go empty (such as C EEG Frame 3 slot 4 in Figure 13.11). Slots must also remain vacant for one frame following the end of transmission from its owner, although this can be overcome by Frame 4 having a header on each frame which announces that the owner of particular slots will cease transmission in the following frame, allowing contention for these frames. Figure 13.11 Illustration of an implicit reservation ALOHA Another disadvantage of this protocol is that the number of users system must be known in advance to allow slots to be allocated to them. An alternative implicit reservation system (Crowther et al., 1973) Each user has priority for transmission on his slot. If there are does not have permanent owners allocated to slots, and it may more slots than users then all users contend for the spare slots, using therefore be used even when the number of users is not known. ALOHA or slotted ALOHA. Whilst an owner is transmitting on his Users seize slots using normal ALOHA contention, but once a slot allocated slot no other user can interfere. However, if a slot falls idle is seized the user becomes its owner and can continue sending data for a frame, then in the next frame other users can contend for its SO long as it has any left. (Rules to prevent a user 'hogging' a slot use. Once a slot has been so seized only the owner is allowed to are often applied.) If the slot falls idle for a frame contention can interfere with the new user. The owner gets his slot back by trans- start on it, all users again being treated as equals. Clearly this system 13/10 Multiple access techniques can be used even when the number of slots is less than the total (13.22) number of users. SRM 1 For no flag (13.23) 13.6.1.2 Explicit reservations SRM 1 For flag In one such technique (Roberts, 1973) one or more slots within a frame is divided into smaller slots, called reservation slots. Users 13.6.2 Carrier Sense Multiple Access content for these slots, using a technique such as ALOHA, and if successful they place a reservation for a slot o