Question about FDDI

[tmaclean]

Hi, I've just begun a networking course in college and we have recently gone over FDDI. One question my instructor was unable to think of an answer to was, how does FDDI know when there's a break in a cable and to use the other channel?

[mbossman2]

FDDI is optical cabling. when the cable has a break the signal strength drops or stops, the concentrators that make up the active part of the ring are able to detect the signal loss and immediately shunt the traffic onto the other ring, isolating the fault and allowing the traffic to flow.

[tmaclean]

Ohh, so could you say if the moderator computer doesn't receive the packet in the amount of time that it's supposed to, it assumes something's wrong and forwards everything to the other ring?

[mbossman2]

yes, a device makes the decision. In the last generation of FDDI concentrators, the concentrator had the "intelligence" to detect the flaw and redirect traffic (if an only if it was a dual attached device), in earlier generations you relied on a PC/Server type device to notify the concentrator to redirect the traffic.

This was one of the earliest forays into self aware networks. that is to say, the devices 1 step up from the physical layer (wires, cables etc) had the ability to make changes to how network traffic flowed to allow for the most efficient and error free transit.