The OSI model: seven layers

Sending a birthday present

Think about posting a birthday present to a friend in another town. Lots of people help, and each does one job:

1. You choose the present and write the card.
2. You wrap it, so nobody can peek.
3. You write part 1 of 2 on the box, because there’s a second parcel coming.
4. You write for Sam and ask for recorded delivery, so you’ll know it arrived.
5. You write the full street address: town, street, house number.
6. The post office puts a label on it for the local van: next stop, the sorting office.
7. The van drives it down the road.

Nobody needs to know how the others do their job. The van driver doesn’t care what the present is; you don’t care which road the van takes.

in one sentence Sending something is lots of small jobs, and each helper only does their own.

Layers

Networks work the same way. Sending one message takes lots of jobs, so they’re split into layers. A layer is one job in a stack of jobs, like one floor of a building. Each layer only talks to the floor just above it and the floor just below it.

The most famous way of splitting the jobs is called the OSI model. A model is a simple picture of how something works, the way a map is a simple picture of a town. OSI stands for Open Systems Interconnection, which means a shared plan so computers made by different companies can connect. It has seven layers, and network engineers use the numbers all the time: “it’s a layer 1 problem” means check the cable.

in one sentence The OSI model splits networking into seven layers, each with one job.

The seven layers

Here they are, matched to the birthday present. Some words in the last column are new; each one gets its own lesson later, so just notice where it sits for now.

A port number is a number that says which program on a computer a message is for, like the name of the person in a house who should open the parcel. Lesson 5.1 covers ports, and the common numbers, in full.

Remember the layers from the bottom up with Please Do Not Throw Sausage Pizza Away: Physical, Data link, Network, Transport, Session, Presentation, Application.

in one sentence From the bottom: Physical, Data link, Network, Transport, Session, Presentation, Application.

Wrapping and unwrapping

On the way out of a computer, a message starts at the top, layer 7, and goes down. Each layer sticks a small label on the front, called a header, with the information that layer needs, like the address label on a parcel. Putting a message inside another layer’s wrapping like this is called encapsulation (it means putting into a capsule).

At the bottom, layer 1 sends it as bits. A bit is the smallest piece of computer data there is: a 1 or a 0, like a light switch that’s on or off. Down a cable, bits are pulses of electricity.

On the way in to the other computer, it goes up the layers, and each layer reads its own label and takes it off, until layer 7 has the original message.

The message has a different name depending on where it is: segment at layer 4, packet at layer 3, frame at layer 2 (a frame is a packet in its local-delivery envelope), and bits at layer 1. They’re all the same message, just wrapped differently.

in one sentence Going down, every layer adds a label; going up the other side, every layer takes its label off.

One honest note

The internet doesn’t really use all seven layers separately. It uses a simpler plan called TCP/IP (named after two of its rules: Transmission Control Protocol and Internet Protocol; a protocol is just an agreed set of rules, like everyone agreeing to write the address on the front of an envelope). TCP/IP squashes layers 5, 6 and 7 into one. People still use the OSI numbers when they talk, so learn them; just don’t be surprised when, in the DHCP lesson, layer 7 comes straight after layer 4.

in one sentence The real internet squashes layers 5 to 7 together, but everyone still talks in OSI layer numbers.