High Speed LAN

HIGH SPEED LAN

The high speed LANs that have emerged can be broadly categorized into three types

1.   Based on token passing :- FDDI and its variations & high-speed token ring
2.    Successors of Ethernet :- fast Ethernet and Gigabit Ethernet
3.  Based on switching technology :- ATM, fiber channel and the Ether switches

FDDI

Fiber Distributed Data Interface (FDDI), developed by American National Standards Institute (ANSI) is a token passing ring network that operates at 100 Mb/s on optical fiber-medium. Its medium access control approach has close similarity with the IEEE 802.5 standard, but certain features have been added to it for higher reliability and better performance.

The standard physical medium is multi-mode 62.5/125 micron optical fiber cable using light emitting diode (LED) transmitting at 1300 nanometers, as the light source. FDDI can support up to 500 stations with a maximum distance of 2 Km between stations and maximum ring circumference of 200 Km. The standard has also been extended to include copper media - Shielded Twisted Pair (STP) and some categories of Unshielded Twisted Pair (UTP) with a maximum distance of 100 m between stations. FDDI over copper is referred to as Copper-Distributed Data Interface (CDDI).

The basic topology for FDDI is dual counter rotating rings: one transmitting clockwise and the other transmitting counter clockwise as shown in the Fig.1. One is known as primary ring and the other secondary ring. Although theoretically both the rings can be used to achieve a data transfer rate of 200 Mb/s, the standard recommends the use of the primary ring for data transmission and secondary ring as a backup.

In case of failure of a node or a fiber link, the ring is restored by wrapping the primary ring to the secondary ring as shown in Fig.1. The redundancy in the ring design provides a degree of fault tolerance, not found in other network standards.

Fig. FDDI dual counter-rotating ring topology