tdm pulse code modulation

7/30/2019 Tdm Pulse code modulation

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MULTIPLEXING TYPES

Frequency Division Multiplexing

Time Division Multiplexing

Wavelength Division Multiplexing


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Categor ies of multiplexing


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FDM

I s the process of tr anslating

individual speech circuits (300- 3400H z) into pre assigned slots within the bandwith of transmission medium. and, the preassigned slots are always available to each user


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F DM


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F DM process


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F DM demulti plexing example


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Example 1


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TDM

The process where a tr ansmission medium is shared by a number of circuits in time domain by establishing a sequence of time slots dur ing which individual channels can be transmittedThus the enti re bandwidth is per iodically available to each channel


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TDM


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TDM fr ames


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PCM PROCESSES

FilteringSampling

QuantizationEncodingLine coding


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SAMPLING


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SAMPLING THEOREM

If a band limited signal is sampled at regular intervals of time and at a rate equal to or morethan twice the highest signal frequency in the

band, then the sample contains all theinformation of the original signal Fs= >2fH


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PULSE CODE MODULATION

Voice Frequency range 0- 4 Khz Sampling the Voice Signal @ 8 Khz

(Double the Max. Frequency as per sampling theorem) i.e. 8000s/sec

Sampling time period Ts=1sec/8000 Ts= 125 microsec Time available for sampling each channel,when we have N total channels=125/N In PCM, Time frame=125microsec ;time

available per chl=125/32 =3.9microsec.


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QUANTIZING

The process of measuring the numericalvalues of the samples and giving them atable value in a suitable scaleThe finite number of amplitude intervals iscalled the quantizing interval likequantizing interval no.1 is 10-20mV; 2 is20-30mV etc. in a case of 1V signal.Linear quantizing is where the quantizingintervals are of the same size


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QUANTIZING LEVELS


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QUANTIZING (one side)


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QUANTIZING

Quantization intervals are coded in binaryform, and so the quantization intervals will bein powers of 2.In PCM, 8 bit code is used and so we have 256intervals for quantizing (128 levels in the

positive direction and 128 levels in negativedirection)


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QUANTIZING (both sides)


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QUANTIZATION DISTORTION

The deviation between the amplitude of samples at the transmitter and receiving endsIn linear quantization, the distortion is moreand to decrease the distortion, the no. of stepsin the given amplitude range has to beincreased.

Due to BW limitations, more quantum levelsin small amplitude region are planned resultsto Non linear (uniform) quantization


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COMPANDING

Is the process where non uniform quantizationis achieved using segmented quantizationIn companding, to specify the location of sample value, the following arenecessarysign of the sample, the segmentno., the quantum level within the segment.


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SEGMENTATION


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PCM ENCODING


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FRAME STRUCTURE

In PCM we have 32 Ts and Ts 0 (FAW)carries the synchronization signals and FAWdigit value is X 0 0 1 1 0 1 1 . FAWtransmitted in alternate frame. In FAW unusedframes, supervisory and alarm signals aretransmitted

Ts 16 carries the signalling information (for 2channels)


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FAW/ALARM DIGIT CODES


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FRAME STRUCTURE

For carrying the signalling for all 30 chls andfor carrying sync. Data for all frames, in PCM16 frame pattern is used and it is known asmulti frameDuration of multi frame is 2msecs.


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PCM Standards

THERE ARE TWO STANDARDS OF PCM NAMELY 1) THE EUROPEAN 2 ) THE AMERICAN.THEY DIFFER SLIGHTLY IN THE DETAIL OF

THEIR WORKING BUT THE PRINCIPLES ARETHE SAME.EUROPEAN PCM = 30 CHANNELSNORTH AMERICAN PCM = 24 CHANNELS

JAPANESE PCM = 24 CHANNELSIN INDIA WE FOLLOW THE EUROPEAN PCM OF30 CHANNELS SYSTEM WORKING.


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EUROPEAN PDH HIERARCHY WITH BIT RATES

1920+/- 15 ppm139.264 Mbps140 Mbps

480+/- 20 ppm34.368 Mbps34 Mbps

120+/- 30 ppm8.448 Mbps8 Mbps

30+/- 50 ppm2.048 Mbps2 Mbps

CHANNELSPARTS PERMILLIONBIT RATEMUX


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Dig. Hier based on 24chl PCM


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MULTIPLEXING OF ASYNCHRONOUS SIGNAL

in order to move multiple ASYNCHRONOUS 2 mbps datastreams from one place to another, they are combinedtogether or multiplexed in groups of four. this is done by taking 1 bit/word from stream #1, followedby 1 bit/word from #2, then #3, then #4.the transmitting multiplexer also adds additional bits inorder to EQUAL or synchronise the bits in themultiplexer and the process adopted for suchsynchronization is called justification bits or pulse stuffing


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JUSTIFICATION TYPES

Positive justification: Commonsynchronization bit rate offered at eachtributary is higher than the bit rate of individual tributary.Positive-negative justification

Negative justification


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DIGITAL MUX CONCEPTS

BYTE INTERLEAVING WORD / BYTE / BLOCK INTERLEAVING:

IF THE CHANNEL TIME SLOT IS LONG ENOUGH TO ACCOMMODATE A GROUP OF BITS THEN THEMULTIPLEXED SIGNAL IS CALLED A BYTEINTERLEAVED OR WORD INTERLEAVED SIGNAL.

D4D3D2D1C4C3C2C1B4B3B2B1A4A3A2A1


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DIGITAL MUX CONCEPTS

BIT INTERLEAVING :

ALTERNATELY EACH CHANNEL CODE CAN BESCANNED ONE DIGIT AT A TIME. THEMULTIPLEXED SIGNAL IS CALLED A BITINTERLEAVED SIGNAL. BIT INTERLEAVING IS USED IN HIGHER ORDER MULTIPLEXING.

D4C4B4A4D3C3B3A3D2C2B2A2D1C1B1A1


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Encoded FDM USA&canada


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Encoded TDM Japanese


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Dig Hier based on 30chl PCM-Encoded TDM European

tdm pulse code modulation

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