information structure, i.e. a set of
standard rate levels. Its basic information structure hierarchy is STM-1
(Synchronous Transport Module) with a corresponding rate of 155 Mbit/s. The
higher-level digital signal series such as 622 Mbit/s (STM-4) and 2.5 Gbit/s
(STM-16) can be formed by synchronously multiplexing the information
module (e.g. STM-1) of basic rate level through byte interleaving. The number
of multiplexing is a multiple of four, e.g. STM-4 = 4×STM-1, STM-16 =
4×STM-4, STM-64 = 4×STM-16.
2. Optical interfaces
The line interfaces (optical interfaces) adopt a universally uniform standard as
well. The line coding of SDH signals only does scrambling and does not insert
redundant code.
The scrambling standard is universally uniform, which enables optical interface
interconnection of SDH equipment from different manufacturers by adding
standard scrambler to the terminal equipment. Scrambling prevents too many
consecutive ―0‖ or ―1‖, making it easy to extract clock signal from line signals.
Since line signals are only scrambled, the optical signal rate of SDH line is the
same as the standard signal rate of SDH electrical interface, thus not increase
the transmission bandwidth of optical channel.
Currently, ITU-T officially recommends scrambled NRZ code to be the uniform
code for SDH optical interfaces.
1.4.2 Multiplexing method
Since lower-speed SDH signals are multiplexed into frame structure of higher-speed
SDH signals through byte-interleaving and multiplexing, the locations of lower-speed
SDH signals in the higher-speed SDH signal are regular and predictable. Thus we can
directly drop/add lower-speed SDH signal such as 155 Mbit/s (STM-1) signal from/to
higher-speed SDH signal such as 2.5 Gbit/s (STM-16) signal. The simplification of
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signal multiplexing/demultiplexing makes SDH particularly suitable for optical fiber
communication system of high speed and bigger capacity.
Besides, SDH uses synchronous multiplexing method and flexible mapping structure,
allowing PDH low-speed tributary signal to be multiplexed within the SDH signal
frame (STM-N). Therefore the locations of low-speed tributary signals in the STM-N
frame are predictable; we can directly drop/add low-speed tributary signal from/to
SDH signal. Thus saves lots of multiplexing/demultiplexing equipment (back-to-back
equipment), increases system reliability, reduces signal damage, reduces equipment
cost and power consumption, and simplifies service dropping/adding.
SDH integrates the advantages in both software and hardware; realizes the ―one-step‖
multiplexing of low-speed tributary signal (e.g. 2 Mbit/s) into STM-N signal; enables
maintenance personnel to schedule services flexibly and conveniently by using only
software. The SDH multiplexing method makes it easier to implement digital cross
connect function; provides the network with strong self-healing ability; and makes it
easier for network operators to dynamically construct network according to actual
needs.
1.4.3 Operation and Maintenance
The SDH frame structure provides abundant overhead bytes for Operation,
Administration, and Maintenance (OAM), which greatly enhance the monitoring
function for the networks and improve the automation of maintenance. PDH signal has
few overhead byte, thus redundant bits need to be added during line coding for
performance monitoring of line. Taking PCM30/32 signal as example, only TS0 and
TS16 timeslots are used for overhead function in its frame structure.
SDH frame has abundant overhead bytes, which account for 1/20 of the whole
bandwidth. Thus OAM functions are enhanced and system maintenance expenses are
reduced. According to statistics, the combined cost of SDH system is only 65.8% of
PDH system, in which the reduction of maintenance expenses plays an important role.
1.4.4 Compatibility
SDH having a strong compatibility can coexist with PDH networks, hence during
construction of SDH network existing PDH equipment can be kept. That is to say, SDH
network can transmit PDH services. In addition, SDH network can also transmit
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Asynchronous Transfer Mode (ATM) signals, and Fiber Distributed Data Interface
(FDDI) signals.
The basic transport module of SDH signal (STM-1) can accommodate various rates of
PDH tributary signals and other digital signals, such as ATM, FDDI, and DQDB; thus
exhibiting forward and backward compatibility. SDH particularly designed application
methods such as STM-N concatenation to adapt to requirements of transmitting new
services such as ATM and IP.
Various patterns (tributaries) are mapped and multiplexed at the network interface (start
point) into the STM-N frame structure, and then demultiplex the tributaries at the SDH
network boundary (end point); thus allowing transmission of signals of various patterns
in SDH transmission network.
1.5 Limitations of SDH
Limitations always come along with benefits. SDH system is not perfect either. It has
the following three limitations.
1.5.1 Low Utilization Ratio of Frequency Band
A major advantage of SDH is its enhanced reliability and enhanced automation of
OAM. This is due to a great number of overhead bytes added in the STM-N frame of
SDH. This will certainly increase the transmission rate and bandwidth, and PDH
signals occupy a lower transmission rate and bandwidth than SDH signals when
transmitting the same valid information. For example, an STM-1 signal of SDH can
accommodate sixty-three 2 Mbit/s or three 34 Mbit/s (equivalent to 48×2 Mbit/s) or one
140 Mbit/s (equivalent to 64×2 Mbit/s) PDH signals. Only when PDH signals are
multiplexed into the frames of STM-1 signals as 140 Mbit/s signals, can the STM-1
signal accommodate the information quantity of 63×2 Mbit/s. However, STM-1 (155
Mbit/s) is higher than the E4 signal (140 Mbit/s) having the same information quantity.
In other words, STM-1 occupies more transmission bandwidth than that of PDH E4
signals, when transmitting the same quantity of information.
1.5.2 Complicated Pointer Justification Mechanism
SDH system allows low-speed signals (e.g. 2 Mbit/s) to be directly dropped from
high-speed signals (e.g. STM-1) in ―one step‖, without the level-by-level
multiplexing/demultiplexing process. Such function is implemented through pointer
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mechanism. The pointers indicate the locations of low-speed signals all the times, so
that the required low-speed signals can be extracted correctly. We can say that the
pointer technology is a major feature of SDH system.
However, the implementation of pointer function increases the complexity of the
system. The most important problem is the generation of a particular jitter to SDH - a
compound jitter resulting from pointer justification. Such jitter often occurs at the
network boundary (SDH/PDH), and has a low frequency and large amplitude causing
low-speed signals to degrade in transmission performance after they are disassembled.
In addition, it is very difficult to filter such jitter.
1.5.3 Impact of Much Use of Software on System Security
A major characteristic of SDH is its high OAM automation, which means that the
software accounts for a great part in the system. On one hand, this makes the system
susceptible to computer viruses, which is predominant nowadays. On the other hand,
man-made incorrect operations and software faults on the network layer could be fatal
to the system. SDH system is heavily dependent on software hence the security for
running SDH system has become an important subject that should be addressed.
SDH system is an emergent novelty. Despite its drawbacks, it has exhibited powerful
vitality in the development of transmission network. Therefore, the shift from PDH to
SDH has become an irreversible trend for the transmission network.
Summary
This chapter describes the technical background and features of SDH, with the main
aim to help you understand the overall concept of SDH.
Exercises
1. Why did SDH becomes the transmission technology used today?
2. What are the limitations of SDH?
3. What services can you transmit through SDH other than PDH?
4. Why it is a disadvantage of SDH to rely heavily on software?
5. What are OAM bytes and why are they used?
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6. Why SDH is not frequency efficient compare to PDH?
7. How many E1, E3, and E4 can be transmitted through a single STM-1?
8. What is meant by Byte Interleave and why it is used in SDH?
9. What are the data rates of STM-64, STM-16, and STM-4?
10. What is Scrambling and why it is used in SDH frames?
11. Is it possible to locate a particular E1 inside a STM-1 frame?
11
2 SDH Frame Structure and Multiplexing
Key points
SDH frame structure
Process of multiplexing 2 M/34 M/140 M PDH signals into STM-N
Concepts of mapping and aligning
2.1 SDH frame structure
SDH signal frame structure arranges low-speed tributary signal evenly and regularly in
one frame, so that it is easy to implement synchronous multiplexing, cross-connection,
adding/dropping, and switching of tributary signals since it aims to conveniently
add/drop low-speed tributary signals to/from high-speed signals. With this, ITU-T
specified STM-N frame in a rectangular block structure with the unit of byte (eight
bits), as shown in Fig. 2.1-1.
Regenerator
Section OverHead
(RSOH)
Administrative Unit
Pointer (AU PTR)
Multiplex Section
OverHead
(MSOH)
STM-N net load (Payload)
9×N columns (bytes)261×N columns (bytes)
270×N columns
9 rows
Transmission
direction
125μs
1
3
5
9
4
Fig. 2.1-1 SDH frame structure
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As shown in the figure, STM-N frame consists of (270×N) columns × 9 rows of bytes,
where N is the N in STM-N with the value range of 1, 4, 16, and 64. N means that the
signal is formed by byte-interleaving and multiplexing of N STM-1 signals. The frame
structure of STM-1 signal is a block of 9 rows × 270 columns. When N STM-1 signals
form STM-N signal through byte-interleaving and multiplexing, only the columns of
STM-1 signals are processed with byte-interleaving and multiplexing, and the row
number is constant.
1. Transmission mode
Serial transmission transmits signals bit by bit in the line and STM-N signal
transmission also conforms to this mode. SDH signal transmit frame bytes from
left to right frame, then top to bottom frame, byte by byte, and bit by bit. After one
row is finished, the next row follows; after one frame is transmitted, the next
frame follows.
2. Frame frequency
ITU-T specified 8000 frames/second as the frame frequency for any level of STM
signal. The time cycle of the frame is 125 μs.
3. Transmission rate of STM-N
The transmission rate of STM-1 is:
270 (270 columns for each frame) × 9 (9 rows all together) × 8 bit (8 bits per byte)
× 8000 (8000 frames/second) = 155.520 Mbit/s
Since the frame’s time cycle is constant, the rate of STM-N signal is regular. For
example, the STM-4 transmission rate constantly equals to four times of STM-1
transmission rate, the STM-16 transmission rate constantly equals to sixteen times
of STM-1 transmission rate. The regularity of SDH signal rate makes it easy to
directly drop/add low-speed tributary signal from/to high-speed STM-N signal
stream. And this is the advantage of byte-based synchronous multiplexing of SDH.
Table 2.1-1 lists the SDH rate levels.
Table 2.1-1 SDH rate levels
STM-1 STM-4 STM-16 STM-64
Rate 155.520 Mbit/s 622.080 Mbit/s 2488.320 Mbit/s 9953.280 Mbit/s
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As shown in Fig. 2.1-1, the frame of STM-N consists of three parts: Section Overhead
(SOH), including Regenerator Section Overhead (RSOH) and Multiplex Section
Overhead (MSOH); Administrative Unit Pointer (AU-PTR); and net information load
(Payload).
We will describe the functions of the three parts.
2.1.1 Payload
The payload area of STM-N frame stores user information block. It functions as the
carriage of the STM-N train, and the cargo in the carriage is the packed low-speed
signal that is to be transported. In order to monitor the cargo (packed low-speed signal)
damage, in real time during transmission, monitoring overhead bytes are added, i.e.
Path Overhead (POH) bytes, into the package of the low-speed signal. POH is loaded
into the STM-N train as part of the payload to be transported in the SDH network; it is
responsible to monitor, manage, and control the path (lower path) performance of
packaged cargo.
2.1.2 Section Overhead (SOH)
SOH are mandatory additional bytes in the STM-N frame to ensure normal
transmission of the payload, and are primarily used for OAM of the network. For
example, SOH can monitor damages of all the cargoes in the STM-N train, while POH
is used to judge which cargo is damaged when cargo damage occurs in the STM-N
train. In other words, SOH monitors the cargoes as a whole, and POH monitors one
specific cargo. SOH and POH also have other management functions.
SOH is divided into RSOH and MSOH, which monitor their corresponding section.
Actually, section functions as a transmission path, and RSOH and MSOH monitor this
transmission path.
RSOH and MSOH have different management range. For example, if 2.5 G signals are
transmitted in fiber, then RSOH monitors the transmission performance of the whole
STM-16; while MSOH monitors the transmission performance of each STM-1 in the
STM-16 signal.
RSOH is located at column (1 ~ 9×N) × row (1 ~ 3), which are 3×9×N bytes all
together. MSOH is located at column (1 ~ 9×N) × row (5 ~ 9), which are 5×9×N bytes
all together.
Transmission_I_03_200909 SDH Basics
14
2.1.3 Administrative Unit Pointer (AU-PTR)
AU-PTR is located at column 1 ~ 9×N of the fourth row, which are 9×N bytes all
together. AU-PTR indicates the exact location of the first byte of the payload in the
STM-N frame, so that the payload can be disassembled correctly according to the
indicator at the receiving end.
Pointers include higher-order and lower-order pointers. High-order pointer is AU-PTR,
and low-order pointer is TU-PTR (Tributary Unit Pointer). The function of TU-PTR is
similar to AU-PTR function, while it points to smaller payload.
2.2 Structure and Process of SDH Multiplexing
There are two cases for SDH multiplexing: one case is multiplexing STM-1 signals
into STM-N signal; the other case is multiplexing PDH tributary signals (such as 2
Mbit/s, 34 Mbit/s, and 140 Mbit/s) into STM-N SDH signal.
1. Multiplexing STM-1 signals into STM-N signal
The multiplexing is implemented by byte-interleaving, with the multiplexing base of
four, i.e. 4×STM-1→STM-4, 4×STM-4→STM-16. The frame frequency is constant
(8000 frames/second) during multiplexing, which means that the rate of upper-level
STM-N signal is four times the lower-level STM-N signal. During byte-interleaving
multiplexing, the payload and pointer bytes of each frame are interleaved and
multiplexed using the original values, while ITU-T specified special standards for SOH.
For the STM-N frame composed by synchronous multiplexing, the SOHs of STM-N
are not formed by multiplexing and interleaving all the SOHs of lower-order STM-N
frames, instead some SOHs of lower-order frames are abandoned, for which there are
special specifications. For SOH details of various level STM-N frame, refer to chapter
3 SDH Section Overhead and Pointers.
2. Multiplexing PDH tributary signals into STM-N signal
The compatibility of SDH network requires SDH multiplexing method satisfy both
asynchronous multiplexing (e.g. multiplex PDH tributary signals into STM-N SDH
signal) and synchronous multiplexing (e.g. STM-1→STM-4), and it should be easy for
dropping/adding low-speed signal from/to high-speed STM-N signal without causing
much signal time-delay or slipping damage. To satisfy these requirements, SDH has a
unique set of multiplexing process and structure. In its multiplexing structure, SDH
uses pointer justification and alignment to replace the 125 μs buffer to justify the
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15
frequency offset of positive tributary signal, and to align the phase. All service signals
will go through the three steps to be multiplexed into STM-N frame: mapping, aligning,
and multiplexing.
ITU-T defines a complete set of multiplexing structure (i.e. multiplexing routes),
through which digital signals of the three PDH systems can be multiplexed into
STM-N signals in multiple ways, as shown in Fig. 2.2-1.
STM-N AUG AU-4 VC-4
AU-3 VC-3
TUG-3
TUG-2 TU-2
TU-12
TU-11
TU-3 VC-3
VC-2
VC-12
VC-11
C-4
C-3
C-2
C-12
C-11
139264kbit/s
44736kbit/s
34368kbit/s
6312kbit/s
2048kbit/s
1544kbit/s
×1
×7
×3×1
×3
×4
×7
×3
×1×N
Pointer Processing
Multiplexing
Aligning
Mapping
Fig. 2.2-1 SDH multiplexing structure defined by ITU-T
In Fig. 2.2-1, the multiplexing structure includes some basic multiplexing unit:
C-Container, VC-Virtual Container, TU-Tributary Unit, TUG- Tributary Unit Group,
AU-Administrative Unit, AUG- AU-Administrative Unit Group. The numbers of these
multiplexing units identify their corresponding signal levels. In Fig. 2.2-1, there are
multiple routes from one payload to STM-N, i.e. multiple multiplexing methods. For
example, there are two routes to multiplex 2 Mbit/s signal into STM-N signal, i.e. there
are two multiplexing methods. As a special note, 8 Mbit/s PDH tributary signal cannot
be multiplexed into STM-N signal.
Although there are multiple routes to multiplex a signal into SDH STM-N signal, the
technical system of Chinese optical synchronous transmission network defined the
PDH system based on 2 Mbit/s signal as the SDH payload, and selected AU-4
multiplexing route, as shown in Fig. 2.2-2.
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STM-N AUG AU-4 VC-4
TUG-3
TUG-2
TU-12
TU-3 VC-3
VC-12
C-4
C-3
C-12
139264kbit/s
34368kbit/s
2048kbit/s
×7
×3
×1
×3
×1×N
Pointer processing
Multiplexing
Aligning
Mapping
Fig. 2.2-2 Basic SDH multiplexing mapping structure of China
2.3 Concepts of Mapping, Aligning, and Multiplexing
In this section, we will describe the three different steps during the process of
multiplexing low-speed PDH tributary signals into STM-N signal: mapping, aligning,
and multiplexing.
2.3.1 Mapping
Mapping is the process to adapt tributary signal into virtual container at the SDH
network border (e.g. SDH/PDH network boundary). The process of mapping involves
rate adjustment of the various PDH tributary signals (140/34/2/45 Mbit/s) first, then
loading these signals to their corresponding standard container C. Then, VC (Virtual
Container) is formed from the containers by adding corresponding path overhead. The
reverse process of mapping is called demapping.
To adapt to various network applications, there are three mapping methods:
asynchronization, bit synchronization, and byte synchronization; and two mapping
modes: floating VC and locked TU.
2.3.2 Aligning
Aligning is the procedure by which the frame offset information is incorporated into
the tributary unit or the administrative unit when adapting to the frame reference of the
supporting layer. Aligning depends on TU-PTR or AU-PTR functions. It is always
accompanied synchronously by pointer justification event.
2.3.3 Multiplexing
Multiplexing is the process to adapt multiple signals of lower-order path layer to
higher-order path layer (e.g. TU-12 (×3) → TUG-2 (×7) → TUG-3 (×3) → VC-4), or
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17
to adapt multiple signals of higher-order path layer to multiplex section layer (e.g.
AU-4 (×1) → AUG (×N) → STM-N). The basic method of SDH multiplexing is to
interleave bytes of lower-order signal first, and then add some stuffing bits and
specified overheads to form the higher-order signal. The reverse process of
multiplexing is called demultiplexing.
