umber of retransmissions required before a message is access system often require a trade off between throughput and successfully transmitted is n., then these are given by Equations delay. 13.10 to 13.12. Message length in bits (13.10) = Channel capacity in bits per second Increasing medium delay Length of medium (m) =17 (13.11) Velocity medium (m/s) (13.12) The random back-off delay is usually an integral number of packets, if fixed length packets are being used. If this varies between 1 and x packets, then the average back-off delay (T) is given by Equation 13.13, and the overall delay or waiting time (T) in the communication channel is given by Equation 13.14. Throughput S 0.184 (13.13) Figure 13.8 Delay-throughput characteristic of an ALOHA channel TwTt + (13.14) In Equation 13. 14 the first bracketed term is the delay experienced 13.5.2 Slotted ALOHA due to the initial transmission and would be the overall delay if no collisions were to occur. The second bracketed term accounts for the In the pure ALOHA system collisions will occur whenever any part delays due to collisions. Delay is also often normalised to the packet of one packet overlaps any part of another packet. The vulnerable length in seconds (which is also the message transmission time), and period is therefore equal to the sum of their two packet lengths, or is denoted by D, as in Equation 13.15. to two packet lengths if all packets are the same size. An alternative version of ALOHA was developed (Roberts, 1972; Kleinrock and Delay D = THAT (13.15) Lam, 1973) which uses fixed length packets and synchronises all transmissions to the start time of the packet. The flow diagram for 13/8 Multiple access techniques this system is as in Figure 13.5 except that transmissions can only occur at the start of a packet period. Pure Aloha The disadvantage of slotted ALOHA is the added complexity of Slotted Aloha requiring a method for synchronising all sources to the start of a packet frame, and the use of fixed length packets. The advantage is that the vulner