12 0 0 Time at A一-+ Node A Ack Node 8 Time at B- Packet 0 or 17 Packet O Point-to-Point Protocols and Links 2.1 INTRODUCTION This chapter first provides an introduction to the physical communication links that constitute the building blocks of data networks.The major focus of the chapter is then data link control (i.e.,the point-to-point protocols needed to control the passage of data over a communication link).Finally,a number of point-to-point protocols at the network,transport,and physical layers are discussed.There are many similarities between the point-to-point protocols at these different layers,and it is desirable to discuss them together before addressing the more complex network-wide protocols for routing. flow control,and multiaccess control. The treatment of physical links in Section 2.2 is a brief introduction to a very large topic.The reason for the brevity is not that the subject lacks importance or inherent interest,but rather,that a thorough understanding requires a background in linear system theory,random processes,and modern communication theory.In this section we pro- vide a sufficient overview for those lacking this background and provide a review and perspective for those with more background. 372 Node A Node B Packet 0 Point-to-Point Protocols and Links 2.1 INTRODUCTION Time at B --------- This chapter first provides an introduction to the physical communication links that constitute the building blocks of data networks. The major focus of the chapter is then data link control (i.e., the point-to-point protocols needed to control the passage of data over a communication link). Finally, a number of point-to-point protocols at the network, transport, and physical layers are discussed. There are many similarities between the point-to-point protocols at these different layers, and it is desirable to discuss them together before addressing the more complex network-wide protocols for routing, flow control, and multiaccess control. The treatment of physical links in Section 2.2 is a brief introduction to a very large topic. The reason for the brevity is not that the subject lacks importance or inherent interest, but rather, that a thorough understanding requires a background in linear system theory, random processes, and modem communication theory. In this section we pro￾vide a sufficient overview for those lacking this background and provide a review and perspective for those with more background. 37