Summary
This chapter describes the SDH frame structure and the functions of its main parts, and
the steps to multiplex PDH (2 M, 34 M, and 140 M) signals into STM-N frame.
Exercises
1. One STM-1 signal can accommodate 140 M signals, 34 M
signals, 2 M signals.
2. Give the different components of the SDH frame and its function.
3. How does a 2M signal mapped to an STM-N frame?
4. Why alignment is required in SDH frames?
5. What are the two types of SOH?
6. How many bytes are used for AU-PTR in a STM-1 frame?
7. How many bytes are contained in a RSOH of a STM-1 frame?
8. What is the difference between AU-PTR and TU-PTR?
9. What is the difference between SOH and POH?
10, How a Container (C) is different from Virtual Container (VC)?
11. What is the frame frequency of a SDH frame?
12. How many rows and columns are contained in a SDH frame of STM-16?
13. How SDH multiplexing structure used in China is different from / similar to
SDH multiplexing structure defined by ITU-T?
18
3 SDH Overhead and Pointer
Key points
SDH overheads
SDH pointers
3.1 SDH Overhead
3.1.1 Concept of Overhead
Overhead is the general designation of overhead bytes/bits. It includes all bytes in the
STM-N frame except payload that carries service information. Overhead is used to
support OAM function of the transmission network. It implements layered monitoring
and management, and it can be divided into section monitoring and path monitoring.
Section monitoring includes monitoring of regenerator section and multiplex section;
while path monitoring includes monitoring of higher-order path and lower-order path.
To illustrate this take 2.5 G system as an example, regenerator section overhead
monitors the whole STM-16 frame, while multiplex section overhead monitors any of
the sixteen STM-1s in the STM-16 frame. Higher-order path overhead monitors VC-4
of every STM-1, while lower-order path overhead monitors any of the sixty-three
VC-12s in the VC-4.
3.1.2 Section Overhead Bytes
Section overhead (SOH) is located at column (1 ~ 9×N) × row (1 ~ 9) except the fourth
row which is the AU-PTR. To describe the various functions of the overhead bytes,
take the STM-1 signal as an example. For STM-1 signal, the SOH includes the
RSOH located at column (1 ~ 9) × row (1 ~ 3), and the MSOH located at column (1 ~
9) × row (5 ~ 9), as shown in Fig. 3.1-1.
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A1 A1 A1 A2 A2 A2 JO ×* ×
*
B1 ? E1 ? F1 × ×
D1 ? ? D2 ? D3
AU-PTR
B2 B2 B2 K1 K2
D4 D5 D6
D7 D8 D9
D10 D11 D12
S1 M1 E2 × ×
? Bytes related with transmission medium (temporarily used)
× Bytes reserved for national use
* Non-scrambled bytes for national use
All the unmarked bytes are to be determined by international specifications (for
medium-related application, additional national use, and other usage)
9 bytes
9 rows
RSOH
MSOH
?
Fig. 3.1-1 Arrangements of STM-1 SOH bytes
Fig. 3.1-1 shows the locations of RSOH and MSOH in an STM-1 frame. As mentioned
previously, each has a different monitoring range. RSOH monitors bigger scope as
STM-N, i.e. it monitors every regenerator section; while MSOH monitors small parts
as STM-1 within the bigger scope, i.e. it monitors every multiplex section.
3.1.2.1 Framing Bytes: A1 and A2
The framing bytes indicate the beginning of the frame, so that the frames of receiving
end and transmitting end are kept synchronous.
3.1.2.2 Regeneration Section (RS) Trace Byte: J0
J0 is used to repetitively transmit a Section Access Point Identifier so that a section
receiver can verify its continued connection from the intended transmitter. Within the
networks of the same operator, J0 can be set as any character; while at the border of
networks from different operators, J0 bytes must match. Through J0 byte, the operator
can find and solve faults in advance to shorten the network recovery time.
3.1.2.3 Data Communication Channel (DCC) Bytes: D1 ~ D12
One of SDH features is its automatic OAM function. SDH can send commands to an
NE and query data of the NE through the network management terminal, and can
Transmission_I_03_200909 SDH Basics
20
perform functions such as real-time service scheduling, alarm/fault locating, and online
test. These OAM functions are all transmitted by D1~D12 bytes in STM-N frame. Data
Communication Channel (DCC) is the physical layer of Embedded Control Channel
(ECC), and it transmits OAM information among NEs, thus constructing a
transmission channel for SDH Management Network (SMN).
D1~D3 bytes are Regenerator Section DCC (DCCR), with the rate of 3×64 kbit/s=192
kbit/s, used to transmit OAM information among regenerator section terminals;
D4~D12 bytes are Multiplex Section DCC (DCCM), with the rate of 9×64 kbit/s=
576 kbit/s, used to transmit OAM information among multiplex section terminals.
The total rate of DCC channel is 768 kbit/s providing SMN with a powerful private
data communication channel.
3.1.2.4 Orderwire Bytes: E1 and E2
E1 and E2 can each provide an orderwire channel of 64 kbit/s for voice
communication.
E1 belongs to RSOH, used for orderwire communication between regenerator sections;
E2 belongs to MSOH, used for direct orderwire communication between multiplex
section terminals.
3.1.2.5 RS User Channel Byte: F1
F1 provides a data/voice channel with a rate of 64 kbit/s. It is reserved for user
(generally network provider) for orderwire communication of specific maintenance
purpose, or for transmission of 64 kbit/s special data.
3.1.2.6 RS Bit Interleaved Parity 8-bit Code (BIP-8) Byte: B1
B1 byte is used to monitor bit errors for the RS layer located at row 2, column 1 of the
RSOH.
3.1.2.7 Multiplex Section (MS) Bit Interleaved Parity (N×24)-bit Code (BIP- N×24) Byte: B2
The working principle of B2 byte is the same as that of B1. But B2 detects the bit
errors for the MS layer. One STM-N frame has only one B1 byte. While B2 byte
monitors bit errors for each STM-1 frame in STM-N frame, and every three B2 bytes
corresponds to one STM-1 frame, thus one STM-N frame has N×3 B2 bytes.
3.1.2.8 MS Remote Error Indication (MS-REI) Byte: M1
M1 byte is a reply message and is sent back to the transmitter from the receiver. M1
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21
byte carries the count of error blocks detected by the receiver through B2 byte, and it
reports MS-REI (MS Remote Error Indication) in the current performance management
of the transmitter, so the transmitter can learn about bit error status at the receiver.
3.1.2.9 Automatic Protect Switching (APS) Channel Bytes: K1, K2 (b1~b5)
K1 and K2 (b1~b5) are used to transmit Automatic Protection Switch (APS)
information, to support automatic switching of equipment in case of fault, so that the
network service can recover automatically (self-healing). They are used specially for
MS APS.
3.1.2.10 MS Remote Defect Indication (MS-RDI) Byte: K2 (b6~b8)
K2 (b6~b8) is used for feedback message of MS far end alarm. The feedback message
is sent back to the transmitter (information source) from the receiver (information
destination), indicating that the receiver detected fault at the receiving direction or that
the receiver is receiving MS alarm indication signal. That is to say, when the received
signal at the receiver degrades, it sends MS-RDI alarm back to the transmitter, so that
the transmitter can learn about the receiver status. If the received b6~b8 bits of K2 are
―111‖, it is MS Alarm Indication Signal (MS-AIS) and indicates that the end itself need
to send MS-RDI to the opposite end by writing ―110‖ into b6~b8 bits of K2.
3.1.2.11 Synchronization Status Message Byte: S1 (b5~b8)
SDH MSOH uses bits 5~8 of S1 byte to represent different clock quality levels
specified by ITU-T, so that the equipment can judge the clock quality received via S1
and then judge whether to switch to a clock source of higher quality. S1 byte structure
is shown in Fig. 3.1-2.
b1 b2 b3 b4 b5 b6 b7 b8
Synchronous Status Message
Fig. 3.1-2 S1 byte structure
The four bits can compose sixteen different codes, which can represents sixteen
different synchronous quality levels, as shown in Table 3.1-1. Among these
combinations, only four combinations are currently used to transmit clock quality
information.
Transmission_I_03_200909 SDH Basics
22
Table 3.1-1 SSM Codes
S1(b5~b8) SDH Synchronous Quality
Level Description S1(b5~b8)
SDH Synchronous Quality
Level Description
0000
Unknown synchronous
quality level (existing
synchronous network)
1000 G.812 local office clock signal
0001 Reserved 1001 Reserved
0010 G.811 clock signal 1010 Reserved
0011 Reserved 1011 Synchronous Equipment
Timing Source (SETS)
0100 G.812 transition office clock
signal 1100 Reserved
0101 Reserved 1101 Reserved
0110 Reserved 1110 Reserved
0111 Reserved 1111 Should not be used for
synchronization
The detailed clock source switching will be described in chapter 6.
3.1.2.12 Bytes Related with Transmission Medium: ∆
Bytes marked with ∆ are especially used for special functions of a certain transmission
medium, e.g. this byte can be used to identify signal direction when using a single
optical fiber to implement bi-directional transmission.
3.1.2.13 Bytes Reserved for National Use: ×
Bytes marked with × are reserved for national use.
Functions of all the unmarked bytes are to be determined by international
specifications.
3.1.3 STM-N Section Overhead
N STM-1 frames form an STM-N frame via byte-interleaving and multiplexing. The
bytes in the AU-4 of each STM-1 are interleaved and multiplexed without changing the
bytes themselves, while the SOH multiplexing obeys a special regulation. Overhead
bytes except A1, A2, and B2 need termination process before being inserted into
corresponding STM-N overhead bytes; while A1, A2, and B2 bytes are interleaved and
multiplexed based on byte into the STM-N. Fig. 3.1-3 shows the SOH structure of
STM-4 frame.
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A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1
B1
A2 A2 A2 A2 A2 A2 A2 A2 A2 A2 A2 A2
1 5 9 13 17 21 25 29 33
J0 *Z0
*Z0
*Z0
* * * * * * * *
E1 F1
D1
K1 K2B2 B2 B2 B2 B2 B2 B2 B2 B2 B2 B2 B2
D2 D3
D4 D5 D6
D7 D8 D9
D10 D11 D12
S1 M1 E2
1 2 3 4 5 6 7 8 9
36 bytes
RS
OH
MS
OH
9 r
ow
s
AU-PTR
Multiplexed columnsReserved bytes for national use
* Non-scrambled bytes
All the unmarked bytes are to be determined by international specifications (for medium-related application, additional national use, and other usage)
Fig. 3.1-3 SOH of STM-4 frame
The SOH bytes of the STM-N frame are a layout of the SOHs of N STM-1 frames after
the interleaving process. During this operation, only the SOH of the first STM-1 is
completely retained, while for the rest N-1 STM-1 frames, only A1, A2 and B2 are
retained. Hence, one STM-N frame only has one B1 byte, but N×3 B2 bytes (since B2
is the result of BIP-24 check, every STM-1 frame has three B2 bytes), one byte each
for D1~D12, one M1 byte, one byte each for K1 and K2. Fig. 3.1-4 shows the byte
arrangements of STM-16 SOH.
A1
B2
A1
B2
A2
E1
D2
K1
D5
D8
D11
A2 A2J0
C1
F1
D3
K2
D6
D9
D12
E2
*
x
x
x
*
x
x
x
A1
B1
D1
B2
D4
D7
D10
S1
A1 A1 A1 A2 A2 A2 Z0
C1
*
x
x x x
B2 B2 B2
x x x
AU-PTR
RSOH
MSOH
9 rows
144 bytes
M1 ...
Note: x Reserved bytes for national use
* Non-scrambled code for national use
Z0 and all the unmarked bytes are to be determined by international specifications (for medium-related
application, additional national use, and other usage)
*
x
x
Fig. 3.1-4 Byte arrangements of STM-16 SOH
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24
3.1.4 Path Overhead
Section Overhead (SOH) is responsible for OAM functions for the section layer, while
Path Overhead (POH) is responsible for OAM functions for the path layer.
POH falls into two categories: lower-order path POH (LP-POH) and higher-order path
POH (HP-POH). HP-POH monitors VC-4 level path, i.e. it can monitor the
transmission status of 140 Mbit/s signals in the STM-N frame. And LP-POH
implements the OAM functions of VC-12 level path, i.e. it monitors the transmission
performance of 2 Mbit/s signals in the STM-N frame.
3.1.4.1 High-order Path Overhead: HP-POH
HP-POH locates at the first column of VC-4 frame, with nine bytes all together as
shown in Fig. 3.1-5.
J1
B3
C2
G1
F2
H4
F3
K3
N1
Higher-order path trace byte
Higher-order path bit error monitoring BIP-8 byte
Higher-order path signal label byte
Higher-order path status byte
Higher-order path user channel byte
TU multiframe location indicator byte
Higher-order path user channel byte
Automatic Protection Switching (APS) channel, allocated for future use
Network operator byte VC-3 or VC-4
Fig. 3.1-5 HP-POH structure
1. J1: Higher-order path trace byte
AU-PTR points at the location of the first byte of VC-4, so that the receiver can
accurately extract VC-4 from the AU-4 according to AU-PTR value. J1 is the
first byte of VC-4, so AU-PTR points at the location of J1 byte.
The function of J1 is similar to that of J0. It is used to repetitively send
identifier for higher-order path access point, so that the receiver in the path can
verify its continued connection to the intended transmitter (this path is
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continuously connected). J1 byte can be configured or modified according to
actual needs, but the requirement is that J1 bytes at the receiver and transmitter
should match.
2. B3: Higher-order path bit error monitoring byte (BIP-8)
Using BIP-8, B3 byte is responsible to monitor bit error performance during
VC-4 transmission, i.e. it monitors the bit error performance during 140 Mbit/s
signal transmission. The monitoring mechanism is similar to that of B1 and B2,
with the difference that B3 performs BIP-8 check for VC-4 frame.
3. C2: Higher-order path signal label byte
C2 is used to indicate the characteristics of multiplexing structure and net
information load, such as if the path is equipped, the loaded services types and
their mapping method. For example, C2=00H indicates that the VC-4 path is
not equipped with signal, and code of all ―1‖s (TU-AIS) should be inserted into
the net load TUG3 of the VC-4 path, so that higher-order path unequipped
alarm i.e. VC4-UNEQ will occur in the equipment. C2=02H indicates that the
net load equipped in the VC-4 is multiplexed via the TUG structure. China
adopts TUG structure to multiplex 2 Mbit/s signals into VC-4. C2=15H
indicates that the net load of the VC-4 are signals of FDDI (Fiber Distributed
Data Interface) format.
4. G1: Higher-order path status byte
G1 is used to convey the path terminating status and performance back to VC-4
originating path equipment. Therefore the status and performance of the
bi-directional path in its entirety can be monitored, from either end or any point
of the path. G1 byte actually transmits the reply message, i.e. the message sent
back to the transmitter from the receiver, so that the transmitter can learn the
status of the VC-4 path signal received by the receiver. Arrangements of G1 bits
are shown in Fig. 3.1-6.
1 2 3 4 5 6 7 8
FEBBE RDI Reserved Standby
Fig. 3.1-6 Arrangements of G1 bits
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26
5. F2, F3: User channel bytes
These two bytes provide orderwire communication between path units (related
to net load) and are seldom used currently.
6. H4: TU multiframe location indicator byte
H4 indicates the multiframe type of the payload and the location of the net load.
For example, it acts as the indicator byte for TU-12 multiframe, or as the
indicator for cell border when ATM net load enters a VC-4.
7. K3: Automatic Protection Switching (APS) channel
Bits b1~b4 of K3 byte are used to transmit the command of higher-order path
protection switching (APS).
8. N1: Network operator byte
N1 is used to provide a Tandem Connection Monitoring (TCM) of higher-order
path.
3.1.4.2 Lower-order Path Overhead: LP-POH
Lower-order path refers to the VC-12 path overhead. It monitors the transmission
performance of the VC-12 level path, i.e. it monitors the transmission status of 2 Mbit/s
PDH signals in STM-N.
Fig. 3.1-7 shows the structure of a VC-12 multiframe, which consists of four VC-12
base frames. LP-POH is located at the first byte of every VC-12 base frame, and one
group of LP-POHs has four bytes: V5, J2, N2, and K4.
V5 J2 N2 K4
VC12 VC12VC12VC12
1 4
1
9
500us VC12
multiframe
Fig. 3.1-7 LP-POH structure
1. V5: Path status and signal label byte
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V5 is the first byte of TU-12 multiframe. TU-PTR points at the location of V5
byte in the TU-12 multiframe.
V5 has the functions of bit error detection, signal label and VC-12 path status
indication. Therefore, V5 functions are similar to those of higher-order path
overhead bytes B3, C2 and G1. Table 3.1-2 lists the V5 byte structure.
Table 3.1-2 Structure of VC-12 POH (V5)
Bit Error Monitoring
(BIP-2)
Far End Background
Block Error
(FEBBE)
Remote Failure
Indication
(RFI)
Signal Label
Remote Defect
Indication
(RDI)
1 2 3 4 5 6 7 8
Bit Error Monitoring:
Transmit Bit Interleaved
Parity code BIP-2: Bit 1
configuration should
enable the parity check
of all odd bits in last
VC-12 multiframe to be
even.
Bit 2 configuration should
enable the parity check
of all even bits to be
even.
Far End Background
Block Error: Send
―1‖ if BIP-2 detects
block error;
otherwise, send ―0‖.
Remote Failure
Indication: Send
―1‖ for fault;
otherwise send
―0‖.
Signal Label:
It indicates the net load
equipment status and
mapping method. It has three
bits which can have eight
binary value.
000: VC path unequipped
001: VC path equipped, but
payload is not specified.
010: Asynchronous floating
mapping
011: Bit synchronous floating
100: Byte synchronous
floating
101: Reserved
110: O.181 test signal
111: VC-AIS
Remote Defect
Indication (equivalent
to FERF used
before):
Send ―1‖ for
receiving failure;
otherwise, send ―0‖.
If the receiver detects block error via BIP-2, it will indicate the block error
number detected via BIP-2 in the local end performance event of V5-BBE.
Meanwhile, it will send V5-FEBBE back to the transmitter via bit b3 of V5.
