In this chapter, we discuss the second layer of the OSI model, namely, the data link layer. This layer handles the transfer of data between the ends of a physical link it is responsible for transferring data from the network layer on the source machine to the network layer on the destination machine. We begin the chapter with an overview of the data link layer and provide information about IEEE, which developed data link layer protocols that serve as the basis for various LAN technologies such as Ethernet and token ring. In the remaining part of the chapter, we examine the data link layer from IEEE's perspective. We also include at the end of the chapter information about the concepts of data prioritization and quality of service (QoS), which are integral to the delivery of time-sensitive data such as real-time voice and video traffic. An outline of the terms and concepts we define and discuss follows:

• Data Link Layer Overview and IEEE's Perspective (Questions 1 6)

• Framing (Questions 7 11)

• Ethernet/802.3 Frames (Questions 12 15, 44 45)

• Flow Control and Flow Control Protocols (Questions 16 27)

• Error Control (Questions 28 43)

• MAC Sublayer (Questions 46 52)

• Random Access and Token Passing Protocols (Questions 53 58)

• Data Prioritization and Quality of Service (QoS) (Questions 59 69)