GCSE · Computer Science · AQA · Spec 8525
Link layer (TCP/IP)
Your laptop sends a file to the printer across the room. One layer asks a tiny question first: who is on the other end of this one connection?
Computer Science · Structure
One small local network, link by link
Every line below is one link. Tap each device to see what its interface is doing, and read what each link is made of.
Tap any part of the diagram to see what it does.
Computer Science · Networks
Whose job is it?
Does each item belong to the link layer (the lowest layer) or to the internet layer (the layer above it)?
Still to sort
Link layer (lowest) (0)
Passing data across one direct connection on a local network.
Where the line is: If it is about one connection between two directly connected interfaces, it is the link layer.
Internet layer (above it) (0)
Getting data across more than one network.
Where the line is: If the data has to cross several networks, that is not the link layer's job any more.
Two layers, seven items. Decide where each one belongs before you look at the reason.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
The lowest layer, and a job exactly one connection long.
What you need to know
- TCP/IP is built from four layers, and the link layer is the lowest one.
- The link layer passes data between devices that are directly connected on the same local network.
- A direct connection can be a cable or a wireless connection.
- Each network interface on a link is identified by a MAC address.
Have a goA laptop has one interface that uses a cable and another that is wireless. What lets the link layer tell the two interfaces apart?
Each interface has its own MAC address.
A MAC address identifies the network interface, not just the whole device, so two interfaces mean two addresses.
- A network interface card (NIC) is the hardware that connects a device to the network.
- NICs, switches, Ethernet and Wi-Fi all work at the link layer.
Have a goSam says: 'Wi-Fi has no wire, so it can't be a proper link-layer thing.' Sam is confidently wrong. What would you tell Sam?
A direct connection can be wireless as well as a cable, and Wi-Fi works at the link layer.
The link layer is about passing data across a direct connection, and that connection is allowed to be wireless.
- One link is one direct connection between two interfaces, and that is as far as the link layer's job reaches.
- Getting data across several networks is the job of the internet layer, the layer above the link layer.
Have a goA file has to travel across three separate networks to reach a server. Can the link layer finish that whole journey on its own?
No. It only covers one direct connection at a time.
Crossing several networks is the internet layer's job, so the link layer's reach stops at the end of one link.
The big picture
The link layer is the lowest of the four TCP/IP layers. Its whole job is passing data across one connection, between two directly connected interfaces on the same local network, over a cable or a wireless connection. Each interface is identified by a MAC address. Getting data across several networks is the internet layer's job.
Key points
Worked example
Problem
A phone is joined to a switch by a wireless connection, and a printer is joined to the same switch by a cable. All three are on one local network. The phone sends a photo to the printer. How many links does the data cross, and what is the link layer doing on each?
⚠ Watch out
Thinking the link layer is just the cable, or that MAC addresses carry data across the internet. The link layer passes data across one direct connection on a local network, cable or wireless, and crossing networks is the internet layer's job.
Memory hook
Link layer = one link. One cable or wireless hop, two interfaces, a MAC address at each end. Then it is the internet layer's turn.
Check yourself
A friend says their MAC address is how a video reaches them from a server several networks away. Which layer's job is that crossing, and why can't the link layer do it on its own?
Flashcards
(9)Which is the lowest of the four TCP/IP layers?
What is the link layer's job?
What kinds of connection can a link use?
What identifies a network interface at the link layer?
What is a NIC?
Name two protocols that work at the link layer.
Which hardware works at the link layer?
True or false: the link layer is just the cable.
Which layer gets data across several networks?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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