omparison of throughput for pure ALOHA, slotted ALOHA, and ALOHA with capture some form of reservation of capacity as a means of minimising mitting on it. A contention occurs and in the next frame only the contention. There are several variants of these as described below, owner is allowed to transmit. and they can be grouped as those needing implicit reservations and In Figure 13.11 in Frame 1 owners A, C, E and H transmit on their those with explicit reservations. allocated slots, the other being free. In the following frame (Frame 2) user A and E successfully seize another slot each, whilst G commences transmission. In addition a collision occurs in slot 4, 13.6.1.1 Implicit reservations either due to the owner and another station or two other stations simultaneously transmitting. In one such system (Binder, 1975b) the transmission slots are In Frame 3 slot 4 is vacant, which means that the owner was not grouped into frames, the number of slots exceeding the number of involved in the collision in Frame 2, so each station had to wait to users. Each user is nominated as being the owner of a slot position make sure. Also a collision has occurred in slot 2, which is due to within the frame. (Frame 0 in Figure 13.11.) the owner wanting his slot back. In Frame 4 the owner B commences transmission in his slot. The Owner ABCDEFGH two users who collided in slot 4 during Frame 2 operate an appro- Frame 0 priate back-off algorithm and will try again. In the meantime other Slot position 12345678 users have decided to contend for slot 4 (one of these may be the Frame owner of the slot) SO again contention occurs. In addition the owner Frame 1 of slot 6 has forced a contention to regain his slot. From this illustration it is seen that the reservation protocol gives high efficiency when users have stream type traffic, with owners Frame 2 transmitting on their slots. However, for bursty traffic, there will be many collisions, and since all users (apart from the owner) need to wait for a frame following a collision, slots can 