Question

Consider HDB3 scrambling used with Bipolar AMI.

a) For the given bit sequence, produce a corresponding signal. Assume even number of pulses since last substitution and the previous pulse was positive for the first substitution you will carry out.

0 0 0 0 0 1 0 0 0 0 0 0 0 0 1

b) When do you use HDB3 over B8ZS? How does the receiver know the difference between a replacement from actual data? Use an example.

c) What are the advantages and disadvantages of using Manchester coding over Bi-Polar AMI?

Answer 1

a) For the given bit sequence, produce a corresponding signal. Assume even number of pulses since last substitution and the previous pulse was positive for the first substitution you will carry out.

0 0 0 0 0 1 0 0 0 0 0 0 0 0 1

b)When do you use HDB3 over B8ZS?

As we have seen HDB3 is more conservative than B8ZS. Patterns of consecutive zeros having upto 7 zeros are not altered in B8ZS. It only replaces a pattern of 8 strings. So, if the data needed to be tightly synchronised, HDB3 is preferred over B8ZS. HDB3 will not wait for 8 zero pattern, but replace once it sees 4 zeros in the message.

How does the receiver know the difference between a replacement from actual data?

We know that in AMI scheme the voltages are inverted, ie after a positive pulse, next 1 is represented as a negative pulse. B8ZS and HDB3 scrambling techniques will cause violation (signal patter that violates rule of AMI ie, +ve pulse after a +ve pulse or -ve pulse after a -ve pulse). These patterns are very unlikely to be caused by noise or transmission impairments. So, in B8ZS, the reciever interprets two code violations as an octet of zeros.

Similarly in HDB3, every fourth zero is a vioaltion. So three bits before the violation and the violation is interpreted as a group of 4 zeros.

The same example from a) is shown here, note how the violations are used to identify the streams of zero at the receiver side

c) What are the advantages and disadvantages of using Manchester coding over Bi-Polar AMI?

Advantages

• Only two levels of voltage for the receiver to interpret - there are three levels in case of AMI
• Midbit transitions provide synchronisation - long sequence of 0s may cause problem with syncronisation in AMI unless scrambling is used.
• Bit error rate is less compared to AMI

Disadvantages

• Bandwidth is more due to midbit transition.
• Additional circuitry or firmware is required to generate and interpret manchester encoding