On the Effectiveness of the FDDI-M Medium Access Protocol for Real-Time Traffic

FDDI networks have been widely deployed to support real-time traffic such as voice and video communications. However FDDI's medium access control (MAC) protocol can transmit synchronous messages up to at most one half of the total bandwidth of the FDDI network. In a recent paper by K. Shin and Q. Zheng [Shin95], a modification to the FDDI MAC protocol, called FDDI-M, has been proposed to double a ring's ability to support synchronous traffic. In this paper we present an analytical study of the timing properties of the FDDI-M protocol. Using the Worst Case Achievable Utilization (WCAU) as the performance metric, we first evaluate the performance of various synchronous bandwidth allocation (SBA) schemes in guaranteeing synchronous message deadlines. It is found that, in comparison with FDDI, the FDDI-M protocol results in a higher WCAU for the normalized proportional SBA scheme. However, for the local SBA schemes, the WCAU values remain zero. Next, other performance metrics such as throughput and percentage of frames missing deadlines are examined using simulation. A series of simulations using actual MEPG video traffic confirms that FDDI-M does provide superior synchronous transmission characteristics.