nse. Node 2 is The cycle time will therefore be large, leading to poor perfor- then probed and, as there is no response, the probe is moved to node mance, if the walk time is large, for example as experienced with 3. A response is now received so node 7 is probed (no response) satellite channels, or if there are a large number of users. This results followed by node 6 (response). Finally user E is polled followed by in large delay. a poll of user F. Therefore, in the worst case, the process of locating Polling all users in turn can be a slow process if there are a large and polling the ready user took 5 probes and two polls. number of users and only a few of these have data to send at any Probing is a good technique to use if the overall loading of the time. In these circumstances a technique called probing is used to system is light (i.e. few users ready to transmit). If there are 2nd users on the system and only one of them is ready to transmit, then the number of probes (including polls) needed to locate it are given by Slotted ALOHA Equation 13.32. Hub polling Number of probes = 2 + 1 (13.32) Pure ALOHA If, however, all the users are ready to transmit then the number of probes (and polls) needed is given by Equation 13.33, which is clearly larger than a simple sequential poll method. It is therefore important, during probing, to choose the initial group size so as to minimise the number of probes. Number of probes = 2n+1-1 (13.33) Number of stations 13.7.2.2 Token passing Figure 13.29 Variation of delay in hub polling compared Token passing is described and analysed in several papers, such as to ALOHA Bux (1989), Black (1989) and Farber and Larson (1972). It is commonly used in a ring topology and is then referred to as a token 13/18 Multiple access techniques ring. It uses active termination points between the users and the within a user node plus propagation delay in the ring) The perfor- network, unlike a polling system which users passive terminators. mance equations are the same as those derived for hub 