Local Area Network (LAN)

Local Area Network (LAN): In a LAN two or more computers or node are directly linked within a small well defined areas such as a room, office, building, campus or a local neighborhood with a range of 10 kilometers. Each hardware device on a LAN such as computer or a printer is called a node. Most LANs are privately owned, controlled and managed by a single person or organization and uses direct high speed cables to share hardware, software and data resources. LAN uses the Institute of Electrical and Electronic Engineers (IEEE-802.5, IEEE 802.3), Ethernet, IBM token ring, etc standardization protocol where as WAN uses TCP/IP, HTTP, FTP, etc.

The main component of LAN are discussed below-

A) Cables and Other Medium for Transmitting Signal: Cables and other medium for transmitting the signal is one of the very important components of any network. The cables can be of the following types-

a) Twisted Pair: It is used in low speed LAN using base hand transmission. In this mode of transmission data is transmitted as simple electrical levels often without any modulation. There is no multiplexing and the entire bandwidth of the medium is used for transmitting signals from one station. It is used for communication up to a distance of 2 km.

            Twisted pair is vulnerable to interference from large machines such as air conditioners. This interference can destroy data.

            Twisted pair cables are generally two types-

i) Shielded Twisted Pair (STP);

ii) Unshielded Twisted Pair (UTP);

            Twisted pair consists of a pair of insulated conductor’s that is twisted together.



b) Co-Axial Cables: It is used for broadband transmission of speeds of 10 Mbps or more. The broadband transmission uses modulation techniques and is suitable for transmitting high speed and multiplexed data. It consists of a solid conductor running coaxially inside a solid or braided outer annular conductor. The two kinds of co-axial cables are


i) Baseband Co-axial Cables: It can carry only one signal at a time, but it is fast (10 million bits per second).

ii) Broadband Co-axial Cables: It can carry more than one signal at a time. Cable TV companies use broad band co-axial cables.

c) Fibre Optic Cables: Fibre optic can be described as a transmission system employing a light-emitting source – turned on and off rapidly by electrical impulses whose emissions are sent through glass pipe to a light sensitive receiver to convert the changing light intensities back into the electrical impulses. The “core” of fibre optic cables is a very thin strand of highly refined cylindrical glass. A second layer of glass called clad surrounds the core. The clad is fured directly to the core so that it is very difficult to see the boundary between the two with naked eyes. Fibre optic cables are unaffected by magnetic or electrical interferences. They are however expensive and hard to install. The fibre optic carries data at the rate of 100 Mbps.


d) Line of Sigh Transmission: Here data are sent into air transmitted by infrared, lasers, microwaves and radio.

i) Wireless LAN: Wireless LAN uses infrared or radio wave transmission. Although wireless LAN are more prone to error and interception they do not require laying cables and moving it when a node is relocated.

e) Satellite Transmission: In many network fibre is used as backbone to distribute the network, while the last mile wiring is still some type of copper. Some network architecture has limitation on cable distances and the number of workstation that can be supported on as single segments. Exceeding these may result in inconsistent poor or absent network service to nodes beyond the limit. In some cases the entire network can be affected.

B) Fibre Connector: There are several different types of fibre connectors that includes the following:-

a) ST: This keyed, bayonet style (twist-lock) connector is widely used.

b) SC: The SC connector used internationally is a snap-lock connector with a duplex connection, one each to transmit and receive.

Other connectors in the fibre arena includes SMA connectors which are rapidly becoming obsolete and proprietary and specialized connectors that are not as widely used.

C) Controlled Mechanism: The controlled mechanism consists of the following units-

a) Cable Interface Unit: Sometimes also called as hub. It sends and receives signals on the network cables. This unit is a box outside the computers.

i) Hub: Hub acts at the data link layer, acting as a breakout box signals they receive and is the common wiring point for a star topology. It forwards packet to all active ports and shares the bandwidth.

ii) Switches: It works at layer 2 of OSI reference model and checks the destination address and makes a virtual path to the destination port. It does not share band width.

iii) Routers: It works at layer 3 of OSI reference model. Its job is to send packets created by higher layer of the network to the ultimate destination.

            If a network has less number of node, the choice should be 10/100 hubs. If it consists of number of nodes and dispersed in different floor / building your best choice will be switch.

b) Network Media Connector: The connector used to attach network media to the networking devices are called networking media connector. Some of the commonly used network media connectors are-

i) Register Jack 11:

ii) Register Jack 45:

iii) BNC (Bayonet Neill Concelman Connectors):

c) Structured Cabling: A structured cabling system consists of outlets which provide the user with RJ45 presentation, which is again enabled back to a Telecommunication Closet (TC) using an individual cable containing four twisted pair. This cabling is known as horizontal cabling which is again connected back to the back  of the user outlet by means of an Insulation Displacement Connection (IDC) connector.

            The maximum length of cables between the hub and any outlet must be 90 metre to comply with EIA/TIA and ISO requirements. The standard allows a further 10 metre for connecting leads and patches leads making a total drive distance of 100 metres.

D) Node: Each hardware device on a LAN such as computer printer is called a node.

a) Network Card: The Network Interface Card (NIC) is the main physical device which sends and receives data from network cables. A network interface card must be installed at every host that wished to connect to network. The NIC is inserted into an expansion slot inside the computer. The card is connected to the cable interface unit by wire.

            LAN can be connected by a bridge, a router or a gateway. If two LANs are similar one can use a bridge to connect them with two or more similar LANs one can use a router to connect them. With two dissimilar LAN one can use a gateway. The gateway translates the LANs difference data format.

In case of LAN, the accesses are generally done through Switched Access, Contention or Multiple Accesses and Token Passing Access.

The Contention or Multiple Accesses is used in bus topology. In this technique if a line is unused a terminal or device can transmit its message at will but if two or more terminals initiate message simultaneously, they must stop and transmit again at different intervals.

The Token Passing Access is used in ring topology. In this system to deliver a message one should hand over addressed note to a rider (the token) on the many go round that would drop it of at the appropriate place.

            There are also other techniques like reservation access, load adaptive access, tree structure based access, etc.

Some of the need and advantages of LAN are-

a) Resource Sharing: Networking is needed for sharing of:

i) Hardware: The Mainframe computer, super computer, Laser Printer, etc.

ii) Software: For sharing costly software, such as LibSys.

iii) Information: Data, text, audios, videos, picture, databases, etc

b) Distance, Cost, Time, and Space: Networking is needed for breaking the barrier of distance, cost, time and space.

c) Organizational Infrastructure: LAN improves the existing organizational infrastructure.

d) High Speed: LAN provides high speed networking when compared with other network.

e) Low Error Rates: LAN provides accurate data transmission.

f) Internet Access: A LAN in turn often connects to other LANs, and to the Internet or other WAN. All users of the LAN network can share a common line to the internet.

Some of the disadvantages of using LAN are -

a) Distance Covered is Limited: The distance that can be covered by LAN is limited.

b) Number of Terminal is Restricted: Number of terminal to be added to the network is also limited.

c) Initial Cost is High: Many people consider the cost in setting up the LAN as high investment.

Most LANs are built with relatively inexpensive hardware such as Ethernet cables, network adapters, and hubs. Wireless LAN and other more advanced LAN hardware options also exist. Specialized operating system software may be used to configure a LAN. For example, most flavors of Microsoft Windows provide a software package called Internet Connection Sharing (ICS) that supports controlled access to LAN resources.