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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)