Computer Networks

1. Introduction

A computer network is a group of computers which are connected together to allow communication between them.

Computer networks can be classified in many different ways. Some important comparisons are :

1.	Over how wide an area the network is spread. Networks in small areas are known as local area networks (LANs) and networks which cover larger areas are known as wide area networks (WANs).

2.	Whether network resources such as file storage space and printers are controlled centrally by a server or whether control is distributed between the network stations on a peer-to-peer network.

3.	How the communication links between the stations on the network are arranged. This arrangement is known as the network topology.

The speed at which data can be sent between different computers on a network is vital. The data transmission rate of a network or connection is commonly referred to as its bandwidth. Many home computers and computer networks have now been linked together to form the Internet.

2. LANs and WANs

Computer networks are usually put into one of two classes depending on how large an area the network covers. The two types of network are :

Local Area Network (LAN ) : A network which covers a small area such as an office or school.
Wide Area Network ( WAN) : A network which connects together computers in a large area or on a number of sites.

Examples of LANs	Examples of WANs
A typical school computer network.	The Internet.
A network of computers used to share book information within a library.	A network of bank cash dispensers.
	A ticket booking system used by a chain of high street travel agents.
Properties of LANs	Properties of WANs
Covers a small area, usually just one site.	Covers a large area, often linking together several sites.
Computers connected by NIC and cables.	Computers connected by modems and telephone lines, ISDN, broadband or similar.
Fast transmission of data.	Slow transmission of data.

3. Connecting to a LAN

Computers which are connected to a network are called stations. Stations on a LAN are normally connected to the network using a network interface card (NIC) and some cable. The cable might be made of copper or optical fibres (made of glass fibres).

Cables must run all the way around a building that has a LAN installed. Every station on the network must be connected to a cable.

Sometimes a radio transceiver can be used to connect a computer to a LAN instead of a cable. Using a radio means that the computer does not have to be plugged into the network, so it can be moved around more easily.

4. Connecting to a WAN

Connecting computers together at different locations on a WAN is more difficult than connecting computers on a LAN. This is because it is not possible to join two computers which may be thousands of miles away from each other by a simple cable.

Therefore connections to a WAN are usually made over a public network such as the Public Switched Telephone Network (PSTN). The cables that connect the computers are owned by a telephone company. Other companies pay the telephone company money to use its cables to build their WANs.

Telephone companies provide many different alternative methods that can be used to connect computers at different locations together. The three most common systems are :

Method	Description	Speed
Modem and Telephone Line	A modem converts digital computer data into analogue form so that it can be sent across a standard telephone line to another computer.	Slow
Terminal Adaptor and ISDN Channel	A terminal adaptor sends data in digital form down a special type of connection called an ISDN channel.	Faster
Broadband	An ADSL or Cable Modem sends data in digital form down a telephone line or cable TV connection.	Very Fast

5. Modems

Modems are often used to connect individual computers to a WAN. A modem can transmit data over a standard telephone line. You can use a modem wherever you can use a telephone.

Because the telephone network was set up before computers were in widespread use, telephone lines were not designed to carry the digital signals that computers use. They were designed to carry human voices in the form of analogue electrical waveforms.

A special device known as a modem must be used to enable a computer to communicate over a telephone line. The modem converts the binary digital data that the computer wants to transmit into an analogue signal for transmission down the telephone line. Another modem must be used at the other end of the telephone line to convert the analogue signal back into a digital one that the receiving computer can understand.

Modulation is the process of converting digital electrical signals generated by a computer into analogue electrical signals which can be transmitted over a telephone line. Demodulation is the process of converting received analogue signals back into digital signals which a computer can understand.

The speed at which data can be transmitted over a telephone line has increased steadily. Most modern modems can receive and transmit data at up to 56.6KBps (kilobits per second). Transmission speeds over a telephone line are very slow compared to other methods. Alternatives such as ISDN and broadband can send data much more quickly.

