| Component | Role | Example |
|---|---|---|
| Message | The information to send | An email, a video |
| Sender | Device that originates the message | Your laptop |
| Receiver | Device that receives the message | A web server |
| Medium | Physical path of transmission | Cable, radio waves |
| Protocol | Set of rules both sides follow | HTTP, TCP |
Without a protocol, two devices may be connected but unable to understand each other, just as two people speaking different languages cannot communicate even in the same room.
Each component in more detail
- Message: the data itself. It can be small (a few bytes for a chat message) or very large (several gigabytes for a film). Large messages are split into many smaller pieces called packets before sending.
- Sender and receiver: any device that can send or receive data: computers, phones, printers, cameras, servers, smart TVs. Most devices are both sender and receiver at different moments. When you open a web page, your laptop sends a request and then receives the page.
- Medium: the physical path. Guided media keep the signal inside a cable (copper or fibre). Unguided media send it through the air as radio waves. Different media have different speeds, distances and costs (see 1.6).
- Protocol: the agreed rules. A protocol defines the format of the message (what each part means), the order of messages (who speaks first), what to do when something goes wrong, and how fast to send.
Why protocols matter so much
A protocol is like the rules of a conversation. When you phone someone you say "Hello", wait for a reply, take turns to speak and say "Goodbye" at the end. Without such rules, both people could talk at once, or never know when the call is over. Computer protocols set rules like these, but much more precisely.
A protocol defines three things:
| Element | Question it answers | Example |
|---|---|---|
| Syntax | What is the format of the data? | The first 6 bytes of an Ethernet frame are the destination address |
| Semantics | What does each part mean, and what action should follow? | A "404" in an HTTP reply means the page was not found |
| Timing | When should data be sent, and how fast? | Wait for an acknowledgement before sending more |
Protocols are written as standards so that equipment from different manufacturers works together. Important standards organisations include the IEEE (Ethernet and Wi-Fi), the IETF (Internet protocols such as TCP, IP and HTTP), the ISO (the OSI model) and the ITU (telecommunications).
Remember
- Five components: message, sender, receiver, medium and protocol.
- A protocol defines syntax, semantics and timing.
- Standards let devices from different vendors communicate.