Approximating the FDDI synchronous mode

We present protocol extensions supporting real-time traffic performance guarantees for Fiber Distributed Data Interface (FDDI) implementations lacking a synchronous mode of operation. We provide a brief description of the FDDI protocol, with an emphasis on the synchronous mode and network parameter tuning to meet real-time application demands, and then discuss hardware limitations of some first-generation FDDI adapters which make the synchronous node ineffective and unable to support the required performance guarantees. By modifying the operating system kernel we can regulate the flow of information to the FDDI adapters and implement guarantees in higher-level software. Specifically, we added a two-tiered regulation facility which, at the lowest level, would place an upper bound on asynchronous transmission rate from the device driver buffers to the adapter. The next level up added the notion of standard and guaranteed performance system queues which user-level traffic could be directed into. Finally, a distributed user-level software mechanism was added to monitor and appropriately regulate the throughput from each adapter on the LAN in response to varying bandwidth requirements