| 摘要 |
A communications system suitable for use as a local area network (LAN) is provided wherein the network has a ring topology comprising a plurality of nodes 10 each of which is connected to adjacent nodes by links 14 and 15. Each node comprises a Link Interface Unit (LIU) 11, a Network Interforce Unit (NIU) 12 and a Terminal Interface Unit (TIU) 13. By providing forward and reverse links 14, 15 between nodes the system is capable of reconfiguring itself after link or node failure such that the failed equipment can be bypassed to minimize system disruption. Link Interface Units (LIU) 11 are also capable of bypassing their own node if a node failure is detected. System control is decentralized with each active node contributing to system control such that prime-failure sites are avoided. System protocol depends upon a token passing scheme where only the node 10 currently holding the token is entitled to transmit data and once it has finished its transmission the node 10 passes the token to the next node 10 in the ring. The system operates under a strict timing regime wherein a system cycle of substantially fixed period is established and the first rotation of the token around the ring during the system cycle is used to initiate transmission of the highest priority data with subsequent rotations of the token initiating transmission of data of progressively lower priorities. Accordingly, the timeslots available for transmission of each data type have variable-boundaries determined by the system load.
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