Second Edition Data Networks

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 provide a sufficient overview for those lacking this background and provide a review and perspective for those with more background. In dealing with the physical layer in Section 2.2, we discuss both the actual communication channels used by the network and whatever interface modules are required at the ends of the channels to transmit and receive digital data (see Fig 2.1). We refer to these modules as modems (digital data modulators and demodulators), although in many cases no modulation or demodulation is required. In sending data, the modem converts the incoming binary data into a signal suitable for the channel. In receiving, the modem converts the received signal back into binary data. To an extent, the combination of mo-dem, physical link, modem can be ignored; one simply recognizes that the combination appears to higher layers as a virtual bit pipe. There are several different types of virtual bit pipes that might be implemented by the physical layer. One is the synchronous bit pipe, where the sending side of the data link control (DLC) module supplies bits to the sending side of the modem at a synchronous rate (i.e., one bit each T seconds for some fixed T). If the DLC module temporarily has no data to send, it must continue to send dummy bits, called idle fill, until it again has data. The receiving modem recovers these bits synchronously (with delay and occasional errors) and releases the bits, including idle fill, to the corresponding DLC module. The intermittent synchronous bit pipe …