Then the performance event V5-FEBBE at the transmitter indicates the
corresponding block error number. Bit b8 of V5 is used for Remote Defect
Indication of VC-12 path. When the receiver receives TU-12 AIS signal, it will
send one VC12-RDI (LP-POH Remote Defect Indication) signal back to the
transmitter.
2. J2: VC-12 path trace byte
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28
The function of J2 is similar to that of J0 and J1. It is used to repetitively send
the identifier of lower-path access point negotiated by the receiver and
transmitter, so that the receiver can verify its continued connection in the path
from the intended transmitter.
3. N2: Network operator byte
N2 is used to provide a Tandem Connection Monitoring (TCM) for the
lower-order path.
4. K4: Automatic protection switching path
3.2 SDH Pointers
Pointers provide alignment function. Through alignment, the receiver can accurately
extract corresponding VC from STM-N signal stream, and then extract low-speed PDH
signal by unpacking VC and C packages; thus implementing the function of extracting
low-speed tributary signals from STM-N signal directly.
Alignment is the procedure by which the frame offset information is incorporated into
the tributary unit or the administrative unit, i.e. using the pointer attached to VC to
indicate and determine the location of the beginning of the lower-order VC frame in the
TU payload (or the location of beginning of higher-order VC frame in the AU payload).
The pointer value is adjusted when the relative frame phase offset causes VC frame
beginning to float, so as to ensure that the pointer always traces and indicates the
beginning of the VC frame. For VC-4, AU-PTR points at the location of J1 byte; for
VC-12, TU-PTR points at the location of V5 byte. TU-PTR or AU-PTR can provide a
flexible and dynamic method for aligning VC in TU or AU frame; because TU-PTR or
AU-PTR can accommodate not only the phase difference of VC and SDH, but also
their rate difference.
Summary
This chapter describes various overheads of SDH frame; the mechanism of layered
monitoring using RSOH, MSOH, HP-POH, and LP-POH; and pointer alignment
principle.
The major points to master are the functions of overhead bytes, and their relations with
alarm and performance detection.
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Exercises
1. MS-AIS and MS-RDI are detected by which byte?
2. What is the detection principle of A1 and A2 byte?
3. What bytes are used to implement monitoring and alarm of bit error at different
SDH layers?
4. What is the use of K1 and K2 bytes?
5. What is the need of S1 byte in a SDH frame?
6. What is the significance of E1 and E2 bytes?
7. How many bytes are contained in a VC-4 POH?
8. What is the use of M1 byte?
9. Which byte indicates the type and composition of VC-4 tributary information?
10. What is the difference between J0 and J1 bytes?
11. What is the purpose of having overhead bytes in SDH?
12. How many VC-12 frames are combined together to form a multi-frame?
31
4 Logic Structure of SDH Equipment
Key points
Common NEs in SDH network
Logical functional blocks of SDH equipment
4.1 Common Network Elements in SDH Network
SDH transmission network is composed of various types of network elements (NE)
which are connected by optical cables. These NEs can perform transmission functions
of SDH network like service add/drop, service cross-connect, and network fault
self-healing. The following contents describe the characteristics and basic functions of
the common NEs in an SDH network.
4.1.1 TM — Terminal Multiplexer
TM is located at the terminal site of the network with only one optical direction, as
shown in Fig. 4.1-1.
TM
2Mbit/s 34Mbit/s
STM-M
140Mbit/s
STM-NW
Note: M<N
Fig. 4.1-1 TM model
The functions of a TM are to multiplex low-speed signals at a tributary port into the
high-speed STM-N signal at a line port, or to drop low-speed tributary signals from
STM-N signals.
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32
Note:
The line port may only input/output one channel of STM-N signal, while tributary port
may input/output multiple channels of low-speed tributary signal.
TM performs the cross-connect function when multiplexing low-speed tributary signals
into the STM-N frame. For example, we can multiplex one STM-1 tributary signal into
any position of a STM-16 line signal, i.e. multiplex STM-1 to any position of the
sixteen STM-1s of STM-16. And we can multiplex one tributary 2 Mbit/s signal into
any position among the sixty-three VC-12s of an STM-1.
4.1.2 ADM — Add/Drop Multiplexer
ADM is used at the transition office in the SDH transmission network, such as the
middle node of a chain or a node in a ring. It is the most frequently used and most
important network element in an SDH network, as shown in Fig. 4.1-2.
ADM
2Mbit/s 34Mbit/sSTM-M
140Mbit/s
STM-N STM-NW E
Note: M<N
Fig. 4.1-2 ADM model
ADM has 2 line sides and 1 tributary side. For convenience of description, we call
them the west (W) line port and the east (E) line port. The ADM tributary side connects
with the tributary ports, and the tributary port signals are the added/dropped services
to/from the line side STM-N signal. The functions of an ADM are to cross-connect and
multiplex low-speed tributary signal to the east/west line, or to drop the low-speed
tributary signal from the line signal received from the east/west line port; in addition,
ADM can cross-connect the STM-N signals of the east/west line. For example,
cross-connect the third STM-1 of east STM-16 with the 15th STM-1 of west STM-16.
ADM is the most important NE in SDH since it may be used as the equivalence of
other network elements, i.e. it can accomplish the functions of other network elements.
For example, one ADM is equivalent to two TMs.
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4.1.3 REG — Regenerator
The characteristic of REG is that it only regenerates optical signals without
adding/dropping electrical line service. There are two kinds of REGs in SDH
transmission network: one is pure optical REG, which regenerates the optical power so
as to extend the optical transmission distance; the other is electrical REG for pulse
regeneration and reshaping, which performs Optical/Electrical (O/E) conversion,
electrical signal sampling, determination, regeneration and reshaping, and E/O
conversion to eliminate accumulated line noise and thus ensures good waveform of the
line signals being transmitted.
Hereinafter we only discuss the latter REG. The REG is an equipment with two sides,
which connect with the west line port (W) and east line port (E) respectively, as shown
in Fig. 4.1-3:
REG STM-NSTM-NW E
Fig. 4.1-3 REG model
The REG processes optical signals at the W/E side by O/E conversion, sampling,
determination, regeneration and reshaping, E/O conversion and sends the processed
optical signal out at the E/W side. Compared with ADM, REG lacks the tributary ports
side. Therefore, ADM is also equivalent to a REG when it does not add/drop local
electrical line service.
REG only processes RSOH in the STM-N frame, and has no cross-connect function (it
only needs to connect W and E directly); while ADM and TM process not only RSOH,
but also MSOH, since they need to multiplex the low-speed tributary signals to the
STM-N frame.
4.1.4 DXC — Digital Cross-Connect Equipment
DXC mainly accomplishes the cross-connection of the STM-N signals. It is an
equipment with multiple ports. ADM is actually equal to a cross matrix completing the
cross-connection of signals, as shown in Fig. 4.1-4.
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DXCm n
Equivalent to
Input lines: m
Output lines: n
Fig. 4.1-4 DXC model
The DXC can cross-connect the input m-channel STM-N signals to the output
n-channel STM-N signals. The figure above indicates there are m lines of input fiber,
and n lines of output fiber. The core function of DXC is to cross-connect. A powerful
DXC can perform the low-level cross-connect (e.g. cross-connect of VC-4 or VC-12
level) of high-speed signals (e.g. STM-16) within the cross-connect matrix.
DXCm/n is generally used to denote the type and performance of a DXC (m≥n), where
m refers to the maximum rate level of the DXC, and n refers to the minimum rate level
that the cross-connection matrix can handle. The bigger m is, the greater the DXC
bearing capacity is; the smaller n is, the stronger the DXC flexibility is. Zero represents
for 64 kbit/s rate of electrical line. The numbers of 1, 2, 3, and 4 respectively represents
for rate of level 1 to level 4 group in PDH system; where 4 also represents for rate of
STM-1 level in SDH system. 5 and 6 respectively represents for rate of STM-4 and
STM-16 level in SDH system. For example, DXC4/1 indicates that the maximum rate
at the access port is STM-1, while the minimum rate of cross-connect is that of the
PDH primary group signal. The values of m and n and their corresponding meanings
are listed in Table 4.1-1.
Table 4.1-1 Relations between rate and m/n
m/n 0 1 2 3 4 5 6
Rate 64 kbit/s 2 Mbit/s 8 Mbit/s 34 Mbit/s 140 Mbit/s
155 Mbit/s
622 Mbit/s 2.5 Gbit/s
DXC with small capacity can be implemented by ADM. For example, ZTE 2.5 G
equipment has the cross-connect capacity equivalent to DXC6/1.
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4.2 Logical Functional Blocks of SDH Equipment
SDH system requires that equipment of different manufacturers be compatible with
each other transversely. In order to realize interconnection, ITU-T standardizes SDH
equipment by adopting a functional reference model to break down the functions
performed by the equipment into basic standard functional blocks. The
implementations of functional blocks are independent of the physical implementation
of the equipment. Different equipments are flexible combinations of these basic
functional blocks to perform different functions. The standardization of the basic
functional blocks enables standardization of equipment and makes the specifications
universal with clear and simple descriptions.
We take the TM equipment as an example to describe its typical functional blocks and
its functions, as shown in Fig. 4.2-1.
SPI RST MST MSP MSA
PPI LPA HPT
PPI LPA LPT LPC HPT
HPC
STM A B C D E F
TTFW
140Mb/s G.703 M L
HO
I
G F
2Mb/s34Mb/s
G.703 KHPA
FGHHIJ
LO
I
HO
A
OHA OHA interface
Note: Takes 2 Mbit/s as example
SEMF MCF
SETS SETPI
P N
Q interface
F interface
External
synchronization
D4-D12 D1-D3
Fig. 4.2-1 Logical functional blocks of TM equipment
The full names of functional blocks in Fig. 4.2-1 are given below:
SPI: SDH Physical Interface TTF: Transmission Terminal Function
RST: Regenerator Section Terminal HOI: Higher-Order Interface
MST: Multiplex Section Terminal LOI: Lower-Order Interface
MSP: Multiplex Section Protection HOA: Higher-Order Assembler
MSA: Multiplex Section Adaptation HPC: Higher-Order Path Connection
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PPI: PDH Physical Interface OHA: Overhead Access Function
LPA: Lower-Order Path Adaptation LPT: Lower-Order Path Terminal
MCF: Message Communication Function LPC: Lower-Order Path Connection
HPT: Higher-Order Path Terminal HPA: Higher-Order Path Adaptation
SETS: Synchronous Equipment Timing Source
SEMF: Synchronous Equipment Management Function
SETPI: Synchronous Equipment Timing Physical Interface
Among them, SPI, TTF, RST, HOI, MST, LOI, MSP, HOA, MSA, HPC, PPI, LPA,
LPT, LPC, HPA and HPT are basic functional blocks of an equipment. Different
equipments can be formed by flexible combination of basic functional blocks, such as
REG, TM, ADM and DXC. SEMF, MCF, OHA, SETS, SETPI are auxiliary functional
blocks which helps basic functional blocks to implement the required functions of an
equipment.
Fig. 4.2-1 shows the functional block diagram of a TM. The signal flow is: the STM-N
signal on the line enters the equipment from reference point A, and are disassembled
into 140 Mbit/s PDH signals by passing through A->B->C->D->E->F->G->L->M; And
then by passing through A->B->C->D->E->F->G->H->I->J->K, it is disassembled into
2 Mbit/s or 34 Mbit/s PDH signals (here we take 2 Mbit/s signal as example). These
two routes are defined as the receiving direction of the equipment. The transmitting
direction is the reverse of these two routes where 140 Mbit/s, 2 Mbit/s, and 34 Mbit/s
PDH signals are multiplexed into the STM-N signal frames on the line.
Summary
This chapter describes the common SDH NEs, and logical functional blocks of SDH
equipment.
Exercises
1. What alarms may cause HP-RDI?
2. What is the function of TTF functional block?
3. What does DXC4/1 mean?
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4. What does DXC4/1 mean?
5. How is it possible to achieve interface compatibility among SDH nodes made by
different vendors?
6. What are the roles of REG?
7. Is it possible to have DXC features embedded in an ADM?
8. Can you add or drop any electrical tributary signal (PDH) into a REG?
9. Is it possible to add or drop any electrical tributary signal (PDH) into a DXC?
10. Why MSOH is not processed by REG?
39
5 Topology and Protection of SDH Network
Key points
Significance of network protection
Basic topologies of SDH network
Concept and categories of self-healing
Chain network protection
Self-healing ring protection
Dual node interconnection protection
Error connection and error squelch
Logical subnet protection
Topologies of complicated network
Overall hierarchy of SDH network
5.1 Significance of Network Protection
Our lives and work become more and more dependent on communication with the
development of technology. According to statistics, communication interruption for one
hour can cause loss of twenty thousand dollars for an insurance company, loss of 2.5
million dollars for an airline company, and loss of 6 million dollars for an investment
bank. Communication interruption for two days can lead to bankruptcy of a bank.
Therefore, the survivability of communication network has become one of the key
factors for modern network design and operation.
5.2 Basic SDH Network Topologies
SDH network is constructed by interconnecting SDH NEs with optical cables. The
geometrical arrangements of network node equipment (NE) and transmission lines
form the network topology. The validity (channel utilization ratio), reliability and
economical efficiency of the network are largely related to the topology.
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There are five basic network topologies: chain, star, tree, ring, and mesh, as shown in
Fig. 5.2-1.
TM DXC/ADM TM
TM TM TM
ADMDXC/ADM ADM TM
TM
TM
ADM
ADM
ADM
ADM
DXC/ADM DXC/ADM
DXC/ADM DXC/ADM
TM ADM ADM TM(a) Chain
(b) Star
(c) Tree
(d) Ring
(e) Mesh
Fig. 5.2-1 Basic network topologies
1. Chain network
The chain network topology is to connect all nodes serially, with the two ends
open. The characteristic of chain network is that it is relatively economical. It is
mostly applied in the early stages of SDH network, and mainly applied in private
networks (e.g. railway network).
2. Star network
The star network topology is to make an NE of the network as the central node
connected with the other nodes, while the other nodes are not connected with
each other. All services need to be transited through this special node. The
characteristic of star network is that it can uniformly manage other network
nodes through the central node, thus facilitates bandwidth allocation and saves
costs. However, the central node has some potential bottleneck problems for
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security protection and processing capacity. The role of the central node is
similar to the tandem office of the switching network. Star topology is mostly
applied in local networks (access network and subscriber network).
3. Tree Network
Tree network topology can be considered as a combination of the chain and star
topologies. Its central node also has some potential bottleneck problems for
security protection and processing capacity.
4. Ring Network
Actually, the ring network topology is to connect the two ends of the chain
network topology, hence any one NE node of the network is not open. Currently,
the ring network topology is very popular because of its powerful survivability,
i.e. powerful self-healing function. The ring network is generally applied in local
networks (access network and subscriber network), inter-office relay network,
etc.
5. Mesh network
The mesh network is to connect all nodes with each other. This network
topology provides multiple transmission routes between two NE nodes, which
improves network reliability and eliminates bottleneck problem and failure
problem. However, high system redundancy will surely reduce the system
validity. Its cost is high and the structure is complicated. Mesh network is mainly
applied in the toll network to improve network reliability.
Currently, the chain and ring networks are employed the most, which can form more
complicated networks through flexible combinations. This chapter mainly describes the
structure and characteristics of chain networks, and the working principles and
characteristics of major self-healing methods.
5.3 Concept and Classification of Self-healing
5.3.1 Overview
According to traffic flow, services on the transmission network can be classified into
unidirectional and bidirectional services.
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Taking the ring network as example, the difference between the unidirectional and
bidirectional services is shown in Fig. 5.3-1.
ADM
ADM
ADM
ADM
Fig. 5.3-1 Ring network
If there is a service between A and C, suppose the service route from A to C is
A→B→C, and the service route from C to A is C→B→A, then the route of A to C and
C to A is identical, This is called consistent route.
In the above example, if the route from C to A is C→D→A, then the route of A to C is
different from the route of C to A. This is called separate route.
The service of the consistent route is called bidirectional service, while the separate
route is called unidirectional service. Service directions and routes of common network
topologies are listed in Table 5.3-1.
Table 5.3-1 Service directions and routes of common network topologies
Network Type Route Service Direction
Chain network Consistent route Bidirectional
Ring
network
Bidirectional path ring Consistent route Bidirectional
Bidirectional multiplex
section (MS) ring Consistent route Bidirectional
Unidirectional path ring Separate route Unidirectional
Unidirectional MS ring Separate route Unidirectional
5.3.2 Self-healing Concept
Self-healing means that network can automatically restore its carried services from a
network fault without manual intervention within a very short period of time (ITU-T
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specifies the recovery time should be no more than 50 ms), so that subscribers will not
realize network fault.
Its basic principle is that networks should be able to find out a substitute transmission
route and re-establish the communication in case of network fault.
The substitute route can make use of the redundancy of the standby equipment or the
currently working equipment to satisfy the recovery demands of all the services or the
designated priority services. Therefore, the preconditions for network self-healing
capability include redundant route, powerful cross capability of the NE and intelligence
of the NE.
Self-healing can only recover the failed services through the standby channel, but
cannot repair or replace the failed components or lines. Thus, the troubleshooting is
still to be completed by manual intervention, e.g. broken cable needs to be connected
manually.
5.3.3 Self-healing Classification
There are multiple ways to classify self-healing network. According to network
topologies, self-healing networks can be classified as follows:
1. Chain network service protection mode
1) 1+1 path protection
2) 1+1 multiplex section (MS) protection
3) 1:1 MS protection
2. Ring network service protection mode
1) Two-fiber unidirectional path protection ring
2) Two-fiber bidirectional path protection ring
3) Two-fiber unidirectional MS protection ring
4) Two-fiber bidirectional MS protection ring
5) Four-fiber bidirectional MS protection ring
3. Inter-ring service protection mode
1) Dual Node Interconnection (DNI protection mode)
2) Multi-node interconnection changed to dual node interconnection
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5.4 Chain Network Protection
5.4.1 Overview
Chain networks has timeslot multiplexing function, that is to say, the VC with certain
sequence number of an STM-N line signal can be reused on different transmission
sections.
TM ADM ADM TM
Tributary
service
Tributary
serviceTributary
service
Tributary
service
A B C D
STM-N
Y
time
slot
X
time
slot
X
time
slot
X
time
slot
X
time
slot
X
time
slot
X
time
slot
Y
time
slot
Fig. 5.4-1 Chain network schematic
As shown in Fig. 5.4-1, there are services between A and B, B and C, C and D, A and D.
