Computer Networks

A network is two or more computers, or electronic devices, connected together so that they can exchange data.

For example a network allows computers to share files, users to message each other, a whole room of computers to share a single printer, etc.

Network connections between computers are typically created using cables. However, connections can be created using radio signals (Wireless/Wifi), telephone lines (and modems) or even, for very long distances, via satellite links.

Using a computer connected to a network allows us to...

• Easily share files and data
• Share resources such as printers and Internet connections
• Communicate with other network users
• Store data centrally for ease of access and back up.
• Keep all settings centrally so anyone can use any workstation.

In particular, if we use a computer connected to the Internet, we can...

• Make use of online services such as shopping or banking.
• Get access to a huge range of information for research.
• Access different forms of entertainment.
• Join online communities.

Computers connected together to create a network fall into two categories:

Servers and Clients

Clients:

Client computers, or workstations, are the normal computers that people sit at to get their work done.

Computers connected together to create a network fall into two categories:

Servers and Clients

Servers:

Servers are special, powerful computers that provide 'services' to the client computers on the network. These services might include:

• Providing a central, common file storage area.
• Sharing hardware such as printers.
• Controlling who can or can't have access to the network.
• Sharing Internet connections.

Servers are built to be very reliable. This means that they are much more expensive than normal computers.

In a small network one server might provide all of these services. In a larger network there might be many servers sharing the work.

A local area network is a network confined to one building or site.

Often a LAN is a private network belonging to an organisation or business.

Because LANs are geographically small, they usually use cables or low-power radio (wireless) for the connections.

A wireless LAN is a LAN that uses radio signals (WiFi) to connect computers instead of cables.

At the centre of the WLAN is a wireless switch or router - a small box with one or two antennas sticking out of the back - used for sending and receiving data to the computers. (Most laptops have a wireless antenna built into the case.)

It is much more convenient to use wireless connections instead of running long wires all over a building.

However, WLANs are more difficult to make secure since other people can also try to connect to the wireless network. So, it is very important to have a good, hard-to-guess password for the WLAN connections.

A wide area network is a network that extends over a large area.

A WAN is often created by joining several LANs together, such as when a business that has offices in different countries links the office LANs together.

Because WANs are often geographically spread over large areas and links between computers are over long distances, they often use quite exotic connections technologies: Optical fibre (glass) cables, satellite radio links, microwave radio links, etc.

Bluetooth is a wireless networking technology designed for very short range connections.

The idea of Bluetooth is to get rid of the need for all of those cables (e.g. USB cables) that connect our computer to peripheral devices such as printers, mice, keyboards, etc.

Bluetooth devices contain small, low power radio transmitters and receivers. When devices are in range of other bluetooth devices, they detect each other and can be paired.

In this type of network, a long, central cable, the 'bus' is used to connect all of the computers together. Each computer has a short cable linking it to the 'bus'. 

A bus network:

• Is cheap to install (just one long cable).
• Can be quite slow since all computers share the same cable when communicating.
• Will stop working if there is a break in the central bus cable.

In this type of network each computer is connected to a loop of cable, the 'ring'.

A ring network:

• Can cope with a break in the ring cable since all of the computers are still joined together. (Turns into a bus network)

In this type of network every computer is connected to a central device. The device passes messages between computers.

At the centre of a star network you might use a hub (cheap, but slower) or a switch (more expensive, but faster).

A star Network:

• Is quite expensive to install (you have to buy lots of cable and the central device)
• Is very fast since every computer has its own cable which it doesn't need to share
• Can cope with a broken cable (Only one computer will be affected)
• Will stop working if the central device breaks
• Is the most common network topology.

A hybrid network is simply one that combines two or more of the other basic network topologies.

E.G. A network that has several star networks linked together is a hybrid network.

A networking interface card is required to connect a computer to a network. Most modern computers have these built into the motherboard but some computers require an extra expansion card to be fitted.

To connect together different devices to make up a network, you need cables. Cables are still used in most networks, rather than using only wireless, because they can carry much more data per second, and they are more secure.

A hub is a device that connects a number of computers together to make a LAN. The typical use of a HUB is at the centre of a star network, the hub has cables plugged into it from each computer.

A hub is a 'dumb' device; if it receives a message, it sends it to every computer on the network. This means that hub based networks are not very secure, every computer can listen in to communications.

A switch, like a hub, is a device that connects lots of computers together to make a LAN. The typical use of a switch is the same as a hub, at the centre of a star network with lots of computers plugged into it.

A switch is a more 'intelligent' device than a hub; if it receives a message, it checks who it is addressed to, and only sends it to that specific computer. Because of this, networks that use switches instead of hubs, are more secure. Switches are more expensive than hubs.

A router is a networking device that connects together two or more networks.

A common use of a router is to connect a personal or business network (LAN) to the Internet (WAN).

A router can make decisions whether a message should be passed between the networks. It can also make the format of the message suitable for the new network.

A router will typically have an Internet cable plugged into it, as well as the cables connecting it to the computers. Alternatively the LAN connection might be wireless, making it a wireless computer.

