Digital Signaling Techniques. Maryam Heidari. Objective. To discuss various number of methods most used for line coding. Introduction/Background. Digital data could be sent across communication lines by various line codes.
To discuss various number of methods most used for line coding.
1-Small Bandwidth : We need small BW in order to be able to send more signals in a communication channel.
2-Enough Timing Content : It should contain enough timing information, so can the receiver can extract the clock information and decode the signal.
3-Small probability of error : The lower probability of error makes the line code more reliable.
4-Good power efficiency : For a specific BW the transmitted power should be small.
5-Power spectral density : Because of the ac coupling in the transformers and repeaters it is desirable to have a ‘0’ dc in the waveform generated by a line code.
6-Transparency : The data should be coded ,so that regardless of ‘0s’ and ‘1s’ the coded signal is received correctly.
Generally there are 2 group of line codes:
1-Level codes : They are independent of the past data and they carry information on their voltage level, 2 common forms of level codes are RZ and NRZ .
RZ:The pulse level will go to zero for a portion of bit duration.
NRZ:The pulse level remains constant during the bit duration.
Unipolar: Only one polarity is used.( + or -)
Bipolar: Both voltage levels are used.
2- Transition codes : The current bit level depends on the previous levels.
These codes have memory.
In This line code a one is represented by a positive voltage level and a zero is represented by no pulse.
The BW is equal to R hertz (data rate).
And the probability of error is :
Advantages : Easy to generate.
Disadvantages : large DC component, not desirable error rate and not transparent.
This line code is most used in magnetic tape recording.
Here a zero is represented by no pulse and a 1 is represented by a level during a portion of the bit length.
Advantages: The dc level is lower than NRZ.
It can be easily generated.
Disadvantages : Non zero dc value.
Higher BW (2R)
This code is most used in baseband data transmission and magnetic tape recording.
In this line code a 1 is represented by +v and a zero is represented by –v or vice versa.
Advantages: Lower bandwidth, better error probability by 3 db and reduced dc value.
Disadvantages: Not capable of error detection, 2 power supply is needed and not transparent.
In This case a 0 is sent by no pulse and a 1 is sent by alternating positive and negative levels depending on whether the previous 1 was sent by + or – voltage level. It is also RZ.
Advantages: low BW, Zero dc value, capable of single error detection,good error probability and capable of recovering clock information.
Disadvantages: not transparent, it needs twice much power as unipolar (3 db) and it needs 2 power supplies.
This code has memory and is most used in telephone systems.
Because the AMI is not Transparent other methods are used to prevent long strings of zeros. HDBN also does not have any dc value and have the same data rate.
In this case when a run of N+1 zeros happens, they will be replaced by a code of length N+1 containing AMI violation.
The most popular form of HDBN is HDB3 ; which uses to special sequences :000V and B00V.
Because violation just happens at the fourth bit of the special code , it can be easily detected and will be replaced by a zero at the receiver.
It is also capable of error detecting because a sign error would make the number of bipolar pulses between violations even instead of odd.
For example in B8ZS a string of 8 zeros will be replaced by 000VB0VB where V’s are bipolar violation and B’s are valid bipolar signals.
B8ZS is most used in DS1 signals and in North America.
Manchester coding(split phase or digital phase)
In this line code transitions occur at the middle of each bit. A high to low transition represents a 1 and a low to high transition represents a 0 or vice versa.
Advantages : No dc Value, extracting timing information is easy and the error rate performance is good.
Disadvantages: High BW and not capable of error detecting.
And twisted pair CSMA /CD bus LANs.
All transition codes have memory and the current bit level depends on the previous bit levels.
Miller code (DM):
A ‘1’ is represented by a transition at the middle of the bit and a ‘0’ is represented by no transition unless it is followed by another zero. In this case another transition will occur at the end of the bit duration between 2 ‘0s’.
Advantages : good timing information recovery, no dc value and lower BW( R/2)
Disadvantages : not capable of error detecting.
Split phase (Mark)
This code is very similar to Manchester but it does have memory.
Here a ‘1’ produces an opposite transition relative to the previous transition but a ‘0’ does not produce and opposite transition.
Split phase(space) is similar to this code but the roles of ‘1s’ and ‘0s’ are changed.
Here a ‘1’ is represented by a mid bit transition but a ‘0’ transition occurs at the beginning of the bit duration.
This code has no dc value and is very easy to synchronize. It is most used in optical communication.
Code Mark Inversion(CMI):
Here a 1 is represented by a NRZ pulse with positive and negative levels alternatively. And a ‘0’ is represented by a mid bit transition.
It does not have dc value, but is has a good synchronization.This is a CCITT chosen code for multiplexers interface.
NRZ(M) and NRZ(S)
NRZ(S) is completely the reverse form of NRZ(M).
Clock information recovery is not good and there is also dc value.
A zero is represented by no transition and one by alternate transitions according to this rule:
Positive if the 1 is preceded by and even number of 0s.
Negative: if the 1 is preceded by an odd number of 0s.
This one is the derivative form of NRZ, so any positive and negative transitions generate + or – half-width pulsed.
We we define BW as null-to-null BW this table shows a good comparison:
In the previous few pages we defined and presented some popular line codes.
Their properties, advantages and disadvantages
Some of their applications was mentioned and finally a comparison of BWs were done.
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