ention and therefore no wastage will read each other's data as it goes around the ring and will know of bandwidth or delays caused by collisions and retransmissions. that a contention has occurred. Both then back off for a random time Because of this the throughput (S) of non-contention systems equals and start again. the offered load (G) if retransmissions due to secondary causes, such When the ring is full, i.e. many users have data to transmit, the as line errors, are ignored. The idealised curve is therefore as in system operates basically as a token passing ring, each user trans- Figure 13.3. mitting in turn. On light loads it operates as a contention ring, with The disadvantage of FDMA systems is that there is wastage of low delay since a ready station does not need to wait for a token to bandwidth, firstly caused by the guard bands and secondly due to arrive before it can begin its transmission. the fact that users can only use their own allocated frequency bands. Therefore if a user does not have any information to transmit its allocated band lies idle, even though other users may have a consid- Non-contention systems erable amount of information to send and are experiencing delays on their channel. FDMA is therefore best for use in systems where all users have a stream of data to send, and it is unsuitable for users The previous two sections have examined systems in which pure with 'bursty' traffic, where contention systems, such as ALOHA, contention is used in order to access a communication channel and perform better. where contention is used but steps are taken to minimise it wherever One of the results of dividing the overall channel capacity possible. The present section will look at the large class of systems amongst multiple users is increased delay (Rubin, 1979). For which avoid contention altogether. example, if there are N users sharing a total channel capacity of C These systems are subdivided into fixed assignment systems, and the total traffic they generate is a with a mean packet length of 