The Data Link Layer uses the services of the Physical Layer to send and receive bits over communication channels.

First, let’s recap the layers we explained in the previous lecture:

In this lecture we will focus on the Data Link layer, that is, sending the data from the source to the first router reliably. Functions:

• Providing a well-defined service interface to the Network Layer.
• Groups bits into frames.
• Dealing with transmission errors
• Regulating data traffic.

There are 2 purposes of the Data Link layer:

• Error control: Deal with bit flips and lost data
• Channel allocation (contention): Deciding how to share medium.

In this lecture we will focus on the first purpose. Next lecture will be about the second purpose.

Frames

If you have a physical layer, this layer can send bits (bit-by-bit). However, only with this we cannot detect errors. We need extra information for reliability. Therefore, what we do is grouping bits and adding information per group/unit.

The name of the unit, including additional bits: Frame.

Structure of a frame: