telecommunications switching systems 16

7/29/2019 Telecommunications Switching Systems 16

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Synchronous Transmission

The synchronization requirements implythat a certain minimum density of signaltransitions is required to providecontinuous indication of signalingboundaries.

Six techniques to recover timing information

Source code restriction

Dedicated timing bits

Bit insertion

Data scrambling

Forced bit errors

Line coding


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Source code restriction

Restrict the code set or data patterns ofthe source so that long-transition-free datasequences do not occur.

00000000 or 11111111

Dedicated Timing Bits

Transition-bearing bits are periodicallyinserted into data stream.

Only 7 out of 8 bits are available for user.The unused bit provides an assurance thatall 8 bits are not 0.

The density of timing pulses ranges from 1in 5 bits to 1 in 20 bits.


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Bit Insertion

Another possibility for precludingunwanted line patterns is to use bit-insertions only when necessary.

A line could be monitored for all 0s in thefirst 7 bits of a time slot. Whenever the 0soccur, a 1 could be inserted into the datastream as the eight bit of the time slot.

Data Scrambling

Data scramblers randomize data patternson their transmission links.

Scrambling is not used on lower ratesystems.


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Forced Bit Errors

Force an occasional bit error in order tointerrupt a long, transition-free datapattern.

Intentional errors are less frequent thanrandom channel errors if transition-freesequences are long and uncommon.

Not recommended with ARQ facility onlink.

Line Coding

Chapter 4


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Purpose of line coding

Extract the DC content from the message

Add synchronization information into line signal

Increase information data rate through thechannel

Change the spectral shape of the message sothat it suits the channel better

Improve performance (error detection and

correction) Compress data

Line Coding Level encoding Bipolar encoding

BNZS

Pair selected Ternary

Ternary coding

Digital Biphase

Differential encoding

Coded Mark inversion Multilevel signaling

Partial response signaling


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Unipolar and polar (NRZ) line codes

DC wander


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Bipolar (AMI) Coding

Spectral Density of Bipolar Coding


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Digital Biphase (Manchester)


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Differential Encoding


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Coded Mark Inversion

CMI encodes 1s as an NRZ level oppositeto the level of the previous one and 0s asa half cycle square wave of one particularphase

CMI


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BNZS: Binary N-ZeroSubstitution

A major limitation of bipolar (AMI) codingis its dependence on a minimum density of1s in the source code to maintain timinginformation.

BNZS augments a basic bipolar byreplacing all strings of N 0s with a special

N-length code containing several purposesthat purposely produce bipolar violations.

B3ZS

Three 0s encoded as 00V or B0V

The decision to substitute with 00V or B0Vis made so that the number of B pulses(unviolated) between violations is odd.


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B3ZS

If an odd number of 1s has been transmittedsince last substitution, 00V is chosen to replacethree 0s.

If the intervening number of 1s is even, B0V ischosen.

In this manner, all purposeful violations containan odd number of intervening bipolar pulses.

Example

Determine B3ZS code for the followingdata sequence:

101000110000000010001


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Solution

HDB3 (high density bipolar 3)

B6ZS


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B8ZS

nBmT codes

Another class of ternary codes are knownas alphabet codes.

In this coding scheme, nbinary digitstaken together are coded into m-digitternary character.

2n

binary characters

3m

ternarycharacters

Generally described as nBmTcodes


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Pair Selected Ternary (PST)

2B2T codes

A balance between positive and negativepulses and a strong timing component aremaintained by switching between the

modes appropriately.

2B2T codes

As the ternary digits are transmitted, sum ofpositive and negative pulses is kept.

If the sum is zero, the mode remainsunchanged,

if a single pulse has been transmitted & if

the sum is positive, -mode is selected and ifthe sum is negative, +mode is selected.


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Example (PST)

Encode the following binary data stream into aPST line code: 01001110101100 (assume initialsum = 0).

Solution

Case1 (+Mode) [0+ -+ +- -0 +0 +- -+]

Case 1 (- Mode) [0- -+ +- +0 -0 +- -+]

Spectrum of Bipolar, B6ZS, and PST line codes


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4B3T code

4B3T

Ternary words in the middle column are dcbalanced.

Codes from first and third columns areselected alternatively to maintain dcbalance.

All 0s code is not selected to maintaintiming component.


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Line Coding

Unipolar, Polar NRZ

Bipolar (AMI)

Digital Biphase (Manchester)

Differential encoding

CMI

BNZS

nBmT (PST)

telecommunications switching systems 16

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