telecommunications switching systems 18

7/29/2019 Telecommunications Switching Systems 18

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TDM

Multiplexing methods


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Two basic Modes of TDM

Synchronous TDM (STDM): repeatedlyassigns a portion of the transmission

capacity to each source

Asynchronous TDM (ATDM) or statisticalTDM or statistical multiplexing (Stat-Mux):assigns capacity as and when needed

Capacity Allocation

Bitwise

Wordwise


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Capacity Allocation

In Bitwise interleaving, the frame size is equal tothe number of channels multiplexed while inwordwise interleaving, it is equal to the productof the word-size and the number of channels.

Bitwise interleaving is natural if delta modulatorcoders/decoders are used and wordwiseinterleaving is natural if PCM codecs are usedfor digitization.

Framing

Frame Synchronization


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Main considerations in choosing a framingprocedure are:

Time required to establish framing

Effects of channel errors in maintainingframe synchronization

Relationships between line clock andsample clocks derived from line clock

Transmission overhead

Complexity of framing circuitry

Framing

The severity of a loss of framing and timerequired to re-establish framesynchronization depend on the nature oftraffic.

For voice traffic, infrequent misframes canbe tolerated if frame synchronization is re-establish rapidly enough to minimize theduration of glitch in the output speech


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Out of frame conditions occur in two ways:

The locally derived sample clock may losesynchronization with the line clock OR (thesource clock has too much jitter or generatesabrupt phase shifts)

Channel errors creating false out-of frameframing patterns

A loss of framing is determined when theoccurrence of framing pattern violationsexceeds some short term densitythreshold.

Considerable redundancy in the framingpattern is required to minimize theprobability of false misframes.


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Frame Sync.

Frame synchronization may be done in numberof ways.

In all the ways, there are one or more framingbits with an identifiable data sequence.

Frame bits may be added additionally to data

bits or some of the data bits may be used asframe bits.

Means of establishing Frame Sync.

Added-digit framing

Added-channel framing

Unique line signal framing

Statistical framing


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Added-Digit framing

The framing bit is added once for everyframe and alternates in value.

Framing is established at receiver bymonitoring first one bit position within N-bitframe and then another, until thealternating pattern is located.

To synchronize with the bit stream, the receiver picks a random bit, andthen examines every 193rd bit for the presence of the special framing pattern.

If too many received bits differ from the pattern, it delays one bit and

begins the search again.

Frame Time

With added-digit framing strategy, the expectedframing time from a random starting point withrandom data is

=

positionframing

anotisposition

ninformatioanthat

determinetobitsof

numberaverage

foundisbit

framingbefore

positionsbitof

numberaverage

TimeFrame


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Frame acquisition time of single-bit framecode

Assume that framing pattern alternates 1s and0s.

Assume that when beginning to test a particularbit position for framing, the value of the firstappearance is saved and compared to thesecond appearance.

Thus the minimum time to reject an invalidframing position is one frame time.

Frame Time

If we denote p by the probability of a 1 and q =1- p the probability that a 0 is received first, theaverage number of frames required to receive amismatch is A0.


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A0


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Frame time

If 1s and 0s are equally likely (p = q = 0.5),A = 2 so frame time = N2 + N bit times.

For N = 193, framing time is 37346 bits, or24.188 msec.

Also of interest is maximum framing time. Unfortunatelythere is no absolute maximum framing time for a T1 system

with random data. It is very unlikely, however, that theframing time would ever exceed the average search time forall bit positions, or 48.25 ms. This latter measure of framingtime is referred to as the maximum average frame time.

Framing time can be reduced by:

Examining one bit at a time, as earlier butduring a reframe the search begins a fewbit positions in front of the present positionunder assumption of small counter offsets.

Monitoring all bit positions simultaneouslyfor the framing pattern (Parallel search).


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The 1.544 Mbps PCM frame

12-frame superframe (SF) structure

Framing pattern of second generation channelbanks (D3, D4) from AT&T

These channel banks provide 8 bits of voice forall time slots except in every sixth frame, whichuses the least significant PCM bit for signaling.

The signaling channels thus derived are dividedinto an A and B sub-channel, implying each subchannel sends a bit in every twelfth frame.


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(F) = 101010;

(M) = 001110;

During the odd frames the framing bitalternates, while during the even frames

the framing bit sequence is00111000111000


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A signaling frame can be identified by a 0-to1transition in the even numbered framesequence.

Correspondingly, a 1-to-0 transition in the evennumbered frame sequence signifies a Bsignaling frame.

Frame acquisition begins by finding the

alternating bit sequence (with 385 interveningbits). Then, the 000111 framing pattern islocated.

telecommunications switching systems 18

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