Advantages of
Modem over ISDN and Broadband

You can use a modem with any telephone line. You do not need a special ISDN line or broadband enabled connection.
You do not have to pay an additional monthly fee to use a modem. Unlike broadband or ISDN you just pay for how much you use the connection, so a modem may be the cheapest option for someone who does not want to connect to a WAN very often.

6. ISDN

A terminal adaptor can be used to connect computers on a WAN together using an Integrated Services Digital Network (ISDN) channel.

ISDN channels are a more modern form of connection than the public switched telephone network. Data can be transmitted digitally over an ISDN channel, so faster transmission speeds can be achieved than when data is sent using a modem and an analogue telephone line. ISDN channels are not available in all countries.

A single ISDN channel transmits data continuously at 64KBps (kilobits per second). This compares to a maximum speed of 56.6KBps for a modem. More than one ISDN channel can be used at a time to send data between two computers. If two channels were used then a speed of 2*64=128KBps could be achieved. When very fast speeds are required, large numbers of channels (e.g. 32) can be used together or (if available) broadband could be used instead.

Advantages of
ISDN over Modem

ISDN channels can transmit data faster than a modem can over a telephone line.
ISDN channels establish a connection to another computer much more quickly than a modem does. The time it takes to connect to a WAN can be reduced to less than a second with ISDN compared to as long as a minute for a modem and telephone line.

7. Broadband

The term broadband is commonly used to refer to high speed connections to a WAN such as the Internet. Broadband connections have become available in the UK in the last few years, but whether or not you can use broadband depends upon where you live. A typical broadband system can transmit and receive data at a speed of 560KBps. This means that data can be transmitted ten times more quickly using broadband than using a modem and telephone line. Because broadband connections are fast it is possible to use them for applications which needs to send a lot of data such as video conferencing.

There are two main methods of connecting a computer to a WAN using broadband. They are :

Method Description
ADSL Modem and Telephone Line This method is only available at locations that have a telephone line which is connected to an ADSL (Asynchronous Digital Subscribe Line) enabled telephone exchange. By connecting an ADSL modem to the computer and the telephone line it is possible to access a WAN through the telephone line and use the telephone to make voice calls at the same time.
Cable Modem and Cable TV This method is only available at locations that have a connection to a cable TV network. By plugging a cable modem into the computer and the cable TV control box it is possible to access a WAN through the cable TV connection.

As well as installing the required hardware it is necessary to contact the company that owns the telephone/cable TV line to get them to enable broadband on a connection before it can be used. A monthly fee will have to be paid to use the service, but this usually does not depend on how much the connection is used.

Advantages of
Broadband over Modem or ISDN
Broadband can transmit data much faster than a modem or ISDN so applications such as video conferencing are possible.
Broadband connections are always available. You do not usually have to "dial up".
Most broadband services are available for a fixed monthly fee. Once this is paid you can use the connection as much as you like without extra charge.

8. Control of Network Resources

One of the most important reasons for connecting computers to a network is to let the computers on the network share expensive resources such as file storage space and printers. Access to these resources must be managed in some way. This can be achieved using either a server based network or a peer-to-peer network.

Server Based Networks

The most common type of network used in a business or school environment is a server based network. On a server based network access to shared resources like storage devices and printers is obtained through special server computers. Three different types of computer are usually connected to a server based network :

Stations : Computers which users work on and are connected to the network.
File Server : A computer which stores and controls access to all the users’ files.
Printer Server : A computer which controls access to one or more shared printers.

The file server must ensure that people can only access files that they are permitted to. Most file servers do this by making each user log on with a user name and password before they can access any files. The user name that a user logs on with will determine which files the user can access.

The printer server controls access to shared network printers. Each station on the network can print out on the printer(s) connected to the printer server. If lots of people try to print at the same time then the printer server puts all the documents that are being printed in a queue. The documents are usually printed by the printer server in the order that they were sent by the users.

A large server based network may have more than one file or printer server connected to it.

Peer-to-Peer Networks

Peer-to-peer networks do not have special computers which are used as servers. Instead any computer on the network can load information from the hard disk of any other computer. A computer on the network can use any printer connected to any other computer.

