Layer 2 — Data Link

What This Layer Does

Layer 1 can move bits, but it cannot tell one device from another. If four computers and a printer share a network, something has to make sure a job sent to the printer reaches the printer and not the computer next to it. That is the Data Link layer's job: delivering data to a specific device on the local network, meaning the group of devices connected together in one place, such as a home, an office floor, or a single Wi-Fi network.

To do this, the Data Link layer wraps the data in a frame. A frame is the data plus a small amount of extra information attached to the front and back of it. The front carries the address of the device the frame is for and the address of the device that sent it. The back carries a check value used to detect damage.

The addresses used here are MAC addresses. Every network interface is given one when it is manufactured, and it is normally written as twelve hexadecimal digits, such as 00:1B:44:11:3A:B7. A MAC address identifies a piece of network hardware. It does not describe where that hardware is, which is why it only works for local delivery — a subject Chapter 4 picks up.

The Data Link layer also checks its own work. The check value at the end of the frame lets the receiving device tell whether the frame arrived intact. A damaged frame is discarded. Note the limit of this: Layer 2 detects damage, it does not repair it. Deciding whether to ask for the data again is somebody else's job.

Common Examples

• Ethernet, the standard for wired local networks
• Wi-Fi, the standard for wireless local networks
• Switches
• Wireless access points
• Network interface cards, which hold the MAC address

You may also hear about VLANs, which let one physical switch carry several separate logical networks, and spanning tree, which stops frames from looping forever between switches. Both are Layer 2 features. Both are subjects for a later book.

Simple Real-World Example

A desktop computer sends a document to a printer on the same home network. The computer builds a frame containing the print data, addressed from its own MAC address to the printer's MAC address, and sends it to the switch built into the home router.

The switch reads the destination MAC address, checks its internal table of which device is on which port, and forwards the frame out that one port only. The other devices on the network never see it.

        ┌──────────┐
        │  Laptop  │  (not the destination — receives nothing)
        └────┬─────┘
             │
   ┌─────────┴──────────┐
   │       Switch       │  reads destination MAC, forwards to one port
   └────┬──────────┬────┘
        │          │
  ┌─────┴────┐  ┌──┴──────┐
  │ Computer │  │ Printer │  <── frame delivered here
  └──────────┘  └─────────┘

What Can Go Wrong?

• Two devices are given the same MAC address. Rare, but it makes local delivery ambiguous and produces intermittent failures.
• The frame is damaged. The check value fails and the frame is discarded. A faulty cable or a noisy wireless environment can cause this repeatedly.
• The device is on the wrong Wi-Fi network or the wrong VLAN. It has a working connection and still cannot reach the devices it expects, because it is in a different classroom.
• A switch port is disabled or misconfigured. The cable is fine, the lights may even be on, and frames still go nowhere.

Remember This

Layer 2 delivers frames to devices on the local network using MAC addresses.