Computer Networks - FDDI

1. FDDI

2. FDDI
• Similar to IEEE802.5, but runs on fiber instead of copper.
Physical Properties
– FDDI consist of dual ring.
• Two independent rings that transmit data in opposite direction.
• Second ring is not used during normal operation. But work when primary link
fails.
– Allows nodes to attach to the network using single cable.
• Called as single attachment station (SAS).
– Concentrator is used to connect multiple SAS

3. FDDI
• Concentrator detects the failure of the SAS.
– By using optical bypass, it keeps the ring connected.
• Network adaptor holds some number of bits between input and
output interface.
– Variable size buffer (never less than 9 bits nor more than 80bits)
• Station transmits an amount equal to half the buffer

4. FDDI
Example: 100 Mbps FDDI
– 10 ns for bit time
• Each station 10 bit buffer – waits until buffer half full before
transmitting
– station introduces 50 ns delay into TRT
• Network size is limited to 500 stations with a maximum distance
of 200km.
• Uses 4B/5B encoding.

5. FDDI
Timed Token Algorithm
• to give a opportunity to the node, to transmit a frame with in a
certain amount of time,
– It sets an upper bound on TRT (Token Rotation time) observed by any
node.
– Target Token Rotation time (TTRT) – all node are agree to live with in the
TTRT
• Each node measures the time between the successive arrival of the
token called measured TRT.
If measured TRT > TTRT
– Token is late and node does not transmit any data.
If measured TRT < TTRT
– Token is earlier and the node can hold the token for the (TTRT – measured
TRT) and transmit data.
– down stream station may not be able to transmit.

6. Provides two class of traffic,
– Synchronous
– Asynchronous
Synchronous:
– Traffic is delay sensitive
– station transmits data whether token is late or early
– Eg., Voice and Video
Asynchronous:
– Station transmits only if token is early.
– Eg., File transfer
Token Maintenance
– Process of setting TTRT.
– Monitor ring to ensure token has not been lost.
– Idle time between valid transmissions that a given node experiences is
• ring latency + time to transmit a full frame
• 2.5 ms maximally sized ring
– Each node sets a timer event that fires after 2.5ms.

7. – If the timer expires, the node suspects a failure
• Then transmits a “claim” frame
• Bid for the TTRT.
• Lowest TTRT- holds the token & inserting a valid token.
– After every valid transmission, node resets the timer back to 2.5ms.
Frame Format: