Network protocols and layers
OCR J277 Paper 1 frequently asks for the role of named protocols and a justification for layering. You need to know each protocol's purpose, the four-layer TCP/IP model, and why layering is helpful.
What is a protocol?
A protocol is a set of rules governing how data is transmitted between devices. Both ends must use the same protocol to communicate.
Key protocols
| Protocol | Purpose |
|---|---|
| TCP (Transmission Control Protocol) | Splits data into packets, ensures reliable delivery, reassembles in order at the destination. |
| IP (Internet Protocol) | Routes packets across networks using IP addresses. |
| HTTP (HyperText Transfer Protocol) | Transfers web pages between a web server and a browser. Plain text, not encrypted. |
| HTTPS | Secure version of HTTP — uses TLS/SSL encryption. Protects logins, banking, etc. |
| FTP (File Transfer Protocol) | Uploading and downloading files between a client and a server. |
| SMTP (Simple Mail Transfer Protocol) | Sending email from a client to a mail server, and between mail servers. |
| IMAP (Internet Message Access Protocol) | Receiving email — keeps messages on the server so they can be accessed from multiple devices. |
(POP3 is also receiving — downloads then deletes from server. OCR may mention it but it is not on the J277 named-protocols list.)
The 4-layer TCP/IP model
Data is built up at the sender (with each layer adding its own header) and stripped down at the receiver.
| Layer | Role | Example protocols |
|---|---|---|
| Application | User-facing protocols that produce/consume the data. | HTTP, HTTPS, FTP, SMTP, IMAP |
| Transport | Splits data into packets, reassembles, error checks, port numbers. | TCP, UDP |
| Internet (Network) | Routes packets between networks using IP addresses. | IP |
| Link (Data link / Network access) | Physical transmission over the medium — cables, Wi-Fi. | Ethernet, Wi-Fi |
Why use layers?
- Self-contained — each layer does its own job and can be changed independently (e.g. replace Wi-Fi with Ethernet without affecting HTTP).
- Standardisation — different vendors' equipment interoperates.
- Easier to develop and debug — a problem can be isolated to a single layer.
- Reusability — TCP can be used by any application-layer protocol; the same Ethernet chip works for HTTP, FTP, SMTP.
Common OCR exam mistakes
- Mixing up SMTP (send) and IMAP (receive). Memory hook: "SMTP for Sending".
- Forgetting that HTTP is not encrypted — HTTPS adds the encryption layer (TLS).
- Listing layers but not explaining the role of each.
- Saying "layers make networks faster" — they do not; they make networks easier to develop and maintain.
AI-generated · claude-opus-4-7 · v3-ocr-computer-science-leaves