August 1997
AN215: ADVANCED NETWORKING

QUESTION 5

Total Marks: 20 Marks

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5. (a) What is the purpose of parity checking? [2]
Parity checking is used to determine whether a transmission error has occurred. [2]
(b) Assume that in a particular transmission protocol, bytes consist of seven bits of data, followed by one parity bit, using even parity. The following bytes are received. In which one has a transmission error occurred? [3]
(i) (ii) (iii) (iv) (v)
01101100 10111011 00010011 11110011 00111010
Explain your answer.
Answer is (iii). [2]
In all the others, the number of '1's is (correctly) even. [1]
(c) Describe the VRC/LRC error-correcting scheme. Explain the limitations of this scheme. [5]
VCR is a simple parity-checking scheme [1]
which permits the detection of single-bit errors. [1]
The longitudinal redundancy check (LRC) provides a further check by transmitting an extra check-byte, the bits of which are formed by computing the parity corresponding bits of the bytes in the message (including the parity bit). [2]
This two-way check also permits a simple error-correcting scheme, provided only single bit errors occur [1]
(d) If the bytes of part (b) are followed by a check byte 00101101, determine which bit was wrong in the transmission, and give the correct form of the byte which was transmitted in error. [4]
The parity is correct for all bits in the check-byte, except the third bit. [2]
Therefore the correct form of byte (iii) is 00110011. [2]
(e) Error-correction coding can entail high overheads. What is the alternative to the use of error-correcting codes? What factors determine which approach is appropriate? [4]
The alternative is error-detection, with retransmission when necessary. (explanation is needed). [2]
If the probability of error is low, then the second method is preferable because it will have over-all a higher efficiency. [2]
If communication is slow (as in satellite comms), the overhead for retransmission will be high, so, despite the low error-rate, error-correcting codes will be appropriate.
(f) In the OSI model, error detection is a function performed by the data link level. Name two services provided by this layer, and two other functions it performs. [2]
[0.5] each for any two from: data link connection, sequencing, error notification, flow control.
[0.5] each for any two from: establishment and release of connections, delimiting and synchronisation, sequence control, flow control, control of data circuit interconnection, management.