FDDI which evolved from the IEEE 802.4 token bus timed token protocol is a fault tolerant 100Mbit/sec token passing counter-rotating dual ring LAN standard that permits data transmission between two end-point devices that can be many tens of kilometers apart.
Physical Transmission Medium
As its name indicates, fiber optic cable is the main form of physical transmission medium used in FDDI. Although a copper-based implementation called, Copper Distributed Data Interface (CDDI) does exist. Although originally conceived as a LAN standard FDDI has also been used extensively for MAN and WAN implementations.
In essence FDDI is a ring network similar to IBM’s Token Ring network but with a number of critical differences. The most noticeable of which is that a FDDI uses a dual-attached, counter-rotating token ring topology.
One ring acts as the primary transmission ring and in the original implementations was capable of delivering transmission speeds of up to 100Mbit/sec. The other or secondary ring was originally intended solely to act as a backup. This meant that the secondary ring was inactive and remained so for as long as the primary ring was functional. In the event of failure of the primary ring the secondary ring would become active. Now all traffic goes to the secondary ring for transmission. It is this built-in redundancy that means FDDI is a fault tolerant technology.
Higher Effective Sustained Data Throughput
Another factor in FDDI’s favor was that it used a much larger frame size than Ethernet which meant that it was capable of much higher effective sustained throughput rates than standard 100Mbit/sec Ethernet. Administrators also had the option of using the secondary ring for data transport rather than having it stand idly by thereby doubling transmission capacity to 200Mbit/sec.
Coverage and Scalability
Not only can FDDI traverse large distances it also scales much better than 100Mbit/sec Ethernet. This means it provides superior support for expanding enterprise networks consisting of hundreds or thousands of users. This is a fact not lost on ISPs now that FDDI-II has added the capacity to carry both voice and video traffic.
Spanning Large Distances
With the fiber optic technologies currently available today signal degradation and regeneration issues are not what they once were and so the distance factor that so limits copper-based media is of negligible consequence where fiber optic transmission is concerned. This makes fiber optic based transmission media the media of choice when it comes to “long haul” applications such as intercontinental, cross-continental and oceanic (marine) backbone links.
Immunity to Noise and the Environment
Environment factors such as moisture and Radio Frequency Interference (RFI) are also not of the same criticality as they are for copper-based and wireless media. The reasons for fiber optic cable as a transmission medium providing a high degree of immunization to noise (EMI) as opposed to other transmission media all stem from the use of light to convey the information (signals) and the construction of the medium (the fiber optic cable).
Due to the degree of difficulty in “tapping” fiber optic transmission lines without being detected, communications and networking technologies such as FDDI that are based on fiber optic transmission media offer a more secure medium than copper-based or wireless technologies.
Fiber Distributed Data Interface II (FDDI-II)
FDDI-II is a more recent development of FDDI that has added support for circuit-switched services thereby enabling FDDI-II to carry both voice and video signals as well. This means that it is admirably suited to large scale ISP Internet backbone implementation scenarios as it provides for scalability, coverage, geographical spanning (particularly now that modern fiber-optic cabling has overcome signal attenuation and regeneration issues), fault tolerance (the redundant secondary ring), speed, bandwidth (multi-mode fiber-optics), cost effectiveness, improved security (particularly in comparison with copper and wireless)
For additional technical and other specifications visit the IEEE website.