slots are allocated 13.7.3.2 Bit-map protocol for exclusive use of a user. However as soon as a user makes a reservation it completes its transmission in a data slot. Also the This uses a structure similar to R-ALOHA, with explicit reserva- reservation slot sequence now does not start from the beginning but tions, except that no contention is used to access the reservation from the last user who transmitted. These therefore overcome the slots. Figure 13.36 illustrates its operation for six users, A to F. two limitations of the bit-map protocol given earlier. However every If there are N users, then initially there will be N reservation slots user still has to wait for a reservation slot, and has an average delay with each user allocated exclusive use of one of these reservation of N/2 reservation slots. slots. If a user wishes to transmit it puts a '1' into its allocated slot, In a variant of the MSAP a user who has just transmitted is given and if it does not wish to transmit then it leaves this slot empty. After the option to transmit immediately in another data slot, and only if all the reservation slots have been SO accessed, all stations know it does not do this does control pass to the next user. The system which users wish to transmit. therefore gives improved performance under conditions of 'stream' Reservation Data slots Reservation Data slots slots slots User ABCDEF Data A Data D ABCDEF Data B Data D Data E Figure 13.36 Illustration of the bit-map protocol References 13/21 Data A BCD Data D EFAB Data B CD Data D E Data E F Figure 13.37 Principle of the MSAP/BRAP protocols traffic, although it does result in an idle slot under conditions of light Users make reservations for a number of slots and are allocated a traffic, if the user does not take up the invitation to transmit again. priority for transmission on data slots. (There can be two types of MASP and BRAP may be considered to be similar in operation to data slots, one reserved for particular users and one shared.) A roll call polling, but they hav