A local area network (LAN) is a communication system that allows a number of independent devices to communicate directly with each other in a limited geographic area such as a single office building, a warehouse or a campus. LANs are standardised by three architectural structures: Ethernet, token ring and fibre distributed data interface (FDDI).

• Ethernet

Ethernet is a LAN standard originally developed by Xerox and later extended by a joint venture between Digital Equipment Corporation (DEC), Intel Corporation and Xerox. The access mechanism used in an Ethernet is called Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

In CSMA/CD, before a station transmits data, it must check the medium where any other station is currently using the medium. If no other station is transmitting, the station can send its data. If two or more stations send data at the same time, it may result in a collision.

Therefore, all stations should continuously check the medium to detect any collision. If a collision occurs, all stations ignore the data received. The sending stations wait for a period of time before resending the data.

To reduce the possibility of a second collision, the sending stations individually generate a random number that determinates how long the station should wait before resending data.

• Token Ring

Token ring, a LAN standard originally developed by IBM, uses a logical ring topology. The access method used by CSMA/CD may result in collisions. Therefore, stations may attempt to send data many times before a transmission captures a perfect link.

This redundancy can create delays of indeterminable length if traffic is heavy. There is no way to predict either the occurrence of collisions or the delays produced by multiple stations attempting to capture the link at the same time.

Token ring resolves this uncertainty by making stations take turns in sending data. As an access method, the token is passed from station to station in sequence until it encounters a station with data to send. The station to be sent data waits for the token.

The station then captures the token and sends its data frame. This data frame proceeds around the ring and each station regenerates the frame. Each intermediate station examines the destination address, finds that the frame is addressed to another station, and relays it to its neighbouring station.

The intended recipient recognises its own address, copies the message, checks for errors and changes four bits in the last byte of the frame to indicate that the address has been recognised and the frame copied. The full packet then continues around the ring until it returns to the station that sent it.

• Fiber Distributed Data Interface (FDDI)

FDDI is a LAN protocol standardised by ANSI and ITU-T. It supports data rates of 100 Mbps and provides a high-speed alternative to Ethernet and token ring. When FDDI was designed, the data rate of 100 Mbps required fibre-optic cable.

The access method in FDDI is also called token passing. In a token ring network, a station can send only one frame each time it captures the token. In FDDI, the token passing mechanism is slightly different in that access is limited by time.

Each station keeps a timer which shows when the token should leave the station. If a station receives the token earlier than the designated time, it can keep the token and send data until the scheduled leaving time.

On the other hand, if a station receives the token at the designated time or later than this time, it should let the token pass to the next station and wait for its next turn. FDDI is implemented as a dual ring. In most cases, data transmission is confined to the primary ring.

The secondary ring is provided in case of the primary ring’s failure. When a problem occurs on the primary ring, the secondary ring can be activated to complete data circuits and maintain service.