Andrew S. Tanenbaum Edition 5th Exercise 3 Question 2 (Page No. 352)

Question

The following character encoding is used in a data link protocol:
A: 01000111 B: 11100011 FLAG: 01111110 ESC: 11100000
Show the bit sequence transmitted (in binary) for the four-character frame A B ESC
FLAG when each of the following framing methods is used:
(a) Byte count.
(b) Flag bytes with byte stuffing.
(c) Starting and ending flag bytes with bit stuffing.

Answer 1

Frame Format: A B ESC FLAG

a. in Byte count method the number of bytes in a frame is mentioned in the beginning of a frame

Format=00000100 01000111 11100011 11100000 01111110

the first Byte has binary value=4 which indicates that next 4 bytes are part of current frame

b. Flag Byte will be inserted at the beginning and end of frame. also escape byte will be inserted before escape and flag bytes occurring in the data part of the frame

Format=FLAG A B ESC ESC ESC FLAG FLAG

=01111110 01000111 11100011 11100000 11100000 11100000 11100000 01111110 01111110

c. flag bytes are inserted at the beginning and of frame along with bit stuffing

01111110 01000111 11100011 11100000 011111010 01111110