Real - Time Performance of TwoToken Ring

When designing real-time communication protocols, the primary objective is to guarantee the deadlines of synchronous messages while sustaining a high aggregate throughput. In this chapter, we study two token ring protocols for their suitability in real-time systems. The priority-driven protocol (e.g., IEEE 802.5) allows implementation of a priority-based real-time scheduling discipline like the rate monotonic algorithm. The timed token protocol (e.g., FDDI) provides guaranteed bandwidth and bounded access time for synchronous messages. We study these two protocols by deriving their schedulability conditions, i.e., the conditions which determine whether a given message set can be guaranteed. Using these conditions, we evaluate the average performance of these protocols under diierent operating conditions. We observe that neither protocol dominates the other for the entire range of the system parameter space. We conclude that the priority-driven protocol performs better for bandwidths in the range of 1-10 Mbit/s, while the timed token protocol has a superior performance at higher bandwidths. 6.1 Introduction Advances in computer and communication technology have led to the widespread use of digital computers in embedded real-time systems such as space vehicle systems , automated manufacturing, industrial process control, and so on. The design and evaluation of such systems is diicult due to the stringent timing requirements of these applications. The challenge is further compounded by the distributed nature of the systems. The success of a distributed real-time system depends on the timely execution of computational tasks which usually reside on diierent nodes and communicate with each other to accomplish a common goal. It is impossible for the individual tasks to meet their timing requirements without the support of an 115 Sec. 6.1 Introduction 116 underlying network which ensures timely delivery of inter-task messages. In this chapter, we study two approaches to the problem of guaranteeing the deadlines of synchronous messages on token ring networks. Synchronous messages are the most common time-critical messages in distributed real-time systems. A real-time synchronous message stream is a sequence of messages arriving periodically at a network node, with each message having a deadline for transmission. The most important goal in building a real-time network is to guarantee the deadlines of synchronous messages; that is, to ensure that they are always transmitted before their deadlines. This study concentrates on token ring networks. Ring-based networks have become popular in recent years because of their simplicity and their potential use of high-speed transmission media. The choice of Media Access …