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PCM , PDH AND SDH(DIFFERENCES)T1, E1, E3 AND DS3 (STANDARDS)
By
Abdul Wahab
OUTLINE
Pulse Code Modulation (PCM) PCM Based TDM Systems T1,E1 etc. Plesiochronous Digital Hierarchy (PDH) Synchronous Digital Hierarchy
PULSE CODE MODULATION
PCM is the most commonly used technique in digital communications
A primary building block for advanced communication systems
Used in many applications: Telephone systems Digital audio recording CD laser disks digital video etc
PULSE CODE MODULATION Based on the sampling theorem Each analog sample is assigned a binary code
Analog samples are referred to as pulse amplitude modulation (PAM) samples
The digital signal consists of block of n bits, where each n-bit number is the amplitude of a PCM pulse
PCM SYSTEM BLOCK DIAGRAM
Sample & Hold Comparator
Ramp Generator
Binary Counter
Parallel to Serial Converter
All pulses have same height and width.
f(t)
0
1
2
3
t
x(t)
Pulse Code Modulation (PCM)
Consider the analog Signal x(t).
0
1
2
3
n
x[n]
Pulse Code Modulation (PCM)
The signal is first sampled
QUANTIZATION
Is the process of converting the sampled signal to a binary value
Each voltage level will correspond to a different binary number
The magnitude of the minimum step size is called the resolution.
The error resulting from quantizing is called the quantization noise. Its value is 1/2 the resolution
0
1
2
3
t
x~(t)Quantized Signal
It is quite apparent that the quantized signal is not exactly the same as the original analog signal. There is a fair degree of quantization error here. However; as the number of quantization levels is increased the quantization error is reduced and the quantized signal gets closer and closer to the original signal
Pulse Code Modulation (PCM)
PCM OF SPEECH SIGNALS (VERY-IMPORTANT) Most of the significant spectral components of speech
signals are contained in the range 300-3400 Hz
Nyquist Rate = 2x3400 = 6.8 kHz
Practical Sampling Rate fs= 8 kHz (WHY..???)
Number of quantization levels = 256
Number of Bits/Sample n = 8 (log2256 )
Data Rate = nfs = 8x8000 = 64 kbps
PCM OF SPEECH SIGNALS (VERY-IMPORTANT)
Bandwidth Requirement
Communication theory tells us that we can transmit errorfree at most two pieces of information per second per hertz bandwidth (lathi pg. 260)
Therefore the minimum required bandwidth for transmission of a PCM speech signal BWmin = 64/2 = 32 kHz
We may require more bandwidth but the signal is now digital and we now have the ability to manipulate, store, regenerate the data. (see advantages of Digital Communication pg 263 of lathi)
PCM BASED TDM SYSTEMS
PCM is widely used in transmission of speech signals in fixed line telephone system.
An example PCM, the T1 carrier system which was developed at Bell labs in the US. And is still in use today in the US and Japan.
A similar scheme called the E1 is used in Europe and Pakistan.
These schemes are used to multiplex the speech from multiple subscribers and transmit them to their destinations over a common “Time Shared” channel. Hence the name time division multiplexing (TDM).
13
PRIMARY MULTIPLEXINGTRUNK NETWORK (T1 = BELL D2)
Digitalswitch
Digitalswitch
n*23*64 Kb/s
n*1544 Kb/s
PCM BASED TDM SYSTEMS T1
The sampling rate used for voice = 8000 samples/sec
Therefore, Sampling Interval = 1/8000 = 125µs
This means that the time between two consecutive samples (from the same source) is 125µs. TDM systems exploit this fact and utilize this interval to sample signals from other subscribers. In T1 systems the signals from 24 subscribers is sampled in 125µs.
The samples are quantized and then converted into a bitstream for transmission over the channel.
PCM BASED TDM SYSTEMS T1
As mentioned previously, sampling rate used for voice = 8000 samples/sec
Every sample is represented by 8 bits Therefore,
Data rate of 1 voice channel = 8x8000 = 64kbps
In the T1 system 24 voice channels are multiplexed in time
therefore,
Data rate of a T1 stream should be = 24x64kbps = 1.536 Mbps
However, the actual data rate = 1.544Mbps
The extra 8000 bps (1.544-1.536=.008Mbps) result from the overhead bits which are inserted alongside the data (details ahead).
PCM BASED TDM SYSTEMS T1
The T1 carrier system multiplexes binary code words corresponding to samples of each of the 24 channels in a sequence. A segment containing one codeword (corresponding to one sample) from each of the 24 channels is called a FRAME.
Each frame has 24x8 = 192 data bits and takes 125µs.
