A photo crosses the building
You send a photo to a printer down the corridor. The cable does not carry a picture. It carries a long line of tiny signals, on or off.
How does the printer know where the photo starts, which part is which, and that nothing was lost? The answer is a set of simple rules and labels.
Key words
| Word | What it means |
|---|---|
| Bit | one signal that is either 0 or 1 |
| Byte | a group of 8 bits |
| Packet | a small labelled piece of a message |
| Header | the labels at the front of a packet |
| Frame | a packet wrapped for one cable link |
| Datagram | a packet sent on its own, with no reply expected |
| Address | a number that says where data comes from or goes to |
| Sequence number | a count that puts packets back in order |
| Encapsulation | wrapping data in a new header at each step |
| Network interface card | the part of a device that joins it to the cable, short name NIC |
| MAC address | the address built into a NIC |
| Network | devices joined so they can share data |
| IP address | an address for a device on a network, such as 192.168.1.20 |
| Ethernet | the common way to join wired devices in an office |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can explain how data is made of bits and bytes, and how encapsulation wraps it in packets and frames for the cable.
Starter: what you already know
Have a go
Starter: what you already know
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Bits and bytes
A bit is a signal that is 0 or 1. On copper it is a small voltage change. On fibre it is a flash of light.
8 bits make 1 byte. One byte can hold 256 different values, from 0 to 255. One letter of text usually takes 1 byte.
Worked example: bits in a message
| Message | Bytes | Bits (bytes times 8) |
|---|---|---|
| The word "cat" | 3 | 24 |
| A text of 100 letters | 100 | 800 |
| A 2000-byte photo piece | 2000 | 16000 |
The cells students fill in are left blank in this preview.
Check: bits and bytes
Have a go
Check: bits and bytes
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.