On the schedulability of synchronous message sets with the minimum message deadline less than 2·TTRT in an FDDI network

We study the problem of guaranteeing synchronous message sets with the minimum message deadline (D_min) less than twice the target token rotation time (TTRT), i.e., D_min<2·TTRT, in an FDDI network. While the restriction of D_min⩾2·TTRT on a synchronous message set is generally accepted as a necessary condition for all message deadlines to be guaranteed, our study shows that this is not always the case. Some synchronous message sets with D_min<2·TTRT may still be guaranteed. We show that there is no need to retain the above restriction on a synchronous message set before determining its schedulability. Furthermore an improved version of a previously-published local synchronous bandwidth allocation scheme, with removal of this restriction, is proposed