Local Area Network (LAN)

A computer network with many computer located in the same building, room or campus is called local area network or LAN. They are widely used to connect personal computers and work stations in company offices and factories to share resources and exchange files of information.

When computers located within a small geographical area such as an office or university campuses (within a radius of 10kms) are connected together we call it a Local Area Network or LAN.

Most LANs connected work-stations and personal computers. Each node (individual computer) in a LAN has its own CPU with which it executes programs, but it is also able to access data and devices anywhere on the LAN. This means that many users can share expensive devices such as laser printers as well as data. Users can also use the LAN to communicate with each other, by sending e-mail or engaging in chat sessions.

There are many different types of LANs-token-ring networks, Ethernets or Ethernet LAN and ARC nets being the most common for PCs. Most Apple Macintosh networks are based on Apple’s Apple Talk network system, which is built into Macintosh computers.

Many people use (incorrectly) the name “Ethernet” in a generic sense to refer to all CSMA/CD protocols, even though it really refers to a specific product that implements 802.3 an IEEE standard for LANs.

Ethernet LAN is a standard developed by Digital Equipment Corporation, Xerox and Inter connecting computers within a small geographic area. This was later redefined and standardized as IEEE standards 802. The standard specifies interconnection of computers using a Bus. The physical layer is a shielding coaxial cable supporting a data rate of 10 million bits/second. The maximum length of cable is 2.8 km and the maximum number of nodes which can be connected is 1024.

The data link layer defines controlling access to the network and how data packets are transmitted between stations connected to the network. Referring to the following figures of Ethernet LAN, we observe that all the stations are connected to the bus and each station connects/communicates with the bus via a transceiver that is combined transmitter and receiver. Each station sends packets as a set of coded bits which are not modulated.

At present Ethernet is one of the most popular Local Area Network used as it is well proven, standardized and supported by all vendors of computers. Cheaper versions of Ethernet are now emerging. One of them is the use of unshielded twisted pair of wires abbreviated as UTP) instead of a coaxial cable. This has become possible due to improvements in electronics technology. The speed of these LANs is also 10 mbps.

Another system using Ethernet protocol is called thin-wire Ethernet. This uses a standard coaxial cable like those used in cable T.V.

LAN are distinguished by the 3 characteristics :

a. their size
b. Their Transmission technology &
c. Their topology

This uses a transmission technology consisting of single cable to which all the machines are attached.

Traditional LANs run at speeds of 10 to 100 mbps and have low delays and make very few errors.

Never LANs can operate at higher speed, upto 100 mbps

Bus and ring are most commonly used to topologies for broadcast LANs

Broadcast network can be either static or dynamic. On a typical static allocation would be to divide up time into discrete intervals and run a round robin algorithms.

Dynamic allocation methods for a common channel are either centralized or decentralized. In centralized channel allocation method, bus are arbitration determines who does next, whereas in decentralized channel allocation machine itself must decide whether or not to transmit.

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