Session 02

What is a Computer Network?

A computer network is two or more computers connected together for the purpose of resource sharing and information transfer.

There are many components that can be part of a network and these components can be grouped into four main categories:

Hosts : Servers, Desktops, Workstations.

Shared peripherals : Printers, Scanners, Multimedia projectors

Networking devices : Hubs and Switches

Networking media: Wires or any other mechanism to transfer data

The equipment that interconnect computers and shared peripherals such as printers are called networking devices.

Cables that are used in connecting the network is called network media.

In early days it was removable media such as floppy disks and magnetic tapes used to carry and transfer the data from computer to computer.

The need for connecting multiple computers was always in mind with computer manufacturers

They have used various methods in connecting computers

The most adopted technique is Ethernet technology which was developed over decades now.

Computer networks can be built and connected using either wired medium or wireless medium.

Wired medium is well developed and fastest way of forming a computer network.

Wireless medium is increasingly getting popular due to distance and terrain challenges.

It is also useful in places like seminars where temporary way of establishing a network to provide services without structured cables.

Use of Computer Networks

Today, based on the requirements of users and companies, computer networks can be located

Within a building,

Within premises that is multiple buildings are connected,

Within branches of same company which are far away by miles and may go across several countries.

The Internet is the World’s largest computer network which have millions of computers connected together permanent or non permanent basis.

It was initially started by connecting computers of few universities in United States.

Types of Computer Networks?

Computer networks can be classified in several ways.

Classification of Computer Networks According to its Ownership

Classification of Computer Networks based on Geographical Location

Classification of Computer Networks based on operations

Classification of Computer Networks According to its Ownership

Private networks

Private networks are networks owned by an institution and the functions belonging to an organization only handled within the network.

No outsiders are allowed to use the company network. All resources belong to the institution.

Public networks

Public networks are networks that are used by many people and organizations in order to communicate with each other.

Internet is a good example of public network.

People pay money to different organizations who handle portions of public network to connect and obtain the services provided by the network. Example

You might be paying Sri Lanka Telecom to get connected to Internet.

virtual private networks (VPN). Larger private organizations which have several branches far

away can create a private tunnel across public network to make all the branches connected.

To form this kind of network company has to pay the owner of that network. Example

If a bank having branches in Jaffna, Galle, Kandy and head office is in Colombo they can easily form a virtual private network with the help of telephone operators if they have the service working within those areas.

Classification of Computer Networks based on Geographical Location

Local Area Network (LAN).

If computers are organized within a room, or within a floor of a building or in a entire building which belong to same institution it is called a Local Area Network (LAN).

This LANs also can be distributed across several buildings in same vicinity which belong to same institution.

Example

A company may have office building and factory building reside in same premises and the Computer network is expanded across these two buildings.

Advantages of LAN:

Very high network speeds and data transfer rates

Protection from the outside world, thus creating a trusted computing environment.

No need to lease telecommunication lines, such as those used for telephone, data and Internet services, once the hardware for connecting all the machines has been bought

Wide Area Network (WAN).

If the connectivity of computers is maintained across public access telephone lines or other wired or wireless communication mechanisms and far away from one location to another is called Wide Area Network (WAN).

As you can see in the figure below two networks are connected using telephone lines. There can be multiple LANs connected forming a group (network) of LANs.

Metropolitan area network (MAN)

The networks that connected across busy suburban areas, belonging to large institutions where offices located across different locations across same city.

Larger than the area covered by a local area network (LAN) but smaller than the area covered by a wide area network (WAN).

Campus Area Network (CAN)

Campuses are having larger LAN networks they are also called as CAN (Campus Area Network).

Larger than the area covered by a local area network (LAN) but smaller than the area covered by a metropolitan area network (MAN).

Classification of Computer Networks based on operations

The host in computer networks can be connected in two methods namely

peer to peer networks

Client / Server networks

peer to peer networks

In a peer to peer network there is no central administrator to the system the host can share its resources with other hosts in the network.

This can be used to build a network with very low cost.

When the number of hosts in network is getting increased, more and more equipment required to connect hosts.

The connected PCs can experience slowness in connectivity.

client server network

In client server network, dedicated computer called server is making its services available for many clients (hosts) in the network.

Since the server is dedicated to a special purpose it can serve many clients without affecting performance.

Since there is no reason for clients to do its resource sharing client computers also give a better performance.

Network Topologies

Topology refers to the shape of a network, or the network's layout.

A computer or equipment connected to computer network is called a node of a network.

How different nodes in a network are connected to each other and how they communicate is determined by the network's topology.

Topologies are either physical or logical.

Physical Topology

Physical Topology means that the way that computers are physically connected will look like description of the topology.

Logical topology

Logical topology is a way that computers communicate via signals or data transmitted irrespective of physical arrangement.

The five most common network topologies

Bus Topology

Ring Topology

Star Topology

Mesh Topology

Tree Topology

Bus Topology

Bus topology uses a common backbone (single wire) to connect all the network devices in a network in a linear shape.

A single cable functions as the shared communication medium for all the devices attached with this cable with an interface connector.

The device, which wants to communicate send the broadcast message to all the devices attached with the shared cable but only the intended recipient actually accepts and process that message.

Ring Topology

All devices are connected to one another in the shape of a closed loop, so that each device is connected directly to two other devices, one on either side of it.

In ring Network, every computer or devices has two adjacent neighbors for communication.

In a ring network, all the communication messages travel in the same direction whether clockwise or anti clockwise.

Any damage of the cable or device can result in the breakdown of the whole network.

Ring topology now has become almost obsolete.

FDDI, SONET or Token Ring Technology can be used to implement Ring Technology.

Ring topologies can be found in office, school or small buildings.

Star Topology

In computer networking the world most commonly used topology for LAN is star topology.

Star topology can be implemented in home, offices or even in a building.

All the computers in the star topology are connected to central device like hub, switch or router.

Computers in a network are usually connected with the hub, switch or router with the Unshielded Twisted Pair (UTP) or Shielded Twisted Pair Cables.

As compared to the bus topology, a star network requires more devices & cables to complete network.

The failure of each node or cable in a star network, won’t take down the entire network as compared to the Bus topology.

However if the central connecting devices such as hub, switch or router fails due to any reason, then ultimately all the network can come down

Mesh Topology

Devices are connected with many redundant interconnections between network nodes.

In a true mesh topology every node has a connection to every other node in the network.

Mesh topology work on the concept of routes.

In Mesh topology, message sent to the destination can take any possible shortest, easiest route to reach its destination.

Router works in find the routes for the messages and in reaching them to their destinations.

The topology in which every device connects to every other device is called a full Mesh topology

Tree Topology

A hybrid topology.

Groups of star-configured networks are connected to a linear bus backbone.

Only the hub devices can connect directly with the tree bus

This bus/star/hybrid combination supports future expandability of the computer networks, much better than a bus or star.