A proxy server is a computer set up to share a resource, usually an Internet connection. Other computers can request a web page via the proxy server. The proxy server will then get the web page using its Internet connection and send it to the computer that asked for it.

Proxy servers are usually used instead of routers as additional software can be installed such as anti-virus, web filtering etc...

A bridge is a device that typically links together two parts of a LAN.

Where a router is used to link two networks such as a WAN and LAN, this is used to link two independant parts of a LAN so they act as a single LAN.

A bridge can only connect networks that use the same rules for handling message.

A firewall is a piece of software that is placed between your device and the rest of the network. If you wish to protect your whole network from hackers, then the firewall should be placed between the LAN and the Internet connection.

A firewall blocks unauthorised connections from being made to your computer or LAN. Normal data is allowed through the firewall (e.g. emails or web pages) but all other data is blocked.

Before broadband connection, computers use to use dial up connections via phone lines to access the Internet. The problem with this is that telephone lines are designed to send analogue signals (voices) whereas Internet signals are digital data.

The solution was to use a device that joins a digital computer with an analogue phone line. This device is known as a modem.

A modem contains a DAC and an ADC.

The DAC converts the computers signals into analogue ones that can be sent down the telephone line. The ADC is then required to turn the analogue signals that arrive back at the modem into digital ones that can then be read by the computer.

So, a modem is required as computers are digital and telephone lines are analogue. The modem will convert the digital signals into analogue ones, then the analogue signals into digital ones.

The Internet is a world-wide network that has grown and evolved from an experimental network created by the US military. The Internet connects millions of people, and thousands of businesses, governments, schools, universities and other organisations.

What can we use the Internet for?

The Internet provides the network connections that links computers together. There are many ways that we can use these connections:

• View web pages on the WWW (World-Wide Web)
• Sending and receiving email messages
• Sharing Files
• Communicating using voice (VOIP) and video (Video Conferencing)
• Playing multi-player games
• Listening to streamed music or watching streamed video

An intranet is the name given to a private network that provides similar services to the Internet; email, messaging, web pages etc...

However, these services are only for the users of the intranet - they are private, not public (unlike Internet services which are generally public).

Businesses and other organisations ofter have intranets for use by their employees.

Typical uses of an intranet would be:

• Viewing internal web pages (e.g company calenders, etc.)
• Internal email and instant messaging between workers
• Sharing of internal documents

If you were asked to build a small, Internet connected network from scratch, what would you need to do?

You would need to buy some hardware:

• One or more switches/hubs to link devices together.
• Network cables to connect devices to the switch, etc.
• A separate wireless access point (or part of the switch) to allow wireless devices to join the network.
• A router to connect your LAN to the Internet.
• A firewall to protect your network from hackers.
• Server(s) to manage network functions such as security, file storage and printers.

You also need to organise some other things:

• Set up an account with an Internet Service Provider (ISP).
• Get an Internet connection installed from the ISP to your location.
• Configure various bits of hardware and software so that everything works in the network.

As soon as your computer is connected to a network, you have to start thinking about security; security of your files, information, etc...

A network allows a person to have physical access to your computer (when they are not sitting in front of it) to gain access all the same. If your computer is connected to a network, other people can connect to your computer.

A person who gains unauthorised access to a computer system is often called a hacker.

There are a number of security measures that you can take to prevent hackers accessing your computer and all of the data stored on it:

• Use a User name and have a good Password
• Always install and use a Firewall

The most common way to protect your computer's data is to setup user accounts with user names and passwords. Anyone not having a user name, or not knowing the correct password will be denied access.

For this to be effective passwords must be chosen that are not easy to guess. Passwords should be a random combination of lowercase letters, uppercase letters and numbers.

Some computers replace the typing of user names and passwords with other forms of user identification such as ID Cards, fingerprint readers, voice print recognition etc...

Biometric Devices are ones which use fingerprints, retina scanners and so on as identification.

A firewall is a device, or a piece of software that is placed between your computer / LAN and the rest of the network / WAN (Where the hackers are).

Often we have data that is private or confidential. This data needs to be protected from being viewd by unauthorised people. This is especially true if the data is to be sent via public network such as the Internet.

The best way to protect data is to encrypt it...

Encryption is the process of converting information into a form that is meaning less to anyone except holders of a 'key'.

For example, if Alice wants to send important, personal messages to Bob, she must...

• First Alice needs to generate a secret 'key'. The key is usually a very long secret number.
• Alice must then give a copy of this key to Bob. She must make sure nobody else can get the key.
• Now when Alice sends Bob a message she starts by encrypting it using special encryption software and the secret key.
• The encrypted message now looks like a jumble of random letters and numbers.
• Alice then sends the encrypted message to Bob.
• She can use a public network like the Internet, since, even if it gets stolen, the encrypted message cannot be read or understood without the key.
• When bob receives the message, he uses a special decryption software and his copy of the secret key to decrypt the secret message.
• Bob can now read the original message from Alice.