Information and Communication Technology


Linking devices together

Many IT devices are able to communicate with each other.

When two or more devices are linked and able to communicate, they form a network.

You may already be familiar with a home network, or one in your school and have some idea about what they can be used for.

For the IGCSE ICT you will look at a number of different types of network, from the very small with just a couple of devices, to the very large with millions.

Network types

Methods of connecting a network

Networks can be wired, wireless or a combination of the two.

Most home, school, and business networks that are cabled use Ethernet cable. An Ethernet cable consists of 8 insulated copper wires held together in a single sheath. The cables are labelled according to industry standard categories that describe their properties.

Most network cable is category 5 or 6 (CAT5, CAT6). Both types allow cable runs of 100m but CAT6 has a higher data transfer rate than CAT5.

Many networks now accept wireless connections to allow for laptops and other portable devices. It is unusual to have a completely wireless network, as components such as wireless access points or wireless routers often require a cable connection as well for administrative purposes.

Wireless networks in most countries use the industry standard known as 802.11 which describe the radio frequencies, power output, etc. that the wireless devices are allowed to use. Different letters after the number, e.g. 802.11g, indicate changes to the standards which allow for improved technology. The most recent one is 802.11n, published in 2009.

802.11n devices have a working range of about 100m.

Network sizes

Network types ending in AN

PAN. Personal area network. This type of network is made from components with short range links, usually less than 10m. The components are often mobile devices such as cellphones, handheld games consoles or iPods. These may linked to a laptop or a nearby PC, but are just as likely to be connecting to another mobile device. Connections are often made using Infra Red or short range radio systems such as Bluetooth.

LAN. Local area network. This type of network is the sort found in the home, a small business, or a school. As the name suggests a LAN is confined to a local area, often a single building. The size restriction is mainly to do with the length of cables that can be used, although with better quality cable and longer range wi-fi connections, a LAN can get larger and may overlap with the size of a CAN or MAN.

CAN or MAN. Campus area network or Metropolitan area network. These are networks run by larger organisations such as universities or city councils. The name reflects the type of organisation, universities have CANs, councils have MANs. But they are just different names for the same thing. A CAN / MAN is made up from two or more LANs, joined by a high speed backbone such as a fibre optic link.

WAN. Wide area network. This is the largest type of network. A WAN may cover a city, a country, or the whole world. It is made up of LANs, or individual computers which are linked by telecommunications links such as phone lines and satellite links. Some of these may be dedicated links used only for the WAN, others are temporary connections using commercial phone systems.

Network components

Which network device does what

The term Server covers any computer which has the function of controlling or providing shared resources for a network. In large networks, servers are typically specialised computers that are designed to run 24/7 with a high workload. They will have features such as multiple CPUs, large amounts of RAM, multiple hard drives, and uninterruptible power supplies. They are often operated through a network connection and may not have graphics cards, or keyboards. This type of server is shown in the top picture.

For smaller networks, the server may just be a standard PC or even a laptop. The name refers to the function rather than the specification of the computer.

A router, shown in the middle picture, is a device that forwards data from one network to another. The most common use of a router in home or small business networks is to connect the home or office LAN to the Internet. In larger organisations, routers may connect LANs or sections of LANs together.

Home routers are often used as network gateways forming the link between a home network and the Internet.

A wireless access point (WAP) is used to connect wireless enabled devices to a network. In home or small office networks, the WAP is often combined with a router as seen in the bottom picture.

Most WAPs and routers, and some switches, contain a signal booster, which will clean up and amplify the received signal before onward transmission. This is often seen in large WiFi networks, where each WAP also acts as a router and booster so that signals can be passed accross the network without losing strength.

A switch is a device for connecting the components of a network together. Each computer, WAP, printer, etc. is connected to the switch by a cable. When one device needs to communicate with another, e.g. when a PC sends a print task to a networked printer, the signals travel via the switch. In home or small business networks, the switch is often combined with the router. In larger networks switches are separate devices and may have extra functions built in that help to manage the network traffic.

Bluetooth enabled devices, e.g. mobile phones, may be attached to a network via a Bluetooth transceiver. Some PCs and laptops have one built in, other require a dongle or other plug-in device.

The use of entertainment, gaming, and media streaming devices in home networks is increasing. It is expected that integration of e.g. games consoles, digital television, video recording, etc. will become more common as the manufacturers of such devices agree on common standards for connections and file formats.

Network addresses

How network devices find each other

As soon as computers are connected in a network, some problems arise. How do they know where to send messages and if a message arrives, how do they know it is meant for them. The answer is that each device in a network, PCs, printers, etc. has to have an address.

Larger networks, ones that are big enough to need a server, use a method called IP addressing. Each device has an Internet Protocol (IP) address. The IP addresses may be allocated manually by the network manager, or automatically by a server. Each device gets given a different IP address.

An IP address is made up of four numbers, separated by dots. e.g. Each number is in the range 0 to 255. This means that the IP address can be held as 4 bytes.

The system was developed for identifying computers on the Internet. See the section below for information on the newer IPV6 addressing system.

Another method of addressing is simply to give each device a name, e.g. Jane's PC, Sunil's laptop, HP Printer.

This method works as long as the network is small and the names can be controlled. Many home and small business networks operate in this way. But as networks get larger, the problem gets bigger. There are probably lots of laptops in the world that are called Sunil's laptop. If two of them try to join a network using names, one of them would have to be renamed first.

Actually the small networks that use the name method also have IP addresses, it's just that the people using the network don't normally see them or use them. The operating system takes care of it.

IPV6 network addresses

When the network gets really large

A major problem with the four byte network address system described above, is that there are not enough addresses to go around.

When the system was first invented, there were not many computers in the world and no one though that they would ever run out of addresses.

Since then, computers have become much more common and many other devices such as printers, mobile phones and games consoles, can now be connected to the Internet and need their own IP address.

In addition, many devices are now connected to the Internet by always-on connections. This stops IP addresses from being shared between a pool of users, as was possible with dial-up connections.

IPV6 addresses are 16 bytes long instead of the 4 bytes for the IPV4 system. This means that there are 2^128 possible addresses, that’s about 34 followed by 37 zeros. Or about 5 followed by 27 zeros addresses for every person in the world.

That may seem rather a lot, but in fact, 7/8ths of the addresses are reserved for future use.

Also, where an organisation is allocated an address, it only has the first 8 bytes as it's address, the other 8 bytes are for the individual machines within that organisation. The default is for the machine address to be the Media Access Control (MAC) address of that machine, but questions have been raised about security issues involved with sending MAC addresses over the Internet.

The MAC address is the unique identification for each network interface. That is, every network card has it's own address, which consists of a number written as six pairs of hexadecimal digits. e.g. 12:3b:ca:98:3a:1f That gives 281,474,976,710,656 possible addressees.

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