Branch-switching in unidirectional channel systems using the IEEE 802.4 token-passing bus access method

In this paper, a distributed access control method is introduced for branched unidirectional channel environments. Branched systems already enjoy wide usage employing conventional CSMA/CD and Token-Passing Bus media access methods. However, in such systems it is well known that performance degrades with an increase in the normalized system end-to-end propagation delay. The proposed method minimizes this effect by employing an efficient modified attempt-and-defer scheme as the intra-branch protocol. An inter-branch protocol runs concurrently and defines which branch is active. A salient characteristic of the proposed method is that the standardized IEEE 802.4 Token-Passing Bus protocol is used for inter-branch switching. This is a significant feature in that the intra-branch protocol design allows all of the IEEE 802.4 token maintenance and error recovery functions to operate according to the existing standard. As a result, the proposed system is easy to implement and highly robust.