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Fiber Distributed Digital Interface (FDDI)

FDDI is a token-passing counter rotating fiber-optic-based ring that has the capacity to interconnect up to 1000 devices over a 100 kilometer path. Due to the maximum distance runs of FDDI, it is often used as a MAN technology in addition to being used as a LAN technology. FDDI standards specify LAN-to-LAN packet-switched backbones, in addition to the counter-rotating ring configuration operating at 100 megabits-per-second. While the token-ring version of FDDI is the most well known, switched FDDI has the most potential to support workstation clusters. Token-ring FDDI still has some life remaining as a LAN technology --- its peak data rate is useful in moving large data files from servers to end-users. However, latency is high in a FDDI ring (you must wait for the token) --- so high that token-ring FDDI generally performs worse in computer clusters than TCP/IP communications over Ethernet (for correctly configured FDDI and Ethernet networks). By collapsing FDDI rings to a single switched connection, the problems with latency are removed. A version of the HP Computational Cluster is offered with switched FDDI as the interconnection network.

Due to the fiber optic-based design of FDDI, there is a question of network performance and cost. The laser diodes cause FDDI implementations to be much more expensive that Ethernet-based networks. FDDI throughput performance is much greater than Ethernet, but the cost/performance ratio must be examined as a function of one's specific application. Early ATM-based LANs used physical connections based on the TAXI standard developed for FDDI, but per-port-costs generally were less for ATM when compared to switched FDDI. The electronics were simpler for the interface cards. With the advent of serial HiPPI and Fibre Channel, there is strong competition from these technologies for significantly higher data rate LANs, and ATM will offer strong competition for both LAN and MAN environments. I believe that FDDI will be supplanted by ATM as a networking technology long before 10 megabit-per-second Ethernet --- mainly for cost considerations. Existing investments in legacy networks will remain in place for extended periods of time, although I see little (cost effective) reason to expand legacy FDDI networks. For high-performance LAN environments, serial HiPPI is much faster for similar investments in fiber-optic-based technology. ATM should also be comparable for high-performance LAN environments with similar investments, and ATM should have a significant cost/performance advantage in the MAN environment. Lastly, ATM can be used transparently throughout either the LAN, MAN, or WAN environments.



next up previous
Next: Ethernet Up: Available Networking Technologies Previous: Asynchronous Transfer Mode



David P. Koester
Sun Oct 22 13:05:27 EDT 1995