Introduction

The contention based protocols, like ALOHA described in previous lessons, permit to achieve relatively low system throughput. The throughput of pure ALOHA is only abut 18 %. In slotted ALOHA the throughput is increased to about 37 %. The reason of relative inefficiency of the ALOHA schemes is the fact that users take no account of what other users are doing when they attempt to transmit data packets. Using CSMA method, in which user listens to the channel before attempting to transmit a packet, the system throughput can be significantly increased.

The basic feature of contention based protocols, which limits their achievable throughput is the high number of packet collisions under heavy load of offered traffic. A key factor limiting throughput in such methods is that when a transmission is initiated, the full time and frequency resources of the channel are being used even though the sender have no knowledge if the transmitted packet will not encounter a collision. In case of collision the channel resources are wasted. There is group of medium access protocols that avoid some of the inefficiencies of the contention based protocols, by adding more control to the access method. They are called controlled random access methods. Two such protocols are described in this lesson: reservation ALOHA (R ALOHA) and polling. R-ALOHA is the example of distributed control random access scheme, poling is the example of centrally controlled random access scheme.