A Study of Data Multiplexing Techniques and Delay Performance

Abstract

This paper is concerned with the study of data multiplexing techniques which provide local distribution for user populations whose source characteristics may be categorized as Inquiry/Response. The techniques studied are Polling, Random Access, and a Loop System. In each method a group of users is multiplexed onto the same line which is connected to a Central Processor. The Central Processor forms the interface between the local system and long haul facilities, but does no computation on arriving messages other than to route them appropriately. The advantage of such systems is flexibility of operation and economy through sharing of equipment at the Central Processor. We focus our attention on the average round-trip message delay and use this as a measure for comparisons of the three techniques. Source models, in terms of calls per busy hour and number of bits per message for users as well as for computer responses, are developed which are appropriate in an Inquiry/Response context. Other factors taken into consideration are transmission rates, synchronization delay time, and allocation of available capacity to the transmission of overhead information, such as polling messages and acknowledgment traffic. The results of the study are presented in graphic form, where the average delay due to traffic in the system is plotted as a function of the number of stations. Principal conclusions can be summarized as follows: Polling — It is found that this system is sensitive to the synchronization delay which takes place each time a user station transmits to the central facility. For reasonable choices of system parameters as many as 100 stations can share the Central Processor without exceeding an average of 1-second round trip message delay. Random Access — This system is not sensitive to synchronization delay. Over most of the range of load parameters, the Random Access system shows lower average delay than the other two systems. Loop System — The study revealed that this system has average delay