FDDI

FDDI

• 1.
  FDDI
• 2.
  FDDI • Similar toIEEE802.5, 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 detectsthe 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 MbpsFDDI – 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 classof 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 thetimer 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: