RESEQUENCING DELAY AND BUFFER OCCUPANCY IN SELECTIVE REPEAT ARQ WITH MULTIPLE RECEIVERS

Of all the automatic-repeat-request (ARQ) protocols, selective repeat (SR) is the most efficient for both single and multiple receivers. However, under this protocol, a receiver may accept packets from the channel out of their original order. To forward the packets in proper order, a receiver must first queue those that arrive out of sequence. In this paper, we consider an SR-ARQ protocol with one source and multiple receivers; each receiver acknowledges all packets and handles its resequencing buffer based only on the packets that it receives error-free. An analysis of the resequencing delay and buffer occupancy at a receiver is presented. We construct a model that enables us to derive steady-state results, taking into consideration such system parameters as number of receivers, propagation delay, packet error probabilities, and acknowledgments. We focus on two measures of occupancy. The first measure corresponds to the number of packets waiting to be resequenced whereas the second measure includes, in addition, the buffer space reserved for packets that cause resequencing delays. The main results are the distribution of the resequencing delay and the distribution of the number of packets occupying the receiver's buffer. We also present an expression for the mean of the second occupancy measure, as well as simple expressions for the mean of the first buffer occupancy in the limit as the packet error probability tends to one. Finally, we provide numerical results to illustrate the effects of system parameters on the resequencing delay and buffer occupancy.