Line coding and decoding - · PDF fileHDB3 (High Density Bipolar of order 3 code) Replacing series of four bits that are to equal to "0" with a code word "000V" or "B00V", where "V"

Line coding and decoding

19-Apr-14 The University of Lahore, Islamabad Communication Systems II 1

Signal element versus data element


Self-synchronization

� Receiver Setting the clock matching the sender’s

� Effect of lack of synchronization


Other properties

� DC components

– Eliminating DC components from the signal power spectrum, the system to be AC coupled

– Magnetic recording system, and system using transformer coupling , have little sensitivity to low frequencies.


� Transmission bandwidth

– Some schemes, such as multilevel codes, increase the efficiency of bandwidth by allowing a reduction in required bandwidth for a given data.

� Power efficiency

– Some codes utilize low power and are power efficient.

� Error detection and correction capability

– Detecting data errors without introducing additional error-detection bits, e.g. duobinarydetection bits, e.g. duobinary

� Self-clocking

– Some PCM codes have inherent synchronizing bits that aid in the recovery of the clock signal. e.g. Manchester codes.


� Transparency

– Some codes are transparent, i.e. they have no long zero components.

� Noise Immunity

� NRZ better than RZ unipolar waveforms� NRZ better than RZ unipolar waveforms


Line coding schemes


Unipolar NRZ scheme


Polar NRZ-L and NRZ-I schemes

• In NRZ-L, the level of the voltage determines the value of the bit. RS232.

• In NRZ-I, the inversion or the lack of inversion determines the value of the bit. USB, Compact CD, and Fast-Ethernet.

• NRZ-L and NRZ-I both have an average signal rate of N/2 Bd.


� NRZ-L and NRZ-I both have a DC component problem.

Example� A system is using NRZ-I to transfer 1-Mbps data. What

are the average signal rate and minimum bandwidth?

� Solution

– The average signal rate is S = N/2 = 500 kbaud. The minimum bandwidth for this average baud rate is Bmin = S


minimum bandwidth for this average baud rate is Bmin = S = 500 kHz.

RZ scheme

� Return to zero

� Self clocking


Polar biphase: Manchester and differential Manchester schemes

� In Manchester and differential Manchester encoding, the transition at the middle of the bit is used for synchronization.

� The minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ. 802.3 token bus and 802.4 Ethernet


Bipolar schemes: AMI and pseudoternary

� In bipolar encoding, we use three levels: positive, zero, and negative.

� Pseudoternary:

– 1 represented by absence of line signal

– 0 represented by alternating positive and negative


– 0 represented by alternating positive and negative

� DS1, E1

PSD of various line codes


HDB3 (High Density Bipolar of order 3 code)

� Replacing series of four bits that are to equal to "0" with a code word "000V" or "B00V", where "V" is a pulse that violates the AMI law of alternate polarity and is rectangular or some other shape. The rules for using "000V" or "B00V" are as follows:

– "B00V" is used when up to the previous pulse, the coded signal presents a DC component that is not null (the number of


presents a DC component that is not null (the number of positive pulses is not compensated for by the number of negative pulses).

– "000V" is used under the same conditions as above when up to the previous pulse the DC component is null.

– The pulse "B" ("B" for balancing), which respects the AMI alternancy rule, has positive or negative polarity, ensuring that two successive V pulses will have different polarity.

HDB3

� The timing information is preserved by embedding it in the line signal even when long sequences of zeros are transmitted, which allows the clock to be recovered properly on reception.

� The DC component of a signal that is coded in HDB3 is


null.

Bipolar 8-Zero Substitution (B8ZS)

� Adds synchronization for long strings of 0s

� North American system

� Same working principle as AMI except for eight consecutive 0s

Amplitude0 0 0 0 0 0 0 01 0 1

10000000001 �� +000+-0-+01 in general 00000000��000V(-V)0(-V)V


� Evaluation

– Adds synchronization without changing the DC balance

– Error detection possible

� Used in T1/DS1

Time

Violation Violation

Coded Mark Inversion (CMI)

� Another modification from AMI: Binary 0 is represented by a half period of negative voltage followed by a half period of positive voltage

� Advantages:

– good clock recovery and no d.c. offset

– simple circuitry for encoder and decoder − compared with HDB3

� Disadvantages: high bandwidth


Multilevel: 2B1Q scheme

� Integrated Services Digital Network ISDN


Multilevel: 4D-PAM5 scheme


Multitransition: MLT-3 scheme


Summary of line coding schemes


Plus HDB3 and B8ZS

Documents

Line coding and decoding - · PDF fileHDB3 (High Density Bipolar of order 3 code) Replacing series of four bits that are to equal to "0" with a code word "000V" or "B00V", where "V"