A token ring local area network for small systems application

Abstract

This research work deals with a new design/ and study of a token ring local area network (LAN) for small systems application like microcomputer network. The salient features of the work carried out are as follows: “ Development of a new access technique and a distributed algorithm to realize a token ring/ “ hardware design and construction of a microprocessor based Ring Interface Unit and an experimental ring net, - construction of a traffic-adaptive simulation model with a cyclic queuing procedure for performance evaluation/ and - study of a multiple token ring which is modelled as separate single rings with a mixed class of stations. Many access techniques have been developed to satisfy the requirements of a local area network/ but the token ring has turned out to be the most suitable scheme for different applications ranging from office automation to real-time systems like process control. Numerous comparative analyses have proved the performance superiority of the token ring over the other schemes. The analysis has also shown the robustness of the token ring to the large variation in the network parameter and traffic. Elowever/ as far as small systems applications are concerned/ the realization of a token ring becomes quite complex in its control procedure/ and relatively expensive mainly due to lack of an LSI implementation of this access scheme. In this work/ a new and simple approach is described for designing and implementing an inexpensive token ring system. Also/ modelling and performance studies of the ring net are done in a more realistic small systems environment. Initially/ in order to find a cost-effective approach to construct a ring node, standard Data Link protocols that are readily available in the form of low-cost single chip hardware are surveyed. As a result/ the IBM-SDLC loop protocol is found to be suitable due to the resemblance of its architecture with the token ring. However/ in the SDLC loop architecture/ the procedure of having a permanent primary station to control the entire loop generates an unbalanced traffic in the network; and poses reliability problems if the primary station fails. These are unsuitable features for a multi-user system like LAN. Therefore/ to avoid these difficulties, in this work/ all the SDLC stations are given an equal priority to control the loop- This is done by suitably modifying the SDLC frame format and the polling procedure. In accordance with these modifications/ a distributed algorithm is developed to convert the centralized SDLC loop architecture into a decentralized network like that of token ring. This algorithm allows the role of the primary station to circulate among all the stations in the loop. That is, it permits any station to acquire this primary status when it has a message to transmit. After the loop-transmit/ the transmitting station is made to pass its primary status to the next access seeking station/ and thus making the loop control become distributed. To realize a loop station/ an INTEL 8085 microprocessor based Ring Interface Unit is constructed and it is tested in an experimental ring net. The network-control part of the system includes a protocol controller/ a DMA controller/ a transceive buffer, a clock recovery and a bypass circuits/ and network command and control software. Although there exist numerous performance analyses on token ring, analyses pertaining to the study of the system in a realistic environment are scarce. In this work, a more general and versatile computer simulation model with a cyclic queueing procedure is constructed. In this model, it is observed that working with a single model (explicit OR implicit polling type) to operate a wide range (0.1 to >0.9) of network traffic is inefficient. For instance, in the case of a low message arrival rate (<0.5), the execution time of the simulation program (of explicit polling) becomes prohibitively high. Because, a light traffic in the ring induces the circulation of idle-token for most of the time: a system state that does not contribute to the servicing of a message. Therefore, in order to improve the program's runtime efficiency, the model is broken into two .different types - low traffic and high traffic models. Depending on the computational time of the previous run, the proper model is switched on for further simulation in the other ranges of traffic. This "traffic-adaptive" simulation model reduces the overall CPU-time of the program for all ranges of traffic. Attempts are made to verify this simulation model by comparing it with known analytical results, and the results are found to be satisfactory. Apart from this varying traffic model, a saturated load (continuously queued) model is also developed. This aims to analyse the sensitivity of the token ring at extreme operational conditions. The simulaton work is largely directed towards the modelling of the ring net in a more realistic small systems environment, i.e., where the network is connected with a mixed class of stations like work-station/ file-server/ and bridge. This unbalanced traffic model is used to analyse multiple rings (2- rings) which are connected by a bridge. The flow of message between rings is characterized by an *'interaction-probability" with which a ring net transfers its messages to the other. This parameter is used to calculate the arrival rate of messages at a bridge-station of the ring net, and the multiple ring net is then modelled as logically separated single rings which have an unbalanced traffic in the net due to the presence of a bridge station. While doing so, the delay history of the messages that arrive at the bridge-station from the other ring is properly accounted for. The above analyses are mainly used to address the problems and to device effective solutions to cope with the natural growth of stations in a ring net of a small systems environment. From different analyses^ it appears that configuring a large single ring net into multiple rings connected by a bridge is a better choice than altering the existing capacity of the growing ring. Besidesf in order to improve the throughput-delay efficiency of the multiple ring net, performance studies for various ringinteraction- probablities are done. The different techniques applied in this effort are: a) giving a "priority-servicing" at the bridge-station/ and b) applying a "resource-balancing" approach in the ring nets to reduce the flow of many messages accross the bridge.