LAN Network

Local Area Network-

LAN is a network of computers that are interconnected in a particular fashion and contained within a limited area say a building.
A typical LAN has two components:
1. A host computer that is called a ‘File server’. Large networks may have more than one
Fileservers.
2. Workstations or the terminals of a LAN are interconnected and connected to the Fileserver.
Workstations in a LAN are personal computers that have their own processing power.

The significant feature of a LAN is that its workstations are intelligent terminals with the capability of distributed processing. This is unlike in a main server or mainframe centralized server-based environment where the connection terminals are dumb without processing power of their own.

The prominent reasons which led to the development and popularity of LANs are:
1. Sharing of expensive resources such as disks, files, printers, plotters, modems, and software was needed.
2. Diskless nodes (workstations) of a LAN provided security and virus protection by not allowing to download of important data from the server or uploading unwanted software.
3. Distributed processing for inter-user communication within an organization and information
the exchange was the most suitable model for MIS development.
4. Data management in a LAN environment is more efficient than the centralized processing systems Since no users directly request to server for data without resorting to file transfer
on a stored media like a floppy or hard disk.
5. LANs are more flexible and modular. This implies that LANs can be expanded easily by adding of workstation with the support of networking software. If more disk storage is needed, another hard disk can be installed or other file server can be connected in a LAN.
More than one LAN can be bridged to share data and communicate.
6. Most LANs have fault tolerant features such that in case of power failures LANs are capable of disk mirroring and disk duplexing to safeguard data.

The most popular types of LAN are as follows:

Ethernet
ARCNet
Token Ring
Switched Ethernet
100VG and LAN
Fast Ethernet
Fibre Distributed Data Interface
ATM

Ethernet

The ethernet network was designed and developed by Xerox and DEC. It was initially designed to use coaxial cable later development enabled it to use other cabling systems. The methodology of communication for ethernet is CSMA/CD (Carrier Sense Multiple Access/Collision Detection).
In the protocol, CSMA/CD, the physical layer of the user's workstation generates a carrier sense signal. This signal is listened to by the other workstation. This other workstation waits and begins communication only if such a signal is not detected. Popular cable connections for ethernet are thick ethernet coaxial and thin ethernet coaxial.

ARCNet

ARCNet network was designed and developed by Datapoint Inc. United States. ARC Net is a baseband token-passing network system. In an ARCNet network transmission speeds are nearly 2.5 Mbps. Generally, ARCNet LAN is on a hybrid topology that is a combination of liner bus and star.

Token Ring

A token ring network is physically a star and electrically a ring. It is based on IEEE 802.5 standard.
In a token ring network token passing scheme is used. It involves hardware known as MAU
(Multistation Access Unit) that is used to connect up to 8 stations in a network.

Switched Ethernet

Switched Ethernet 10 Mbps uses a switch to segment a large LAN into smaller LANs. This ensures
a dedicated 10 Mbps connection to a particular PC at all times. Though the switched Ethernet hub is an inexpensive and effective solution, it has a drawback. In some of situations, it can move the bandwidth bottleneck to another point in the network.

100VG any LAN

This LAN was developed by Hewlett Packard and IBM and standardized under IEEE 802.12
specifications. This is based on DPAM (Demand Priority Access Method) which is somewhat
similar to Token ring or FDDL This LAN has not become much popular because other technologies
like ATM and Fast Ethernet provide better compatibility with existing networks.

Fast Ethernet

Fast Ethernet or 100BaseT is an extension of Ethernet specification as approved under IEEE 802.3.
These networks provide higher bandwidth with data rates as high as 100Mbps. These networks preserve core Ethernet protocols CSMA/CD and are compatible with existing ethernet wiring
types, media, and applications.

Fibre Distributed Data Interface

FDDI is an ANSI standard technology for fiber optic networks mostly suitable for high end
workgroups requiring data rates around 100 Mbs.

CDDI (Copper Distributed Data Interface) uses similar technology and is a relatively much cheaper networking solution. Since FDDI is an expensive technology, it is being used mostly as a backbone interconnect between low-speed LANs.

The backbone is a wire that stretches between networks. Other application areas where it has been found useful are data intensive and bandwidth-hungry such as multimedia, CAD/CAM (Computer
Aided Design and Manufacturing), 3D modelling, etc.

ATM
Asynchronous Transfer Mode is promoted by ITU (International Telecommunication Union) as a high-speed communication link for desktops as well as a high-performance backbone for LANs
and WANs. The most salient feature of ATM is its scalability in speed varying from 25 Mbps to multiple Gigabits per second. The other feature of ATM is that it allows simultaneous transmission
and receipt of data without interruption (duplex mode). ATM also allows simultaneous transmission of video, image, audio, and data over a single line. Despite the high potential of ATM technology
for high bandwidth networking, the prevalence of ATM is currently severely restricted due to several factors. Firstly, ATM is a considerably expensive technology. Secondly, many of the
applications need alteration to be successfully run on high bandwidth provided by ATM.
ATM may be considered if the volume of transactions on a network is very high and a seamless integration of LANs and WANs is required.

Transmission Media

The main transmission media used in LANs are
1. Twisted pair,
2. Coaxial cable
3. Fibre Optic.

Twisted Pair Cable

It consists of two wires which provide a signal path and a return polyethylene dielectric and is shielded by braided metal. This is covered in a plastic jacket.

Twisted Pair Cable

Coaxial Cable

There are many types of coaxial cables reflecting the differing number and types of protection shields used.
Data rates, as high as 500 Mbps, are supportable, but normally it is up to 50 Mbps. Error rates may be in the order of 10e-9. Distances up to 500m can be covered without signal degradation. Coaxial cable support baseband as well as broadband transmission. Baseband transmission possesses
digital signals from one workstation to another usually at a speed of 10 Mb/s up to a distance of nearly 10000 feet. Broadband transmission enables transmitting of voice, audio, video, and data signals(e.g., television and cable television transmission) at a speed of nearly 5 Mb/s over distance of nearly 10 Km.

Coaxial Cable

The cost of cable installation is usually higher than the cost of the cable itself. Flexible coaxial cable is used in thin-wire Ethernet. Therefore, it is relatively easy to install. Rigid coaxial cable is used in the original thick wire Ethernet. Therefore it is more difficult to install for it requires more careful handling.

Fibre Optic Cable

It consists of hair-like grass strands, covered with cladding and an outer jacket. This is often used for backbone networks and is the foundation for the over 100Mbps FDDI network.

Fiber Optic Cable

These span long distances without signal degradation. Though these are highly reliable, installation of these cables is much more expensive than other types of available cables.

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