Fiber Distributed Data Interface (FDDI)

FDDI has been touted as the next generation of LAN/MAN. Currently many companies are making FDDI boards. Although the scale of economy didn't play a favorable role in that the cost of FDDI ports is not as affordable as people had expected. Furthermore, as you will see, that FDDI still relies on shared media transmission, which limites the aggregate capacity of the whole network to 100. Recently, it appears that FDDI is overshadowed by the overwhelming acceptance of ATM. (I wonder what will be next before ATM goes out of favor :-)

Key features of FDDI:

• Optical fiber is used as the main transmission media, which makes it completely different from all the previous LAN systems (a variation of FDDI exists that uses coaxial cable, but has similar MAC layer protocol to FDDI).
• Distance up to 200, which is significantly longer than what CSMA/CD can cover. Therefore, FDDI can be used either as backbones connecting other LANs (Ethernet, Token Ring, etc) or used as a high speed MAN;
• Two fibers are used to form a dual ring. Stations can be connected to both ring or just one. The rings are NOT in one piece, but disjoint segments of fibers linking stations. Therefore, FDDI is NOT an All-Optical-Network.
• Maximum frame size is 36,000 bits.
• FDDI protocol is very similar to that of token rings.
• FDDI standard covers both physical layer and MAC layer aspects:
  • Physical layer: Low cost multimode optic fiber, LEDs and photodiodes are used. Error rate no more than . The ring is synchronized by a common clock. Data rate on each ring is 100 , this is much lower than the real capacity of optical fibers, but can be implemented using low cost components; 4B5B coding scheme is used to code the bit streams. In this scheme, blocks of 5bits are used to represent 4bit information, i.e., for each of the 16 four bit data block, a 5bit block is used. The other 16 5bit blocks are never used.
  • MAC layer: To be described below.

How FDDI works:

• FDDI uses release after transmission (RAT). Therefore, at any time, there may be more than one frame flying around the ring, especially when the ring is long and data rate is high (100 is high). Transmitting node is also responsible for removing its own frames when they come around the ring.
• There are two priorities. High priority is for voice, video, real-time control, etc.
• Define as target token rotation time: it is the upper bound on the time-average intertoken arrival time.
• Define as allocated time for node to send its high prority traffic, including delay to reach the next node. (Assuming there are nodes)
• Condition must be satisfied;
• Let be the time when the token arrives at node . is the time elapsed between two token visits to node . The following conditions must be met:
  1. If , the node is allowed to send low priority traffic for up to seconds, provided high priority traffic is transmitted first (up to seconds) and does not use all of the seconds. Therefore, we have .
  2. If , no low priority traffic is allowed, but high priority traffic can be sent within the next seconds. Therefore, .
  3. Combine the above two, we have in general:
• From the above discussion we can show that the worst delay for any use is about . To see how this can happen, assume that token header processing is neglible, at one token rotation, there is no transmission request, then . Now when the token arrives at station , station realizes that the token got back in zero time, in other words, node is seconds ahead of schedule, he can use the entire to transmit his high and low priority data ( seconds for high priorty and for low priority data) and still beat the target token rotation time. When the next node gets the token, it also happens to be fully loaded with high priority traffic just before the token arrives, it has to transmit at least seconds. Continuing this way, and assuming all the nodes are fully loaded with high priority traffic, each one of them will be behind schedule but still has to transmit their share of the high priority traffic. The time it takes the token to get back at node will be . If is very small, .
• FDDI guarantees service to high priority traffic, but not low priority traffic.