The timed-token protocol for real-time communications

The timed-token protocol is a token-passing protocol in which each node receives a guaranteed share of the network bandwidth. Partly because of this property, the timed-token protocol has been incorporated into a number of network standards, including the IEEE 802.4 token bus, the Fiber Distributed Data Interface (FDDI), and the Survivable Adaptable Fiber Optic Embedded Network (Safenet). Networks based on these standards are becoming increasingly popular in new generation real-time systems. In particular, the IEEE 802.4 standard is included in the Manufacturing Automation Protocol (MAP), which has been widely used in computer-integrated manufacturing and industrial applications. Meeting message deadlines requires proper control of medium access. In the timed-token protocol, access to the communication medium is controlled by a token that is passed among the nodes in a circular fashion. Messages are segregated into two separate classes: synchronous and asynchronous. Synchronous messages, used for real-time communication, can have deadline constraints and thus are given a guaranteed share of the network bandwidth. The authors focus on meeting the deadlines of synchronous messages.<>