The services between A and B occupy timeslot X of optical cable section A—B (VC
with sequence number of X, for example, the 48th
VC-12 of 3rd
VC-4), and the service
between B and C occupy timeslot X of optical cable section B—C (the 48th VC-12 of
the 3rd
VC-4), and the services between C and D occupy timeslot X of optical cable
section C—D (the 48th VC-12 of the 3
rd VC-4). The above illustration is called timeslot
reuse. Since timeslot X of the optical cable has been occupied, the services between A
and D can only occupy timeslot Y of the optical cable, for example, the 49th VC-12 of
the 3rd
VC-4, or the 48th VC-12 of the 7
th VC-4.
The time slot reuse function of chain network can expand network service capacity.
Network service capacity refers to the total service amount than can be transmitted on
the network. It is related to the network topology, network self-healing mode, and
service distribution relation between the NEs.
The minimum traffic of the chain network occurs when the end stations of the chain
network act as the service host station. The service host station exchanges services with
all the other NEs, and all the other NEs do not exchange services with each other.
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Taking Fig. 5.4-1 as example, if A is the service host station, there is no service among
B, C, and D. Yet B, C, D can communicate with A . Since the maximum capacity of the
optical cable section from A to B equals to STM-N (suppose the system rate level is
STM-N), the service capacity of the network is STM-N.
The condition for chain networks to reach maximal service capacity is that service only
exists between neighboring NEs. As shown in Fig. 5.4-1, services only exist between A
and B, B and C, C and D, and no service exists between A and D. At this time, timeslot
can be reused, and the service on each optical cable section can occupy all the time
slots of the whole STM-N. Provided that the chain network has M NEs, the maximum
service capacity of the network would reach (M-1)× STM-N, where (M-1) refers to the
number of optical cable sections.
Common chain networks include:
Two-fiber chain: It cannot provide the service protection (self-healing) function.
Four-fiber chain: It generally provides 1+1 or 1:1 service protection. Two optical fibers
act as the active transmitting/receiving channel, and the other pair acts as the standby
transmitting/receiving channel.
5.4.2 Basic Chain Network Protection Types
5.4.2.1 1+1 Path Protection
1+1 path protection is based on the path. Whether to switch or not is determined by the
signal quality of each path.
1+1 path protection adopts the principle of ―Concurrent Transmission and Preferred
Receiving‖. When adding services, the path service signal will be sent simultaneously
to the working and protection channels. When dropping services, it will be received
simultaneously two path signals from the working and protection channels. In both
situations, the signal with better quality will be added or dropped.
It generally adopts PATH-AIS signal as the switching proof without APS protocol. The
switching time should be no more than 10 ms.
5.4.2.2 1+1 Multiplex Section Protection
Multiplex section protection is based on the multiplex section. Whether to switch or not
is determined by the signal quality of the multiplex section between two stations. When
the multiplex section is faulty, the service signal in the whole station will be switched
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to the protection channel for protection purpose.
In 1+1 multiplex section protection mode, the service signal simultaneously crosses
over the working and protection channels for transmission.
Under normal status, the signal of the working channel is used. When the system
detects LOS, LOF, MS-AIS, or the alarm of bit error >10E-3, it will switch to the
protection channel to receive the service signal.
5.4.2.3 1:1 Multiplex Section Protection
In 1:1 multiplex section protection mode, the service signal does not always cross over
the working and protection channels simultaneously. Thus, it can transmit the
additional low priority service in the protection channel.
Upon fault of the working channel, the protection channel will discard the additional
service, and perform cross-over and switching to protect service signals according to
the APS protocol.
When working normally, 1:1 protection is equivalent to 2+0 protection.
5.5 Self-healing Ring Protection
5.5.1 Self-healing Ring Classification
The self-healing ring can be classified according to different standards:
1. According to the service direction of the ring:
Unidirectional ring and bidirectional ring.
2. According to the number of the optical fibers between NE nodes:
Two-fiber ring (one pair of receiving/transmitting optical fibers) and four-fiber
ring (two pairs of receiving/transmitting optical fibers).
3. According to the protected service level:
Path protection ring and multiplex section protection ring.
The differences between the path protection ring and multiplex section
protection ring are as listed in Table 5.5-1.
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Table 5.5-1 Differences between the path ring and the multiplex section ring
Path Protection Ring Multiplex Section (MS) Switching Ring
Protection Unit Service protection is based on the path, that
is, protect one VC of the STM-N signal.
Whether to switch or not is determined by
the signal transmission quality of the path
of the ring.
Service protection is based on the multiplex section.
Whether to switch or not is determined by the signal
quality of the multiplex section of the ring.
Switching
Condition
PATH-AIS;
Whether to switch or not is generally
determined by the receiver when it detects
TU-AIS signal.
Switching is started by the APS protocol carried via K1
and K2 bytes. The switching conditions of the MS
protection are LOF, LOS, MS-AIS or MS-EXC alarm
signals.
Switching
Mode
Taking the STM-16 ring as example, if the
48th TU-12 of the 4th VC4 received has
TU-AIS, only this TU-12 path is switched
to the standby channel.
When the MS is faulty, the whole STM-N or 1/2
STM-N service signals of the ring will all be switched
to the standby channel.
Optical Fiber
Utilization
Ratio
The path protection ring is generally a
dedicated protection. In normal
circumstances, the protection channel is
also used to transmit the active service (1+1
service protection), so the channel
utilization ratio is low.
The MS protection ring adopts public protection. In
normal circumstances, the primary channel is used to
transfer the primary service. Adopting 1:1 protection
mode, the standby channel is used to transmit additional
service, so the channel utilization ratio is high.
Note:
As the STM-N frame has only one K1 and one K2, the multiplex section protection
switching is not to switch only one of the paths, but to switch all primary STM-N
(four-fiber ring) or 1/2 STM-N (two-fiber ring) services to the standby channel.
5.5.2 Two-fiber Unidirectional Path Protection Ring
The two-fiber unidirectional path protection ring consists of two rings made up by two
optical fibers, one is S1 which is the primary ring, and the other is P1 which is the
standby ring. The service flow directions of the two rings must be opposite. The
protection function of the path protection ring is realized through the switching
function of the NE tributary card. The tributary card concurrently transmits the
tributary service to S1 and P1. Services of the two rings are identical but the flow
directions are opposite. Normally the NE tributary card drops tributary service from the
primary ring as shown in Fig. 5.5-1.
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CA AC
CA AC
S1
P1
P1
S1
A
DC
B
Fig. 5.5-1 Two-fiber unidirectional path switching ring
If there is service between NEs A and C in the ring network, A and C will concurrently
transmit the tributary services to the S1 and P1 rings. Services are transmitted to C
through S1 optical fiber (primary ring service) via NE D, and are concurrently
transmitted to C by P1 optical fiber (standby ring service) through NE B. Under normal
conditions, tributary card at NE C chooses to receive the service from the S1 ring,
which is the primary ring. The service transmission from C to A is similar to that from
A to C, like S1: C->B->A, and P1: C->D->A.
Receiving end selects service on S1 ring: C->B->A
Even if the optical fibers between B and C are cut off, the concurrent transmission
function of the NE tributary card will not change, that is to say, the services on S1 and
P1 are still identical, as shown in Fig. 5.5-2.
CA AC
CA AC
S1
P1
P1
S1
AD
C
B
Switching
Fig. 5.5-2 Two-fiber unidirectional path switching ring (in case of fault)
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The service from A to C is concurrently transmitted to S1 and P1 optical fibers by the
tributary card of NE A, of which, service on S1 is transmitted to C via D, and service
on P1 is transmitted via B. As the optical cable between B and C is cut off, the service
on P1 cannot reach C. However, since C selects to receive the service on S1 by default,
the service from A to C is not interrupted, and the tributary card of NE C will not
perform protection switching.
The tributary card of NE C concurrently transmits the service to NE A onto S1 and P1
rings. The service from C to A on P1 is transmitted to A via NE D. The service from C
to A on S1 cannot be transmitted to A due to the broken optical cable between B and C.
NE A chooses to receive the service on S1 by default. As the service from C to A on S1
cannot reach A, the tributary card of NE A will receive the TU-AIS alarm signal from
S1 ring, and it will immediately switch to receive the service from the P1 ring, thus the
service from C to A is transmitted to A and the ring service path protection is
completed. At this time the tributary card of NE A is in the path protection switching
status, that is, switches to receive the standby ring signals.
The advantage of two-fiber unidirectional path protection ring is its fast switching
speed. Since services on the rings are concurrently transmitted and preferred received,
the path service protection mode is actually 1+1 protection. The service flow direction
is simple and clear, and the service is easy to configure and maintain.
The disadvantage of this protection mode is that the network service capacity is not
large. The service capacity of two-fiber unidirectional ring constantly equals to STM-N,
which is irrelevant to the node number of the ring and the service distribution among
the NEs.
For instance, when certain service between NE A and NE D occupies timeslot X. Since
the service is unidirectional, the service from A to D will occupy timeslot X of the
optical cable section from A to D of the primary ring (and occupy timeslot X of the
optical cable section of A to B, B to C, and C to D of the standby ring). The service
from D to A will occupy timeslot X of D to C, C to B, and B to A of the primary ring
(and occupy timeslot X of optical cable section from D to A of the standby ring.). In
other words, the service occupying timeslot X of A to D will occupy the timeslot X of
all optical cables of the ring (both the primary ring and standby ring), and other
services cannot use this timeslot (there is no function of timeslot reuse). When the
traffic between A and D equals to STM-N, other NEs cannot transmit service with each
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50
other any more, because all the timeslot resources of the STM-N are occupied already.
Therefore, the largest service capacity of the unidirectional protection ring is STM-N.
The two-fiber unidirectional path ring is mostly applied when the ring has a service
host station or service concentration station.
Note:
The service flow direction of S1 and P1 must be opposite when making up the path
ring. Otherwise the ring network has no protection function. The path protection ring
only switches one path.
5.5.3 Two-fiber Bidirectional Path Protection Ring,
The service of two-fiber bidirectional path protection ring is bidirectional (consistent
route), and the protection principle is ―Concurrent Transmission and Preferred
Receiving‖. It adopts the 1+1 service protection mode. The service capacity equals to
that of the two-fiber unidirectional path protection ring. It is shown in Fig. 5.5-3.
Fig. 5.5-3 Two-fiber bidirectional path protection ring
The path protection rings of ZTE equipment are non-revertive.
Tips:
When self healing occurs in the network, service will switch to the standby channel for
transmission. There are two modes for switching: revertive mode and non-revertive
mode.
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Revertive mode means that when the primary channel is faulty, service will switch to
the standby channel; and when the primary channel recovers, the service will switch
back to the primary channel. Generally, it is necessary to wait for a while (a few
minutes or so) so as to switch the service back from the standby channel to the primary
channel until the transmission performance of the primary channel becomes stable.
Non-revertive mode means that when the primary channel is faulty, service will switch
to the standby channel; and when the primary channel recovers, the service will not
switch back to the primary channel. The original primary channel now serves as
standby channel, and the original standby channel now serves as primary channel. Only
when the original standby channel has fault, will the service switch back to the original
primary channel.
5.5.4 Two-Fiber Bidirectional MS Protection Ring
The two-fiber bidirectional MS protection/switching ring (also known as two-fiber
bidirectional MS shared ring) adopts timeslot protection method. It uses the first half of
timeslots in each fiber (e.g. 1st ~ 8th AU4s in STM-16) as working timeslots to
transmit primary service; and the other half (e.g. 9th ~ 16th AU4s in STM-16) as
protection timeslots to transmit additional service and protect primary service. In other
words, it uses the protection timeslots of one fiber to protect the primary service of
another fiber. Therefore, there are no dedicated primary or standby fibers in a two-fiber
bidirectional MS protection ring. Instead, the first half of timeslots in each fiber are
primary channel, and the other half are standby channel, and the service flow directions
of the two fibers are opposite.
When the network is normal, the service flow directions are shown in Fig. 5.5-4.
Fig. 5.5-4 Two-fiber bidirectional MS protection ring
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When the network is normal, the primary service from A to C is transmitted using
timeslots S1 of S1/P2 fiber (for STM-16 system, primary service can only use 1st ~ 8th
AU4s); it is transmitted to C through B in S1/P2 fiber, and NE C receives the service in
timeslots S1 of S1/P2 fiber. The primary service from C to A is transmitted using
timeslots S2 of S2/P1 fiber; it is transmitted to A through B in S2/P1 fiber, and NE A
extracts the service from timeslots S2 of S2/P1 fiber.
When the optical cable between B and C is cut off, the service flow directions are
shown in Fig. 5.5-5.
Fig. 5.5-5 Two-fiber bidirectional MS protection ring (in case of fault)
When the optical cable between B and C is cut off, the primary service from A to C is
transmitted to B in S1/P2 fiber; And NE B performs switching (the NE adjacent to the
fault location performs switching), which switches all the service in timeslots S1 of
S1/P2 fiber to timeslots P1 of S2/P1 fiber (e.g. in STM-16 system, it switches all the
service in 1st ~ 8th AU4s of S1/P2 fiber to 9th ~ 16th AU4s of S2/P1 fiber). And then
the primary service is transmitted to NE C through NE A and D via S2/P1 fiber. NE C
(fault end point) will also perform switching, which switches the primary service from
A to C in timeslots P1 of S2/P1 fiber back to the timeslots S1 of S1/P2 fiber; then NE C
will extract service from timeslots S1 and completes receiving of primary service from
A to C.
The primary service from C to A in timeslots S2 is first switched by NE C to timeslots
P2 of S1/P2 fiber; and then it is transmitted to NE B through D and A via S1/P2 fiber;
NE B will perform switching, which switches the primary service from C to A in
timeslots P2 of S1/P2 fiber back to the timeslots S2 of S2/P1 fiber; then NE A will
extract service from timeslots S2 and completes receiving of primary service from C to
A.
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Through the above method, the self-healing of ring network is completed.
Timeslots P1 and P2 can be used to transmit additional service under normal condition.
In case of fault, the additional service is interrupted, and timeslots P1 and P2 are used
as protection timeslots to transmit primary services.
Compared with path protection ring, MS protection ring needs to use APS protocol,
which costs more protection switching time. As per ITU-T specifications, the
protection switching time should be less than 50 ms.
The service capacity (i.e. the maximum traffic amount) of the two-fiber bidirectional
MS protection ring equals to (K/2)×STM-N, where K refers to the number of NEs
(K≤16). This is the maximum traffic amount when there is service only between
adjacent nodes. Under this circumstance, every optical cable section is used privately
by the two adjacent NEs. For instance, cable section A-D only transmits the
bidirectional service between A and D, and cable section D-C only transmits the
bidirectional service between D and C. The service between two adjacent NEs does not
occupy timeslot resource of other optical cable section, so that every cable section can
transmit maximum traffic of 1/2×STM-N (timeslot can be reused). And the number of
optical cable section equals to that of the nodes in the ring network, therefore, the
service capacity under this circumstance reaches the maximum traffic amount: (K/2) ×
STM-N.
The MS protection mode of ZTE equipment is revertive, with the default protection
switching recovery time as eight minutes.
5.5.5 Four-fiber Bidirectional MS Protection Ring
The four-fiber bidirectional MS protection ring consists of four fibers: S1, P1, S2, and
P2. Among which, S1 and S2 are primary fibers which transmit the primary service; P1
and P2 are standby fibers which transmit protected service. In other words, P1 and P2
protect the primary service of S1 and S2 in case of primary fault. Please pay attention
to the service flow directions of these four fibers: the service flow directions of S1 and
S2 are opposite (consistent route, bidirectional ring); the service flow directions of S1
and P1 are opposite, and those of S2 and P2 are also opposite; the service flow
direction of S1 and P2 are the same, and those of S2 and P1 are also the same.
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Note:
Each node of the four-fiber ring is configured as double ADM system. Because one
ADM only has two line ports of E/W (east/west), but the NE node of four-fiber ring has
two line ports each for E/W direction. Therefore, the NE node needs to be configured
as double ADM system.
As shown in Fig. 5.5-6, when the ring network works normally, the primary service
from A to D is transmitted to D through B via S1 fiber; and the primary service from D
to A is transmitted to A through B via S2 fiber (bidirectional service). NE A and D
exchanges primary services by receiving the service in the primary fibers.
Other nodes and
corss-sections
Node A Node B
Node CNode D
Add/drop
service
Add/drop
service
W
P
W
P
W
P
W
P
WW
PP
P
P
P
P
W
W
W
W
S1
S2P2
P1
Fig. 5.5-6 Normally the service between A and D passes B and C
If faults occur to optical cables between B and C, the cross-section switching or
cross-ring switching will happen to the service in the ring. The trigger conditions and
switching procedures are as follows:
1. Cross-section switching
For the four-fiber ring, if the fault only affects working channel, service can be
recovered by switching to the cross-section protection channel.
As shown in Fig. 5.5-7, when the working fiber S1 between B and C is broken
and the other three fibers wok normally, the service from A to D will be
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transmitted to B via S1 fiber, then B will perform cross-section switching to
switch service from S1 to P2; when the service reaches C, C will perform
cross-section switching to switch service from P2 back to S1; then the service
will be transmitted via S1 to D and dropped at D. The service from D to A will
also be switched cross-section at node C and B. Therefore, the service routes are
the same before/after cross-section switching, which are: A→B→C→D and
D→C→B→A.
Other nodes and
cross-sections
Node A Node B
NodeCNode D
Add/drop
service
Add/drop
service
Cross-
section
BrCross-
section
Sw
Cross-
section
Br
Cross-
section
Sw
W
P
W
P
W
P
W
P
X
WW
PP
P
P
P
P
W
W
W
W
Fig. 5.5-7 Route example for the cross-section switching in case of fault
2. Cross-ring switching
For the four-fiber ring, if the fault affects both working channel and protection
channel, the service can be recovered by cross-ring switching.
