Data Link Layer

The physical layer only moves bits. It does not care whether they arrive intact, and it does not decide who may transmit when. The job of the data link layer is to turn an unreliable physical connection into a reliable point-to-point link as far as the layers above are concerned.

This chapter is organized around three questions. First, how bits are organized into meaningful units — frames, and why the frame header carries so many control fields. Second, what happens when transmission goes wrong — error detection and correction, why CRC can detect but not correct errors, and how sequence numbers distinguish a lost frame from a duplicated one. Third, what happens when several devices compete to transmit — the core of medium access control: how Ethernet’s CSMA/CD handles a shared medium with “listen while transmitting plus collision detection”, and how switched Ethernet sidesteps the problem entirely.

The chapter closes by contrasting LAN and WAN protocols to show the difference in design philosophy between wide-area protocols such as PPP and HDLC, which assume a poor-quality link that must be negotiated, and Ethernet, which assumes the link is essentially reliable.

After this chapter you should be able to explain what Ethernet gave up and what it gained in moving from a shared medium to switched operation.