57426430-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 nbit number is the amplitude of a PCM pulse

PCM SYSTEM BLOCK DIAGRAM

f(t)

Sample & Hold

Ramp Generator

Comparator

Binary Counter

Parallel to Serial Converter

All pulses have same height and width.

x(t)

Pulse Code Modulation (PCM)

3

2

1

0 t Consider the analog Signal x(t).

x[n]

Pulse Code Modulation (PCM)

3

2

1

0 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

Pulse Code Modulation x~(t ) (PCM) Quantized Signal 3

2 1

0 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

t

PCM OF SPEECH SIGNALS (VERYIMPORTANT) 

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 (VERYIMPORTANT) 

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).

PRIMARY MULTIPLEXING TRUNK NETWORK (T1 = BELL D2)

13

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 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 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 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.

PRIMARY MULTIPLEXING TRUNK NETWORK (E1 = CEPT30)

19

Digital switch

Digital switch

n*30*64 Kb/s n*2048 Kb/s

HIGHER ORDER MULTIPLEXING 20

Digital switch

Digital switch

Optical Fiber or Microwave Link

SYNCHRONOUS MULTIPLEXING OF ALMOST SYNCHRONOUS DATA FLOWS

F

E

D

C

B

1 Frame

A T F S E

T

S

R

Q

P

D 1 0 R C Q B P A

S C fout > n * MAX(fin )

21

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 

PLESIOCHRONOUS DIGITAL HIERARCHY  Each

23

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 > Full demultiplexing required at each node !

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

PDH HIERARCHIES level 0

64 kbps *

30

*

24

*

24

1

E1

2.048 Mbps * 4

T1

1.544 Mbps * 4

J1

1.544 Mbps * 4

2

E2

8.448 Mbps * 4

T2

6.312 Mbps * 7

J2

6.312 Mbps * 5

J3

32.064 Mbps * 3

J4

97.728 Mbps

3

E3 34.368 Mbps *

4

E4

4

139.264 Mbps CEPT

T3 44.736 Mbps *

T4

6

274.176 Mbps N.A.

Japan

Y(J)S SONET

FRAMING AND OVERHEAD In 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

PDH OVERHEAD digital signal

data rate

voice

overhead percentage

(Mbps)

channels

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 %

Overhead always increases with data rate ! Y(J)S SONET

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

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

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

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 Hierarchy  different terminology but interoperable  Overhead doesn’t increase with rate  OAM designed-in from beginning Y(J)S SONET

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

SYNCHRONOUS DIGITAL HIERARCHY – The entire trunk network has one clock – Different channels can each have their own asynchronous clock. – Add-drop multiplexers STM-1

STM-1

Up to 63 channels at 2 Mb/s

34

– Multiplexed stream based on 125 µ S frames

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

implify interconnection between network operators expand the compatibility

mperfection 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 bitrate are the same as STM-1

SONET/SDH SOLUTION

• Modularity 51.84

OC-1

155.52

OC-3

STM-1

155.52

622.08

OC-12

STM-4

622.08

2488.32

OC-48

STM-16

2488.32

9953.28

OC-192

STM-64

9953.28 Speed Unit (Mbps)

SONET/SDH SOLUTION (DS3)

• Fixed percentage overhead DS1 1.544Mbps × 28

Mux

OC-1 51.84Mbps ×3

Mux

OC-3 155.52Mbps ×4

Mux

OC-12 622.08Mbps

OH • Overhead insertion for PDH signals Voice 64Kbps × 24

Mux OH1

DS1 1.544Mbps ×4

Mux OH2

DS2 6.312Mbps ×7

Mux OH3

DS3 44.736Mbps

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

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

SONET ARCHITECTURE regenerat ADM

Path

Line

Termination

Termination

ADM

or Section Termination

Line

Path

Termination

Termination

path line secti on

line section

line section

SONET (SDH) has at 3 layers: 

secti on

path – end-to-end data connection, muxes tributary signals path section 

there are STS paths + Virtual Tributary (VT) paths



line – protected multiplexed SONET payload



section – physical link between adjacent elements

multiplex section regenerator section

Each layer has its own overhead to support needed functionality SDH terminology

Y(J)S SONET

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

*3

STS-12

OC-12

622.080M

*4

STS-48

OC-48

2488.32M

*4

STS-192

OC-192

9953.28M

*4 Y(J)S SONET

SONET/SDH TRIBUTARIES SONET

SDH

STS-1

T1

T3

E1

E3

28

1

21

1

E4

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

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

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. 

SDH RINGS 52

34 Mb/s

2 Mb/s

SDH RINGS 53

SDH RINGS 54

SDH RINGS 55

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?