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Basis is the sampling theoremNyquists rate of sampling

Extension of pulse modulationPCM coding

Useful for digital data(mainly for digitized speech

signals)Digitization is done by CODEC (coder/decoder)generates

8000 samples /sec satisfying Nyquists rate of 4 KHz telephone

channel BW. Then PCM is used to convert analog signal to

digital form

Time Division Multiplexing(TDM)

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Samples are separated at 125sec

In this interval the samples of other speech signals are

sent.

All clocks are synchronized.

T1 carrier system widely used (Japan &US) has 24

voice channels multiplexed and coded using PCM.

Principle of Time Division

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Ch-1

Ch-24

Clk-1

Clk-2

Ch-2

Muxed

signal

N-channel Multiplexing

Clk-24

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Techniques of T1 telephony

24 voice signals are sampled sequentially and resulting analogsignal is fed into CODEC

Every sample is coded with 7 bit binary code and the 8th bit is

used for control

So the PCM signal from each channel is an 8-bit word.

For 24 channels we have 248 =192 bits. One extra bit used

for frame synchronization- so 193 bits in total.

Speed of PCM is calculated as 193/125sec=1938000=1.544 Mbps. It is the digital signal(DS)-1

----

8(control bit)Frame125 sec (193 bits)

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International standard for PCM to use 30 speech channels with

8 bit coding for each sample.

It contains 32 time slots in a frame, each slot having an 8 bit

code.30 slots for channels and 2 for other purposes (0- flag &16-

signaling info) as shown below

Standards for PCM channels

1

0 1 3130

125 second frame

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Compaction techniques

Used to reduce the number of bits/channel afterdigitization

Following methods may be used

Differential PCM, Delta modulation, DPCM withpredictive coding

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Digital Hierarchy

One higher order multiplexed signal is obtained from

4 immediate lower order signals

Total bit rate is not exact multiple of lower order dueto additional flags and alarms

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Standards for Digital Multiplexing

Hierarchy

Order No of

channels

Bit

rate(Mbps)

composition

1 30 2.048 3064 Kbps

2 120 8.448 42.048 Mbps

3 480 34.368 48.448 Mbps

4 1920 139.264 434.368 Mbps

5 7680 564.992 4139.264Mbps

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T-series hierarchy is as follows :

4 T1 streams of 1.544 Mbps are Multiplexed to a stream of

T2 with speed 6.312 mbps

6 T2 carriers are multiplexed to a T3 stream of 44.736 Mbps 7 T3 carriers are multiplexed to a stream of T4 with speed

274.176 Mbps

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T-series table for Synchronous

TDM(time slot allotted regardless of

data sent or not)

Carrier No. of channels Bit rate(mbps)

T1 24 1.544

T2 96 6.312

T3 576 44.736

T4 4032 274.176

Every frame consists of a cycle of time slots . Each frame has one or more slot dedicated toeach data source.