TABLE OF CONTENTS Multiple access techniques are primarily for continuous applications like voice and video, where a dedicated channel facilitates good performance. However, most data applications do not require continuous transmission: data are generated at random time instances, so dedicated channel assignment can be extremely inefficient. Moreover, most systems have many more total users (active plus idle users) than can be accommodated simultaneously, so at any given time channels can only be allocated to users that need them. Random access strategies are used in such systems to efficiently assign channels to the active users.
All random access techniques are based on the premise of packetized data or packet radio. In packet radio, user data is collected into packets of N bits, which may include error detection/correction and control bits. Once a packet is formed it is transmitted over the channel. Assuming a fixed channel data rate of R bps, the transmission time of a packet is ? = N/R. The transmission rate R is assumed to require the entire signal bandwidth, and all users transmit their packets over this bandwidth with no additional signaling to separate simultaneously transmitted packets. Thus, if packets from different users overlap in time a collision occurs, in which case both packets may be decoded unsuccessfully. Packets may also be decoded in error as a result of noise or other channel impairments. The probability of a packet decoding error is called the packet error rate. Analysis of random...
