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INTERNATIONAL TELECOMMUNICATION UNION

TELECOMMUNICATIONSTANDARDIZATION SECTORSTUDY PERIOD 1997 - 2000

COM 13-30-EMarch 1998

Original: English

Question: 19/13

STUDY GROUP 13 CONTRIBUTION 30

SOURCE*: RAPPORTEUR OF Q.19/13

TITLE: DRAFT RECOMMENDATION G.OTN (VERSION 3.0)#

ABSTRACT

This contribution reports the version 3.0 of Draft Rec. G.otn as produced at the last

Rapporteurs Meeting (Geneva, 5-10 February, 1998).

SUMMARY

Draft Rec. G.otn describes the functional architecture of optical transport networks

using the modelling methodology described in Rec. G.805. The optical transport

t k f ti lit i d ib d f t k l l i i t t ki i t

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network functionality is described from a network level view point taking into

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Draft ITU-T Recommendation G.otn (version 3.0)

ARCHITECTURE OF OPTICAL TRANSPORT NETWORKS

(Geneva, February 1998)

Page

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ARCHITECTURE OF OPTICAL TRANSPORT NETWORKS

Scope

This Recommendation describes the functional architecture of optical transport

networks using the modelling methodology described in Recommendation G.805. The

optical transport network functionality is described from a network level view point,

taking into account an optical network layered structure, client characteristic

information, client/server layer associations, networking topology, and layer network

functionality providing optical signal transmission, multiplexing, routing, supervision,

performance assessment, and network survivability.

This Recommendation is restricted to the functional description of optical transport

networks that support digital signals. The support of analogue or mixed

digital/analogue signals is outside of the current scope.

It is recognised that the design of optical networks is subject to limitations imposed by

the accumulation of degradations introduced by the number of network elements andtheir network topology. However, many of these degradations and the magnitude of

their affects, are associated with particular technological implementations of the

architecture described in this recommendation and are therefore subject to change as

technology progresses. As such the description of these effects is outside the scope of

this Recommendation.

References

The f ll i ITU T Re e d ti d the efe e e t i i i

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2 Client Specific Overhead Information is associated with a particular

client/server relationship and is therefore processed by a particular

adaptation function.

3 Auxiliary Channel Overhead Information is information that may be

transferred by an optical network layer but which does not by necessity

have to be associated with a particular connection. An example of such an

auxiliary channel is a datacommunications channel for the purposes of

transferring management data between management entities. NOTE -

These management entities are not trail termination and adaptation

functions

4 Reserved Overhead Information for national use.

5 Unassigned Overhead Information. This overhead may of types 1,2, 3 and

4 defined above.

Optical Supervisory Channel:The optical supervisory channel associated with the

optical transmission section is an optical carrier that transfers overhead information

between optical transmission section transport entities. The supervisory channel maysupport more than one type of overhead information. This overhead information may

be used by one or more transport network layers.

Abbreviations

For the purposes of this Recommendation the following abbreviations are used:

AIS Alarm indication signal

AP A i t

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OMS_SN Optical multiplex section subnetwork

OMS_SNC Optical multiplex section subnetwork connectionOSS Optical SDH Section

OTDM Optical time division multiplexing

OTM Optical transport module

OTM-p Optical transport module of order p

OTN Optical transport network OTS Optical transmission section

OTS-p Optical transmission section of order p

OTS/OMS_A Optical transmission section/Optical multiplex section adaptation

OTS_LC Optical transmission section link connection

OTS_NC Optical transmission section network connection

OTS_T Optical transmission section termination

OTU Optical transport unit

OTUG-n Optical transport group of order n

PDH Plesiochronous digital hierarchy

RS Regenerator section

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In the following functional description, optical signals are characterised by

wavelength (or central frequency) and may be processed per wavelength or as a

wavelength division multiplexed group of wavelengths. The functional description ofother optical multiplexing techniques (e.g. optical CDM) in optical networks is for

future study.

