Protocols and layers
What is a protocol?
A protocol is a set of rules for transferring data between devices: how the data is formatted, how it's addressed, and how errors are spotted. Both devices must follow the same protocol to understand each other, just as two people need a common language.
Data sent across a network is split into small pieces called packets, each sent separately and put back together when they arrive.
Different protocols are used for different jobs: one for web pages, another for sending email, another for transferring files.
Key terms
The common protocols
| Protocol | What it's for |
|---|---|
| TCP/IP (Transmission Control Protocol / Internet Protocol) | The rules the internet runs on. TCP splits data into packets, checks they all arrive, and puts them in order. IP addresses and routes each packet |
| HTTP (Hyper Text Transfer Protocol) | Requesting web pages from a web server, and sending them to the browser |
| HTTPS (HTTP Secure) | HTTP with encryption, so data such as passwords and card details can't be read if intercepted |
| FTP (File Transfer Protocol) | Transferring files between computers, such as uploading a website's files to its web server |
| SMTP (Simple Mail Transfer Protocol) | Sending email, from your device to your email server, and between email servers |
| POP (Post Office Protocol) | Downloading email from the server to one device, usually removing it from the server |
| IMAP (Internet Message Access Protocol) | Reading email that stays on the server, so every device sees the same inbox |
Key terms
Email: SMTP, POP and IMAP
Email uses three protocols. SMTP sends a message on its way; POP or IMAP fetch it for the person receiving it.
| POP | IMAP | |
|---|---|---|
| Where emails are kept | Downloaded to one device, usually deleted from the server | Kept on the server |
| Several devices | Each device gets its own copies, out of step | Every device sees the same inbox: read on your phone shows as read on your laptop |
| Offline | Downloaded emails can be read offline | Needs a connection, unless the device keeps copies |
| Server space | Little needed | More needed |
Most people now read email on several devices, which is why IMAP is more common.
Exam tip
Name the protocol and say what it does in the scenario: 'IMAP, so that the emails stay on the server and every device sees them'.
Layers
Sending data across a network is a big job, so it's split into layers. Each layer does one part of the job, using its own protocols, and only passes data to the layer directly above or below it. Data goes down the layers to be sent, and back up them at the other end. The diagram shows the four-layer TCP/IP model as an example; you don't need to learn the names or jobs of the layers.
Why layers help:
- Each layer is a smaller problem, easier to design and understand: this is decomposition.
- One layer can be changed without changing the others, as long as it passes data on in the same way.
- Different companies can make hardware and software for different layers that still work together.
- Problems are easier to find and fix, one layer at a time.