As shown in Fig. 5.5-8, when S1 and P2 fibers between B and C are both broken,
the service between A and D is transmitted to B via S1 fiber, and B will perform
cross-ring switching to switch service from S1 to P1; then the service is
transmitted back to A via P1, and is transmitted on to D and C; C performs
cross-ring switching again to switch the service from P1 back to S1, and the
service is transmitted via S1 till it reaches D where it is dropped. Therefore, the
routes of the bi-directional service between A and D change after cross-ring
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switching, which are respectively A→B→A→D→C→D and
D→C→D→A→B→A.
Other nodes and
cross-sections
Node A Node B
Node CNode D
Add/drop
service
Add/drop
service
Cross-
ring Br
Cross-
ring Sw
Cross-
ring Br
Cross-
ring Sw
W
P
W
P
W
P
W
P
X
WP
P
P
P
P
W
W
W
W
X
Fig. 5.5-8 Route example of the cross-ring switching in case of fault
The cross-section switching has higher priority than the cross-ring switching. If one
fiber section has requests for both of them, the system will respond to the cross-section
switching, because the service will reach the designated end via longer path after
cross-ring switching, which will seize protection path of other service. Therefore,
cross-section switching request is prioritized. Only when the service cannot be
recovered using cross-section switching, will cross-ring switching is used.
The service capacity, that is the maximum traffic amount of the four-fiber bidirectional
MS protection ring equals to K×STM-N, where K is the number of NEs (K≤16).
5.5.6 Comparison of Common Self-healing Rings
Among the above five protection modes of self-healing ring, three modes are
commonly used for networking. Table 5.5-2 compares these three protection modes
and lists their protection methods and characteristics.
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Table 5.5-2 Comparisons of three commonly used modes for self-healing ring
Item Two-fiber Unidirectional Path
Ring
Two-fiber Bidirectional MS
Ring
Four-fiber Bidirectional MS
Ring
Node number K K K
Line rate STM-N STM-N STM-N
Ring transmission
capacity
STM-N K/2×STM-N K×STM-N
APS protocol No Yes Yes
Switching time <30 ms 50 ms ~200 ms 50 ms ~200 ms
Node cost Low Medium High
System complexity Simple Complicated Complicated
Major application
occasion
Access network, relay network
(centralized service)
Relay network, toll network
(distributed service)
Relay network, toll network
(distributed service)
5.6 Dual Node Interconnection (DNI) Protection
5.6.1 Terminologies
Drop-and-Continue: It refers to a function of the ring node, where the service signal
will be dropped from the working channel (Drop) of the ring and also continue to be
transmitted forward along the ring (Continue).
Dual Hubbed: The dual hubbed service can be led to the two central offices or any one
of the offices (or similar sites). Once one of the two junction points is faulty, the dual
hubbed service can be recovered.
Dual Node Interconnection: It refers to the structure between two rings. Two nodes of
one ring are interconnected with two nodes of the other ring.
Hold-off Time: It refers to the time interval from claiming the signal failure or signal
deterioration to starting the protection switching algorithm.
Path Selector: It is used in the SNCP architecture to select the working channel from
one side of the node or from the other side of the node to drop the service according to
the channel level criteria.
Primary Node: It is the node used in the MS-Ring interconnection architecture to
provide service signal selection and D&C function for certain tributary. Different
tributaries can have different primary nodes.
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Propagation of Switching: One switching results in another switching. From the point
of maintenance, the propagation of switching is always unwelcome.
Ring Interconnection: It refers to an architecture between two rings, where one or
several nodes of each ring are interconnected with the other ring.
Ring Interworking: It refers to a network topology, where two rings are
interconnected via two nodes on each ring. This topological operation mode can
prevent any service loss on the ring when any one of the nodes is faulty, as shown in
Fig. 5.6-1.
Secondary Circuit: It is the replaceable route for the service to be transmitted from
one ring to another ring in the MS shared protection ring interworking architecture. It is
used when the service circuit is interrupted.
Secondary Node: It is the node that can provide replaceable interworking route for the
tributary in the MS shared protection ring interworking architecture.
Service Circuit: It is the original route preferentially selected for the service to be
transmitted from one ring to another ring in the MS shared protection ring interworking
architecture.
Service Selector: It is used for node function of ring interworking in the MS shared
protection ring architecture. It determines to select the service from one side of the
node or another side of the node according to some criteria.
Single Node Interconnection: It refers to an architecture between two rings. One node
of each ring is interconnected with one node of another ring.
Termination Node: It refers to the node where one tributary enters into or leave the
ring. (it can not be the primary node or secondary node.)
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5.6.2 DNI Principle
A
Fig. 5.6-1 Ring interworking
Dual Node Interconnection (DNI) is a structure between two rings where each ring
provides two nodes to interconnect with the other ring. It provides protection for the
services of one ring crossover another ring by allocating the two interconnections
between the two rings. One special mode of dual node interconnection is called ring
interworking. The ring interworking is a network topology where the two rings are
interconnected via two nodes on each ring. The topological operation mode can prevent
any service loss on the ring if any one of the nodes is faulty. As shown in Fig. 5.6-1,
one tributary can be added and dropped at node A of the upper ring, or at node Z of the
lower ring. The meanings of the characters are described as follows:
· TA = the transmitted signal at node A.
· RA = the received signal at node A.
· TI1 = the transmitted signal of one node of the two interconnection nodes.
· RI1 = the received signal of one node of the two interconnection nodes.
· TI2 = the transmitted signal of another node of the two interconnection nodes.
· RI2 = the received signal of another node of the two interconnection nodes.
In the ring interworking, the interface relationship of the two sets of interconnection
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nodes are described as follows:
· RI1=RI2=TA
· TI1=T I2
· RA=T I1 or T I2.
In other words, the signal from node A to node Z is transmitted to the two
interconnection nodes. Similarly, the signal sent back from node Z to node A is also
transmitted to the two interconnection nodes. Finally, only one of the two mutually
repetitive signals at the two interconnection nodes is selected by node Z or A.
5.6.3 Application Instance
Fig. 5.6-2 shows a DNI network consisting of two MS rings and a DNI network
consisting of one MS ring and one path ring.
1. Two MS rings
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SS
SS
P S
SS
P S
MS shared protection ring
P Primary node
S Secondary node
Service Selector
MS shared protection ring
Fig. 5.6-2 Interworking of two MS shared protection rings
2. MS ring and path ring
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P S
SS
PS
PS
PS
PS
SS
MS shared protection
ring
P Primary node
S Secondary node
Service selector
Path selector
SNC protection ring
Fig. 5.6-3 Interworking of MS shared protection ring and path ring
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5.7 Error Connection and Error Squelch
5.7.1 Error Connection
As shown in Fig. 5.7-1, there are two services in the ring: A<-> C and A<->F.
A<->C: A and C both use the 1st AU 1st TU12 of the optical board.
A<->F: A and F both use 1st AA 1st TU12 of the optical board (A uses another optical
board).
A
Fig. 5.7-1 Error connection example: 2.5G MS Ring
When node A is faulty, F will switch to the protection timeslot 9th AU 1st TU12 of the
protection optical board to receive and transmit the services of A<->F. As C is not the
end of the disconnected optical fiber, the service of A<->C is switched to the protection
time slot 9th AU 1st TU12 at node B. Now C is connected with F and becomes an error
connection.
5.7.2 Error Squelch of Error Connection
The principle for handling error connection is to disconnect it.
The methods for handling error connection: For one service, if it has been detected that
the target point does not exist already, insert AIS in the protection timeslot of the
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protection optical board. Then for node F and B (the two ends of the disconnected
optical fiber), insert AIS in the protection timeslot 9th AU1 1st TU12. Although error
connection occurs, the service that is wrongly connected will not go through, so as to
squelch the error connection.
5.8 Logical Subnet Protection
5.8.1 Overview
Logical subnet is a method of splitting network based on the logical topology of the
network. It is a subnet developed after splitting the channel layer and section layer
horizontally based on the service and function features of the circuit-layer network.
In view of the disadvantage of splitting the network based on the physical topology of
the network, we regard the SDH transmission network from a logical viewpoint, and
split the network based on the logical topology of the network. This way, a
large-capacity complex physical network is split into several logical subnets with
independent functions and services, so that it is far easier for the system to manage and
protect the logical subnets.
The SDH network in the intersection ring structure can be simplified into: Use the
over-ring service and non-ring service as a basis of splitting the logical subnet, and
split the physical ring logically into different logical subnets according to the service or
protection mode. Now we use the intersection ring shown in Fig 5.8-1 as an example to
describe the logical splitting of intersection ring. Fig 5.8-1 splits the intersection ring
into two independent MS rings. The two logical MS rings can apply the bidirectional
shared protection mode to configure and protect the services. This highly improves the
resource utilization.
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+
Fig 5.8-1 Split the Intersection Ring into Independent Logical Rings
5.8.2 Basic Principles
In the actual networking of SDH, due to limitation of fiber distribution, multiple
transmission network may share one fiber section, and the MS ring protection mode is
configured for each transmission network. Therefore, this shared fiber section should
be split for each transmission network, so as to build multiple logical subnets logically,
and provide the MS protection for multiple logical subnets.
The logical subnet protection is only limited to the expansion of the MS-Spring mode.
Logical MS-Spring means changing the rule of ―splitting the line bandwidth into
working channel and protection channel evenly (measured in AUG)‖ to this rule:
Define some AUGs as working channels as specifically required, and define other
AUGs as protection channels, still measured in AUG, but the splitting rule is: It can be
split flexibly only if the number of AUGs of the protection channels of any logical NE
in the logical subnet is not less than the number of AUGs of the working channel of
any logical NE.
Specifically, the logical subnet protection function is to carry multiple logical optical
ports in one physical optical port. Each logical optical port can combine with other
logical optical ports or SDH devices to form MS ring network, i.e., logical subnet. The
logical subnet implements the corresponding MS protection according to the
corresponding MS protection excitation, and tries to provide protection for the services
affected by line faults.
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A maximum of 4 logical subnets can be carried on a fiber section.
5.8.3 Categorization
From perspective of application mode, the logical subnet can be divided into three
types:
· Dedicated mode
· Shared mode
· Extra service mode
After combining with the MS ring network type, the logical subnet of each mode can
be implemented in the four following ways:
· 2-fiber ring + 2-fiber ring
· 4-fiber ring + 4-fiber ring
· 2-fiber ring + 4-fiber ring
· 4-fiber ring + 2-fiber ring
5.8.4 Application Instance
5.8.4.1 STM-16 Rate Two-fiber Ring and STM-4 Rate Two-fiber Ring
Network consisting of one high-rate two-fiber ring and one low-rate two-fiber ring is
shown in Fig. 5.8-5. NE A, B, C, D and NE E, F, C, D respectively compose two logic
multiplex section rings. The two rings share the optical fiber cross-section connecting
C with D, with the bandwidth of STM-16.
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A
D
E
B
C
F
Logic MS ring 1
(STM-16)
Logic MS ring 2
(STM-4)
2#2#
2#
2#
7#7#
7#
7#
1#
1#
1#
8#
8#
8#
Fig. 5.8-5 Combination of high-rate two-fiber ring and low-rate two-fiber ring
The allocation for the bandwidth of the shared cross-section is shown in Fig. 5.8-6.
From left to right in the figure, they respectively represents for: the bandwidth
allocation when configuring only one MS ring, the dedicated bandwidth allocation
when configuring two logic MS rings, the optimized dedicated bandwidth allocation
when configuring two logic MS rings, and the shared bandwidth allocation when
configuring two logic MS rings.
W
P
W1
P1
W2
P2
W1
W2
P1
P2
W1
W2
P0
Fig. 5.8-6 Allocation modes for the shared bandwidth when high-rate two-fiber ring combines with
low-rate two-fiber ring
When adopting optimized dedicated allocation to allocate the shared cross-section
bandwidth, the detailed configurations of the two logic subnets are shown in Fig. 5.8-7.
In the logic MS ring 1, the 1st~8th AUs of the optical board in slot #7 between A and B
are the working AUs, while the 9th~16th AUs are the protection AUs. The 1st~8th AUs
of the optical board in slot #2 between A and D are the working AUs, while the 9~16
AUs are the protection AUs. The marked shadow area refers to the detailed bandwidth
allocation mode of the physical shared cross-section.
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Fig. 5.8-7 Protection configuration of each NE in the two logic MS rings when the high-rate two-fiber ring combines with
the low-rate two-fiber ring
5.8.4.2 STM-16 Rate Four-fiber Ring and STM-4 Rate Four-fiber Ring
Network consisting of one high-rate four-fiber ring and one low-rate four-fiber ring is
shown in Fig. 5.8-8. NE A, B, C, D and NE E, F, C, D respectively compose two logic
multiplex section rings. The two rings share the optical fiber cross-section connecting
C with D, with the bandwidth of (STM-16)×2.
A
D
E
B
C
F
Logic MS ring 1
(STM-16)
Logic MS ring 2
(STM-4)
2#2#
2#
2#
7#7#
7#
7#
1#
1#
1#
11#8#
8#
8#
11#16#
16#
16#
16# 11#
11#
10#
10#
10#
17#
17#
17#
Fig. 5.8-8 Combination of high-rate four-fiber ring and low-rate four-fiber ring
The allocations for the bandwidth of the shared cross-section are shown in Fig. 5.8-9.
From left to right, they respectively represents for: the bandwidth allocation mode
when configuring only one MS ring, the dedicated bandwidth allocation when
configuring two logic MS rings, the optimized dedicated bandwidth allocation when
configuring two logic MS rings, and the shared bandwidth allocation when configuring
two logic MS rings.
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69
W
P
W1
W2
P1
P2
W1
W2
P2
P1
W1
W2
P0
Fig. 5.8-9 Allocation modes for the shared bandwidth when high-rate four-fiber ring combines with
low-rate four-fiber ring
When adopting the optimized dedicated allocation to allocate the shared cross-section
bandwidth, the detailed configurations of the two logic subnets are shown in Fig.
5.8-10. In the logic MS ring 1, the 1st~16th AUs of the optical board in slot #7 between
A and B are the working AUs, while the 1st~16th AUs of the optical board in slot #16
are the protection AUs. The 1st~16th AUs of the optical board in slot #2 between A and
D are the working AUs, while the 1st~16th AUs of the optical board in slot #11 are the
protection AUs. The marked shadow area refers to the detailed bandwidth allocation
mode of the physical shared cross-section.
Fig. 5.8-10 Protection configuration of each NE in the two logic MS rings when the high-rate four-fiber ring combines with
the low-rate four-fiber ring
5.8.4.3 STM-16 Rate Four-fiber Ring and STM-4 Rate Two-fiber Ring
Network consisting of one high-rate four-fiber ring and one low-rate two-fiber ring is
shown in Fig. 5.8-11. NE A, B, C, D and NE E, F, C, D respectively compose two logic
multiplex section rings. The two rings share the optical fiber cross-section connecting
C with D, with the bandwidth of (STM-16)×2.
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A
D
E
B
C
F
Logic MS ring 1
(STM-16)
Logic MS ring 2
(STM-4)
2#2#
2#
2#
7#7#
7#
7#
1#
1#
1#
11#8#
8#
8#
11#16#
16#
16#
16# 11#
11#
Fig. 5.8-11 Combination of high-rate four-fiber ring and low-rate two-fiber ring
The bandwidth allocation of the shared cross-section is shown in Fig. 5.8-12. From left
to right, they respectively represents for: the bandwidth allocation when configuring
only one MS ring, the ordinary dedicated bandwidth allocation when configuring two
logic MS rings, the optimized dedicated bandwidth allocation when configuring two
logic MS rings.
W
P
W1
P1
W1
W2
P2
P1
W2
P2
Fig. 5.8-12 Allocation modes for the shared bandwidth when high-rate four-fiber ring combines
with low-rate four-fiber ring
When adopting optimized dedicated allocation to allocate the shared cross-section
bandwidth, the detailed configurations of the two logic subnets are shown in Fig.
5.8-13. In the logic MS ring 1, the 1st~16th AUs of the optical board in slot #7 between
A and B are the working AUs, while the 1st~16th AUs of the optical board in slot #16
are the protection AUs. The 1st~16th AUs of the optical board in slot #2 between A and
D are the working AUs, while the 1st~16th AUs of the optical board in slot #11 are the
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71
protection AUs. The marked shadow area refers to the detailed bandwidth allocation
mode of the physical shared cross-section.
Fig. 5.8-13 Protection configuration of each NE in the two logic MS rings when the high-rate four-fiber ring combines with
the low-rate two-fiber ring
5.8.4.4 STM-16 Rate Two-fiber Ring and STM-4 Rate Four-fiber Ring
Network consisting of one high-rate two-fiber ring and one low-rate four-fiber ring is
shown in Fig. 5.8-14. NE A, B, C, D and NE E, F, C, D respectively compose two logic
multiplex section rings. The two rings share the optical fiber cross-section connecting
C with D, with the bandwidth of STM-16.
A
D
E
B
C
F
Logic MS ring 1
(STM-16)
Logic MS ring 2
(STM-4)
2#2#
2#
2#
7#7#
7#
7#
1#
1#
1#
11#8#
8#
8#
16#10#
10#
10#
17#
17#
17#
Fig. 5.8-14 Combination of high-rate two-fiber ring and low-rate four-fiber ring
The bandwidth allocation of the shared cross-section is shown in Fig. 5.8-15. From left
to right, they respectively represents for: the bandwidth allocation when configuring
only one MS ring, the dedicated bandwidth allocation when configuring two logic MS
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72
rings, the optimized dedicated bandwidth allocation when configuring two logic MS
rings, and the shared bandwidth allocation when configuring two logic MS rings.
W
P
W1
P1
W2
P2
W1
W2
P2
P1
W1
W2
P0
Fig. 5.8-15 Allocation modes for the shared bandwidth when high-rate two-fiber ring combines with
low-rate two-fiber ring
When adopting optimized dedicated allocation to allocate the shared cross-section
bandwidth, the detailed configurations of the two logic subnets are shown in Fig.
5.8-16. In the logic MS ring 1, the 1st~8th AUs of the optical board in slot #7 between
A and B are the working AUs, while the 9th~16th AUs are the protection AUs. The
1st~8th AUs of the optical board in slot #2 between A and D are the working AUs,
while the 9th~16th AUs are the protection AUs. The marked shadow area refers to the
detailed bandwidth allocation mode of the physical shared cross-section.