At the receiver it is also necessary to know where a frame starts in order to separate information bits correctly. For this purpose, a Framing bit is added at the beginning of each frame.
Therefore,
Total number of bits/ frame = 193
PCM BASED TDM SYSTEMST1 FRAME FORMAT
Along with voice data, frames should also contain: Framing bits and Signaling bits.
Framing Bits: Indicate start of frames.
Signaling Bits: Contain control information such as Routing Information, On-Hook/ off-Hook signals, Alarm signals etc.
PRIMARY MULTIPLEXING E1
The international standard for primary rate telephone multiplexing uses 2048 Kb/s (E1) links. Each E1 link carries 32 channels at 64 Kb/s each. 30 channels are used for carrying voice, one for signaling and one for synchronization and link management.
19
PRIMARY MULTIPLEXINGTRUNK NETWORK (E1 = CEPT30)
Digitalswitch
Digitalswitch
n*30*64 Kb/s
n*2048 Kb/s
20
HIGHER ORDER MULTIPLEXING
Optical Fiber or Microwave Link
Digitalswitch
Digitalswitch
21
SYNCHRONOUS MULTIPLEXING OF ALMOST SYNCHRONOUS DATA FLOWS
D C B AE
S R Q PT
F
EST D C B AR Q P01F
1 Frame
S C
fout > n * MAX(fin)
Primary rate dataflows to be multiplexed can be derived from independent clocks !
PLESIOCHRONOUS DIGITAL HIERARCHY (PDH) The Plesiochronous Digital Hierarchy (PDH) is a
technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems. The term plesiochronous is derived from Greek plēsios, meaning near, and chronos, time, and refers to the fact that PDH networks run in a state where different parts of the network are nearly, but not quite perfectly, synchronised.
PDH is typically being replaced by Synchronous Digital Hierarchy (SDH) or Synchronous optical networking (SONET) equipment in most telecommunications networks.
PDH allows transmission of data streams that are nominally running at the same rate
23
PLESIOCHRONOUS DIGITAL HIERARCHY
Each multiplexed section has its own clockEach level of multiplexing has its own clockFrame structure from multiplexed signals is
not explicitly present in the multiplexed stream
> Full demultiplexing required at each node !
PDH PRINCIPLEIf we want yet higher rates, we can mux together TDM signals
(tributaries)
We could demux the TDM timeslots and directly remux them but that is too complex
The TDM inputs are already digital, so we must insist that the mux provide clock to all tributaries (not always possible, may already be locked to a network)
OR somehow transport tributary with its own clock
across a higher speed network with a different clock (without spoiling remote clock recovery)
Y(J)S SONET Slide 24
PDH HIERARCHIES
Y(J)S SONET Slide 25
64 kbps
2.048 Mbps
1.544 Mbps
1.544 Mbps
6.312 Mbps
6.312 Mbps
8.448 Mbps
34.368 Mbps
139.264 Mbps
44.736 Mbps 32.064 Mbps
97.728 Mbps
274.176 Mbps
CEPT N.A. Japan
4
3
2
1
0
level
* 30* 24
* 24
* 4
* 4
* 4
* 4
* 7
* 6
* 4
* 5
* 3
E1
E2
E3
E4
T1
T2
T3
T4
J1
J2
J3
J4
FRAMING AND OVERHEADIn addition to locking on to bit-rate
we need to recognize the frame structure
We identify frames by adding Frame Alignment Signal
The FAS is part of the frame overhead (which also includes "C-bits",
OAM, etc.)
Each layer in PDH hierarchy adds its own overhead
For example E1 – 2 overhead bytes per 32 bytes – overhead 6.25 % E2 – 4 E1s = 8.192 Mbps out of 8.448Mbps
so there is an additional 0.256 Mbps = 3 % altogether 4*30*64 kbps = 7.680 Mbps out of 8.448 Mbps
or 9.09% overhead
What happens next ? Y(J)S SONET Slide 26
PDH OVERHEAD
Overhead always increases with data rate !Y(J)S
SONET Slide 27
digital
signal
data rate
(Mbps)
voice
channels
overhead
percentage
T1 1.544 24 0.52 %
T2 6.312 96 2.66 %
T3 44.736 672 3.86 %
T4 274.176 4032 5.88 %
E1 2.048 30 6.25 %
E2 8.448 120 9.09 %
E3 34.368 480 10.61 %
E4 139.264 1920 11.76 %
OAManalog channels and 64 kbps digital channels
do not have mechanisms to check signal validity and quality
thus major faults could go undetected for long periods of time hard to characterize and localize faults when reported minor defects might be unnoticed indefinitely
Solution is to add mechanisms based on overhead
as PDH networks evolved, more and more overhead was
dedicated toOperations, Administration and Maintenance (OAM) functions
including: monitoring for valid signal defect reporting alarm indication/inhibition (AIS)
Y(J)S SONET Slide 28
LIMITATIONS OF PDH Three incompatible PDH standards are used globally
(North American, Japanese, European) No worldwide optical interface standard (vendor
specific) Insufficient capacity for network management Complex de-multiplexing structure to extract a
particular tributary signal (e.g extracting E1 from E4) PDH based networks do not meet present & future
telecom demands (maximum BW offered by PDH is E4) Overhead percentage increases with rate Inability to identify individual channels in a higher-order
bit stream.
