rk. Using Equation 13.5 gives the value of a as in Equation 13.7. 2. The average arrival rate (a) at each node, assumed to have a Poisson distribution and to be the same for all nodes. This is also the rate of data generated by each node. 3. The capacity (C) of the transmission medium usually specified in bit/s, and also referred to as its bandwidth. 4. The velocity of propagation (v) within the medium. For example, for coaxial cable this is about 65% the velocity of Propagation time in the medium (13.7) light, or 2 X 108 m/s. Transmission time of the message 5. The average length (L) of the message frame in bits. Classification of multiple access techniques 13/5 For satellite systems a is usually large, of the order of 10-100, due 13.4 Classification of multiple access to the round trip delays, whilst for terrestrial systems it is normally relatively small, usually less than 1. techniques The utilisation factor (U) is therefore given by Equation 13.8, and the theoretical curve of Figure 13.2 can be modified to take factor Figure 13.4 provides a classification for the many different multiple a into account, as in Figure 13.3. access techniques which have been developed and gives a few examples, described later in this chapter. In pure contention systems a source transmits its data independently of other users, i.e. does not take any action to avoid collision with data being transmitted by a = 0.1 other users. If collisions occur these are sensed and the data is retransmitted. In contention minimisation systems the source takes some action to minimise the occurrence of collisions but does not avoid these altogether, whilst in non-contention systems each source takes due regard of all other sources transmitting on the common line, so that data collisions are avoided. These three major classifications can be subdivided further, as described in the following sections. Offered load (G) Pure contention systems The pure contention multiple access techniques are the simplest to implement since a source having data to tra