Network Protocols and Layers
A protocol is an agreed set of rules for how data is transmitted between devices, so that equipment made by different manufacturers can communicate. Each named protocol has a specific purpose: TCP breaks data into packets and reassembles them in order at the far end, checking that none are missing; IP routes packets across networks using addresses; HTTP transfers web pages and HTTPS is the encrypted version; FTP transfers files; SMTP sends email; POP downloads email and deletes it from the server, while IMAP leaves it on the server so it stays synchronised across devices. The TCP/IP model organises these into four layers: application, transport, network and link (or data link). Layering means each layer only has to do its own job and talk to the layer directly above and below, so a protocol can be changed or improved without rewriting the whole stack, and developers can work on one layer in isolation.
Before you start
Make sure you're comfortable with these topics first:
Method
- Read the scenario and pick out what is actually happening: fetching a web page, sending an email, downloading email, or uploading a file. That tells you which application layer protocol is in use.
- Name the protocol and say what it does in that context, rather than just expanding the abbreviation. Marks come from the purpose, not the words behind the letters.
- Distinguish POP from IMAP by asking where the message is stored afterwards: POP downloads and removes it from the server, IMAP keeps it on the server so several devices see the same mailbox.
- For HTTPS questions, say that it encrypts the data in transit so an interceptor cannot read it, and link that to why it matters for the data in the scenario, such as card details or passwords.
- For layer questions, place the protocol in the right layer first, then explain the layer's job in one sentence.
- When asked for the benefits of layering, give a reason and a consequence: self-contained layers mean one can be changed without affecting the others, different developers can specialise, and manufacturers can build compatible hardware and software.
Worked example
A user logs into an online shop, adds an item to the basket and pays by card. She then checks her order confirmation email on both her phone and her laptop. Name the protocols involved and place each in a TCP/IP layer, explaining what each one does here.
- Start with the web browsing. The pages are transferred using HTTP, but because card details and a password are involved the site will use HTTPS, which encrypts the data so an interceptor cannot read it.
- HTTP and HTTPS sit in the application layer, which is where protocols that talk directly to the software the user sees operate.
- Underneath, TCP splits the page and form data into packets, numbers them, and reassembles them in the correct order at the other end, requesting any that go missing. TCP sits in the transport layer.
- IP addresses each packet and routes it across the internet from router to router. IP sits in the network (internet) layer.
- The confirmation email is sent from the shop's server using SMTP, which is the protocol for sending mail. SMTP is an application layer protocol.
- Because she reads the same email on two devices and it stays in her mailbox, her devices are using IMAP rather than POP. IMAP leaves messages on the server and synchronises them, so the phone and laptop both show the message as read once she opens it.
Practice questions
Try each question, then tap to reveal the answer.
Q1Define the term protocol.Show answer
Answer: An agreed set of rules for how data is formatted, transmitted and received, so that different devices and manufacturers can communicate reliably.
Q2State the difference between HTTP and HTTPS, and give one situation where HTTPS is essential.Show answer
Answer: HTTP transfers web pages as plain text; HTTPS encrypts the data in transit. HTTPS is essential wherever sensitive data is sent, such as a login page, an online banking session or a card payment.
Q3A user reads the same email on a phone and a tablet, and deleting it on one deletes it on the other. Which protocol is in use and why?Show answer
Answer: IMAP. It stores messages on the mail server and synchronises the state across all devices, rather than downloading and removing them like POP.
Q4Explain the role of TCP when a large file is downloaded.Show answer
Answer: TCP splits the file into numbered packets at the sender, checks that every packet arrives at the receiver, requests any that are missing or corrupted, and reassembles them in the correct order to rebuild the original file.
Q5Name the four layers of the TCP/IP model in order from the one closest to the user.Show answer
Answer: Application, transport, network (internet), link (data link).
Q6State the layer that IP operates in, and describe its job.Show answer
Answer: The network (internet) layer. IP adds source and destination addresses to packets and routes them across networks towards their destination.
Q7Give two reasons why network communication is divided into layers.Show answer
Answer: Any two of: each layer is self-contained, so one can be changed or improved without altering the others; it breaks a complex system into manageable parts; developers can specialise in one layer; it allows hardware and software from different manufacturers to work together as long as they follow the standards.
Exam-style questions
Written in the style of a GCSE Computer Science exam paper, with a full mark scheme.
A school is moving its email from a system where messages are downloaded to one computer to a system where staff can read their mail on a laptop, a phone and a desktop. (a) Name the protocol the new system should use and justify your choice. (b) Name the protocol used to send the mail out. (c) Explain one reason why the school should also move its intranet from HTTP to HTTPS.
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Explain what is meant by a layered network model, and discuss the advantages of designing network communication in layers.
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See real GCSE Computer Science past-paper questions, with official mark schemes →
Free printable worksheet
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