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Computer Networks - AQA Computer Science Cheat Sheet by [deleted]

What is a Computer Network?

A digital teleco­mmu­nic­ations network which allows nodes (i.e. computers) to share resources.

Pros and Cons of Networks

Comm­uni­cat­ion: becomes easier as a result of technology like texting, emailing, etc.
Flex­ibi­lity: if inform­ation is stored on a network, it means users can access it from anywhere in the world.
Sharing resour­ces: sharing files and inform­ation over a network, including software (can be streamed using web applic­ations) and access to printers.
Hard­ware: routers, network cards etc are required to set up a network. This is expensive and requires profes­sional expertise to set up.
Vuln­era­bil­ity: hackers can break into networks. Malware can spread and damage files on many computers via a network.
Depe­nde­nce: users relying on a network might be stuck without access to it.

Wired vs Wireless

Wired Networks
The computers are physically connected by wires (e.g. coaxial copper cables, fibre optics). They are arranged in topo­log­ies.
Wireless Networks
There is no physical connec­tion, as radio waves (e.g. in the form of WiFi) are used to commun­icate data instead.
Which is Better?
It is dependent on situation. Wired networks are more reliable, as there is less interf­erence. Security is also easier to manage. However, cabling and other physical components can make it very expensive. Wireless networks allow mobility and flexib­ility, but are much less secure and interf­erence can occur. Data transfer may also be slower.

Why is network security important?

Network security is a broad term for any measures that protect a network from unauth­orized access, misuse, destru­ction, or the sharing of confid­ential inform­ation. It is important because otherwise sensitive data may be shared or lost. It is also essential from a legal persep­ctive. For busine­sses, corporate espionage is another a potential issue.

Methods of Network Security

Checking the identity of a user, usually by requiring them to input a password or biometric ID.
Encoding data it using a key, meaning that the same key is needed to decrypt the data. This is how HTTPS works.
Protects a network from unauth­orised access.
MAC Address Filter­ing
Allows devices to access or be blocked from accessing a network based on their physical address embedded within the device’s network adapter.


Tr­ans­mission Co­ntrol Pr­otocol, a protocol dictating how to establish and maintain a network conver­sation.
In­ternet Pr­otocol
A 4-layer model that is essential to networ­king.
Appl­ication Layer
Where the network applic­ations, such as web browsers or email programs, operate. Examples: HTTP, HTTPS
Tran­sport Layer
Sets up the commun­ication between the two hosts and they agree settings such as ‘language’ and size of packets.
Network Layer
Addresses and packages data for transm­ission. Routes the packets across the network.
Data Link Layer
This is where the network hardware such as the NIC (network interface card) is located. OS device drivers also sit here.

TCP/IP Diagram


Network Protocols

A family of protocols that dictate how devices on the same network segment format and transmit data.
Wi-Fi or WLAN
A family of protocols that deal with wireless transm­ission.
Tr­ans­mission Co­ntrol Pr­otocol: splits (and later reasse­mbles) data into packets. Also involves error checking, as expects an acknow­led­gement transm­ission within a set time frame.
User Da­tagram Pr­otocol:
In­ternet Pr­otocol: each device has an IP address. Packets are 'addre­ssed' to ensure they reach the correct user.
Hy­pertext Tr­ansfer Pr­otocol: used to access a web-page from a web server.
Hy­pertext Tr­ansfer Pr­otocol Se­cure: u7ses encryption to protect data.
File Tr­ansfer Pr­otocol: handles file uploads and downloads, transfers data and programs.
Simple Mail Tr­ansfer Pr­otocol: handles outbound email. SMTP servers have databases of user's email addresses.
In­ternet Me­ssage Access Pr­otocol: handles inbound emails.
A network protocol is a set of rules/­con­ven­tions that dictate how a network operates.

Network Topologies

What is network topolo­gy?
The way that a network is physically struct­ured.
What is star topolo­gy?
A network where there is a central server that all of the computers and periph­erals are connected to.
Advantages of star toplogy:
1. If a computer fails, there is no impact on the other devices.
2. Security is good, because the data only passes through the server, not any other devices.
3. There are no data collis­ions.
Disadv­antages of star topology:
1. If the server fails, it's a catast­rophe.
2. Lots of cabling is need to connect all the devices indivi­dually, so it's quite expensive.
What is bus topolo­gy?
A network where there is a central backbone of cable connecting every computer. At each end of the cable is a term­ina­tor to stop data from contin­ually being moved around.
Advantages of bus toplogy:
1. Cheap.
2. Easy to add more devices.
Disadv­antages of bus topology:
1. Only approp­riate for small networks, otherwise data transm­ission is too slow.
2. Data collisions are likely.
3. If the backbone is severed, all computers are impacted.

Bus Topology

Star Topology

Types of Network

Pe­rsonal Area Ne­twork - a network comprising only a small number of devices belonging to only one individual (e.g. Blueto­oth).
Local Area Ne­twork - a network that encomp­asses a small area (e.g. one company's network).
Wide Area Ne­twork - a network comprising many devices and covering a large area (e.g. the Internet). Often under collective ownership.

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