where the channel capacity is divided out amongst users and is 1/u, then the delay is given by Equation 13.28. fixed; demand assignment systems, where the capacity is divided into segments, but is allocated to users according to their needs at (13.28) any time; and reservation systems, where the capacity is not rigidly Delay D1 divided and can by reserved in advance by potential users. If the channel is now divided equally amongst the N users, each 13.7.1 Fixed assignment systems. having exclusive use of its capacity (C/N) only, and the total traffic each user generates is a/N, then the delay is given by Equation Three techniques are covered under fixed assignment systems: 13.29, which shows the increased delay over the case where the total frequency division multiple access (FDMA) which uses the same channel resource is shared amongst all users, using some form of principles as frequency division multiplexing described in Chapter contention system. 20; time division multiple access (TDMA) which again is based on the time division multiplexing principles of Chapter 21; and code (13.29) division multiple access (CDMA) also sometimes referred to as Delay D. 2 spread spectrum multiple access (SSMA). 13/14 Multiple access techniques Another disadvantage of fixed assignment systems, such as FDMA, is that the number of users cannot easily be changed. This Two frame group Two frame group Two frame group would require the overall channel frequency band to be redivided amongst the new users. ABCDABEFABCDABEFABCDABEF FDMA is also not suitable for use in systems which require broadcast of data to many users. Since each user is allocated a single Frame Frame Frame Frame Frame Frame frequency band, it sends on this band and the receiver also monitors this band. Figure 13.21 Unequal slot assignment within TDMA 13.7.1.2 Time division multiple access TDMA works on the principle that the complete bandwidth of the so that user data is correctly timed with the start of their allocated channel is allocated to all users, but these us