SONET/SDH
MOTIVATION AND HISTORY
Y(J)S SONET Slide 30
COMPARING CLOCKS
A clock is said to be isochronous (isos=equal, chronos=time)
if its ticks are equally spaced in time
2 clocks are said to be synchronous (syn=same chronos=time)
if they tick in time, i.e. have precisely the same frequency
2 clocks are said to be plesiochronous (plesio=near
chronos=time)
if the same frequency but are not locked
Y(J)S SONET Slide 31
IDEA BEHIND SONET
Synchronous Optical NETwork Designed for optical transport (high bitrate) Direct mapping of lower levels into higher
ones Carry all PDH types in one universal
hierarchy ITU version = Synchronous Digital
Hierarchydifferent terminology but interoperable
Overhead doesn’t increase with rate OAM designed-in from beginning
Y(J)S SONET Slide 32
SYNCHRONOUS DIGITAL HIERARCHY (SDH)
Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber
Lower data rates can also be transferred via an electrical interface
Difference from PDH SONET/SDH are tightly synchronized across the entire network Greatly reducing the amount of buffering SONET and SDH can be used to encapsulate earlier digital
transmission standards
34
SYNCHRONOUS DIGITAL HIERARCHY
STM-1 STM-1
Up to 63 channels at 2 Mb/s
– The entire trunk network has one clock
– Multiplexed stream based on 125 S frames
– Different channels can each have their own asynchronous clock.
– Add-drop multiplexers
STANDARDS AND APPLICATIONS OF SDH
• Why SONET/SDH?
• SONET/SDH solution
• SDH format
• SDH mapping/multiplexing
• SDH pointer application
WHY SONET/SDH
• SONET/SDH’s goal simplify interconnection between network operators expand the compatibility
• Imperfection of PDH Three different regional digital hierarchies Rate & Format conversion induces extra high cost to customers
• Demanding broadband services To the high speed signals, the processing time for performing conversion between PDH region is not long enough
BASIC UNIT OF FRAMING IN SDH The basic unit of framing in SDH is a STM-1 (Synchronous
Transport Module, level 1), which operates at 155.52 megabits per second (Mbit/s). SONET refers to this basic unit as an STS-3c (Synchronous Transport Signal 3, concatenated) or OC-3c, depending on whether the signal is carried electrically (STS) or optically (OC), but its high-level functionality, frame size, and bit-rate are the same as STM-1
SONET/SDH SOLUTION
• Modularity
OC-1
OC-192
OC-12
OC-3
OC-48
STM-1
STM-4
STM-16
STM-64
51.84
155.52
622.08
2488.32
9953.28
155.52
622.08
2488.32
9953.28
Speed Unit (Mbps)
SONET/SDH SOLUTION (DS3)
• Fixed percentage overhead
Mux Mux Mux
DS1 OC-1 OC-3 OC-12
28 3 4
OH
51.84Mbps1.544Mbps 155.52Mbps 622.08Mbps
• Overhead insertion for PDH signals
Mux Mux Mux
Voice DS2 DS3
24 4 7
OH1
64Kbps 6.312Mbps 44.736Mbps
OH2 OH3
DS11.544Mbps
SONET/SDH BENEFITS
• Reduce costs simplified standard interfaces eliminate vendor proprietary interfaces
• Integrated network elements enhanced operations capabilities
• Survivability grants upgradability (modularity)
• No bandwidth bottlenecks
SONET/SDH
ARCHITECTURE
Y(J)S SONET Slide 41
LAYERS
SONET was designed with definite layering concepts
Physical layer – optical fiber (linear or ring) when exceed fiber reach – regenerators regenerators are not mere amplifiers, regenerators use their own overhead fiber between regenerators called section (regenerator
section)
Line layer – link between SONET muxes (Add/Drop Multiplexers) input and output at this level are Virtual Tributaries (VCs) actually 2 layers
lower order VC (for low bitrate payloads) higher order VC (for high bitrate payloads)
Path layer – end-to-end path of client data (tributaries) client data (payload) may be
PDH ATM packet data Y(J)S
SONET Slide 42
SONET ARCHITECTURE
SONET (SDH) has at 3 layers: path – end-to-end data connection, muxes tributary signals path section
there are STS paths + Virtual Tributary (VT) paths
line – protected multiplexed SONET payload multiplex section section – physical link between adjacent elements regenerator section
Each layer has its own overhead to support needed functionality
SDH
terminology Y(J)S SONET Slide 43
Path
Termination
Path
Termination
Line
Termination
Line
Termination
Section
Termination
path
line line line
ADM ADMregenerator
section
section section
section
STS, OC, ETC.
