Multiple Logical Token-Rings in a Single High-Speed Ring*t

The present paper describes a new media access control protocol, called the multiple token protocol, for high-speed ring networks. The purpose of this protocol is to increase the throughput under a heavy load and to decrease the access delay under a light load ofthe token-ring protocol. The protocol maintains several logical token-rings in the single physical ring. The number of logical rings can be changed dynamically by the ring stations according to the ring load: when the load increases, the number of logical rings should be decreased, and vice versa. Each logical ring has its own token, and its operation is close to a token ring with early-token-release and destination removal. Multiple tokens enable multiple simultaneous transmissions of new frames by different stations and decrease the waiting for a token delay. However, the integration of several token rings in a single media is not straightforward since they interfere with the operation of each other. The present protocol is not the first to enable multiple simultaneous transmissions of frames to a ring network. However, unlike previous protocols, that use the buffer insertion concept combined with some fairness mechanism, the new one uses tokens in order to regulate the access to the ring and to ensure fairness. The token concept introduces two major advantages over the buffer insertion one. First, unlike in buffer insertion rings where data.-frames may experience a delay of one data.-frame at every intermediate station, in the new protocol the delay in the intermediate stations does not depend in the number ofsuch stations (i.e, in the distance between the source and the destination). Moreover, this delay can be reduced even to 0, if needed, in order to support the transmission of time critical frames. Second, the new protocol ensures the removal of a data.-frame even if both the destination and the source of the frame fail to remove it from the ring. The formal description of the new protocol is also unique. There are 3 finite state machines: for the receiver, for the transmitter and for token manipulations. The combination of these machines creates a single one that represnts the station algorithm. ·This work was supported in part by Fibronics Ltd. fRevised version of Technical Report # 680, July 1991 lThis work was performed in pInt when the author was with the Dept. of Computer Science, Technion, lIT o T ec hn io n C om pu te r Sc ie nc e D ep ar tm en t T eh ni ca l R ep or t C S0 73 8. re vi se d 19 92