Optical Transport Network Layered Structure

The optical transport network layered structure is comprised of the optical channel,

optical multiplex section and optical transmission section layer networks, as

illustrated in Figure 1. Motivation for this three-layer structure is as follows.Optical Channel Layer Network: This layer network provides end-to-end networking

of optical channels for transparently conveying client information of varying format

(e.g. SDH STM-N, PDH 565 Mbit/s, cell based ATM, etc.). The description of

supported client layer networks will be given in G.onr. To provide end-to-end

networking, the following capabilities are included in the layer network:

5 optical channel connection rearrangement for flexible network routing;

6 optical channel overhead processes for ensuring integrity of the optical channel

adapted information;

7 optical channel supervisory functions for enabling network level operations and

management functions, such as connection provisioning, quality of service

parameter exchange and network survivability.

Optical Multiplex Section Layer Network: This layer network provides functionality

f t ki f lti l th ti l i l N t ki biliti i l d d

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continuous data stream with associated overhead information which includes an

integrity check.This characteristic information is an Optical Transport Unit (OTU).

The optical channel layer network contains the following transport functions andtransport entities (see figure 2)

11 Optical channel trail

12 Optical channel termination source (OCh_T source)

13 Optical channel termination sink (OCh_T sink)

14 Optical channel network connection (OCh_NC)

15 Optical channel link connection (OCh_LC)

16 Optical channel subnetwork (OCh_SN)

17 Optical channel subnetwork connection (OCh_SNC)

17.1.1 Optical Channel Termination

The following generic processes may be assigned to the optical channel trail

termination:

18 validation of connectivity integrity

19 assessment of transmission quality

20 transmission defect detection and indication

The means of providing these processes is described in Section 6.2.

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21 OMS trail

22 OMS termination source (OMS_T source)23 OMS termination sink (OMS_T sink)

24 OMS network connection (OMS_NC)

25 OMS link connection (OMS_LC)

26 OMS subnetwork (OMS_SN)

27 OMS subnetwork connection (OMS_SNC)

28 Optical Multiplex Section Termination

The following generic termination processes may be assigned to the optical multiplex

section termination:

29 validation of connectivity integrity

30 assessment of transmission quality

31 transmission defect detection and indication

The means of providing these processes is described in Section 6.2.

OMS bidirectional termination: consists of a pair of co-located optical multiplex

section termination source and sink functions.

Optical multiplex section termination source: accepts adapted information from a

client layer network at its input, inserts the OMS overhead and presents the

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The following generic processes may be assigned to the optical transmission trail

termination:

37 validation of connectivity

38 assessment of transmission quality

39 transmission defect detection and indication

The means of providing these processes is described in Section 6.2.

OTS bidirectional termination: consists of a pair of co-located optical transmission

section termination source and sink functions.

OTS termination source: accepts adapted information from a client layer network at

its input, inserts the OTS overhead into the optical supervisory channel, and adds the

optical supervisory channel to the main signal. The termination function conditions

the information for transmission over the physical medium and ensures that the

optical signal meets the physical interface requirements. The output of the OTS

termination source is the characteristic information of the optical transmission section

layer network. This characteristic information is referred to as an optical transportmodule (OTM).

OTS termination sink: accepts the characteristic information of the transmission

section layer network at its input, reconditions the information to compensate for

signal degradation resulting from transmission over the physical medium, extracts the

optical supervisory channel from the main optical signal, processes the OTS overhead

contained within the optical supervisory channel and presents the adapted

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the adaptation may include processing such as scrambling and channel coding (e.g.

NRZ). For a digital mapping the adapted information is a continuous data stream

of defined bit rate and coding scheme.

41 management specific processes are described in section 6.2.

The OCh/Client adaptation sink (OCh/Client_A_Sk) performs the following processes

between its input and its output:

42 recovery of the client signal from the continuous data stream. The processes are

dependent upon the particular client/server relationship and can be null. For a

digital client the adaptation may include processes such as timing recovery,decoding and descrambling.

43 management specific processes as described in section 6.2.

44 OMS/OCh Adaptation

The OMS/OCh adaptation source (OMS/OCh_A_So) performs the following

processes between its input and its output:

The bidirectional OMS/OCh adaptation (OMS/OCh_A) function is performed by a

co-located pair of source and sink OMS/OCh adaptation functions.

