It should be noted that multiplexers are normally required at both ends of a shared tele- phone line, thus allowing the eight channels multiplexed at one line to be demultiplexed at the other, thus providing a transparent con- nection between each of the computer ports to each of the remote terminals. The word transparent means that the multiplexer does not interrupt the flow of data and EIA RS-232 interface signals, which New York Denver Remote terminals Central computer New York Denver Remote terminals Central Multiplexer computer Multiplexer Fig. 2-4. A typical multiplexer system. 24 TELECOMMUNICATIONS normally pass through the cable between the computer port and the attached modem or terminal. When multiplexers are installed, neither the computer nor the mo- dems located in Denver know the multiplexer system is being used, whether the telephone lines attaching the terminals to the mulitplexer are leased lines or dial-up lines. As a result, neither terminal equip- ment nor computer software need to be changed when multiplexer systems are installed to reduce telephone line rental or long-distance dial charges. 2.7 MULTIPLEXER TECHNOLOGY Multiplexers combine multiple communications paths by one of two methods: frequency division or time division. Each technique com- bines multiple paths on a single link in a different manner to satisfy diverse needs. Frequency division multiplexing (FDM) is the earliest and least so- phisticated form of multiplexing. Applicable to low-speed telegraph and data transmissions as well as voice, FDM divides the bandwidth of a voiceband line into multiple frequency bands or derived chan- nels. A narrow band of frequencies called a guard band is required to separate adjacent channels to prevent signal interference between channels. Data transmission is FSK modulated. FSK (frequency shift keying) employs a pair of frequencies to correspond to a binary one and binary zero. The sending and receiving multiplexer channels can use the same set of frequencies for full-duplex operation over fo