The demand assignment systems are part of non-contention systems minimum delay time irrespective of whether any data is transmitted. in which the communication channel is allocated to a user as Three types of polling are considered here: roll call polling, hub demanded, rather on a fixed basis, as in the previous section. polling and probing. Figure 13.27 illustrates the principle of roll call polling. A central Four types of demand assignment multiple access techniques are station, which may be a specified user who has taken on this task, described here: polling, token passing, slotted ring and register insertion. polls each of the other users in turn. When a user receives a poll he puts his messages on the common line and the addressed user retrieves it. When all messages have been completed the user sends a 'complete' message to the central station, which then polls the next 13.7.2.1 Polling user in its planned sequence. The central station can be programmed to poll some users more often than others, giving them priority, or it Polling techniques are ideally suited to star, bus or tree structures can be made to poll users who consistently transmitting large although, in practice, any of the systems of Figure 13.1 may be used. volumes of data more often, so that it acts as a demand assignment The principles of polling have been described and analysed in system. several texts, such as Betsekas and Gallager (1987), Hammond and In hub polling the central station commences the poll cycle by O'Reilly (1986), Hayes (1984), Schwartz (1987) and Chou (1983). sending a poll to the first user. If this user has any data to transmit In polling each user in turn is interrogated to see if they have any it puts this onto the common line for the addressed user to retrieve. data to transmit. If they have data then they will commence trans- When all data has been completed (of if there was no data in the first mission and stop when their buffers are empty. At the end of this place) a 'complete' signal is sent to the next user in 