Sending a parcel
You send a gift to a friend in another country. You write a note. You put it in a box. You write the address on the box. A van takes it to a post office. A plane flies it. Another van brings it to your friend.
Nobody does the whole job. Each person does one small job. Then they pass the parcel on. Computers send data in the same way, in layers.
Key words
| Word | What it means | Example |
|---|---|---|
| Encryption | Scrambling data so that only the right people can read it. | HTTPS |
| Network | Computers joined together to share data. | The college network |
| Router | A device that joins networks and sends data the right way. | A home router |
| Port | A number that says which program on a computer should get the data. | 80 for a web page |
| ASCII | A code that gives each letter and symbol a number. | A is 65 |
The cells students fill in are left blank in this preview.
More key words
| Word | What it means | Example |
|---|---|---|
| Layer | One level in a model. Each layer does one job. | The transport layer |
| OSI model | A model with seven layers that describes how data moves across a network. | Layers 1 to 7 |
| TCP/IP model | A model with four layers. The internet is built on it. | Application, transport, internet, network |
| Protocol | A set of rules that computers follow to talk to each other. | HTTP |
| Header | Extra information a layer adds to the front of the data. | The address on the box |
| IP address | A number that names a device on a network. | 10.0.12.1 |
| MAC address | A number built into a network card. It names the card. | Six pairs of letters and numbers |
| Frame | The data with the last header added, ready to go on the cable or the air. | [Frame][IP][TCP]Hi |
| Ethernet | A common way to join computers in a building, usually with a copper cable. | The cable behind a desk PC |
| OSPF | Open shortest path first. A routing protocol: routers share a map to find the best path. | Routers in a large network |
| DNS | Domain name system. It turns a name such as example.com into an IP address. | Finding a website |
| ARP | Address resolution protocol. It finds the MAC address that goes with an IP address. | Who has 10.0.12.1? |
The cells students fill in are left blank in this preview.
Put the parcel jobs in order
Have a go
Put the parcel jobs in order
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Why use layers?
A layer does one job and talks to the layer above and below it. You can change one layer and leave the rest as it is.
For example, you can change from a cable to Wi-Fi. Only the bottom layers change. Your web browser does not notice.
The seven OSI layers
| Layer | Name | Its job | Examples |
|---|---|---|---|
| 7 | Application | Gives network services to the program you use | HTTP, HTTPS, FTP, SMTP, DNS |
| 6 | Presentation | Changes the format of data. Handles encryption and compression (making data smaller) | HTTPS (the S means encrypted) |
| 5 | Session | Starts, keeps and ends a conversation between two programs | Keeping a video call connected until you hang up |
| 4 | Transport | Breaks data into parts and delivers them safely | TCP |
| 3 | Network | Gives IP addresses and chooses the route across networks | IP, OSPF |
| 2 | Data link | Moves data between two devices that are joined by one cable or one radio link. Uses MAC addresses | Ethernet, Wi-Fi, switches |
| 1 | Physical | Sends the 1s and 0s as electricity, light or radio | Ethernet cables, Wi-Fi radio, USB |
The cells students fill in are left blank in this preview.
A way to remember the order
data-callouts.tip.label
From layer 7 down to layer 1: "All People Seem To Need Data Processing". Application, Presentation, Session, Transport, Network, Data link, Physical. You can make your own sentence.
Quick check: the layers
Have a go
Quick check: the layers
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.