Fig. 5.8-16 Protection configuration of each NE in the two logic MS rings when the high-rate two-fiber ring combines with
the low-rate four-fiber ring
5.9 Topology and Features of Complicated Network
The combinations of chains and rings can compose some more complicated network
topologies. This section describes several topologies commonly used for networking.
5.9.1 T Network
T network is actually a tree network, as shown in Fig. 5.9-1.
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TM
TM
TMADM ADM
ADM
ADM
STM-16
STM-4
A
Fig. 5.9-1 T network topology
Suppose it is an STM-16 system on the trunk, and STM-4 system on the tributary lines.
The function of T network is to add/drop tributary STM-4 services to/from the trunk
STM-16 system via NE A. Tributary lines are connected to the tributaries of NE A.
These tributary services are regarded as low-speed tributary signals of NE A, and are
added/dropped via NE A.
5.9.2 Ring-chain Network
The network topology of ring-chain network is shown in Fig. 5.9-2.
TMADM
ADM
ADM ADM
ADM
STM-4
STM-16
C
A B D
Fig. 5.9-2 Ring-chain network topology
The ring-chain network consists of basic topologies of ring and chain networks, which
are connected together via NE A, as shown in Fig. 5.9-2. The STM-4 service of the
chain is the low-speed tributary service of NE A, and is added/dropped via NE A. The
STM-4 service on the chain has no protection, while the service on the ring is protected.
For example, suppose there is service between NE C and NE D in the figure. If the
optical cable between A and B is broken, the service on the chain will be interrupted; if
the optical cable between A and C is broken, the service between C and D will not be
interrupted because of the ring protection function.
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5.9.3 Tributary Cross-Over of Ring Subnets
The network topology is shown in Fig. 5.9-3.
ADM
ADMADM
ADM
ADM
ADM
ADM
ADM
STM-1/4STM-16STM-16
AB
Fig. 5.9-3 Network topology of tributary cross-over of ring subnets
Two STM-16 rings are connected together through the tributary path between A and B.
Any NE in the two rings can exchange service with each other through the tributary
between A and B, there are multiple routes for choices, and the system redundancy is
high. Sine all services between the two rings must be transmitted through the
low-speed tributary between A and B, the speed bottleneck of low-speed tributary and
security problem exist.
5.9.4 Tangent Rings
The network topology is shown in Fig. 5.9-4.
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75
ADM
ADM ADM
ADM
ADM
ADM
ADM
ADM 2500
STM-1 STM-1
STM-1
ADM
STM-16
STM-16
STM-16
A
Ring Ⅱ
Ring III
155
622
STM-4
STM-4
STM-4
DXC/ADM
RingⅠ
Fig. 5.9-4 Tangent rings topology
The three rings are tangent with each other through NE A. A DXC or an ADM (ring II
and ring III are both low-speed tributaries for NE A) can be used. This networking
mode can enable NEs of the rings to exchange services freely, with greater service
dispatching ability than a ring network with tributary cross-over. It provides services
with more routes for choice, and the system redundancy is higher. However, this
networking mode has problems of security and protection for the central node (NE A).
5.9.5 Intersected Rings
The tangent rings can be extended to be intersected rings in order to provide the backup
central (important) node with more routes for choice, and improves system reliability
and redundancy. The intersected rings topology is shown in Fig. 5.9-5.
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ADM
ADM
ADM
ADM
ADM
ADM
STM-16
STM-16
STM-16
622
STM-4
STM-4
STM-4
DXC/ADM
DXC/ADM
Fig. 5.9-5 Intersected rings topology
5.9.6 Hinge Network
The hinge network topology is shown in Fig. 5.9-6. NE A is the hinge node, and chains
or rings of STM-1 or STM-4 can connect to the tributary side of NE A. Through the
cross-connect function of NE A, the tributary service can be added/dropped to/from the
trunk, and tributaries can exchange services between each other; thus avoiding adding
direct route and equipment between tributaries, and avoiding occupying resource of the
trunk network.
ADM
ADM ADM
ADM
ADM
ADM
ADMADM
ADM
ADM
STM-16STM-16
STM-1
STM-1
STM-16
STM-4
STM-4
DXC/ADM
A
STM¡ ª4/1
Fig. 5.9-6 Hinge network topology
5.10 Overall Architecture of SDH Network
SDH has great advantages compared with PDH. However these advantages can only be
exhibited when constructing SDH network.
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77
In traditional networking, improving the utilization ratio of transmission equipment is
the most important issue. To improve utilization ratio and security of lines, many direct
routes are added between the nodes, which results in too complicated network structure.
With the development of modern communication, the most important task is to
simplify network structure, build a strong OAM functions, reduce transmission cost,
and support the development of new services.
The SDH network structure of China includes four layers, as shown in Fig. 5.10-1.
DXC16/16
DXC16/16
DXC64/64
DXC64/64
DXC4/4
ADM
DXC16/16
DXC4/4
DXC4/1 ADM ADM DXC4/1 ADM ADM
ADM
ADM ADM
OLT OLT
ADM
OLT
OLT OLT
ADM
OLT
OLT
DXC4/1
Second-level trunk network
User access network
Ring
Star
First-level trunk network
STM64 or STM16
ADM
OLT
Relay network
Fig. 5.10-1 SDH network structure of China
The top layer is the long-distance first-level trunk network. Major capital cities of
provinces and the tandem cities with large traffic are equipped with DXC4/4, and
high-speed optical fiber lines of STM-4/STM-16 connect these cities, giving the
national mesh backbone network with bigger capacity and high reliability, assisted by a
small amount of linear networks. Since DXC4/4 has 140 Mbit/s interfaces for PDH, the
original PDH 140 Mbit/s and 565 Mbit/s systems can also be accommodated into the
long-distance first-level trunk network managed uniformly by DXC4/4.
The second layer is the second-level trunk network. Its major tandem nodes are
equipped with DXC4/4 or DXC4/1, STM-1/STM-4 transmission chains connect these
nodes, thus composing province internal mesh network or ring backbone network,
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assisted by a small amount of linear networks. Since DXC4/1 has 2 Mbit/s, 34 Mbit/s,
and 140 Mbit/s interfaces, the original PDH system can also be accommodated into the
second-level trunk network managed uniformly, with flexible circuit dispatching
ability.
The third layer is the relay network (i.e. the transmission part between toll terminating
office and local office, or between local offices), which can be divided into several
rings according to different regions. ADMs compose self-healing ring of
STM-1/STM-4, or dual-node ring in route backup mode. These rings have strong
survivability and service dispatching function. The ring network mainly adopts MS
protection switching ring, whether to use four-fiber or two-fiber is determined by the
traffic amount and cost. The rings communicate with others through DXC4/1, to
dispatch services and implement other management functions. The rings can serve as
the gateway or interface between toll network and relay network, and between relay
network and user network. They can also serve as the gateway between PDH and SDH.
The bottom layer is the user access network. It’s located at the boundary of the whole
network, there are few service capacity requirements for it, and most of its traffic is
concentrated on one node (terminating office), therefore path switching ring and star
network are both good applications to this layer. The equipment needed for this layer
include ADM and OLC (Optical User Loop Carrier system). The rate is STM-1/STM-4.
The interface can be: STM-1 optical/electrical interface; 2 Mbit/s, 34 Mbit/s, or 140
Mbit/s interface; ordinary telephone user interface; small-scale switch interface; 2B+D
or 30B+D interface; and metropolitan network interface.
The user access network is the largest and most complicated part of SDH network,
which occupies 50% of the whole communication network investment. Applying
optical fibers to the user access network is a step-by-step process. FTTC (Fiber To The
Curb), FTTB (Fiber To The Building) and FTTH (Fiber To The Home) are different
stages of this process. China currently needs to consider adopting the integrated
SDH/CATV network when popularizing optical fiber user access network, which is to
provide not only telecommunication services, but also CATV service.
Summary
This chapter describes the basic topologies, self-healing principle, and networking and
features of complicated networks of SDH network. They key points to master are:
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79
working principles, application scopes, and service capacities of the unidirectional path
protection ring and two-fiber bidirectional MS protection ring.
Exercises
1. The switching condition of unidirectional path protection ring is alarm.
2. The switching condition of two-fiber bidirectional MS protection ring
are: , , alarms.
3. Relate two fiber unidirectional path protection ring and two fiber bidirectional
path protection ring.
4. Which one is faster; path protection or multiplex section protection?
5. What is the difference between Cross-ring Switching and Cross-connection
Switching?
6. How returnable mode is different from non-returnable mode?
7. How DNI works?
8. What are the advantages of logical subnet protection?
9. Why it is not possible to provide protection over a two fiber chain network?
10. What is the maximum and minimum number of nodes that can be protected by
multiplex section protection method in a ring topology?
.
81
6 Timing and Synchronization
Key points
Synchronization methods
Working mode of the clock in master/slave synchronization network
Synchronization methods of SDH network
Protection switching principles of the clock in SDH network
6.1 Synchronization Modes
Network synchronization is one of the major problems to be solved in digital network,
because we need to ensure that the transmitter put the pulse at a certain time position
(timeslot) when transmitting the digital pulse signal, and the receiver should be able to
read this pulse from the certain timeslot, so that the transmitter and receiver can
communicate with each other normally. The above function is implemented by
synchronizing clocks of the transmitter and receiver. Therefore, the purpose of network
synchronization is to restrict the frequency and phase of clock at each node within the
pre-defined allowable range to avoid transmission performance degradation
(impairment) caused by inaccurate synchronization of transmitter/receiver in the digital
transmission system.
There are different modes for digital network synchronization, among which two
modes are commonly used: pseudo synchronization and master/slave synchronization.
6.1.1 Pseudo Synchronization
It means that the clock of each digital exchange in the digital switching network is
independent from each other and without any relation, while the clock of each digital
exchange is of extreme precision and stability. Usually the cesium atom clock is used
due to its high precision. Since the clock has high precision, although the clock of each
exchange in the network is not completely the same (in frequency and in phase), the
error is very small and the network is close to synchronization, so it is called
pseudo-synchronization.
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This method is generally applied in international digital network, that is, the digital
network between one country and another country. For example, there is one cesium
atom clock each at China international exchange and U.S.A international exchange,
and these two clocks adopt pseudo-synchronization method.
6.1.2 Master/Slave Synchronization
Master/slave synchronization is to set up a clock master office equipped with
high-precision clock in the network, and each exchange within the network is
controlled by the master office (i.e. track the clock of the master office and use it as
timing reference). The upper-level exchanges control the lower-level exchanges, until
reaching the end network element in the network – the terminal exchange.
The master/slave synchronization method is generally used for the internal digital
network of a country or region. Its characteristics are: there is only one master office
clock in the country or region, other network elements within the network all rely on
this master office clock as their timing reference.
The principles of master/slave synchronization and pseudo-synchronization are shown
in Fig. 6.1-1.
To overseas international
exchang
International
exchange
International
exchange
Local
exchange
National
exchange
National
exchange
National
exchange
National
exchange
Local
exchange
Local tandem
exchange
Local tandem
exchange
Local tandem
exchange
Local tandem
exchange
Terminal
exchange
Terminal
exchange
Terminal
exchange
Terminal
exchange
MS MS MS MS
MS MS MS MSMS
MS MSMS MS MS
MS: Master/Slave
synchronization
Pseudo-
synchronization
.
.
.
.
.
.
Fig. 6.1-1 Master/slave synchronization and pseudo-synchronization
In order to enhance the reliability of the master/slave synchronization system, a vice
clock may be set up in the network by adopting the hierarchical master/slave control
mode. Both clocks adopt the cesium clock. In normal cases, the primary clock works as
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83
the timing reference of the network, and the vice clock also relies on the primary clock
for timing reference. When the primary clock encounters fault, the vice clock will
provide timing reference for the whole network. After the primary clock recovers, it
will switch back to the primary clock to provide the network timing reference.
The synchronization mode adopted by China is the hierarchical master/slave
synchronization mode of which the primary clock is in Beijing and the vice clock is in
Wuhan. When adopting the master/slave synchronization, the timing signals of the
upper level NEs are transmitted to the lower level NEs via a certain route – via the
synchronous link or by affixing to the line signals. The NE of the upper level extracts
the clock signal, tracks and locks the clock using its own phase-locked oscillators, and
uses the clock as reference to generate local clock signal for itself. Meanwhile it
transmits the clock via the synchronous link or the transmission line (i.e. affixing the
clock information to the line signals for transmission) to the lower level NEs for clock
tracking and locking. If one NE fails to receive the reference clock transmitted from the
upper level NE, it can use its external timing reference or start its internal crystal
oscillator to provide the local clock used by itself, and it will transmit the local clock
signal to its lower level NEs.
Besides the pseudo synchronization and master/slave synchronization modes, there are
other synchronization modes in digital network, including mutual synchronization,
external reference implantation.
The external reference implantation mode backs up the clock at important nodes in the
network to avoid situation when the primary clock reference at the important node is
lost, and the quality of its internal clock is not good enough, so that the normal work of
a wide range of NEs will be influenced. The external reference implantation mode
adopts the GPS (Global Position System), and sets up GPS receivers at important NE
offices to provide high-precision timing, and to form the Local Primary Reference
clock (LPR). Once the primary clock reference is lost, other lower level NEs in the
region still adopts the master/slave synchronization mode to track the reference clock
offered by the GPS.
6.2 Working Modes of Sub-Clock in Master/Slave Synchronous Network
In the master/slave synchronous digital network, the clocks of sub-stations (lower level
stations) have three types of working modes.
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6.2.1 Normal Working Mode - Track and Lock the Upper Level Clock
The clock reference tracked and locked by the sub-station is transmitted from the upper
level station which may be the primary clock of the network, or the internal clock of
the upper level NEs transmitted to this NE, or the local GPS clock.
Compared with the other working modes of sub-clocks, this type of working mode
offers sub-clocks with highest precision.
6.2.2 Hold-on Mode
When all the timing references are lost, the sub-clocks will enter the holdover mode
where the clock sources of the sub-stations will use the last frequency information
saved before losing the timing reference signals for its timing reference. In other words,
the sub-clocks have the ―memory‖ function which can offer the timing signals
relatively matching the original timing references to ensure a fairly small frequency
error between the sub-clock frequency and the reference clock frequency for a long
time. However, the inherent oscillation frequency of the oscillator will gradually
wander, so the relatively high precision offered by this working mode cannot last very
long. The clock precision of this working mode is just less than that of the normal
working mode.
6.2.3 Free Run Mode – Free Oscillation Mode
When the sub-clock loses all its external timing references including the timing
reference memory or it remains in the holdover mode for too long, the internal
oscillator of the sub-clock will work in the free oscillation mode.
This mode offers the lowest clock precision.
6.3 Network Synchronization Requirements of SDH
The synchronization performance of digital network is critical to the normal work of
the network. The introduction of SDH network raises more requirements for network
synchronization. When the network is in normal working mode, every NE is
synchronized to one reference clock, there is only phase offset and no frequency offset
between NEs clocks, therefore only occasional pointer justification event will occur
(point justification event does not happen often when the network is synchronized).
When one NE loses its synchronous reference clock and enters holdover mode or
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free-oscillation mode, the frequency offset between its local clock and the network
clock will occur, resulting in continuous pointer justifications and affecting normal
transmission of service in the network.
SDH and PDH networks will coexist for a long time. Jitter and wander at the
SDH/PDH boundary mainly comes from pointer justification and payload mapping
process. The pointer justification frequency at SDH/PDH border is closely related to
the synchronization performance of the gateway node.
If the SDH input gateway which execute asynchronous mapping loses its
synchronization, the frequency offset and wander of the clock at this node will result in
continuous pointer justification of the whole SDH network, and deteriorate the
synchronization performance.
If the last node connecting with SDH network loses its synchronization, the SDH
network output will still have pointer justification and affect synchronization
performance.
If the middle network node loses synchronization, as long as the input gateway is still
synchronized with the PRC (Primary Reference Clock), the network unit that is next to
the faulty node and is still synchronized or the output gateway can correct the pointer
move of the middle network node, so that there will be no net pointer move at the last
output gateway and will not affect the synchronization performance.
6.4 Clock Source Types of SDH NE
The clock sources of SDH NE include four types as follows:
· External clock source: input interface provided by SETPI functional block
· Line clock source: extracted from the STM-N line signals by SPI functional
block
· Tributary clock source: extracted from PDH tributary signals by PPI functional
block, but this clock is seldom used since the pointer justification at the
boundary of SDH/PDH networks will influence the clock quality
· Internal clock source of equipment: provided by SETS functional block
Meanwhile, SDH NE provides the external with the output interface of clock source
through the SETPI functional block.
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6.5 Selection Principle of Clock in SDH Network
The service add/drop and rerouting functions of SDH network bring unprecedented
flexibility and high survivability to network applications, and makes network
synchronous timing selection more complicated. In SDH network, the timing reference
allocation between nodes is realized through a large number of lower level SDH NE
clocks. Therefore, the quality of the timing reference must be properly identified. The
Synchronization Status Message (SSM) is used to indicate the quality of timing
reference.
SDH MSOH makes use of the 5th to 8th bits of byte S1 to transmit SSM message,
which can represents sixteen different synchronization quality levels. Refer to ―3.1.2.11
Synchronization Status Message Byte: S1 (b5~b8)‖ for details.
In SDH network, the timing reference allocation between nodes is realized through a
large number of SDH NE clocks. With the increasing number of NEs in the
synchronous link, the quality of the timing reference signal is gradually deteriorating.
Therefore, if there are multiple synchronous paths of the same quality level for an NE
to choose from, adopting the synchronous path that passes through the least number of
NEs will help to improve the timing performance of the SDH network. According to
this principle, ZTE designed the S1 byte patent algorithm which enables NEs to choose
the clock reference signal of the highest quality level and shortest synchronous path.
The clock selection should follow the rules below:
1. If an NE can select from multiple valid clock sources, it will first select the
clock with the highest quality level according to the quality level information of
the clock sources.
2. If the quality levels of clock sources are the same, the NE will select the clock
source passing through the least number of NEs along the transmission path.
3. The NE forwards to the downstream NE the quality level information of the
currently adopted clock source and the quantity of passed NE via the S1 byte,
and sends the ―unavailable‖ status information to the upstream NE.
Note:
The upstream and downstream NEs are relative. If NE B extracts clock from NE A, NE
A is the upstream NE of NE B, and NE B is the downstream NE of NE A.
