Fibre Distributed Data Interface (FDDI) | Networking | Computers

After reading this article you will learn about the concept of fibre distributed data interface (FDDI).

FDDI is a high-performance Token Ring LAN running at 100 Mbps over distances up to 200 km handling stations of the order of 1000. It can be used like copper wire in IEEE 802 LANs but is sometimes used as a backbone to connect copper LANs. This is mainly done to save money, because Fibre Optic cables cost many times the cost of UTP copper cables. FDDI uses multimode fibres because of cost considerations.

Single mode Fibres are not needed for networks running at speeds of 100 Mbps. It also uses LEDs instead of lasers because these cost much less and also because FDDI may sometimes be asked to connect directly to user workstations.

There is the possibility that curious users may occasionally unplug the fibre connecter and look directly into it to watch the bits travel at 100 Mbps. With a laser the curious onlooker might end up with a hole in his retina.

LEDs are weak enough to not be able to damage the retina and yet are strong enough to transfer data accurately at 100 Mbps. The FDDI requirements state that error level should be less than 1 bit in 2.5 x 10¹⁰ bits. The FDDI cabling consists of two fibre rings, one transmitting clockwise and the other transmitting anticlockwise.

If there is trouble or a breakdown with either ring, the other can be used as a backup. If both rings break at the same point due to an accident or a fire or some similar reason, then both ring can be joined into a single ring approximately twice as long. FDDI consist of two classes of stations, A and B.

Class A stations connect to both rings and class B stations connect to only one the two rings. Depending upon the fault tolerance requirements and cost, a station can choose class A and class B rings or numbers of both suitably.

The physical layer uses an encoding scheme called 4 out of 5. Each group of 4 Medium Access Control symbols is encoded as a group of 5 bits on the medium. 16 of the 32 combinations are for data, 3 are for delimiters, 2 are for control, 3 are for hardware signaling and 8 are so far unused.

The advantage of this scheme is that it saves bandwidth but the loss is that it lacks self-checking capability (this is available in Manchester encoding which is not used because this would require 200 mega-baud for 100 Mbps transmission. To compensate for this lack of self-checking capability, a long preamble is used to synchronize the receiver to the sender’s clock.

The basic FDDI protocols are very similar to IEEE 802.5 standard protocols. To transmit data a station must first obtain the token. Then it transmits a frame and removes it when it comes around the next time. A difference between FDDI and IEEE 802.5 is the fact that a station may not generate a new token until its frame has gone all the way around and come back.

In FDDI, with potentially 1000 stations and 200 km of fibre, the amount of time wasted waiting for the frame to go around the ring can be substantial. For this reason, it was decided to allow a station to put a new token back into the ring as soon as it has finished transmitting its frames. In a large ring, several frames might be on the ring at the same time.

FDDI has since been upgraded with a new version called FDDI II. The earlier protocol is some­times referred to as FDDI I. FDDI II which is the successor modifications have been made to handle synchronous circuit-switched PCM data for voice or ISDN traffic, in addition to ordinary data.

The protocols concerned deal with the physical layer and the Medium Access Control sub layer which is a sub layer of the Data Link Layer. Normally, however, all the versions of FDDI including version FDDI I and FDDI II are known simply as FDDI.

Distributed Queue Dual Bus:

While the workings of a Distributed Queue Dual Bus invariably referred to by its acronym DQDB. However, we shall now look at it in a new light—as a protocol for propagating a high-speed MAN.

We have looked at various protocols suitable for LANs, but none of the protocols that we have seen so far are suitable for use in MANs. For networks that cover an entire city IEEE has defined one MAN—DQDB as a standard 802.6.

In metropolitan areas, two parallel unidirectional buses cover the various areas of the city (or metropolitan area) and stations are attached in parallel to both buses. Each bus has a head-end which generates a steady stream of 53-byte cells. Each cell travels downstream from the head-end.

When it reaches the tail-end of the bus it falls off the bus. Each cell carries a 44-byte data field and holds two protocol bits Busy and Request. Busy indicates that a cell is occupied and Request is set when a station wants to make a request.

Since the two buses are unidirectional, to transmit a cell, a station has to know whether a destination is to the left or to the right so that it can use the appropriate bus. Data is inserted into the appropriate bus using a wired-OR circuit, therefore, the failure of a station takes down the entire network.

The IEEE 802.6 protocol queues up in the order in which they become ready to transmit and on the basis of the first in first out principal.

To simulate the FIFO queue each station maintains two counters—a Request Counter (RC) and CD. RC counts the number of downstream requests pending. At this point RC is copied on to CD and then RC is reset to zero and then counts the number of requests made after the station became ready.

For example, if CD = 3 and RC = 2 for station k, the next three empty cells that pass by station k are reserved for downstream stations. For the moment, assume that a station can have only one cell ready for transmission at a time. To send a cell, a station must first make a reservation by setting the Request bit in some cell on the reverse bus.

As it travels down the reverse bus, every station that it passes notes it and increments its RC. The head end of the on the other bus then creates an empty cell. As it passes by the requesting station, it may not fill the cell unless its CD = 0. But it will fill the cell if its CD = 0, since that means it is at the head of the queue. DQDB systems are now being installed by many carriers throughout cities as MANs.