Another protocol gaining in popularity is the fiber optic distributed data interface (FDDI). The current specification dictates a maximum throughput of 100 Mb/sec, although progress is being made toward a 200 Mb/sec synchronous standard. Each FDDI packet can be up to 4,352 bytes long, approximately 20 of which are for control. As with ATM and Ethernet, FDDI makes no provisions for guaranteed delivery or data integrity. Unlike Ethernet and ATM, however, FDDI is token based. Any machine wishing to transmit a message must first acquire the token through a broadcast operation. Also, messages are explicitly forwarded from node to node, instead of being allowed to propagate along the entire medium. This strategy has the positive effect of eliminating collisions, but it can cause higher than normal latencies for networks with many nodes.
Richard Martin of the University of California at Berkeley
implemented active messaging on an FDDI network of four HP PA-RISC
workstations. The FDDI card in each workstation was memory mapped
into each communicating process and a special process called the
scheduler. Through mutual exclusion constructs, the scheduler allowed only one
user process to access the board at any given time. The ``active''
user process forwarded any packets not destined for it to the
scheduler, which distributed them accordingly. Outgoing packets from
``excluded'' processes were handled in a similar manner. Mr. Martin
achieved a sustained transfer rate of 12 MB/sec with large packets and
measured round-trip latencies as low as 29 
sec for packets with
no payload. This is certainly quite an accomplishment, especially when
compared with the performance of interconnection networks on today's
massively parallel processing systems. Even though the code is
virtually unportable, the algorithms and protocols are universal in
their applicability. These will be outlined later in the paper.