Computer Networks

                   

Network

                  

Network is a telecommunications network that allows computers to exchange data. In computer networks, networked computing devices pass data to each other along data connections. The connections (network links) between nodes are established using either cable media or wireless media.

• Towns and cities interconnected by roads and motorways

• Computers exchanging data through cables

• People gossiping

           Most experts agree that a computer network is formed when two or more computers communicate.      However, there are some people who insist that you need three computers to make it a ‘real’ network.

Eg: LAN

      WAN

      CAN  

Local Area Network (LAN)

A  LAN is a computer network that interconnects computers within a limited areas using network medias..

• School

• office network

Connections are made by copper or fibre-optic cable, infrared or short-distance microwave radio

Wide Area Network (WAN)

A WAN is a network that covers a broad area ( ie, any telecommunication network that links across metropolitan,region,national.
.

• The telephone system

• A bank’s secure ATM (cash machine) network

• Satellite links between TV companies Connections are made by copper or fibre-optic landlines, line-ofsight     microwave radio, orsatellite relay

Metropolitan Area Network (MAN)

   MANs connect sites around a town or city.

• Town council offices

• University campus network

• Cable TV company

Connections are made by copper or fibre-optic landlines, line-ofsight microwave or high-frequency radio

Other terms

Campus Area Network (CAN): somewhere between a LAN and a MAN, this describes connections over a village-sized area.  Usually restricted to larger universities.

Tiny Area Network (TAN): used by some people to describe small networks within a house or small office.

Topology

The study of geometrical properties and spatial relation unaffected by the continuous change of shapr or size of figures.

Star

In a star layout, all computers are connected to a central hub. Star networks allow computers to be connected & disconnected with ease, so long as enough ports are available.  However, if the hub breaks then the entire network goes down.

Ring

In a ring layout, each computer connects to two neighbours to forma loop.  Messages are passed from computer-to-computer until they reach their destination. Ring networks are efficient and cheap but if one of the computers breaks or is removed then the entire network goes down.

Mesh

Mesh networks connect many computeers directly to each other.This means faster communication, and provides alternative routes if a computer or link breaks.  However, more connections means mesh networks are expensive to build.

Bus

A bus provides a common electrical connection for a number of computers, but only two computers may communicate at the sametime otherwise there will be a signal collision. Buses are very cheap and allow easy connection & disconnection.  However, buses tend to slow down with increased message traffic.

Extended Star (Tree)

The extended star design breaks the network into smaller starconfigurations, each minor hub connected to a major hub.  Extended stars offer an economical, modular way of organising networks.  They are very common in modern network designs.

Relationships

Client-Server

A server program carries out work for client programs.  

Server software is often located on a powerful, high-specification machine, allowing it to serve many clients at once.  For this reason, the computer itself is usually referred toas a server.  A range of server programs are available:

• File server: stores files centrally

• Print server: processes all printing for the network and sends work to selected printers.

• Internet proxy server: connects to an ISP on behalf of other computers on the network, passing messages       to-and-from the Internet.

• Database server: allows simultaneous access to large database files.

• Webserver: hands out webpages on request.

• Mail server: stores and processes email; like a post office.

• Domain controller: this manages security for the network, processing passwords and deciding who can             access services.

•Client software is located on normal-specification computers, known as workstations. 

 .

Peer-to-Peer

When a computer is capable of acting as both client and server, we call it a peer.  Peer-to-peer networks contain computers that are requesting services from each other.

             Network Media

Electrical media

Plain copper wire

A pair of plain copper wires is a poor medium for electronic signals, and is therefore rarely used in computer networking.

Electro-Magnetic Interference (EMI): When a signal current passes through a wire or circuit it creates small electrical and magnetic fields.  These fields can affect signal currents in other wires, or other circuits, adding electrical noise to the signals, degrading them. Long, plain copper wires are easily affected by EMI — hence they should not be used to carry computer signals more than a few of metres.

Co-axial Cable (Co-ax)

In co-ax a copper wire is surrounded by a copper screen.  This screen (usually a fine braid or mesh of copper filaments) is connected to signal ground and it absorbs EMI, protecting the signal wire from electrical noise.  Screen and signal wire are separated by plastic insulation.

Bandwidth

The closeness of the screen and signal wire creates an effect called capacitance.  This limits the range of frequencies — bandwidth — that may be carried through the cable
.

Attenuation

This capacitance and electrical resistance of the copper creates an impedance to signals, measured in ohms per metre (Ω/m), reducing the strength of the signal over a long distance.  This reduction is called attenuation and it is measured in decibels (dB).

Connectors

In networking, RG-8 ‘thick’ co-axial cable is terminated with N connectors.  These bulkyconnectors are also used with radio equipment.

RG-58 ‘thin’ co-ax is usually terminated with BNC connectors.These are also associated with oscilloscopes, older video recorders and radio equipment.

Twisted Pair (UTP & STP)

If pairs of plain copper wires are wrapped around each other this helps cancel-out EMI.  Twisted pair cables offer good bandwidth but are not as good as co-axial cable.  However,this kind of cabling is very cheap to buy and install. Twisted pair comes in two flavours
 UTP (unshielded twisted pair) and STP (shielded twisted pair).  STP costs more but offers better electrical characteristics. The most popular kind of UTP used for networking is Category 5e (“Cat5”) cable.

Plenum cable

If you are installing a cable in an air duct, you must use a special fire-resistant, low-smoke plenum version of the cable. Connectors The standard connector used with Cat5 cable is the RJ45.  After sorting individual wires into appropriate channels the connector is crimped onto the wires.

Optical media

Fibre-Optic

Optical fibre transmits signals as pulses of light along a flexible glass tube.  It does not use electricity, except to power the transmitting and receiving circuitry at either end. Fibre optic cables have exceptional bandwidth, zero EMI and very low attenuation and can carry signals for miles.  However, they are expensive to buy and install.

Fibre optic cables come in two types: single-mode or multi-mode.  Multi-mode cablescarry multiple signals at once and is cheaper but can only carry signals about a mile.  Single-mode cables are more expensive but are used for long-distance (up to 24 miles).