45 modulation of an optical carrier by the optical transport unit signal by means of a

defined modulation scheme

46 wavelength (or frequency) and power allocation to the optical carrier

47 optical channel multiplexing to form an optical multiplex

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The OTS/OMS adaptation sink (OTS/OMS_A_Sk) performs the following processes

between its input and its output:

52 Processes associated with the OTS/OMS adaptation sink are for further study

53 Connection Functions

The OCh connection function, OCh_C, and OMS connection function, OMS_C,

provide flexibility within their respective network layers. In each case characteristic

information is routed between input (termination) connection points ((T)CPs) and

output (T)CPs. The connection functions may be used by the network operator to

provide routeing, grooming, protection and restoration.

Optical Network Topology

Optical network layers can support unidirectional and bidirectional point-to-point

connections, and undirectional point-to-multipoint connections.

Unidirectional and Bidirectional Connections and Trails

A bidirectional connection in a server layer network may support either bidirectional

or unidirectional client layer networks but a unidirectional server layer network may

only support unidirectional clients.

A bidirectional optical transmission section layer network connection may be

supported by one optical fibre for both directions (single fibre working) or each

direction of the connection may be supported by different fibres.

[editors note: OAM and overhead transfer in single fibre working is currently not

id d i G t d i f f th t d ]

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- within and between administrative domains

It shall provide a means of detection and notification in the event of a misconnection.The optical transport network shall provide facilities to:

- ensure interconnection of transport network entities that have compatible adapted or

characteristic information.

- Detect faults, isolate faults and initiate recovery actions where applicable.

In the event of a signal within the server layer being interrupted, upstream and

downstream network entities in the server layer shall be notified.

The optical transport network shall provide monitoring within/between administrative

boundaries.

Generic performance management

The optical transport network shall be able to detect performance degradations to

avoid failures and verify quality of service:

- end-to end;

- within and between administrative domains

Generic management communications

The optical transport network shall support communications between:

- personnel at remote sites

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reporting connectivity and transmission performance defects for a given connection.

Recommendation G.805 defines four types of monitoring techniques for connections.

The connection supervision process can be applied to network connections and

connection segments, where the latter is defined as an arbitrary series of subnetwork

connections and link connections.

Inherent Monitoring

Connections may be indirectly monitored by using the inherently available data from

the server layers and computing the approximate state of the client connection from

the available data. The use of this monitoring technique for optical network layerconnections is for further study.

Non-Intrusive Monitoring

The connection is directly monitored by use of listen only (non-intrusive) monitoring

of the original data and overhead. The approximate state of the connection can be

determined by the information provided at each of the monitoring points.

Non-intrusive monitoring of the characteristic information transported by aconnection is an application that can be used to provide fault localisation. If a trail

termination sink function detects a disturbance it may not be immediately obvious

where this disturbance first originated. The trail termination sink function therefore

indicates that there is a disturbance of a certain kind but not where it is. In order to

locate such a disturbance, the trail is viewed as a series of link connections. At the end

of every link connection a non-intrusive monitoring termination sink function (TTm)

may be used to monitor the characteristic information at that point. The TTm does

t id d t d i f ti t it t t A l f th li ti f

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Segment Connection Monitoring

The intended role of connection segments is to represent that portion of a trail thatexists within a particular administrative region. Segment connection monitoring in

optical transport network layers is for further study.

Optical Network Survivability Techniques

This section describes the architectural features of network strategies that may be

applied to enhance the survivability of optical transport networks from network link

and node impairments. The survivability techniques considered for optical transport

networks encompass both protection and network restoration capabilities.

Protection

A protection application makes use of pre-assigned capacity between nodes. The

simplest architecture has 1 working and 1 protection capacity (1+1), the most

complex architecture has n working and m protection capacities (m:n).

Two types of protection architecture are considered: trail protection and sub-network

connection protection.

Trail Protection: Trail protection is a dedicated protection mechanism that can be

used on any physical structure (i.e. meshed, ring or mixed). It can be applied in both

the OCh and OMS layers. Trail protection is not recommended for use in the OTS

layer. Generically it is an end to end protection mechanism. A working trail is

replaced by a protection trail if the working trail fails or if the performance falls

below the required level.

T il t ti t i idi ti l bidi ti l

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process and the client layer receives Server Signal Fail (SSF) generated by the server

layer. Subnetwork connection protection using non-intrusive monitoring (SNC/N)

uses client layer information to protect against failures in the server layer and failuresand degradations in the client layer.

