December 1998
AN215: ADVANCED NETWORKING

QUESTION 4

Total Marks: 20 Marks

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LAN are able to deliver data quickly and reliably over short distances. One of the most frequently used LAN Medium Access Control methods is CSMA/CD.

 
(a) What is the full form of the acronym CSMA/CD? Describe the basic operation of this method of access. [7]
Carrier Sense Multiple Access with collision detection (1 mark). This method uses the contention scheme to access the media (1 mark). In the first part CDMA, the station uses the listen-before-talk scheme. In the second part i.e. CD the station uses the listen-while talking scheme (1 mark). In this scheme the station which wants to transmit listens to the medium to check if it is idle, if it idle then it will begin transmission (1 mark). If a collision is detected then the station immediately stops transmission and then transmits a jamming signal to inform other stations that a collision has occured (1 mark). The station then waits for a random period of time and then tries to re-transmit (1 mark). This is called binary-exponential back-off (1 mark).

 
(b) What is the standard defined by IEEE for this type of Access Control called? [2]
IEEE 802.3 (2 marks).

 
(c) Give the standard frame format for CSMA/CD [6]
 
Bytes
7 1 2 or 6 2 or 6 2 0-1500 0-46 4
Preamble Start of delimiter Destination address Source address length data pad check-sum

If all entries correct inclusive of the size (6 marks). If size of field is absent but all other entries correct (4 marks). For missing entries, or incorrect extra ones, deduct (1 mark) each.

 
(d) What is the IEEE definition for token-ring networks? Describe this type of network with the aid of diagram? [5]
IEEE 802.5 (1 mark).
When a station wants to transmit it seizes the token by inverting a single bit in the 3 byte token (1 mark). This will convert the token into the first 3 bytes of the frame which the station will transmit (1 mark). As there in only one token this will allow only one station to transmit at a time, hence eliminating the problem of channel access (1 mark).
Award (1 mark) for a diagram. Award marks for other relevant points.