line (which need they send a 'complete' message and the next user in turn is polled. not be the next adjacent user). The 'complete' signal is treated as a If a user does not have any data to send he simply returns the poll by the recipient, and SO on, as shown in Figure 13.28. Finally 'complete' message. This is therefore a demand assignment system the last user in the chain returns the 'complete' signal to the central since the more data a user has the longer he is able to transmit. station, SO ending the poll cycle. (There may be some rules to prevent a station from hogging all the Hub polling can result in reduced walk time between users, since transmission time.) control is not passed back and forth to a central site, although it is Since only one station is polled at any time and is therefore able more difficult to implement since each user needs greater intel- to transmit, there is no collision. It is also possible to introduce a ligence. Complete Message Central Message station Complete Figure 13.27 Illustration of roll call polling Non-contention systems 13/17 Complete Complete Message Central Message station Complete Figure 13.28 Illustration of hub polling Assume that there are N identical users, and that all data is held at reduce the number of polls required and so reduce the overall delay these user stations in buffers, the buffers being emptied at each poll. in the system. The average cycle time (t) is related to the average walk time (t) Probing is a technique in which groups of users are polled at a by Equation 13.31, where the traffic intensity (p) is as in Equation time, to find the users who are ready for transmission. It is similar 13.1. Figure 13.29 shows the delay curves. to the tree walk protocol illustrated in Figure 13.17. The N users are divided into groups and these are probed simultaneously. The users (13.31) who have data ready for transmission reply to this probe. For example in Figure 13.17 suppose user F is the only ready user and that node 1 is probed and it provides a ready respo