57426430-PCM-PDH-and-SDH

PCM , PDH AND SDH

(DIFFERENCES)

T1, E1, E3 AND DS3 (STANDARDS)

By

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

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

PCM SYSTEM BLOCK DIAGRAM

Sample & Hold Comparator Ramp Generator Binary Counter Parallel to Serial Converter

All pulses have same height and width.

0 1 2 3 t x(t)

Pulse Code Modulation

(PCM)

0 1 2 3 n x[n]

Pulse Code Modulation

(PCM)

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 f_s= 8 kHz (WHY..???)

 Number of quantization levels = 256

Number of Bits/Sample n = 8 (log₂256 )

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 BW_min = 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).

1

3

PRIMARY MULTIPLEXING

TRUNK NETWORK (T1 = BELL D2)

Digital

switch

Digital

switch

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

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

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,

PCM BASED TDM SYSTEMS

T1 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

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.

1

9

PRIMARY MULTIPLEXING

TRUNK NETWORK (E1 = CEPT30)

Digital

switch

Digital

switch

n*30*64 Kb/s

n*2048 Kb/s

2

0

HIGHER ORDER

MULTIPLEXING

Optical Fiber or Microwave Link

Digital

switch

Digital

2

1

SYNCHRONOUS

MULTIPLEXING

OF ALMOST SYNCHRONOUS DATA FLOWS

D C B A

E

S

R

Q

P

T

F

E S T F D 1 0 R C Q B P A

1 Frame

S C

f

_out

> n * MAX(f

_in

)

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

2

3

PLESIOCHRONOUS DIGITAL

HIERARCHY



Each multiplexed section has its own clock



Each level of multiplexing has its own clock



Frame structure from multiplexed signals is not

explicitly present in the multiplexed stream

PDH PRINCIPLE

If 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 OVERHEAD

In addition to locking on to bit-rate

we need to recognize the frame structure

We identify frames by adding

F

rame

A

lignment

S

ignal

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 %

OAM

analog 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 to

Operations, 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

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

S

ynchronous

O

ptical

NET

work



Designed for optical transport (high bitrate)



Direct mapping of lower levels into higher

ones



Carry all PDH types in one universal

hierarchy



ITU version =

S

ynchronous

D

igital

H

ierarchy



different 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

3 4

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

_µ

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

51.84Mbps

1.544Mbps 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 DS1 1.544Mbps

SONET/SDH BENEFITS

• Reduce costs

simplified standard interfaces

eliminate vendor proprietary interfaces

• Integrated network elements

enhanced operations capabilities

• Survivability

grants upgradability (modularity)

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 (

A

dd/

D

rop

M

ultiplexers)



input and output at this level are

V

irtual

T

ributaries (

VC

s)



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 regenerat ADM or secti on section secti on section

STS, OC, ETC.

A SONET signal is called a

S

ynchronous

T

ransport

S

ignal

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 –

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.

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

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.

5

2

SDH RINGS

5

3

5

4

5

5

SDH RINGS

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