The following SNC protection architectures have been identified for optical networks:

1+1 Unidirectional SNC/N for optical line systems

In this architecture a permanent bridge is utilised at the transmit end. At the receive

end of the trail a protection switch is effected by selecting one of the signals based on

purely local information. This architecture can be applied in the OTS network layerwhere its application is restricted to network connections rather than subnetwork

connections. It is therefore suitable for short haul optical line systems without in line

amplifiers. This architecture is illustrated in figure 8. It may be used without an

automatic protection switching protocol.

Other architectures including SNC/I and SNC/N in the OCh and OMS layers using

connection functions is for further study.[editors note - the use of non-associated

OAM needs to be considered for these architectures]

Network Restoration

Optical network restoration techniques are based on optical channel and optical

multiplex section cross-connection. In general, the algorithms used for restoration

involve rerouteing. To restore an impaired connection alternative facilities may be

chosen among the available capacity of the optical layer network.

[Edi N O i l k i h i i f h

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OCH_T_So

OCH AP

OCH_T_Sk

OCH trail

OCH_NC

OMS_T_So

OMS AP

OMS_T_Sk

OMS trail

OMS_NC

OTS_T_Sk

OTS AP

OTS_T_Sk

OTS trail

OTS_NC

OTS/

OMS_A_So

OTS/

OMS_A_Sk

OMS/

OCH_A_So

OMS/OCH_A_Sk

OCH_A_So OCH_A_Sk

OCH TCP

OMS TCP

OTS TCPOTS TCP

OTS AP

OMS TCP

OMS AP

OCH TCP

OCH AP

OCH layer

network

OMS layernetwork

OTS layernetwork

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FIGURE 1Client Server associations in an optical transport network

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AP

OCH_T_So

TCP

AP

TCP

OCH_T_Sk

OCH_SNC OCH_SNC

OCH_LC

CP CP CP CP CP

OCH_LC OCH_LC

OCH trail

OCH_SNC

OCH_NC

FIGURE 2OCH layer network example

AP

OMS_T_So

TCP

AP

TCP

OMS_T_Sk

OMS_LC

CP CP CP CP CP

OMS_LC OMS_LC

OMS trail

OMS_NC

FIGURE 3OM S layer network example

OMS_SNCOMS_SNC OMS_SNC

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OCH

OMS

OTS

OCH

OCH_SN

leaf

root

leafMPCP

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FIGURE 5Point-to-multipoint optical channel connection

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Trail

T-So T-Sk

AP AP

TTm

SN A SN B

TTm

SN C SN D

FIGURE 6

An example of a trail monitored by non-intrusive monitoring; the 2 intermediate TTm functions are

non-intrusive monitoring termination sinks. This technique can be used to monitor the trail at

intermediate matrix connections within the OCH and OMS layers. Misconnections or faults detected at

the trail termination sink can be traced backward from the sink.

AP APProtected Trail

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FIGURE 71+1 Unidirectional trail protection

MCp MCp

TCP

TCP TCP

TCP

Network connection

Network connection

SF Signal fail

MCp Matrix connection

TTm Monitor trail termination

TCP Termination connection point

AP Access point

AP

TCP

AP

TCP

Protected network connection

OTS_T

OTS/OMS_A OTS/OMS_A

TTm TTm

SF SF

TTm TTm

SF SF

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FIGURE 8

Subnetwork connection protection with non-intrusive monitoring in the optical transmissionsection layer network.

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Appendix I

Examples of Optical Network Functionality

This appendix describes examples of functional groupings that may be applied to the

optical network.