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6.5.1 Synchronization Principle of SDH Network
6.5.1.1 Master/Slave Synchronization Mode
Synchronous digital network in China adopts hierarchical master/slave synchronization
mode. That is using a single reference clock to control the synchronization of the whole
network through the synchronous link of the synchronous distribution network. Within
the network, a series of hierarchical clocks are adopted, and the clocks of each level are
synchronous with the clocks of the upper level or of the same level.
According to the precision level, the master/slave synchronous clocks of SDH network
can be divided into four types (levels) corresponding to different application scopes.
ITU-T standardizes each level of clocks and the quality levels of clocks are listed
below in an order from high to low:
1. Primary reference clock: compatible with G.811 specifications, as the timing
reference of the whole network
2. Transit exchange clock: compatible with G.812 specifications, as the vice clock
for transit exchange
3. Terminal exchange clock: compatible with G.812 specifications, as the vice
clock for terminal exchange (local office)
4. SDH NE clock: compatible with G.813 specifications, as the internal clock of
SDH equipment
If the NE works in the normal working mode, the performance of various clocks
transmitted to corresponding exchanges is mainly determined by the performance of
the synchronous transmission link and the timing extraction circuit. If the NE works in
the protection mode or the free-run mode, the performance of various clocks mainly
relies on the performance of the clock sources generating the clocks (the clock sources
located at various NE nodes accordingly). Therefore, high-level clocks must adopt
high-performance clock sources.
6.5.1.2 Precautions when Transmitting Clock Reference in Digital Network
1. There should be no loop during synchronous clock transmission.
In Fig. 6.5-1, if NE2 tracks the clock of NE1, NE3 tracks the clock of NE2, and
NE1 tracks the clock of NE3, the transmission link of the synchronous clock
will form a loop, and when the clock of one NE deteriorates, the synchronous
performance of all the NEs in the whole loop will deteriorate as a domino
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effect.
NE1
NE2 NE3
Fig. 6.5-1 Network diagram
2. Reduce the length of the timing transmission link as much as possible to avoid
the influence on the quality of the transmitted clock signal due to distance.
3. The clock of the sub-station should acquire its reference from the equipment of
higher level or of the same level.
4. The primary/standby clock references should be acquired through distributed
routes, to prevent losing the clock reference when the primary clock
transmission link is interrupted.
5. Choose the transmission system with high usability to transmit clock reference.
6.5.2 Instance
An application instance of SSM is shown in Fig. 6.5-2.
A
D
C
B
Unavailable
PRC
PRC
PRC
PRC
PRC
PRC
Synchronous path (in use)
Synchronous path (not in use)PRC }Synchronization status
message
( a )
Unavailable
Unavailable
Unavailable
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A
D
C
B
PRC
PRC
PRC
PRC
PRC
PRC
}
PRC
( b )
Unavailable
Synchronous path (in use)
Synchronous path (not in use)Synchronization statusmessageUnavailable
Unavailable
Unavailable
A
D
C
B
SETS
SETS
SETS
SETS
SETS
SETS }
( c )
Unavailable
Synchronous path (in use)
Synchronous path (not in use)
Synchronization statusmessageUnavailable
Unavailable
Unavailable
Fig. 6.5-2 Instance of SSM application
In Fig. 6.5-2, each NE has two synchronous clock sources to choose from. The
configurations of the synchronous sources of each NE are listed in Table 6.5-1.
Table 6.5-1 Settings of NE synchronous sources
NE Clock Source List
NE A External clock source and internal clock source
NE B Line clock 1 and line clock 2
NE C Line clock 1 and line clock 2
NE D Line clock 1 and line clock 2
During normal operation, the available synchronous source of NE A includes the
external access clock of Primary Reference Clock (PRC) and internal clock source.
According to rule 1, NE A will automatically choose the external clock source PRC and
send its synchronous quality level information to other NEs. The available synchronous
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sources of NE B are the A-B line clock and A-D-C-B line clock. According to rule 2,
NE B will automatically choose the A-B line clock as its synchronous source. Similarly,
NE D will automatically choose the A-D line clock as its synchronous source. NE C
may choose the A-B-C line clock or A-D-C line clock. In Fig. 6.5-2 (a), NE C chooses
the A-B-C line clock. Each NE will send the ―unavailable‖ status message to its
upstream NE according to rule 3.
In case of line interruption, as shown in Fig. 6.5-2 (b), when the line between NE B and
C is broken, NE C will choose the A-D-C line clock and send the ―unavailable‖ status
message to its upstream NE D.
If there is no external clock source, as shown in Fig. 6.5-2 (c) where the external clock
source of NE A is interrupted, NE A will enter the clock holdover mode, and then enter
the free-oscillation mode after the time of the holdover mode is over. At this time, each
NE is still synchronous with NE A, the clock source level will degrade to the
equipment clock SETS of the NE.
Summary
This chapter describes the synchronization methods and structure of SDH synchronous
network, various working modes of NE clock sources, and the SDH network
synchronization when the external clock changes.
Exercises
1. What are the working modes of a clock source?
2. What kinds of clock sources are commonly used for SDH NE?
3. Give the guidelines used in the process of synchronization of an NE.
4. What is meant by Free Running Clock and why it is not recommended for use?
5. What do you understand by the term tracing the clock?
6. Where Pseudo Synchronization is used?
7. Which byte is used for clock quality information exchange purposes?
8. What clock message is conveyed by a downstream NE to an upstream NE?
9. Why line clock tracing is better than tributary clock tracing?
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10. What is meant by frequency offset and phase offset?
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7 Optical Interfaces
Key points
Types of optical interfaces
Parameters of optical interfaces
Optical interfaces are the most characteristic part of synchronous optical cable digital
line system. Since they are standardized, they can directly connect different NEs
through optical lines; thus saves unnecessary optical/electrical conversion, avoids
signal impairment (such as pulse distortion) brought by the O/E conversion, and saves
network operation cost.
7.1 Optical Interface Types
Optical interfaces can be classified into three types according to different applications:
optical interface for intra-office communications, optical interface for short-haul
inter-office communications, and optical interface for long-haul inter-office
communications. The optical interfaces of different applications have different
identifiers, as shown in Table 7.1-1.
Table 7.1-1 Optical interface identifiers
Application Intra-office Inter-office
Short-haul Long-haul
Operating Wavelength (nm) 1310 1310 1550 1310 1550
Optical Fiber Type G.652 G.652 G.652 G.652 G.652 G.653
Transmission Distance (km) ≤2 ~15 ~40 ~80
STM-1 I-1 S-1.1 S-1.2 L-1.1 L-1.2 L-1.3
STM-4 I-4 S-4.1 S-4.2 L-4.1 L-4.2 L-4.3
STM-16 I-16 S-16.1 S-16.2 L-16.1 L-16.2 L-16.3
The first character of the identifier indicates the application:
· I represents for the intra-office communications
· S represents for short-haul inter-office communications
· L represents for long-haul inter-office communications
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The first digit following the dash after the characters represents the STM rate: e.g. 1
represents for STM-1, 16 represents for STM-16.
The second digit following the dash after the characters represents the working
wavelength window and optical fiber type:
· 1 and blank indicates that the working wavelength is 1310 nm, and the optical
fiber type is G.652
· 2 indicates that the working wavelength is 1550 nm, and the optical fiber type is
G.652 or G.654
· 3 indicates that the working wavelength is 1550 nm, and the optical fiber type is
G.653
7.2 Optical Interface Parameters
The locations of optical interfaces in SDH network system is shown in Fig. 7.2-1.
Tra
nsm
it
CTX
S
Plug
Optical cable
facilities
Rec
eiv
e
CRX
R
Plug
Fig. 7.2-1 Locations of optical interfaces in SDH network
In Fig. 7.2-1, point S is the reference point on the optical fiber just after the transmitter
optical connector (CTX) of the transmitter (TX), and point R is the reference point on
the optical fiber just before the receiver optical connector (CRX) of the receiver (RX).
Parameters of optical interfaces can be classified into three categories: optical
parameters of the transmitter at reference point S, optical parameters of the receiver at
reference point R, and optical parameters between point S and point R.
All values specified are worst-case values, i.e. the bit error ratio of each regenerator
section (optical cable section) should be no more than 1×10-10
for the extreme (worst)
case of optical path attenuation and dispersion conditions.
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7.2.1 Optical Line Code Pattern
There are abundant overhead bytes for system OAM functions in the frame structure of
SDH system. The line code pattern of SDH system adopts the scrambled NRZ code,
and the line signal rate equals to the standard STM-N signal rate. ITU-T G.707
specified the scrambling method for NRZ code, which is the standard 7-level scrambler,
with the scramble generation polynomial of 1+X6+X
7, and the scramble sequence
length of 27-1=127 (bits). The advantages of this method are: the code pattern is simple
and does not add the line signal rate; there is no optical power cost or requirement of
coding; the transmitter only needs one scrambler, and the receiver can receive services
from the transmitter by simply adopting the same standard decoder, so that the optical
lines of equipment from different manufacturers can connect with each other. The
adoption of scrambler aims to prevent too many continuous ―0‖ or ―1‖ of signals
during transmission, and to make it easy for the receiver to extract the timing
information (done by the SPI functional block) from signals. In addition, when the
pseudo random sequence generated by the scrambler is long enough, i.e. when the
relevancies of scrambled signals are little, the relevancies of regenerators’ jitters can be
reduced quite a bit.
7.2.2 S Point Specifications-Specifications of Optical Transmitter
1. Maximum -20 dB width
Since the main power of Single-Longitudinal Mode (SLM) laser concentrates
on the peak mode, the spectral width of SLM laser is specified by the maximum
full width of the central wavelength peak, measured 20 dB down from the
maximum amplitude of the central wavelength under standard operating
conditions. The spectral characteristics of SLM laser are shown in Fig. 7.2-2.
Fig. 7.2-2 Spectral characteristics of SLM laser
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2. Minimum Side Mode Suppression Ratio (SMSR)
The minimum SMSR is specified as the minimum ratio of the mean optical
power (P1) of the peak longitudinal mode to the mean optical power (P2) of the
most distinguished side mode, measured under full-modulated and worst
reflection conditions.
SMSR=10 log (P1/P2)
G.957 specifies that the value of SMSR should be no less than 30 dB.
3. Mean launched power
The mean launched power at reference point S is the average optical power of a
pseudo random signal sequence transmitted by the transmitter.
4. Extinction ratio (EX)
The extinction ratio is defined as the minimum ratio of the average optical
power (P1) of the logical ―1‖ (Mark) to the average optical power (P0) of the
logical ―0‖ (Space).
EX=10 log(P1/P0)
ITU-T specifies the extinction ratio to be 10 dB for long-haul transmission
except for L-16.2, and to be 8.2 dB for other cases.
7.2.3 R Point Specifications-Specifications of Optical Receiver
1. Receiver sensitivity
Receiver sensitivity is defined as the minimum acceptable value of average
received power at point R to achieve a 1×1010
BER. Typical margins between
a beginning-of-life, nominal temperature receiver and its end-of-life, worst-case
counterpart is in the 2 to 4 dB range. The actual measured receiver sensitivity is
usually 3 dB (sensitivity floating value) greater than the specified minimum
value (worst-case value).
2. Receiver overload
Receiver overload is the maximum acceptable value of the received average
power at point R to achieve a 1×1010
BER. When the received optical power
is greater than the receiver sensitivity, the improvement of signal-to-noise ratio
reduces the BER; but with the continuous increase of received optical power,
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the receiver will enter the non-linear working area, thus causing the BER to get
worse, as shown in Fig. 7.2-3.
BER
光接收功率
1×10¯10
A B Received optical
power
Fig. 7.2-3 BER graph
In the figure, the optical power at point A is the receiver sensitivity, the optical
power at point B is the receiver overload, the range between A and B is the
dynamic range where the receiver can work normally.
Summary
This chapter describes types and parameters of optical interfaces in SDH system.
Exercise
1. Where in the SDH equipment, optical interfaces are located?
2. What are optical interface types?
3. Decode the meaning of ―S-4.2‖.
4. What type of scrambled line code pattern is adopted by SDH?
5. What is receiver sensitivity?
6. What is mean by receiver overload?
7. What is SMSR?
8. What is the purpose of using optical line code patterns?
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9. What is the disadvantage of frequent optical to electrical conversions?
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8 Transmission Performance
Key points
Concepts of bit error, jitter, and wander
Specifications of bit error and jitter
Specification of wander
8.1 Bit Error Characteristics
Bit error means that errors occur to certain bits of the data flow after signal receiving,
judgment, and regeneration; resulting in impairment of the transmitted information
quality.
8.1.1 Generation and Distribution of Bit Error
The influence of bit error on services is mainly determined by service type and bit error
distribution.
The ideal optical fiber transmission system has very stable transmission channels, and
is almost free from external electromagnetic interference.
1. Bit error generated internally
The bit errors generated inside the optical fiber transmission system include bit
errors caused by various noise sources; by alignment jitters; by multiplexers,
cross-connect equipment, and switches; and by inter-bits interference generated
by the optical fiber dispersion, which can be represented by long-term system
bit error performance.
2. Bit error caused by pulse interference
Bit error of this kind is generally caused by burst pulse such as electromagnetic
interference, equipment fault, and transient interference on the power supply. It
features burst and large quantity, and can be represented by short-term system
bit error performance.
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8.1.2 Measurement of Bit Error Performance
The bit error performance specified by ITU-T G.821 refers to the bit error performance
of the 64 kbit/s path in the digital reference circuit which is 27500 km in total and is
connected end to end. It is based on the bit error state. With the increase of
transmission rate, the bit error performance measurement system based on the unit of
bit is getting more limited.
Currently the bit error performance of path with high bit rate is measured based on the
unit of block (B1, B2, and B3 all monitors block error). This measurement generates a
group of parameters based on block, which are mainly used to monitor continuous
services.
Block refers to a sequence of bits related to the path.
The parameters are defined as follows.
1. Block error
It is the block in which bit error occurs during transmission.
2. Errored Second (ES) and Errored Second Ratio (ESR)
If one or more block errors are detected in one second, this second will be
considered as an Errored Second (ES). The ratio of the number of ESs to the
total available time in the stipulated test period is called Errored Second Ratio
(ESR).
3. Serious Errored Second (SES) and Serious Errored Second Ratio (SESR)
When no less than 30% block error or at least one defect is detected in one
second, this second will be considered as a Serious Errored Second (SES).
The ratio of the number of SESs to the total available time in the stipulated test
period is called Serious Errored Second Ratio (SESR).
SES is generally the burst block error caused by pulse interference. Therefore,
SESR can usually indicate the anti-interference capability of the equipment.
4. Background Block Error and Background Block Error Ratio (BBER)
Background Block Error (BBE) refers to the block error detected during the
period other than the unavailable time and SES period. The ratio of the number
of BBEs to the total number of blocks during the period other than the
unavailable time and SES period is called BBER.
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BBER obtained via long-term test can generally indicate the bit error status of
the equipment internally, which is usually related to the performance stability of
the component employed in the equipment.
5. Defects
When the abnormality occurrence density has caused finite interruption of the
ability to execute one function, it is considered that a defect occurs. The main
network defects include Loss of Signal (LOS), Loss of Frame (LOF), Loss of
Pointer (LOP), various levels of alarm indications, and Signal Label Mismatch
(SLM).
8.1.3 Bit Error Specifications Related to Digital Section
ITU-T uses hypothetical digital reference link with the total length of 27500 km to
make equivalent of digital link, and allocates maximum bit error performance
specification for each section in the link; so that when the bit errors of each section in
the main link compose one link without exceeding the specifications, the performance
can satisfy the performance requirements of the digital signal end-to-end transmission
(27500 km).
Table 8.1-1, Table 8.1-2, and Table 8.1-3 respectively lists the bit error performance
specifications for 420 km, 280 km, and 50 km.
Table 8.1-1 Bit error performance specifications of HRDS for 420 km
Rate (kbit/s) 155520 622080 2488320
ESR 9.24×10-4 To be determined To be determined
SESR 4.62×10-5 4.62×10-5 4.62×10-5
BBER 2.31×10-6 2.31×10-6 2.31×10-6
Table 8.1-2 Bit error performance specifications of HRDS for 280 km
Rate (kbit/s) 155520 622080 2488320
ESR 6.16×10-4 To be determined To be determined
SESR 3.08×10-5 3.08×10-5 3.08×10-5
BBER 1.54×10-6 1.54×10-6 1.54×10-6
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Table 8.1-3 Bit error performance specifications of HRDS for 50 km
Rate (kbit/s) 155520 622080 2488320
ESR 1.1×10-4 To be determined To be determined
SESR 5.5×10-6 5.5×10-6 5.5×10-6
BBER 2.75×10-7 2.75×10-7 2.75×10-7
8.1.4 Measures to Reduce Bit Error
· To reduce internal bit error
Currently the average bit error ratio of the regenerator section is under the order of
magnitude 10-14
, and, thus, can be considered in the operating status of ―no bit
error‖. To improve signal-to-noise ratio is the main measure to reduce system
internal bit errors. Besides, selecting the appropriate extinction ratio for
transmitter, improving the balance characteristic of receiver, reducing alignment
jitters can all help to improve the system internal bit error performance.
· To reduce external bit error interference
The basic measure is to enhance the anti-EMI (Electro Magnetic Interference)
ability and ESD (Electro-Static Discharge) ability. For example, enhance the
grounding.
In addition, allocating enough redundancy when designing the system is a simple
and feasible measure.
8.2 Availability Parameters
· Unavailable time
If the digital signal of any transmission direction has a bit error ratio per second
worse than 10-3
for consecutive ten seconds, from the first second of the ten
seconds on, it is considered to enter the unavailable time.
· Available time
If the digital signal of any transmission direction has a bit error ratio per second
better than 10-3
for consecutive ten seconds, from the first second of the ten
seconds on, it is considered to enter the available time.
· Availability
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Availability refers to the percentage that the available time occupies in the total
time. Certain availability specifications need to be satisfied to ensure normal
working system, as shown in Table 8.2-1.
Table 8.2-1 Availability specifications of hypothetical digital section
Length (km) Availability Unavailability Unavailable Time/Year
420 99.977% 2.3×10-4 120 minutes
280 99.985% 1.5×10-4 78 minutes
50 99.99% 1×10-4 52 minutes
8.3 Jitter/Wander Performance
Jitter and wander are related to the system timing characteristics.
