38 Point-to-Point Protocols and Links Chap.2 In dealing with the physical layer in Section 2.2,we discuss both the actual com- munication 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 is another form of bit pipe in which the sending DLC module supplies bits synchronously to the modem when it has data to send,but supplies nothing when it has no data.The sending modem sends no signal during these idle intervals,and the receiving modem detects the idle intervals and releases nothing to the receiving DLC module.This somewhat complicates the receiving modem, since it must distinguish between 0,1,and idle in each time interval,and it must also regain synchronization at the end of an idle period.In Chapter 4 it will be seen that the capability to transmit nothing is very important for multiaccess channels. A final form of virtual bit pipe is the asynchronous character pipe,where the bits within a character are sent at a fixed rate,but successive characters can be separated by Network Network layer layer Packets Data link Data link control control Frames Virtual synchronous unreliable bit pipe Physical Physical interface interface Communication link Figure 2.1 Data link control (DLC)layer with interfaces to adjacent layers.38 Point-to-Point Protocols and Links Chap. 2 In dealing with the physical layer in Section 2.2, we discuss both the actual com￾munication 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 is another form of bit pipe in which the sending DLC module supplies bits synchronously to the modem when it has data to send, but supplies nothing when it has no data. The sending modem sends no signal during these idle intervals, and the receiving modem detects the idle intervals and releases nothing to the receiving DLC module. This somewhat complicates the receiving modem, since it must distinguish between 0, 1, and idle in each time interval, and it must also regain synchronization at the end of an idle period. In Chapter 4 it will be seen that the capability to transmit nothing is very important for multiaccess channels. A final form of virtual bit pipe is the asynchronous character pipe, where the bits within a character are sent at a fixed rate, but successive characters can be separated by Packets Frames Virtual synchronous unreliable bit pipe Communication link Figure 2.1 Data link control (DLC) layer with interfaces to adjacent layers