Figure I.1 Example of G.957/G.otn conversion

Figure I.2 Example of optical wavelength conversion

Figure I.3 Example optical line system with optical channel and optical multiplex

section cross-connection

Figure I.4 Application of functional architecture to cases of single and multi-channel

1-R regeneration (amplification) and channel crossconnection

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OTSp/

OMSn_A

OMSn/

OCH_A

OMSn_T

OSS

OCH_T

OCH/RS_A

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FIGURE I.1

Example of G.957 to G.otn conversion

OTS1/

OMS1_A

OMS1/

OCH_A

OMS1_T

OTS1_T

OMS1/

OCH_A

OMS1_T

OTS1_T

OTS1/

OMS1_A

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OCH_TSource

OCH AP

OCH_T

OCH trail

OCH_LC

OMS_T

OMS AP

OMS_T

OMS trail

OMS_NC

OTS_T

OTS AP

OTS_T

OTS trail

OTS_NC

OTS/OMS_A

OTS/

OMS_A

OMS/OCH_A

OMS/OCH_A

OCH/

Client_A

OCH/Client_A

OCH TCP

OMS TCP

OTS TCPOTS TCP

OTS AP

OMS TCP

OMS AP

OCH TCP

OCH AP

FIG I.3Example optical line syst em with op tical channel and op tical multiplex section cross-connection

OMS trail

OMS_LC

OTS trail

OTS_NC

OCH_SNC

OCH_LC

OMS_LC

OTS trail

OTS_NC

OMS_SNC

OCH-XC OMS -XCLT LT

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FIG I.4Application of functional architecture to cases of single an d multi-channel 1-R regeneration (amplification) and channel cross connection

Note: Line terminals and t rails etc are no t sh own for simplification

OTSn/OMSn_A

OMSn/

OCH_A

OMSn_T

OTSn/OMSn_A

OTSn/

OMSn_AOTS1/OMS1_AOTS1/

OMS1_A

OTS1_T OTS1_T OTSn_T OTSn_T OTSn_T

Single Channel 1-R Multi-Channel 1-ROptical channel cross-connection

OMS1/

OCH_A

OMS1_T

OTS1_T

OCH_SNC

OTS1/OMS1_A

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Requirement Explanation, Example Func. OCh OMS OTS Comments

Connectivity supervision Trail trace identifier TT R R R The trail trace must be integral to theconnection that it is validating. The

connectivity of each layer network shall

be verifiable

Types of connection

monitoring

See section 6.xx _ _ _ _ See section 6.xx

Accidental openconnection

TT R R may also be achieved using TMN

Continuity supervision Indicate presence or

absence of signal

TT R R R

Signal quality supervision Layer specific supervision

of optical parameters such

as OSNR, power,

frequency

TT R R R For localisation of transmission

problems and indication of signal

degradation. Parameters are for further

study

Payload type Type of client signals

(signal label) e.g.channel wavelengths,

no. active tributaries,

maximum number of

tributaries,

payload status,

test signals, idleall are ffs

A R R R For channels might need to indicate

number and wavelengthsIdentify client of OTN

Identify tributaries

Maintenance information Forward defect indication A/T R R A maintenance information signal

which indicates towards a trailtermination that the signal transported

over a trail has been interrupted due to a

defect in a link connection supportingthe trail

Adaptation generates FDI, trail

termination detects

Remote information Remote defect/failure TT R ffs ffs Further work is required to identify in

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indication which scenarios this may apply e.g. pt-

mpt

Protection control Automatic protection

switching protocol

A/T ffs ffs ffs APS is protection architecture

dependent and may not be required in

all circumstances or in each layersee section 7

Managementcommunication

Message based channelDCC (Auxiliary channel)

A ffs R OTS DCC could be used to manage all3 layer networks. OMS is

reconfigurable and may not allow a 2ndDCC

Engineering order wire Auxiliary channel A O Not required by all operators, function

can be replaced with a mobile phone

Optical supervisory

channel

(OTS)

Fault, performance,

configuration

A/T R LOS, LOF (if applicable) EDC, EDV

performance monitoring

Optical mux section

overhead

Fault, performance of

overhead carrier

A/T R

Optical channel overhead Fault, performance of

overhead carrier

A/T R

TCM/subnetwork

monitoring using sublayers

correlation of monitoring

information

A/T R ffs

Reserved/unassigned A/T R R R Reserve for future requirements

A Adaptation process

TT Trail termination processA/T may be assigned to one or both, allocation is for further study

ffs for further study

D Desirable

O Optional

R Required

Not required

Note: The table does not describe any OAM for client specific processes in the Och/Client adapatation

Table 1: OAM requirements for the optical transport network

ITU-T\COM-T\COM13\C\030E.DOC