ers have use of this for time slots. a limited time period only (Lam, 1976; Lam, 1977; Hayes, 1984; As illustrated in Figure 13.19 guard bands (time periods) must be Stevens, 1990). This is shown pictorially in Figure 13.19. used to prevent interference between users, caused by variations in the synchronisation times. This is shown in Figure 13.22. User A is Guard bands allocated the first slot and transmit data which is correctly timed but which only occupies a part of its allocated slot. (This again illus- trated the inefficiency of fixed assignment systems with fixed packet lengths, when a user does not have data ready to transmit during his allocated time, or only has data which partially fills his allocated time slot.) User A Guard User B Guard User C Slot 1 Slot 2 Slot 3 Data 1 Data 2 Data 3 Timing Timing error error Figure 13.19 User assignment within TDMA Figure 13.22 Effect of timing variations in TDMA (hypothetical case) The user time slots are combined into frames, as in Figure 13.20, which shows a six user system (users A to F). The frames repeat after Users B and C have data which completely fill their time slots but, a frame period of This is the most usual assignment method due to timing variations, they do not start at the correct times. The where each user has equal allocation of the channel. Alternatively guard band period now absorbs this timing variation and prevents some users can have a greater share, and this is accommodated by interference between adjacent transmissions. grouping the frames and varying the user repetition period. This is The advantages and disadvantages of TDMA are similar to those illustrated in Figure 13.21 where users A and B are allocated for FDMA. It is most suited to systems where the number of users increase capacity (with frame repetition period TF) over users C to is fixed and each user has a continuous stream of traffic to send. F (with frame repetition period To). However, since all users are continuously listening to the channel, but only accepting data during 