On the stability of timed token rings

We analyze in this paper the stability of two types of timed-token rings: the existing Fiber Distributed Data Interface (FDDI) token ring protocol, and a new variant of the FDDI that we propose. The FDDI supports two classes of traffic: synchronous and asynchronous. The time constraint mechanism of FDDI guarantees the transmission delay of synchronous traffic: a Target Token Rotation Time (TTRT) being fixed, the FDDI protocol ensures that the token rotation time is always bounded above by twice TTRT. We consider the stability of the asynchronous traffic, for both FDDI and for the new proposed protocol, for which the token rotation time is also bounded by twice TTRT. We obtain sufficient and necessary stability conditions, which indicate how the choice of parameters affects the stability of the system. We show that the stability conditions of the new protocol are weaker (and thus it enables us to transmit more asynchronous packets). This protocol is also easier to implement as it requires less timers.