PCM , PDH and SDH

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]


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


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.


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).


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


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)


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


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


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


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


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


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)


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?

Documents

PCM , PDH and SDH