Chapter 6: Link Layer and LANs

Kurose and Keith W. Ross examines the link layer, which operates directly below the network layer and is responsible for moving datagrams across individual communication links. The chapter begins by describing how network-layer datagrams are encapsulated into link-layer frames for transmission and how different links—wired, wireless, and hybrid—can use different link-layer protocols along an end-to-end path. It distinguishes between broadcast channels, where multiple hosts share the same medium (e.g., wireless LANs, satellite networks, hybrid fiber-coaxial cable), and point-to-point links. For broadcast channels, the authors detail multiple access protocols such as ALOHA, slotted ALOHA, CSMA, and CSMA/CD, explaining how they resolve collisions and share bandwidth. The chapter covers link-layer addressing using MAC addresses and explores the relationship between link-layer and network-layer addressing, including the role of ARP (Address Resolution Protocol) in mapping IP addresses to MAC addresses. It also explains error detection techniques like parity checks, checksums, and cyclic redundancy checks (CRC). The discussion extends to switched local area networks (LANs), covering Ethernet frame structure, physical topologies, and the operation of Ethernet switches, including self-learning and forwarding tables. The chapter concludes with an overview of link virtualization through technologies like VLANs, illustrating how the link layer enables efficient, reliable communication over diverse physical media.