DQDB^+/-: A Fair and Waste-free Media Access Protocol for Dual Bus Metropolitan Networks

Abstract

The distributed queue dual bus (DQDB) protocol, the media access protocol of the IEEE 802.6 standard for metropolitan area networks, does not fully take advantage of the capabilities of a dual bus architecture. Although a fair bandwidth distribution among the stations is guaranteed when using the so-called bandwidth balancing mechanism, the protocol requires a considerable amount of time to adjust to changes in the network load. Additionally, the bandwidth balancing mechanism leaves a portion of the available bandwidth unused. In this study it is shown that the shortcomings of the IEEE 802.6 standard can be overcome by adopting a different fairness mechanism. A new media access protocol for dual bus networks is presented that achieves a fair distribution of the bandwidth in one round-trip delay. The protocol is based on the unique solution to a so-called fair and waste-free bandwidth allocation. This bandwidth allocation can be implemented in a distributed manner. A comparison of the new protocol with the DQDB protocol shows considerable advantages regarding the transmission delay of messages and the time a station needs to obtain a fair portion of the available bandwidth. The advantages of the protocol become more apparent for large networks and high transmission speeds. In addition, the new protocol can provide services that do not distribute the bandwidth equally among the stations, i.e., it can perform nonuniform bandwidth allocations. Examples of several nonuniform bandwidth allocations are presented, such as a peruser bandwidth allocation, a guaranteed bandwidth mechanism, and a (quasi-) priority scheme. © 1993, Walter de Gruyter. All rights reserved.