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Local Area Networks LAN

Why use a LAN?
The two basic reasons for developing a LAN are information sharing and resource sharing. Information sharing refers to business needs that require users to access the same data files ,exchange emails or search the internet for information. Resource sharing refers to one computer sharing a hardware device (for e.g a printer) or software package with other computers on the network. The main benefit of resource sharing is cost savings, while the main benefit  of information sharing is improved decision making.
image courtesy Cray Networks

Dedicated Server versus Peer-to-Peer Networks
A dedicated server LAN has one computer that acts as the network server. It can connect with almost any other network,handle very large databases and use sophisticated LAN software. Moreover,high -end dedicated server LANs can be interconnected easily to form enterprise wide networks or in some cases replace the host mainframe central computer. Four common types of dedicated server LANs are file servers, database servers,print servers, and communication servers. All computers on a peer-to-peer LAN run special-network software that enables them to function both as a client and as a server.

LAN Components
The Network Interface card (NIC) enables the computer to be physically connected to the network cable,which provides the physical layer connection among the computers in the network. Most LANs are formed with un-shielded twisted pair (UTP) wires, shielded twisted pairs (STP) wires,coaxial cable, and fiber optic cable. Network Hubs provide an easy way to connect network cables and act as repeaters or amplifiers. Most buildings built today have a seperate LAN cable plan, as they do for telephone cables and electrical cables. Wireless LANs use similar protocols (Ethernet , for example) as other LANs,but they transmit data through air rather than by cable. The Network operating system (NOS) is the software that performs the functions associated with the data link and the network layers and interacts with the application software and the computer's own operating system. Every NOS provides two sets of software:one that runs on the network server(s) and one that runs on the network client(s). A network profile specifies what resources on each server are available for network use by other computers and which devices or people are allowed what access to the network.

Ethernet (IEEE 802.3)
Ethernet is the most commonly used LAN in the world, accounting for almost 70 percent of all LANs. Ethernet uses a bus topology and a contention based media access technique called carrier sense multiple access with collision detection (CSMA/CD). Many different types of Ethernet use different network cabling (e.g . 10Base-2, 10Base-5,10Base-T and 10Broad-36).

Token Ring (IEEE 802.5)
Token ring is the second most popular type of LAN, with almost 25 percent of the worldwide market. Token ring uses a ring topology with a controlled access technique called token passing.

Other LANs
Arcnet and AppleTalk are two older protocols not commonly used today. Wireless LANs(IEEE 802.11) are rapidly  growing type of LAN and can use infrared,or more commonly radio transmission( either direct-sequence spread spectrum or frequency -hopping spread-spectrum). Although wireless LANs enable communication with wires, they suffer from interference and low transmission rates (usually less than 2 Mbps).

image courtesy KSI
Improving LAN Performance
Every LAN has a bottleneck, a narrow point in the network that limits the number of messages that can be processed. Generally speaking , the bottleneck will lie either in the network server or the network circuit. Server performance can be improved with a faster NOS that provides better disk caching and disk elevatoring,by buying more servers and spreading applications among them,or by upgrading the server's CPU,memory,NIC and the speed and number of its hard disks. Circuit capacity can be improved by using token ring rather than Ethernet, and by segmenting the network into several separate LANs. Overall LAN performance also can be improved by reducing the demand for the LAN by moving files off the LAN, using disk caching on the client computers, and by shifting the user's routines.

Selecting a LAN
It should satisfy the Network Needs. ,it should also meet the Technology issues which include protocol,configuration/topology, network operating system,vendor service and support, application systems,number and type of servers, cleint computers and users, and Network Interface Cards.
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