A SIIIFIULATION STUDY OF DQDB AND FDQ MAC PROTOCOLS FOR HIGH SPEED MANs

Abstract

Distributed Queue Dual Bus (DQDB) is a Media Access Control (MAC) protocol which is being standardized as an IEEE 802.6 Metropolitan Area Network (MAN). This dissertation implements several alternative solutions that can improve the network's fairness and also a new protocol, Fair Distributed Queue (FDQ) suitable for very high speed MAN's. Fairness improvement methods for DQDB include 1) Bandwidth Balancing(BB) Scheme : Under which the system wastes some part of channel bandwidth and distribute rest of it equally among all the nodes 2) the proportional assignment scheme (PR) : Under which bandwidths assigned to the individual stations are in proportion to• their offered traffic during heavy load conditions 3) the (multiple - request) FCFS - message - queue - based DQDB scheme (MD) : Under which a station follows the regular DQDB protocol, but it is allowed to make multiple outstanding requests, one for each of its queued segments, in order to create a distributed FCFS message queue rather than a FCFS segment queue and 4) a combination of . MD and PR denoted by MP. Simulation examples are employed to compare the performance of the above schemes and to gain insights into their characteristics. FDQ has similarities to DQDB including bus topology and same nodal hardware. Like DQDB, FDQ achieves full throughput efficiency independent of the bus lengths, the transmission speed, and the number of nodes. Unlike DQDB, FDQ allocates equal bandwidth under heavy load to all active users in a time period less than or equal to the round-trip propagation delay without wasting bandwidth. Its delay characteristics are studied via simulation and compared to DQDB. The simulation study has shown that FDQ has lower average delays under poisson Ioad than DQDB with or without the bandwidth balancing mechanism.