17.4 MEDIUM ACCESS CONTROL PROTOCOLS IN LANS

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The MAC sublayer differs according to the topology of the LAN. In general, how do different nodes access the medium? There are two possibilities: centralized control, in which a control station grants permission to different nodes, or a decentralized network, in which stations collectively perform medium access control function. In decentralized networks, there are three types of categories: round robin, reservation, contention.

Round robin: Each station is given an opportunity to transmit for a fixed time. Control of the sequence of the nodes may be centralized or distributed. This mechanism is useful when all the stations have data to transmit.

Reservation: Each node is given a time slot (as in TDMA). Reservation of the slots may be centralized or decentralized.

Contention: This is a distributed control mechanism wherein all stations contend for the medium. When two or more stations contend for the medium simultaneously, it may result in collision. CSMA/CD, which was described earlier, is an example of this protocol.

The CSMA/CD protocol is used in bus/tree and star topologies.

The CSMA/CD protocol differs based on whether the LAN is broadband or baseband. In baseband LANs, carrier sensing is done by detecting voltage pulse train. In broadband LANs, carrier sense is done by detecting RF carrier. For collision detection, in baseband LANs, if the signal on the cable at the transmitter tap point exceeds a threshold, collision is detected (because collision produces voltage swings). Because of the attenuation in the cable, a maximum length of the segment is specified in the standard—mainly to ensure collision detection using the threshold. In broadband LANs, head-end can detect the garbled data, or a station can do bit-by-bit comparison between transmitted and received data.

Because multiple nodes have to share the medium, the choice of the MAC protocol is very important in LANs. Though CSMA/CD is the most popular MAC protocol, variations of this protocol have been developed to increase the throughput.

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