raffic is heavy the token is seized and marked as busy by each user in turn. As only one token circulates at any time there is no contention. Number of users Token passing is similar to hub polling where no central station is used to commence the poll. Also hub polling is usually applied to bus structures whereas token passing is more often used in ring structures. The scan time in a token passing system is the time it Figure 13.31 Variation of throughput with number of takes for the token to be passed around all users, whilst the walk users for token passing and CSMA/CD time is the time taken for a bit to go around the ring (i.e. one bit delay Non-contention systems 13/19 IEEE 802.5 describes the token ring protocols, and IEEE 802.4 the and if that is the case the data is loaded from the buffer into the token bus. The bus structure can be considered to be a logical ring, register and clocked out onto the line. as in Figure 13.32, the token moving between users as shown by the If the loading is light the user can continue to send data, but if it dotted line. is heavy it must wait between transmissions for gaps in the line, and for these to build up in the register. This method therefore automat- Logical ring ically prevents users from 'hogging' the line. It also permits variable length packets (as in token passing) and several frames to be on the ring at any time (as in slotted ring). 13.7.3 Reservation systems There are a large number of different multiple access techniques which are based on some form of reservation. One such system has already been described, that of Reservation ALOHA. This was Figure 13.32 Illustration of a logical ring introduced under contention systems, since some form of contention is used to access the reservation slots, although once this has been done the use of the data slots is contention free. Reservations can be made using any of the techniques described earlier, pure contention, contention minimisation, or non-contention 13.7.2.3 Slotted ring (fixed assignment or demand assignment