Local Area Networks Components

Local Area Networks

A local area network (LAN) is a collection of computers, usually personal computers, that share hardware, software, and data. In simple terms, local area networks hook personal computers together through communication media so that each personal computer can share the resources of the other. As the name implies, Local Area Networks cover short distance, usually one office or building or a group of buildings that are close together.

Local Area Network Components

Local Area Networks do not use the telephone network. Networks that are LANs are made up of a standard set of component.

• All networks need some system for interconnection. In some LANs the nodes are connected by a shared network cable. Low-cost LANs are connected with twisted wire pairs, but many LANs use coaxial cable or fiber optic cable, which are both more expensive and faster. Some local area networks, however, are wireless, using infrared or radio wave transmission instead of cables. Wireless networks are easy to set up and reconfigure, since there are no cable to connect or disconnect, but they have slower transmission rates and limit the distance between nodes.
• A network-interface card, sometimes called a NIC, connect each computer to the wiring to the network. A network-interface card is a circuit  board that fits in one of the computer's internal expansion slots.
• Similar network can be connected by a bridge, which recognizes the messages on the network and passes on those  addressed to nodes in other networks. For example,  a fabric designer whose computer is part  of a department Local Area Network for a textile manufacturer could send cost data via a bridge, to some one in the accounting department whose computer is part of another company LAN, one used for financial matters.
• A gateway is a collection of hardware and software resources that lets a node communicate with a computer on another dissimilar network. A gateway, example, could connect an attorney on local area network to a legal service offered through a wide area network.

Local Area Network Types

Two ways to organize the resources of a Local Area Network are client/server and peer-to-peer.

• Client / Server
  A client/server arrangement involves a server, which is a computer that controls the network.In  particular, are server has the hard disks holding shared files and often has the highest-quality printer, which can be used by all nodes. The client are all the other computers on the network. Under the client or server arrangement, processing is usually done by the server, and only the results are sent to the node. Sometimes the server and the node share processing. For example, a server, upon request from the node, could search a data base of cars in the state of Maryland and come up with a list of all jeep Cherokees. This data could the passed on to the node computer, which could process the data further, perhaps looking for certain equipment or license plate letters. This method can be contrasted with a file server relationship, in which the server transmits the entire file to the node, which does all its own processing. Using the Jeep example, the entire car file would be sent to the node, instead of just the extracted Jeep Cherokee records.

  

  Peer-to-peer Network

  
  
  Client/server has attracted a lot of attention because a well-designed system reduces the volume of data traffic on the network and allows faster response at each node. Also since the server does most of the heavy work, less expensive computers can be used as nodes.
• Peer-to-Peer 
  All computers in a peer-to-peer arrangement have equal status; no one computer is in control. With all files and peripheral devices distributed across several computers, users share each other's data and devices as needed. An example might involve a corporate building in which marketing wants its files  kept on its own computer, public relations wants its files kept on its own computer, personnel wants its files kept on its own computer, and so on; all can still gain access to the other's files when needed. The main disadvantage is lack of speed-most peer-to-peer networks slow down under heavy use. Many networks are hybrids, containing elements of both client or server and peer-to-peer arrangement.

Local Area Network Protocols

We have already noted that networks must have a set of rules-protocols-to access the network to send data. Recall that a protocol is embedded in the network software. The two most common network protocols for LANs are Ethernet and the Token ring network.

Ethernet, currently the most common network protocol, uses a high. speed network cable. Ethernet uses a bus topology and is inexpensive and relatively simple. Since all the nodes-computers-in a Local Area Network use the same cable to transmit and receive data, the nodes must follow a set of rules about when to communicate; otherwise, two or more nodes could transmit at the same time, causing garbled or lost messages. Operating much like a party line, before transmitting data a node "listens"to find out if the cable is in use. If the cable is in use, the node must wait. When the cable is free from other transmission, the node can begin transmitting immediately. This transmission method is called by the fancy name of carrier sense multiple access with collision detection, or CSMA/CD.

If, by chance, two nodes transmit data at the same time, the messages collide. When a collision occurs a special message, lasting a fraction of a second, is sent out over the network to indicate that it is jammed. Each node stops transmitting, waits a random period of time and then transmits again. Since the wait period for each node is random, it is unlikely that they will begin transmitting at the same time again.

A token  Ring network, which is closely associated with IBM, works on the concept of a ring network topology and a token-a kind of electronic signal. The method of controlling access to the shared net work cable is called token passing. The idea is similar to the New York City subway: If you want to ride-transmit data-you must have a token. The token circulates from node to node along the ring-shaped Local Area Network.

Only one token is available on the network. When a node on the network wishes to transmit, if first captures the token; only then can it transmit data. When the node has sent its message, it releases the token back to the network. Since only one token is circulating around the network, only one device is able to access the network at a time.

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