signal with a much wider bandwidth than was required subscriber (the phone user) and an exchange or end office. for voice transmission. A technique called FDM was de- (Before digital dialing, end offices had names such as veloped that allowed a trunk line with only two pairs of Prospect or Elgin, SO telephone numbers began with an wires to carry many voice conversations (many voice abbreviation of the end office name, for example, channels) simultaneously. Because of limitations in the PR6-6178 or EL3-1978.) An enterprise could install technology first developed for FDM, a voice channel was a Private Branch Exchange (PBX) to switch calls within standardized to be 4000 Hz of bandwidth. The additional the enterprise. PBXs were connected to local offices, 600 Hz between 3400 Hz and 4000 Hz was necessary to which connected to trunks. separate the channels on a multiplexed line SO that one The electrical signal varied in frequency from about channel would not interfere with the next. 600 to 3400 hertz (Hz, cycles per second). Although humans can hear frequencies from about 20 to 20,000 Hz, most speech energy is concentrated in the range of The addition of data 600-3400 Hz. Early telephone end offices accomplished switching Today's digital telecommunications networks evolved out with human operators, but in the 1930s switching was of the analog switched long-distance networks that were automated. Before digital dialing, electromechanical created in the early part of the 20th century. The rotary switches (in smaller, especially rural, end offices) telephone system was designed for voice communica- and crossbar switches connected subscriber lines. tions (see Figure 3.1-7 Leased Lines diagram). Some Telephones had rotary dials that generated electrical errors can be introduced into a voice signal without pulses, as many pulses as the number that was dialed causing problems for the people at either end of the (one pulse for "1," two pulses for "2," and SO on through conversation. For example, a shift in the phase of ten fo