Peer-to-peer networks are cheaper to set up and easier to manage than server based networks. They are however less secure. Because of this peer-to-peer networks are used mainly by small companies who trust their employees and do not have the necessary technical staff to maintain a server based network.

9. Topology

Network topology refers to the layout used to wire the computers together. There are four common topologies. Any topology can be used regardless of whether a network is local or wide area, server based or peer-to-peer.

Bus (or Line)	Ring

Each computer is connected to a single cable which connects all of the computers. This is the cheapest network topology as the smallest amount of cabling is required. If the network cable breaks anywhere then none of the computers can access the network. Also known as a line network.	Each computer is connected to the two computers on either side of it. The last computer is linked to the first to form a ring. If the network cable breaks anywhere then none of the computers can access the network.
Star	Hierarchical

All communication takes place via a central computer. If the central computer fails the whole network will break down. If one of the network cables breaks only the computer connected to that cable is affected.	The network is organised hierarchically (in different layers). A hierarchical network is composed of several independent sections linked by hubs. If one of the independent network sections fails the others can continue to function.

10. Transmission Media

Data transmitted through a network or between two hardware devices is carried by a medium. The most common media are described here :

Wired
Copper Cable	Copper cable is used to electronically connect together computers on most LANs and in telephone lines. It allows reasonably fast transmission and extra connections can easily be added to a copper cabled network. Data transmitted along a copper cable can be corrupted by electrical interference.
Optical Fibre	Optical fibre can transmit data much more quickly than copper cables and across longer distances. This is because the data is carried as light rather than electrical signals. Optical fibres are interference free and transmit data more securely than copper cables. The hardware required to use optical fibres is more complex and expensive than that needed to connect to a network using copper cable.
Wireless
Radio	Radio transmission is sometimes used as a method to directly connect computers (usually laptops) to a LAN. Radio connections are slower than wired connections and more vulnerable to interference. It is easy to add extra computers to a radio network as no new wiring is required. Computers connected by radio can be moved around whilst remaining connected to the network.
Microwave	Microwave transmission is used to connected together two different sites that belong to a WAN. For two sites to be connected by a microwave link there must be a line-of-sight between them (you must be able to see one site from the other) and they must be within about 30 miles of each other. Operating a microwave link is cheaper than paying to use the telephone network if you have a lot of data to send but transmissions can be affected by poor weather conditions.
Satellite	Satellites provide a versatile but expensive means of transmitting data. Satellites can transmit data very quickly and (subject to a suitably located satellite) to/from anywhere in the world. A suitable satellite dish and transceiver are required. Data transmitted by satellite is less affected by the weather than data sent by microwave as the signal travels through space rather than through the atmosphere.
Infra-Red	Infra-red transmission sends data through the air as invisible light. Infra-red transmission speeds are very slow so this method is not usually used in networks. Instead it is used by hardware devices to communicate with each other. For example a remote control communicates with a televsion using infra-red and a laptop could print to a printer by infra-red.

11. Hubs and Switches

The most common network topology is the bus (or line) topology. It is cheap and easy to set up a bus network because relatively little cable is required but bus networks are not as reliable as other topologies. If the main cable breaks anywhere along the network the computers connected to it will no longer be able to communicate with each other.

One way to reduce the effect of a cable break is to split the network up into sections using hubs or switches. This diagram shows a bus network that has been split up into three sections :

The main network cable is known as the backbone. It will use a fast transmission medium such as fibre optic cable.
Devices called hubs or switches are connected to the backbone.
Each computer is connected to a hub/switch by its own cable. Through the hub the computers can communicate with other computers on the network.

A sectioned network is more reliable than a standard bus topology. If a cable connecting a computer to a hub breaks then only that one computer will be affected. All of the other computers will continue to be able to use the network (note however that if the main backbone cable fails none of the computers will be able to use the network).

Hubs and switches are also used to connect together the computers at different levels in a hierarchical network.

