LAN technologies

LAN (Local Area Network) refers to a group of computers interconnected into a network so that they are able to communicate, exchange information and share resources (e.g. printers, application programs, database etc). In other words, the same computer resources can be used by multiple users in the network, regardless of the physical location of the resources.

Each computer in a LAN can effectively send and receive any information addressed to it. This information is in the form of data 'packets'. The standards followed to regularize the transmission of packets, are called LAN standards. There are many LAN standards as Ethernet, Token Ring , FDDI etc. Usually LAN standards differ due to their media access technology and the physical transmission medium . Some popular technologies and standards are being covered in this article.

Media Access Control methods

There are different types of Media Access Control methods in a LAN, the prominent ones are mentioned below :

• Ethernet - Ethernet is a 10Mbps LAN that uses the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol to control access network. When an endstation (network device) transmits data, every endstation on the LAN receives it. Each endstation checks the data packet to see whether the destination address matches its own address. If the addresses match, the endstation accepts and processes the packet. If they do not match, it disregards the packet. If two endstations transmit data simultaneously, a collision occurs and the result is a composite, garbled message. All endstations on the network, including the transmitting endstations, detect the collision and ignore the message. Each endstation that wants to transmit waits a random amount of time and then attempts to transmit again. This method is usually used for traditional Ethernet LAN.
  
• Token Ring - This is a 4-Mbps or 16-Mbps token-passing method, operating in a ring topology. Devices on a Token Ring network get access to the media through token passing. Token and data pass to each station on the ring. The devices pass the token around the ring until one of the computer who wants to transmit data , takes the token and replaces it with a frame. Each device passes the frame to the next device, until the frame reaches its destination. As the frame passes to the intended recipient, the recipient sets certain bits in the frame to indicate that it received the frame. The original sender of the frame strips the frame data off the ring and issues a new token.
  
• Fast Ethernet - This is an extension of 10Mbps Ethernet standard and supports speed upto 100Mbps. The access method used is CSMA/CD .For physical connections Star wiring topology is used. Fast Ethernet is becoming very popular as an upgradation from 10Mbps Ethernet LAN to Fast Ethernet LAN is quite easy.
  
• FDDI (Fiber Distributed Data Interface) - FDDI provides data speed at 100Mbps which is faster than Token Ring and Ethernet LANs . FDDI comprise two independent, counter-rotating rings : a primary ring and a secondary ring. Data flows in opposite directions on the rings. The counter-rotating ring architecture prevents data loss in the event of a link failure, a node failure, or the failure of both the primary and secondary links between any two nodes. This technology is usually implemented for a backbone network.

LAN Topologies

In the late 1960s and the early 1970s researchers developed a form of computer communication known as Local Area Networks (LANs). These are different from long-distance communications because they rely on sharing the network. Each LAN consists of a single shared medium, usually a cable, to which many computers are attached. The computers co-ordinate and take turns using the medium to send packets.

Unfortunately, this mechanism does not scale. Co-ordination requires communication, and the time to communicate depends on distance - large geographic separation between computers introduces longer delays. Therefore, shared networks with long delays are inefficient. In addition, providing high bandwidth communication channels over long distances is very expensive.

There are a number of different LAN technologies. Each technology is classified into a category according to its topology, or general shape. The first of these is a star topology, as illustrated in Figure 3.

The hub accepts data from a sender and delivers it to the receiver. In practice, a star network seldom has a symmetric shape; the hub often resides in a separate location from the computers attached to it.

A network using a ring topology arranges the computers in a circle - the first computer is cabled to the second. Another cable connects the second computer to the third, and so on, until a cable connects the final computer back to the first. This is illustrated in Figure 4.

Once again, the ring, like the star topology, refers to logical connections, not physical orientation.

A network that uses a bus topology consists of a number of computers all connected to a single, long cable. Any computer attached to the bus can send a signal down the cable, and all computers receive the signal. This is illustrated in Figure 5.

LAN Components
Basic LAN components
There are essentially five basic components of a LAN

Network Devices : such as Workstations, Printers, File Servers which are normally accessed by all other computers

Network Communication Devices : i.e. devices such as hubs, routers, switches etc., used for network operations

Network Interface Cards (NICs) : for each network device required to access the network .
Cable as a physical transmission medium.

Network Operating System - software applications required to control the use of the network LAN standards.

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