A SONET signal is called a Synchronous Transport Signal
The basic STS is STS-1, all others are multiples of it - STS-N
The (optical) physical layer signal corresponding to an STS-N is an OC-N
Y(J)S SONET Slide 44
SONET Optical rate
STS-1 OC-1 51.84M
STS-3 OC-3 155.52M
STS-12 OC-12 622.080M
STS-48 OC-48 2488.32M
STS-192 OC-192 9953.28M
* 3
* 4
* 4
* 4
SONET/SDH TRIBUTARIES
E3 and T3 are carried as Higher Order Paths (HOPs)E1 and T1 are carried as Lower Order Paths (LOPs)
(the numbers are for direct mapping)
Y(J)S SONET Slide 45
SONET SDH T1 T3 E1 E3 E4
STS-1 28 1 21 1
STS-3 STM-1 84 3 63 3 1
STS-12 STM-4 336 12 252 12 4
STS-48 STM-16 1344 48 1008 48 16
STS-192 STM-64 5376 192 4032 192 64
NO COMMON STANDARD
Before SDH there were no standards to ensure that equipment from different vendors interworked on the same system.
Vendors can have their own unique designs which means we have to buy the same vendor’s equipment for both ends of the line.
Ideally we would like to shop around for the most suitable equipment, without having to keep to the same supplier.
ADVANTAGES OF SDH Designed for cost effective, flexible telecoms
networking – based on direct synchronous multiplexing.
Provides built-in signal capacity for advanced network management and maintenance capabilities.
Provides flexible signal transportation capabilities – designed for existing and future signals.
Allows a single telecommunication network infrastructure – interconnects network equipment from different vendors
ADVANTAGES OF SDH
SDH integrates three major digital hierarchies of the world
SDH offers standard optical interfaces (ITU-T based)
Simple and direct multiplexing / de-multiplexing method for adding or dropping electrical signals
Rich overhead bytes (OAM=4%) for management, maintenance, and operation. Supports powerful network management system.
Support flexible and self-healing networks (protection)
ADVANTAGES OF SDH
Both synchronous and plesiochronous operations are possible.
Bit rates exceeding 140Mb/s are standardized on a worldwide basis.
All current PDH signals can be transmitted within the SDH except 8 Mb/s (E2) which has no container.
A reduction in the amount of equipment & an increase in network reliability.
DISADVANTAGES OF SDH
Bandwidth utilization is comparatively poor than PDH (waste of BW due to various management overhead bytes)
SDH equipments are complicated to deal with due to variety of management traffic types and options.
SDH adopts large-scale software control which makes it vulnerable to man-made mistakes, software bugs, configuration problems, etc.
.
WHERE IS SDH USED ?
SDH can be used in all of the traditional network application areas.
A single SDH network infrastructure is therefore possible which provides an efficient direct interconnection between the three major telecommunication networks.
52SDH RINGS
34 Mb/s 2 Mb/s
53
SDH RINGS
54SDH RINGS
55SDH RINGS
CUT !
NOTES ON SDH RATES
The most common SDH line rates in use today are 155.52 Mbps, 622.08 Mbps, 2.5 Gbps, 10 Gbps.
SDH is a structure that is designed for the future, ensuring that higher line rates can be added when required.
SUMMARY PCM is widely used in transmission of speech signals in fixed
line telephone system. Example of is PCM, the T1 and E1 The nominal data rate on the multiplexed (T1) link is 1544
Kb/s which is the result of multiplexing 24 channels at 64 Kb/s Each E1 link carries 32 channels at 64 Kb/s each. 30 channels
are used for carrying voice, one for signaling and one for synchronization and link management.
Digital Signal 3 (DS3) is a digital signal level 3 T- Carrier. It may also be referred to as a T3 line. The data rate for this type of signal is 44.736 Mbit/s.
PDH allows transmission of data streams that are nearly running at the same rate replaced by SDH
Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber
QUESTIONS?