Data Communications Networking Devices - 4th Ed.pdf

174 __________________________________________________________ WIDE AREA NETWORKSshifted upward in frequency by a ®xed amount of frequency. This frequencyshifting places the voice conversation into a prede®ned channel of the FDMmultiplexed circuit. At the opposite end of the circuit, another FDM demultiplexesthe voice conversations by shifting the frequency spectrum of each conversationdownward in frequency by the same amount of frequency in which it waspreviously shifted upward.As previously mentioned, the primary use of FDM equipment by communicationscarriers was to enable those carriers to carry a large number of simultaneousvoice conversations on a common circuit routed between two carrier of®ces. Theactual process for allocating the bands of frequencies to each voice conversation hasbeen standardized by the ITU. ITU FDM recommendations govern the channelassignments of voice multiplexed conversations based upon the use of 12, 60 and300 derived voice channels.ITU FDM recommendationsThe standard group as de®ned by ITU recommendation G.232 occupies thefrequency band from 60 to 108 kHz. This group can be considered as the ®rst levelof frequency division multiplexing and contains 12 voice channels, with eachchannel occupying the 300 to 3400 Hz spectrum shifted in frequency.Figure 2.2 Standard ITU FDM groups. ITU FDM recommendations govern the assignmentof 12, 60, and 300 voice channels on wideband analog circuits