FDDI (Fiber Distributed Data Interface) is another ring-based network, and unlike Token Ring, it is implemented without hubs, although you can use devices called concentrates to perform a similar function. FDDI uses fiber optic cables to implement very fast, reliable networks.
What is Fiber Distributed Data Interface (FDDI)?
FDDI is a high-performance fiber optic token ring LAN running at 100 Mbps over distances up to 200 km with up to 1000 stations connected. it can be used in the same way as any of the other LANs, but with its high bandwidth, another common use is a backbone to connect copper LANs, as shown in the given below image.
|An FDDI ring being used as a backbone to connect LANs and computers|
The high speed and high capacity of FDDI enable it to function as a very reliable backbone to connect a large number of LANs. FDDI uses multimode fibers because the additional expense of single-mode fibers is not needed for networks running at only 1000 Mbps. it also uses LEDs rather than lasers, not only due to their lower cost but also because FDDI may sometimes be used to connect directly to user workstations.
The FDDI cabling consists of two fiber rings, one transmitting clockwise and the other transmitting counterclockwise, as shown in the given below image. if either one breaks, the other can be used as a backup. if both break, the other can be used as a backup. if both break at the same point.
For example, due to a fire or other accident in the cable duct, the two rings can be joined into a single ring approximately twice as long, as shown in the given below image. each station contains relays that can be used to join the two rings or bypass the station in the event of station problems. wire centers can also be used, as in IEEE 802.5 protocol.
|FDDI consists of two counter-rotating rings and two rings joined together|
How does FDDI work?
FDDI (like Token Ring) uses token-passing schemes to control network access. Unlike Token Ring, several FDDI devices can transmit data simultaneously.
FDDI uses an even more sophisticated method of accessing the network than does Token Ring. Like Token Ring, a token is passed around the ring, and the possessor of the token is allowed to transmit FDDI frames. Unlike Token Ring, an FDDI network may have several frames simultaneously circulating on the network. this is possible because the possessor of the token may send multiple frames, without waiting for the first frame to circulate all the way around the ring before sending the next frame.
The possessor of the FDDI token is also allowed to release the token and send it to the next station in the ring as soon as it is through transmitting frames, rather than having to wait for the frames to make it all the way around the ring. this means that the next station may begin transmitting while the frames from the first station are still circulating, as can be seen in the given below image.
|Dual counter-rotating rings allow FDDI to operate in spite of network faults|
Advantages of FDDI
- High bandwidth: It has a bandwidth as high as 250 Gbps. High bandwidth allows for tremendous speed. FDDI implementation can handle data rates of 100 Mbps.
- Security: It is difficult to eavesdrop on fiber optic cable transmission.
- Physical durability: Fiber optic cable does not break as easily as other kinds of cables.
- Resistance to EMI: Fiber optic cables are not susceptible to Electromagnetic Interference.
- Cable distance: Fiber optic cables transmit signals over 2 kilometers without needing any repeaters.
- Weight: Fiber optic cable weighs much less than copper wire with similar bandwidth.
- Use of multiple tokens: FDDI uses multiple tokens to improve network speed.
- Ability to prioritize workstations: FDDI can designate some workstations as low-priority workstations. this allows FDDI to bypass low-priority workstations when necessary, providing faster service to high-priority stations.
- System fault tolerance: FDDI can isolate faulty nodes with the use of wiring concentrators for instantaneous rerouting. wiring concentrators function as centralized cabling connection devices for workstations.
Disadvantages of FDDI
FDDI is a complex technology, and installation and maintenance require a great deal of expertise.
FDDI is costly, in addition to the fiber-optic cable cost, the adapters and concentrators are also very expensive.