e high attenuation requires high amplification in the exchanges, the noise that accompanies the analog signal is also highly amplified, degrad- ing quality. Chapter 1 Analog Digital/analog Digital/analog Analog Figure 1-11 terminal conversion conversion terminal Analog local loop. Analog Digital Analog local loop transmission local loop Multiplexing limitations are a real drawback for the network and helped to spur migration away from FDM. The advent of new high-bandwidth transmission media makes multiplexing essential for efficient utilization of all the bandwidth available. Time Division Multiplexing (TDM), now the most widely employed multiplexing mechanism in digital transmissions, is easier to implement and cheaper to run than its predecessor. In brief, here is how TDM systems work. Connecting exchanges allocate time slots between them to certain channels (see Figure 1-12). Only during that time slot can a switch transmit data from the allocated channel. Hence, information belonging to the first channel is transmitted in the first time slot. The second time slot is assigned to the next channel, and SO on. When all channels have had a time slot-an opportunity to transmit-the cycle begins over with the first channel. The receiving side is synchronized with the sender side, SO it can mark the end of one time slot and the begin- ning of another. TDM too requires the use of buffers in the exchanges. An exchange has to store the information ahead of transmission in anticipation of the proper time slot's arrival. This, as explained previously, adds some small, but not onerous, delay to the switching process. Note that the predominant transmission technique for telephony pro- vides 64-kbps channels, exactly the same data rate as the output of PCM codecs. This makes these channels perfectly suitable for the digital trans- mission of PCM-encoded voice. Digital Signalling Systems The appearance of digital transmission made upheaval in signalling sys- tems almost inevitable. Consider the fact that analog systems use ton