12. Interconnections

Sometimes a number of LANs can be connected together to make a WAN. For example a company with LANs in each of its offices might choose to connect all these LANs together to make a WAN. If this is done then the workers at the different offices will be able to share information across the network. To connect two networks together one of two pieces of hardware is required :

Bridge	If the two networks are of the same type they can be connected together using a bridge. The bridge simply copies data that is being transmitted from one network onto the other.
Gateway	If the networks are of different types then the job of passing data between them is more complicated. Data that is sent from one network to the other will need to be converted from the format that is used on the sending network into the format that is used on the receiving network. This job is done by a gateway.

There is no limit to the number of networks you can connect together in this way. In the example below three networks are connected together. Note that computers on Subnet A can talk to computers on Subnet C by sending messages through Subnet B.

In complex network such as the Internet there are many different routes that data could be sent down to travel from one computer or subnet to another. Devices called routers are used to determine what route to send data along.

13. Security

Computers connected to a network are more vulnerable to security threats than stand-alone computers. This is because :

Data stored on a computer on a network can be accessed from another computer at a different location on the network.
Data transmitted between two computers could be intercepted by another computer on a network.
It is hard to control physical access to computers on a network as they are usually spread over a wide area.
Viruses can spread across a network.

For this reason extra security measures are usually applied to computers connected to a network. These may be implemented by a network operating system. Likely network security measures include :

Network Security Measures
User Names and Passwords	The use of a user name and password system restricts who can access what data on a networked computer system. Users often have to log on to a file server before they can use a computer connected to a network.
File Access Rights	File access rights specify what actions a user can take on a file or directory. The rights that a user has will depend upon the user name he is logged on with.
Logs	When a user name and password system is in use a log can be kept of who has done what on a computer system. This may deter unwanted activity and will help track down anyone who has made unauthorised used of a computer.
Encryption	Encryption is the coding of data so that it can not be read without being decoded first. Data is often encrypted as it is transmitted around a network so that if it is intercepted it can not be read.
Firewall	A firewall is used to control and monitor data sent between two networks or between a home computer and a network. Firewalls can limit the types of data that are allowed to pass between networks and identify and block both viruses and attempts to hack into a network.

Ensuring security is even more essential when a computer is connected to a public network such as the Internet. Standard security measures such as backing up and using a virus checker should also be used on computers connected to a network.

14. Advantages and Disadvantages

Here are some of the advantages and disadvantages of networking a group of computers togther :

Advantages

Expensive resources such as printers and storage devices can be shared between many computers.
A user can use can load and edit files that they have created on any station, providing the station has the software required to use the files.
You can use a network to send electronic mail or to access huge databases such as the World Wide Web or a database of all the products a company sells.
If data is stored centrally on a file server then it can be easily and reliably backed up as is is all stored in one place.

Disadvantages

There are additional costs involved in setting up a network. These include cabling, interface cards, file servers.
If one computer, cable or interface breaks the whole network may stop operating and you may not be able to use any of the computers until the fault is repaired. In practice this rarely happens. The exact effect of a failure will depend on the toplogy of the network.
Viruses stored on a computer on a network can spread to other computers over the network.
The files stored on computer networks can be accessed, stolen and edited more easily that files stored on a non-networked computer could be. Appropriate security measures must be implemented to prevent this.
If the network is complicated then it may be necessary to employee a network manager to run it. This will be an ongoing cost.

GCSE ICT Companion 04 - (C) P Meakin 2004

Method	Description
ADSL Modem and Telephone Line	This method is only available at locations that have a telephone line which is connected to an ADSL (Asynchronous Digital Subscribe Line) enabled telephone exchange. By connecting an ADSL modem to the computer and the telephone line it is possible to access a WAN through the telephone line and use the telephone to make voice calls at the same time.
Cable Modem and Cable TV	This method is only available at locations that have a connection to a cable TV network. By plugging a cable modem into the computer and the cable TV control box it is possible to access a WAN through the cable TV connection.