RATE-BASED HEADEND REQUEST CONTROL PROTOCOL FOR GIGABIT/S METROPOLITAN AREA NETWORKS

Abstract

A number of access protocols for high-speed LANs and MANs have been proposed in the research literature. Among them, the Distributed Queue Dual Bus (DQDB) protocol has been standardized as the IEEE 802.6 protocol for MANs. However, recent research has shown that the DQDB protocol has a number of problems. Chief among these problems is its unfairness, which makes it difficult to employ the protocol in networks which have large propagation delays. In view of the above, we present a new protocol called Rate-based Headend Request Control (RHRC), which is suitable for use in MANs operating in the Gbit/s range. RHRC overcomes to a great extent the problems faced by DQDB. This is achieved by employing a multiple requests operation and a bandwidth allocation mechanism. In addition, it retains most of the positive features of DQDB which include full bandwidth utilization and small access delays at very low offered load. RHRC also has two attractive features. Firstly, the transmission mechanism that controls access to the data channel provides equal opportunities to messages rather than to stations so that stations experience equal mean message delays under ideal conditions of zero propagation delay and an infinite bandwidth request channel. Secondly, the bandwidth allocation mechanism achieves fair bandwidth sharing among stations when the network is overloaded, and maintains fairness in terms of mean message delays among the underloaded stations. In this thesis, the delay characteristics of the RHRC protocol are studied under different sets of network assumptions to show that the protocol solves the delay unfairness problem of DQDB. The bandwidth allocation mechanism is then examined in terms of its convergence speed. It is shown that RHRC is capable of converging much faster than DQDB with BWB and to a lesser extent, HCBA. Next, three priority schemes are described for RHRC. Finally, the robustness of RHRC under transmission errors is discussed. Generally, the performance of RHRC is found to be little affected by increasing network size or transmission speed or both. Hence, RHRC is particularly suitable for high speed metropolitan area networks.