Learning to Appreciate Tokens: How do FDDI internals work?

FDDI Networks feature a counter-rotating, dual fiber optic ring.
The primary ring circulates in one direction; the secondary ring circulates in the opposite direction

The FDDI ring conforms to the following rules

o	All stations are repeaters
o	In order to send data, a station must have the token
o	When holding a token, a station is not repeating
o	The designation station copies a frame, repeats it and
	sets appropriate status flags within the FDDI frame upon
	receipt
o	The transmitting station strips the frame off the ring
o	Management is distributed, thre is no master or monitor station


The FDDI token passes around the ring.	Because the FDDI standard uses the "append token" technique, more than one message can circulate on the ring at any one time, resulting in the efficient use of the available bandwidth.

As node A awaits the token, it acts as a repeater.  After seizing the token (a process called "absorption") and initaing the transmission of Frame F1 destined for node C, node A appends the token to the end of the frame

Node B repeats F1 until the token appears, then it absorbs the token and transmits frame F2 destined for node D.  Node C copies F1, sets the copy bit for node A to see, and then repeats F1, F2, and the token.

Node D repeats F1, which is not intended for it, copies F2, sets a copy bit for node B to see, and repeats F2 and the token.  Node A finally absorbs its original F1 frame and repeats F2 and the token.  Node B absorbs its F2 frame and repeats the token.

Normally, the backup ring is idle.  If a DAS device fails, the FDDI primary and secondary rings "wrap" to maintain network operation and integrity.   The failure of just two DAS devices, however, can result in a disjointed ring and segmented network.

On the other hand, a SAS device failure does not disrupt network operation.  Network integrity is preserved by automatic bypass of the failed device or connection by the concentrator.