upergroup is translated in frequency to the supergroup frequency band by mixing with the appropriate carrier frequencies. The carrier frequencies are 420 kHz for group 1, 468 kHz for group 2, 516 kHz for group 3, 564 kHz for group 4, and 612 kHz for group 5. In the mixing process, in each case, the difference is taken (i.e., the lower is selected). This frequency translation process is illustrated in Figure 4.20. 4.5.2.4 Line Frequency. The band of frequencies that the multiplexer applies to the line, whether the line is a radiolink, wire pair, or fiber-optic cable, is called the line frequency. Some texts use the term high frequency (HF) for the line frequency. This is not to be confused with HF radio, which is a radio system that operates in the band of 3 MHz to 30 MHz. The line frequency in this case may be the direct application of a group or supergroup to the line. However, more commonly a final frequency translation stage occurs, particularly on high-density systems. 15 An example of line frequency formation is illustrated in Standard Carrier groups frequencies (see CCITT Rec. G.241) Group 1 Figure 4.20 Formation of the standard CCITT supergroup. The vertical arrows show the frequencies of the group level regulating pilot tones. (From CCITT Rec. G.233, courtesy of ITU-T Organization, Ref. 6.) "High-density" meaning, in this context, a system carrying a very large number of voice channels. TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK Carrier frequency 312 552 312 552 312 s 552 3124 552 312 s 552 312 552 Supergroup Figure 4.21 Makeup of the basic CCITT 15-supergroup line frequency assembly. (From CCITT Rec. G.233, courtesy of the ITU-T Organization, Geneva, Ref. 6.) Figure 4.21. This figure shows the makeup of the basic 15-supergroup assembly. Its capacity is (15 X 60) 900 voice channels (Ref. 7). 4.5.3 Pilot Tones Pilot tones are used to control level in FDM systems. They may also be used to actuate maintenance alarms. In Figures 4.20 and 4.21, the pilot tones are indicated by the vertical lines wit