ds users who have information to transmit, and limits Figure 13.16 Delay-throughput curves for CSMA and their number SO that transmission is successful. To ensure fairness ALOHA users on either side of the tree are searched in turn following a successful transmission or if there is no data available for trans- method for allocating users to the available slots within a trans- mission on one side. mission system. Dynamic allocation is usually used, where the If the loading on the channel is heavy, i.e. many users waiting to number of users per slot decreases as the load increases. transmit, then clearly it is more efficient and quicker to start search- A dynamic allocation method which has been described (Capeta- ing at an intermediate level rather than at Level 1. It can be shown nakis, 1979a and 1979b) is known as the adaptive tree walk proto- that this optimum level is related to the number of users 'n' who col, and is illustrated in Figure 13.17 for eight users A to H. have data to transmit and is given by Equation 13.27. Initially all users who wish to transmit data contend for a slot, and the protocol is at Level 1 and node 1. If a collision occurs then the Optimum search level = log, n (13.27) protocol moves up the tree to Level 2 and node 2. Now only the users below this node (A to D) are permitted to contend for the slot. Other asymmetrical protocols have been developed, but are not If a transmission is successful then the slot in the next packet is considered here. For example Kleinrock and Yemini (1978) de- reserved for users under node 3. If, however, there was contention scribe the urn protocol. All these have the same aim, to limit the Non-contention systems 13/13 number of users per slot SO that the probability of transmission 13.7.1.1 Frequency division multiple access increases, ideally reaching one, with one ready user per slot. FDMA works on the principle of dividing the total bandwidth of the 13.6.4 Contention rings communication channel into a number of discrete segments, and allocating each s