Question

For the bit stream 1000000010110000000001, sketch the waveform for B8ZS and HDB3. You may make any assumptions.

Answer 1

Let's understand some background first:

AMI - it stands for Alternate Mark Inversion. Here, there are three voltage levels +v, 0 and -v. Binary 0 is represented using 0 voltage. And binary 1 is represented as +v if last 1 was represented as -v and vice versa. It means no two consecutive 1's can be represented as same voltage level, either +v or -v. Two consecutive 1's must be represented using opposite polarity.

Scramling is a technique used in case of 8 consecutive 0's.

B8ZS means Binary 8 Zero Substitution and is used for scrambling. 8 consecutive 0's are replaced by 000VB0VB.

V - It means violation of AMI rule. That means if previous 1 was represented using +v, then this bit will also be +v, and if previous 1 was represented using -v, then this bit will also be -v.

B - It means Bipolar, it simply means opposite voltage or polarity of previous bit.

HDB3 means High density bipolar 3 zero. Here 4 consecutive 0's are replaced by 000V or B00V according to following rule:

000V - This pattern will be used when the no. of non-zero (+v or -v) pulses are odd after last substitution.

B00V - This pattern will be used in case of no. of non-zero (+v or -v) pulses being even after last substitution.

Here, odd and even refers to the number of non-zero pulses after last substitution. For 1st substitution, there was only 1 non-zero pulse, hence odd.

For 2nd substitution, there were 3 non-zero pulses after 1st substitution hence, odd. For 3rd substitution, there were 0 non-zero pulses hence even.