· Timing jitter (hereinafter referred to as jitter) refers to the short-term deviation
between the ideal instant and the specified instant (such as the optimum
sampling time) of the digital signal. The ―short-term deviation‖ is the phase
change with the change frequency higher than 10Hz.
· Wander refers to the long-term deviation between the ideal instant and the
specified instant of the digital signal. The ―long-term deviation‖ is the phase
change with the change frequency lower than 10Hz.
8.3.1 Generation Principles of Jitter/Wander
In the SDH network, there are the same jitter sources as the other transmission
networks, including various noise sources, unbalance of timing filter, regenerator
defects (such as inter-bits interference, threshold wander of amplitude limiter). In
addition, SDH network introduces new jitter mechanism:
1. Mapping jitter of the plesiochronous tributary
Since fixed stuffing bits and control stuffing bits are inserted when loading
tributary signals into VC, these bits needs to be removed when dropping the
tributary signals. At this time, these signals with interspaces will result in clock
gap, and will generate pulse buffing jitter which is the remained jitter.
2. Pointer justification jitter
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This kind of jitter is caused by negative/positive justification and de-justification
of pointers.
For the mapping jitter of the plesiochronous tributary, we can take measures to
reduce it to an acceptable level; while the jitter caused by pointer justification
(with the unit of byte, happens every three frames) has low frequency and large
amplitude, so it can not be filtered using ordinary measures.
Temperature change of environment is the general reason that causes wander of
SDH network. It can change the transmission characteristics of optical cable, and
result in signal wander and clock system wander.
Finally, the combination of pointer justification and network synchronization in
SDH NE also generates jitter and wander of very low frequency. However,
wanders of SDH network generally come from clocks of different levels and the
transmission system, especially the transmission system.
8.3.2 Jitter Performance Specifications
The major parameters to measure jitter performance in SDH network are listed as
follows.
· Input jitter tolerance
The input jitter tolerance includes jitter tolerances of PDH input interface
(tributary interface) and STM-N input interface (line interface).
The input jitter tolerance of the PDH input interface (tributary interface) is the
maximum input jitter value that the PDH input interface can endure without
causing bit error in the equipment. Since SDH and PDH networks coexist, in
transmission network there requirement is that PDH services can be added to SDH
NE. In order to satisfy this requirement, the tributary input interface of the SDH
NE must be able to tolerate the maximum jitter of PDH tributary signals, that is,
the jitter tolerance of this tributary interface can bear the jitter of the transmitted
PDH signal.
The input jitter tolerance of the STM-N input interface (line interface) is defined
as the sinusoidal peak-to-peak jitter value which can enable the optical equipment
to generate 1dB optical power penalty. This parameter specifies that the input jitter
tolerance of a certain level NE should be able to tolerate the output jitter tolerance
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generated by the upper-level NE when the SDH NEs are interconnected to
transmit STM-N signal.
· Output jitter tolerance
Similar to input jitter tolerance, output jitter tolerance includes tolerances of PDH
tributary interface and STM-N line interface. It is the maximum jitter of the output
interface when there is no jitter at the equipment input interface.
When dropping PDH service from SDH NE, the output jitter of the PDH tributary
interface should guarantee that the equipment receiving the PDH signal can
endure the output jitter. The output jitter of the STM-N line interface should
guarantee that the SDH network receiving the STM-N signal can endure the
output jitter.
· Mapping jitter and combined jitter
Pointer justification and mapping at the PDH/SDH network boundary will result
in special jitter that only exists in SDH. To specify this kind of jitter, mapping
jitter and combined jitter are employed together to describe it.
Mapping jitter refers to the maximum jitter of the output PDH tributary signal
from the PDH tributary interface of the SDH equipment when PDH signals with
different frequency offsets are inputted into the PDH tributary interface of the
SDH equipment and the STM-N signal has no pointer justification.
If the input at the SDH equipment line interface complies with the pointer testing
sequence signal specified in G.783, the combined jitter refers to the maximum
jitter of the output signal tested at the PDH tributary interface of the system when
the frequency offset of the input signal is properly changed after the pointer
justification occurs in the SDH equipment.
· Jitter transfer function – characteristic of jitter transfer
This function specifies the restriction capability (jitter gain) of the jitter of the
output STM-N signal on the jitter of the input STM-N signal, hence controlling
the jitter accumulation of the line system and preventing the rapid accumulation of
system jitter.
Jitter transfer function is defined as the relation between the frequency and the
ratio of the STM-N output signal jitter to the STM-N input signal jitter, where the
frequency is the jitter frequency.
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8.3.3 Measures to Reduce Jitter
1. To reduce jitter of line system
Jitter of the line system is the main jitter sources in SDH network. Taking
measures to reduce it is one of the critical factors to ensure the network
performance.
The basic measure to reduce jitter of line system is to reduce jitter (output jitter)
of one single regenerator, control characteristic of jitter transfer (improve the
restriction ability of output signal on input signal jitter), improve the jitter
accumulation method (adopt scrambler and jitter reducer to randomize
information transmitted and reduce the relevancy between system jitters generated
by regenerators, thus improving the jitter accumulation characteristic).
2. To reduce output jitter of PDH tributary interface
Since pointer justifications adopted by SDH may cause great phase jump (pointer
justification is in the unit of byte) accompanied by jitter and wander, the
desynchronizer is used at the tributary interface of SDH/PDH network boundary
to reduce jitter and wander.
Desynchronizer has the functions of buffering and phase smoothing. It is usually
implemented by the phase-locked loop with buffer. The important techniques
include self-adapting technique and bit leakage technique.
8.3.4 Notes
1. What is optical power penalty?
Jitter, wander, and optical fiber dispersion will reduce the signal-to-noise ratio and
thus increase bit errors. This can be compensated by increasing the optical power
of the transmitter. That is to say, jitter, wander, and dispersion degrade the system
performance to be worse than some certain specification; to make the system
performance reach the certain specification; we can increase the optical power of
the transmitter. And the optical power increased is the optical power penalty
needed to satisfy the certain specification.
The optical power penalty of 1 dB is the maximum value that the system can
tolerate.
2. Hypothetical Reference Connection (HRX)
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HRX is a hypothetical connection with specified structure, length, and
performance in telecommunications network. It can be used as a model for
network performance research, can be compared with the network performance
specifications, and thus export the specifications of every smaller entities.
The longest standard HRX consists of fourteen circuits connected serially, with
two terminating office having twelve sections of circuits all together. This is the
all-digital 64 kbps connection between two subscribers at the two ends of
communication, with the full length of 27500 km.
Summary
This chapter describes the bit errors and specifications of jitter and wander, which are
used to measure the transmission performance.
The key points of this chapter are system bit error measurement, meanings of the
commonly used parameters for jitter performance.
Exercises
1. What are the different error events defined by ITU for transmission networks?
2. How is ―Availability‖ and ―Unavailability‖ determined in a period?
3. What are the possible causes of bit errors?
4. What is Jitter?
5. What measures should be taken to reduce bit errors?
6. What do you understand by the term ―Availability Parameters‖?
7. Why it is necessary to reduce jitter?
8. What are the causes of jitter generation?
9. What is Wander and how it can be minimized in a transmission line?
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9 Test
Key points
Issues covered in SDH tests
9.1 SDH Test Method
SDH tests can be performed by dedicated test instruments, or by functions (via
overhead bytes) of SDH network management system. The differences between these
two testing methods are: the test using network management system mainly aims to the
SDH system maintenance; the tested items are not as comprehensive as the test using
dedicated test instruments; and it is generally performed by OAM personnel. The test
using dedicated test instruments covers various test items; it is mainly applied in
science research, manufacturing, installation and debugging, check and acceptance of
project.
9.2 SDH Tested Items
SDH test generally covers regular tested items owned by SDH only. The regular tested
items are similar to those of PDH, e.g. jitter test, wander test, and transfer characteristic
test. The tested items owned by SDH only can be classified into four categories:
1. Test of transmission ability: includes BER test, mapping/demapping test. It
aims to test the ability of SDH to transmit the payload.
2. Test of pointers: includes tests of timing offset and payload output jitter. It aims
to test the ability of SDH to accommodate asynchronous work.
3. Test of embedded overhead: includes tests of alarms and performance
monitoring function, protocol analysis. It aims to confirm the overhead
function.
4. Test of line interfaces: includes a series of tests for parameters of electrical
interface and optical interface. It aims to ensure the transverse compatibility of
optical path.
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Summary
This chapter describes the testing methods and tested items of SDH.
Exercise
1. Why functional testing method is not as powerful as test equipment method?
2. What is the need of SDH test methods?
3. What type of test items can be achieved by using dedicated test instruments?
4. What are the four tested items which are owned only by SDH?
5. Which items are classified as the regular tested items?
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10 Introduction to Network Management
Key points
Basic concepts of TMN
Basic concepts of SDH management network
Management ability of SDH
OSI model and ECC protocol stack
10.1 TMN Fundamentals
10.1.1 TMN Management Frame
To implement the integrated, unified and efficient management of telecommunications
network, ITU-T recommended the concept of Telecommunications Management
Network (TMN). The basic concept of TMN is to provide an organizational hierarchy
to realize the interworking between various operating systems (network management
systems) and the telecommunication equipment, and to use a universal hierarchy with
standard interfaces (including protocols and information specifications) to exchange
management information, thus realizing automatic and standard management of the
telecommunications network. In concept, TMN is network independent from the
telecommunications network and specializes in network management. It has some
various interfaces connected with the telecommunications network to receive
information from telecommunications network and control the operation of
telecommunications network. TMN often utilizes part of the facilities in the
telecommunications network to provide communication. Therefore, some parts of the
two networks may overlap. The relation between TMN and telecommunications
network is shown in Fig. 10.1-1.
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TMN
Operation system Operation system Operation system
Data communications network
Switch Transmission system Switch Switch
Telecommunications network
Transmission system
Workstation
Fig. 10.1-1 Relation between TMN and telecommunications network
10.1.2 Physical Structure of TMN
The physical structure of TMN mainly describes the physical entities and interfaces
inside the TMN. The simplified physical structure is shown in Fig. 10.1-2.
OS
DCN
MD
DCN
QA NE
Q3/F/X
WS
TMN
Q3
Q3
NEQA
Q3/F
Qx
Qx Qx
Fig. 10.1-2 Physical structure of TMN
OS in Fig. 10.1-2 is the operating system, i.e. the network management system that
executes the OSF. In fact, it is a large-scaled system program that manages the network
resources. MD is the coordinating equipment which executes MF and implements the
coordination between OS and NE. In addition, it also provides QAF and WSF, or even
OSF sometimes. MD can be realized via the hierarchical mode. QA is the Q adapter
which implements the adaptation and interconnection between the NE and non-TMN
interfaces.
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Data Communications Network (DCN) is the telecommunications network in the TMN
that supports DCF. It mainly provides the functions of the three lower layers of the OSI
reference model, but not the functions from layer 4 to layer 7. The DCN can be formed
by connecting the subnets of different types (such as X.25 or DCC).
NE consists of the telecommunication equipment (or part of it) which executes NEF
and the supporting equipment. It can contain other TMN function blocks, generally the
MF. An NE usually has one or more standard Q interfaces, and sometimes F interfaces.
The workstation (WS) is the system to perform WSF. It mainly translates information
at the f reference point to a displayable format at the g reference point, and vice versa.
10.1.3 TMN Interfaces
Standard TMN interfaces need to be specified in order to simplify the interconnections
between the equipment of different manufacturers. It is the key point for TMN. The
standard interfaces need to give a universal specification to the protocol stack and the
messages carried by the protocol.
10.1.3.1 Q Interface
Q interface generally corresponds to Qx interface. Qx interface connects MD with MD,
NE with MD, QA with MD, and NE with NE (at least one NE has MF function). In
traditional PDH system, the Qx interface usually only provides the functions at the
three lower layers of the OSI reference model. Therefore, it is suitable to connect
simple equipment such as multiplexer and line system. Either A1 or A2 protocol stack
specified in ITU-T Recommendation G.773 is applicable for the Qx interface, where
A1 is for the connection mode, and A2 is for the connectionless mode (LAN
technology). In SDH system, Qx interface generally contains the functions of all seven
layers. Its protocol stack can be the CONS1, CLNS1, or CLNS2 specified in ITU-T
Recommendations Q.811 and Q.812; where CONS1 is the interface of the X.25 packet
network, CLNS1 is the connectionless interface that employs LAN technology, and
CLNS2 is the connectionless interface that employs the interworking protocol on the
basis of the X.25 protocol.
10.1.3.2 F/G/X Interface
F interface corresponds to the f reference point. It can connect a remote workstation to
OS or MD via the DCN. G interface corresponds to the g reference point, and X
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interface corresponds to the x reference point. In general, X interface has higher
security requirement than Q interface.
10.1.4 TMN Layers Division
According to ITU-T Recommendation M.3010, the management layer model of the
TMN is divided into Network Element Layer (NEL), Element Management Layer
(EML), Network Management Layer (NML), Service Management Layer (SML) and
Business Management Layer (BML).
Fig. 10.1-3 displays the management layer division of the TMN with the highest layer,
the Service Management Layer. NE can be an SDH equipment, or any other
manageable equipment such as PDH equipment or switch.
Network Manager
Layer (NML)
NMS
NE
NE
NE
NE
EMS EMS
NENE
NE
Element Management
Layer (EML)
NMS
SMSService Management
layer (SML)
Network Element
Layer (NEL)
Fig. 10.1-3 TMN management layers
10.2 SDH Management Network (SMN)
10.2.1 SMN and TMN
SDH management network (SMN) is a subset of TMN that manages SDH NEs. SMN
can be further divided into series of SDH Management Subnets (SMS). These SMSs
consist of series of separate Embedded Control Channel (ECC) and intra-station data
communication links, and form an organic part of the whole TMN. The significant
characteristics of SMN are its intelligent network elements and embedded ECC. The
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combination of these two characteristics greatly reduces transmission time and
response time of TMN information. In addition, it can download the network
management function to the network element via the ECC, thus realizing distributed
management. The basic characteristic of SDH is its powerful and efficient network
management capability.
The relationships among TMN, SMN, and SMS are shown in Fig. 10.2-1.
TMN
SMN
SMS-1 SMS-2 SMS-n
Fig. 10.2-1 Relationships among SMS, SMN, and TMN
Unitrans ZXONM Network Management System (NMS) can be an SDH Management
Subnet (SMS), or an SDH Management Network (SMN). Its relationship with the
Telecommunications Management Network (TMN) is described below. As shown in
Fig. 10.2-1, TMN belongs to the most general management network category. SMN
consisting of multiple SMSs is a subset of TMN, and is responsible for managing SDH
NE. Because Unitrans ZXONM network management system is part of the TMN, it
can provide standard interfaces to accept management by the upper-layer network
management center.
The logic channel that transmits NMS messages in SDH system is ECC whose physical
channel is DCC. DCC employs bytes D1~D3 of SDH regenerator section overhead
(RSOH) and bytes D4~D12 of the multiplex section overhead (MSOH) to compose
channels of 192 kbit/s and 576 kbit/s, which are respectively called DCC (R) and DCC
(M). DCC (R) can access the regenerator (REG) and the terminal multiplexer (TM),
and DCC (M) is the express channel of the NMS information between the TMs.
10.2.2 SDH Management Interfaces
The major operation and running interfaces related to SDH management network are
the Qx and F interfaces. SMS communicates with the TMN via the Qx interface.
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10.3 SDH Management Functions
ITU-T specifies five major functions for network management system: Configuration
Management, Fault Management, Performance Management, Security Management,
and Accounting Management.
1. Configuration Management: To configure resources and services of the
transmission network. It includes configuration of network data, equipment
data, link channels, protection switching function, synchronous clock source
distribution strategy, orderwire equipment, line interface parameters, tributary
interfaces, and NE time; query, backup, and restoration of configuration
information; and query and statistics of path resources.
2. Fault Management: To detect, analyze and locate equipment faults. It includes
setting of alarm levels; real-time display of alarms; alarm settings of
confirmation, shielding, filtering, reversion, and sound; query of history alarms;
locating alarm; and alarm statistics and analysis.
3. Performance Management: To perform effective check and analysis of various
performances of the equipment. It includes settings of performance thresholds,
query of current and history performance data, performance data analysis.
4. Security Management: To provide security guarantee for equipment
maintenance. It includes setting of user levels, operation rights and
management areas; and management of user login and user operation log.
5. Accounting Management: To provide the basic information related to
accounting. The information includes time for circuit establishment, duration,
and quality of service (QoS).
Maintenance management is sometimes listed as an independent functional block. It
provides measures for normal equipment operation and locating fault including
loop-back control, alarm insertion, and bit error insertion.
10.4 OSI Model and ECC Protocol Stack
10.4.1 OSI Concept
The Open System Interconnection (OSI) hierarchical model is the standard computer
network functional structure specified by International Standardization Organization
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(ISO). OSI aims at enabling interconnection of different information process system. It
is a conceptual and functional structure, and does not involve detailed implementation
methods or technologies. However, it has profound and lasting effect on the new
communication field related with computer communication. Fig. 10.4-1 shows the OSI
model.
Application layer
Presentation layer
Session layer
Transport layer (TCP)
Network layer (IP)
Data link layer (ATM)
Physical layer
Fig. 10.4-1 OSI model
10.4.2 ECC Protocol Stack Description
Application layer CMISE,ROSE,ACSE
Presentation layer X.216, X.226
Session layer X.215, X.225
Transport layer ISO8073/AD2
Network layer ISO8473
Data link layer ITU-T Q.921
Physical layer SDH DCC
Fig. 10.4-2 ECC protocol stack
Summary
This chapter describes the hierarchical structure of SDH network management, the
compositions and protocols of SDH Management Network (SMN).
Exercises
1. What is TMN and why it came into being?
2. List all the parts which are included in the physical structure of TMN.
3. What is the role of Qx interface?
4. What is the difference between Network Management Layer and Element
Management Layer?
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5. What do you know about UNITRANS ZXONM network management system?
6. What are the five major functions of a network management system according to
ITU-T?
7. List all (seven) layers of ECC protocol stack.
8. What is covered by Security Management?
9. QoS is addressed in which type of SDH management?
10. What is the function of F interface?
11. What layers are included in the management layer model of TMN?