Report Number: CSL-TR-77-137
Institution: Stanford University, Computer Systems Laboratory
Title: Performance analysis of computer communication networks via random access channels
Author: Yu, Philip S.
Date: April 1977
Abstract: The field of computer communication networks has grown very rapidly in the past few years. One way to communicate is via multiple access broadcast channels. A new class of random access schemes referred to as the Mp-persistent CSMA scheme is proposed. It incorporates the nonpersistent CSMA scheme and the 1-persistent CSMA scheme, both slotted and unslotted versions, as its special cases with p=0 and 1, respectively. The performance of the Mp-persistent CSMA scheme under packet switching is analyzed and compared with other random access schemes. By dynamically adjusting p, the unslotted version can achieve better performance in both throughput and delay than the currently available unslotted CSMA schemes under packet switching. Furthermore, the performance of various random access schemes under message switching is analyzed and compared with that under packet switching. In both slotted and unslotted versions of the M0-persistent CSMA scheme, the performance under message switching is superior to that under packet switching in the sense that not only the channel capacity is larger but also the average number of retransmissions per successful message under message switching is smaller than that per successful packet under packet switching. In dynamic reservation schemes, message switching leads to larger channel capacity. However, in both slotted and unslotted versions of the ALOHA scheme, the channel capacity is reduced when message switching is used instead of packet switching. This phenomenon may also happen in the Mp-persistent CSMA scheme as p deviates from 0 to 1 for certain distributions of message length. Hence, the performance under message switching may be superior to or inferior to that under packet switching depending upon the random access scheme being used and the distribution of message length (usually a large coefficient of variation of message length implies a large degradation of channel capacity in this case) for certain random access schemes. Nevertheless, for radio channels, message switching can achieve larger channel capacity if appropriate CSMA schemes are used. A mixed strategy which is a combination of message switching and packet switching is proposed to improve the performance of a point to point computer communication network when its terminal access networks communicate via highly utilized radio channels.