What rules let different devices communicate over a network?
Understand the purpose of network protocols, the common protocols and ports, and the four-layer TCP/IP model and why layering is used.
A focused answer to AQA GCSE Computer Science 3.5.3, covering the purpose of network protocols, the common protocols and ports, and the four-layer TCP/IP model and why layering is used.
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What this dot point is asking
AQA wants you to explain the purpose of network protocols, name the common protocols and what they do, and describe the four-layer TCP/IP model and the benefits of layering.
The purpose of protocols
Common protocols
TCP and IP do different jobs within TCP/IP: IP handles addressing and routing each packet across the network, while TCP handles splitting the data into packets, checking they all arrive, requesting any that are lost, and reassembling them in the right order. This split is itself an example of why layering is useful.
The four-layer model
Why layering is used
Packets and why they are used
TCP/IP works by splitting data into small chunks called packets, each carrying a piece of the data plus a header with the source and destination addresses and a sequence number. Packets are used because they let many conversations share the same network, can each take the best available route, and can be re-sent individually if lost, which is far more efficient and robust than sending one huge block. At the destination the sequence numbers let TCP reassemble the packets into the original data in the right order, even if they arrived out of order. This packet model is exactly why the transport and internet layers exist as separate jobs in the four-layer model.
Try this
Q1. State the purpose of a network protocol. [1 mark]
- Cue. A set of agreed rules that lets devices communicate over a network.
Q2. Name the four layers of the TCP/IP model. [2 marks]
- Cue. Application, transport, internet (network) and link (data link).
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksIdentify a suitable protocol for each of the following and state what it does: requesting a web page securely, transferring a file to a server, and sending an email.Show worked answer →
Requesting a web page securely: HTTPS, which requests and sends web pages with the data encrypted, protecting sensitive information such as logins. Transferring a file to a server: FTP (File Transfer Protocol), which uploads and downloads files between a client and a server. Sending an email: SMTP (Simple Mail Transfer Protocol), which sends email from a client to a mail server and between mail servers.
Markers reward the correct protocol for each task and a brief statement of its job. Naming HTTP instead of HTTPS for the secure case would lose the mark for the security point.
AQA 20224 marksExplain what is meant by layering in the TCP/IP model, name the four layers in order, and give one benefit of using a layered model.Show worked answer →
Layering means dividing the complex task of networking into separate, self-contained layers, each with its own job, that pass data to the layer above or below through standard interfaces.
The four layers are: application (protocols programs use, such as HTTP and SMTP), transport (TCP, splitting data into packets and reassembling them), internet or network (IP, addressing and routing packets), and link or data link (the physical connection and hardware such as Ethernet).
A benefit: each layer can be developed or changed independently without affecting the others, as long as the interfaces stay the same, which also lets equipment from different makers work together.
Markers reward the meaning of layering, the four layers in the correct order, and a valid benefit.
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A focused answer to AQA GCSE Computer Science 3.5.1, covering what a network is, the difference between a LAN and a WAN, and the star and bus network topologies.
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A focused answer to AQA GCSE Computer Science 3.5.2, covering the comparison of wired and wireless connectivity, how Wi-Fi works, and the role of encryption in wireless networks.
- Understand the methods used to keep a network secure, including authentication, encryption, firewalls and MAC address filtering.
A focused answer to AQA GCSE Computer Science 3.5.4, covering the methods used to keep a network secure, including authentication, encryption, firewalls and MAC address filtering.
- Understand why data is compressed, and the difference between lossy and lossless compression including run-length encoding and Huffman coding.
A focused answer to AQA GCSE Computer Science 3.3.8, covering why data is compressed and the difference between lossy and lossless compression, including run-length encoding and Huffman coding.
Sources & how we know this
- AQA GCSE Computer Science (8525) specification — AQA (2020)