3.3 Network Availability

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Availability is a measure of performance dealing with the LAN’s ability to support all users who wish to access it. A network that is highly available provides services immediately to users, whereas a network that suffers from low availability typically forces users to wait for access. The topology of the LAN influences availability.

Availability on the bus topology is dependent on load, the access control protocol-used, and length of the bus. With a light load, availability is virtually assured for any user who wishes to access the network. As the load increases, however, so does the chance of collisions. When a collision occurs, the transmitting nodes back off and try again after a short interval. The chance of collisions also increases with bus length.

A network based on a star topology can only support what the central hub can handle. In any case, each LAN module in the hub can handle only one request at a time, which can impact other users on that segment during heavy load conditions. Hubs equipped with multiple processors and LAN modules can alleviate this situation somewhat, but even with multiple processors, there will not usually be a one-to-one correspondence between users and processors. Such a system would be cost-prohibitive.

In terms of network availability, the ring topology scores higher than either the bus or star topology. This is because each node on the ring has an equal chance at accessing the network, which is governed by the token. However, since each node on the ring must wait for the token before transmitting data, the time interval allotted for transmission decreases as the number of nodes on the ring increases.

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