THROUGHPUT ENHANCEMENT SCHEMES FOR DQDB MAC PROTOCOL

Abstract

Metropolitan Area Networks(MANs) are a relatively new concept in networking.MANs have high speed,longer distances and multiservice capability. MANS are conceived as a backbone networks for lower speed LANs, as an access network to future Asynchronous Transfer Mode(ATM) based B_ISDN. DQDB (Distributed Queue Dual Bus) is a Media Access Control(MAC) protocol which is being standardized as an IEEE 802.6 MAN. The DQDB MAC protocol offers two types of services to its users: Isochronous and Asynchronous. This dissertation analyzes the asynchronous part of the DQDB MAC protocol in underload & overload conditions and in saturation conditions (i.e. when each DQDB MAN node is trying to size all the medium capacity). In DQDB MAN, it should be noted that transmission slots are never erased halfway the bus and only terminated at the end of bus. At first, a special node called "erasure node" is introduced in a DQDB network for slot reutilization in the downstream side. Secondly, an extension of DQDB i.e. SP-DQDB (Slot Pre-use DQDB), is introduced which is fully compatible with IEEE 802.6, which allows multiple use of a slot for segment transmission before it get used according to the basic DQDB protocol.The compatibility of SP_DQDB with DQDB is investigated by integrating performance improvement schemes of DQDB with SP-DQDB. Simulation technique is used to study the throughput, delay, and fairness performance and it is demonstrated that SP-DQDB is much more efficient than DQDB and integration of SP-DQDB with "erasure node"give still better results.