r "0"). The pulses "stepped" the switches to make the signal (essentially a delay in the transmission of the the interconnection. Trunk lines connected end offices. signal) will have little or no effect on voice quality. But As the number of end offices grew, it quickly became such errors can cause problems for data transmission. necessary to organize the phone system into a hierarchy; When the telephone network was an analog network, too many trunk lines would have been required to electromechanical switches could inject a certain amount interconnect all of the end offices in even a metropolitan of noise into circuits. Other instruments in the trans- area, let alone the whole country (about 20,000 end mission path, such as multiplexers, could corrupt the offices are in use today). By connecting each end office to signal further. At lower data rates, say 300 or 1200 bps, a toll center and connecting toll centers together with modems can correctly transmit data with few, if any, trunk lines, far fewer trunks were required even though errors even when the line is noisy, but as data rates any subscriber could reach any other subscriber in the increase, noise causes more problems. As a practical Customer B Customer A Voice services T-1/FT-1 Customer C CLASS Customer D Backbone SWITCH FRAME SWITCH LARGE HUB Customer E Customer F MULTI-TENANT Figure 3.1-7 Voice communications network. CHAPTER 3.1 Data and voice traffic matter, in the analog network, data could not be trans- TDM architecture mitted reliably over long distances on switched lines at rates over 4800 bps. Therefore, it became common TDM replaced FDM. TDM is a scheme in which nu- practice for the telephone companies to "lease" lines to merous signals are combined for transmission on a single companies for continuous, unswitched use. A company communications line or channel. Each signal is broken up with two computer sites could, for example, lease a line into many segments, each having a very short duration. or a set of lines to interconnect the sites. TDM m