Read the introduction to chapter 6 and the beginning of section 6.1, "Principles", on pages 211–212. Stop at section 6.1.1 "Framing", which you will read below.
The datalink layer uses the service provided by each of the different technologies found in the physical layer to send and receive bits between directly connected devices. The datalink layer receives packets from the network layer. What are the two main services provided by the datalink layer? Why are these services necessary?