What is a computer network?

A computer network is a group of (2 or more) computer systems that are linked together. The main issue that comes up in networking is how the computer network is set up; the way they are linked is a large issue. We connect computer systems to networks so we can share access to files and external devices (such as printers).


A computer that is not connected to any network is a 'standalone' computer. A 'client' is a user on a network. Being able to connect different standalone computers via a network is useful, for many reasons, including:

  • Users can access files across the network, from different computers. For instance, a network allows a school to have different computers, and sync the files from one to another automatically.

  • Users can all share a single peripheral, such as a printer. This means that, using the school example, each computer can be connected to the same printer.

  • Users can communicate with one another. Within the school example, emails can be used to share news between students and teachers, and can give students a direct communication line with their teachers out of hours.

  • Administrators can track what each computer is used for, on the network. This means any content not suitable for the network can be tracked.

However, there are some drawbacks of networking too. These include:

  • Viruses and malware are a major risk, especially as many users means many more methods of entry into a system.

  • If a piece of networking hardware fails, this can cause an outage across the entire network. If this network is relied upon for important tasks, this can disrupt work.

  • The computer systems can run slower if many people are utilizing the network at the same time.

  • Shared data means more risk of accidental and malicious deletion of important data over the network.


LAN networks/WAN networks

A Local Area Network is a group of computers that are connected together over a small geographical area such as a single building or single site. This includes, but is not limited to, home networks, school networks or a network in a shop.

A Wide Area Network is a group of computer that are connected together over a large geographical area such as town-wide or country-wide. The biggest WAN would be a global collection of networks, also known as the internet.

LAN has its own infrastructure of cabling and network hardware. This is due to distance and practicalities of the network. WAN, on the other hand, uses external hardware and external infrastrucutre, such as the use of satellite, phone lines or the internet.

LAN networks are wired up using fibre optic cables, UTP cables or via a wireless protocol such as WiFi. WAN networks are connected using fibre optic cables, telephone lines, or satelite links.

Dedicated lines are created and maintained by the company. This is what all LAN networks run on. These are much more expensive, as the company owns the hardware to make the network work. However, up-time is prioritised on dedicated lines, as a networking administrator would handle all the upkeep of the network inhouse (within the company, rather than an external company).

Leased lines, on the other hand, are what most WANs are based upon. They use lines rented from an ISP such as BT or Sky. This is a cheaper way of creating a WAN than placing your own cables, but it means you rely on an external company to upkeep those connections.

Client-Server Model

Client-server networks have a dedicated server, that clients (other computers) can connect to and access the resources on that network. The server manages access and security to other devices, manages access to the internet, manages access to files and can also make backups of data.

Client-Server Model

There are many benefits to this kind of network:

  • It is easy to manage; the central server creates all the permissions to the resources on the network. For example, in a school example, a student account can be given access to only certain files, whilst a teacher has access to their own shared resources drive.

  • Since all the files are stored on the server, the files are all in one place. This means backups are easy to make, as they aren’t scattered across the network.

  • Shared peripherals such as printers are controlled via one server, making it easy to access and send requests to use.

  • It is easy to send software updates across the network. You can force-update every computer on the network.

However, there are some drawbacks, including:

  • A server can be expensive to setup, and even more expensive to maintain, as it requires specialist IT professionals to manage the network.

  • Specific software is required to setup the network. These software packages can be expensive, and difficult to set-up.

  • The server is a single point of failure. If the server goes down, then every client will lose access to all resources.

Peer-to-Peer Model

A peer-to-peer network has no central server. Each workstation shares its files equally with others. Each 'peer' serves their own files to other computers on the network, and each peer is responsible for it’s own security, backup, and resources. Peripherals such as printers are usually connected to a single peer, but other users can request usage of that peripheral. However, that other computer has to be turned on for access to be given.

Peer-to-Peer Model

There are many benefits to peer-to-peer networks:

  • No expensive server is required to be built, as each individual workstation hosts its own files and resources.

  • No specific operating systems are required for the network to run, making it cheaper to setup.

  • If a single computer fails, it won’t disrupt anyone else on the network; only the files hosted by that peer are unavailable.

  • No specialist IT professionals are required to maintain the network, as each user sets their own permissions.

  • They are much easier to setup on a network, as it doesn’t require specific software packages.

However, there are some major drawbacks too, including:

  • Peer-to-peer networks are unruly to manage; each user sets their permissions, and as more computers are added to a network, these permissions have to be modified too.

  • All the files are scattered across the network, making backups of this data more difficult.

  • If a peer computer is offline, then any resources and peripherals such as printers also go offline with it.

  • Security, in terms of viruses, is difficult, as each user is responsible for their own computers security.

  • Permissions can be set wrong, meaning that certain users have more permissions than they should.


To be able to connect up standalone computers to a network, you need special networking hardware that give them the ability to network.

A Network Interface Controller (NIC, also often known as a network interface card) is a device that connect a computer to a wired or wireless network. Protocols define how this controller handles networking (see chapter 5 for more information on protocols). On modern computers, a NIC is built into the motherboard, instead of requiring a separate add-in card.

Network Interface Controller

A Wireless Access Point allows wireless enabled devices to connect to a network without any cables. Although bandwidth is lower whilst using wireless devices, it is much more convenient for portable devices such as phones. Wireless Access Points can extend the range of a network to other parts of a building, if required.

Wireless Access Point

A switch sends data between different computers on a network. These separate data, and only forward the relevant data to the correct device. They read the 'from' address from each internet packet, and learn about which devices are on the network. If the 'to' address is recognized, it will send the data to the correct device only. The addresses utilized for the to and from addresses are based upon addresses built into the NIC.

Network Switch

Beyond GCSE Information

An older method to complete the same task as a switch, is by using hubs and bridges. A hub doesn’t create an index of devices on the network, and sends the data to every computer on a network, even when that data was only required to be sent to one single device.

A router is a device that connects one network to another. It creates a WAN from a bunch of smaller LAN networks; an example of this is connecting a local LAN at a school, to the internet. A router uses an IP address for their to and from addresses.


Transmission Media

Unshielded Twisted Pair cables (UTP cables) are copper cables that usually connect up computers to a switch or router within a LAN. This cable is made from pairs of wires twisted around each other, to reduce any interference. These wires are usually made up of one for transmitting data, and one for receiving data.

Shielded twisted pairs also exist, where copper shielding is used around twisted pairs to make them less susceptible to interferance. Finally, coaxial cables are also copper cables, that are less used in networking.

Most LAN networks use copper cables. All copper cables have signal degradation, meaning over a long distace, that signal can worsen. However, they are cheaper to setup.

Copper cable

Fibre optic cables are made from glass, where light can pass through and reflect to the signal. Light pulses represent binary digits.

The backbone of the internet is made of fibre optic cables. There is less interference in fibre optic cables than in copper, and less chance for degeneration of the signal. However, it is extremely expensive to setup.

Fibre optic cables

The Internet

The internet is the most widely known wide-area network, and is a bunch of small local-area networks connected together.


Every computer has an IP address, which is a unique string of numbers separated by full stops that identifies each computer over a network. It is similar to a postcode to identify a house. However, remembering an IP address is difficult. This is why a DNS server exists.

A URL, or Uniform Resource Locator, is an address that declares where data is located. A domain name is the part of a network address that identifies it to belonging to a particular domain or server (for example "google.com").

A Domain Name Server (or DNS) is a server that sits between your home network, and the server you are trying to access. A client computer sends a domain name to the DNS server, and the DNS server looks up the domain in a table, if a match to an IP is found the client is sent back the IP and a connection is made. This means that a domain name URL can be turned back into the IP address, making it easier on the end user.

If no record exists of that particular domain name on that DNS server, it will query other DNS servers until a match is found which is then sent back to the user. Otherwise, the request will give an error.

DNS Server

Hosting and the Cloud

Websites are designed to be online 24/7, for any user to connect and access its resources. As such, they have to be stored on a server that is online constantly. These servers are dedicated to the purpose of hosting a website, and this process on these servers is called 'hosting'.


Cloud storage is another term frequently used. These servers are placed remotely from the location that the service is being accessed, and is used over the internet. Access can be used anytime, anywhere. This is what is referred to as the 'cloud'.

Virtual networks

A virtual network is a software barrier on a physical network. They divide devices on a LAN into smaller groups, as if they were separate networks. Every device is connected via the client-server model, yet not every device has permissions to access all the resources on the network.

The benefits of a virtual network, is that it is scalable. Adding more devices means you only have to configure the permissions for that single new device, and the permissions are handled by the main server. This is also secure, as the server handles all permissions and makes sure it can access only certain resources. Finally, this is cheaper than creating separate networks to separate out users

Virtual network

Virtual networks are powerful. In an office, for instance, the finance staff require access to the accounting software. As such, they will be on a separate virtual network than the programming staff, who require an entirely different set of software and permissions.

Another example is in a shopping center or mall. Free WiFi might be available to visitors, which has it’s own permission to only access the internet, and certain websites. However, staff won’t need access to the internet, and instead need access to local data such as employee records.

Performance of networking

The bandwidth is the amount of data that can be sent and received correctly within a given timeframe. This is about how much data can be sent via the transmission media, not how fast data travels. This is measured in bits per second, or the bit rate.

The error rate can increase when using a less reliable connection method, such as wireless connections. This means that data is transferred wrong, and has to be re-transmitted until it arrives successfully. Copper cables can degrate over long distances, and this can affect the error rate. Similarly, wireless connections can be affected by range, and other surrounding factors such as other devices on the network, and physical items such as walls in a house.

Latency is the delay from sending data, to receiving it. This is caused by a bottleneck in the network, such as high error rates, or too much traffic on a network. Different devices have different latencies.

If a network has too many users, it can cause a network to slow down. If there is insufficient bandwidth, too little data can be sent through the switch. Similarly, if a hub is used instead of a switch, it sends out information to all devices, bottlenecking the network (as irrelevant data is sent, using up valuable resources).

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1.5 Systems Software

2.2 Network topologies, protocols and layers