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# Character Encoding in Data Link Protocol & Other Questions

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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 method is used:

(a) Character count.
(b) Flag bytes with byte stuffing.
(c) Starting and ending flag bytes, with bit stuffing

The following data fragment occurs in the middle of a data stream for which the byte-stuffing algorithm described in the lecture is used: A B ESC C ESC FLAG FLAG D. What is the output after stuffing?

One of your classmates, Scrooge, has pointed out that it is wasteful to end each frame with a flag byte and then begin the next one with a second flag byte. One flag byte could do the job as well, and a byte saved is a byte earned. Do you agree?

To provide more reliability than a single parity bit can give, an error-detecting coding scheme uses one parity bit for checking all the odd-numbered bits and a second parity bit for all the even-numbered bits. What is the Hamming distance of this code?

A block of bits with n rows and k columns uses horizontal and vertical parity bits for error detection. Suppose that exactly 4 bits are inverted due to transmission errors. Derive an expression for the probability that the error will be undetected.

A channel has a bit rate of 4 kbps and a propagation delay of 20 msec. For what range of frame sizes does stop-and-wait give an efficiency of at least 50 percent?

Imagine a sliding window protocol using so many bits for sequence numbers that wraparound never occurs. What relations must hold among the four window edges and the window size, which is constant and the same for both the sender and the receiver.

Suppose that we model protocol 4 using the finite state machine model. How many states exist for each machine? How many states exist for the communication channel? How many states exist for the complete system (two machines and the channel?) Ignore the checksum errors.

##### Solution Summary

6 questions inclduing character encoding, data streams, flags and parity.

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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 method is used:

(a) Character count.
(b) Flag bytes with byte stuffing.
(c) Starting and ending flag bytes, with bit stuffing

(a) 00000101 01000111 11100011 11100000 01111110
(b) 01111110 01000111 11100011 11100000 11100000 11100000 0111111001111110
(c) 01111110 01000111 110100011 111000000 011111010 01111110

(Ref: http://www.cs.mu.oz.au/522/Assignments/tute-w3-model-2005.pdf)

The following data fragment occurs in the middle of a data stream for which the byte-stuffing algorithm described in the lecture is used: A B ESC C ESC FLAG FLAG D. What is the output after stuffing?

Answer: After stuffing we get: A ...

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