INTRODUCTION TO MULTIPLEXING
3400-Hz spectrum. The group is formed by mixing each of the 12 voice channels with
a particular carrier frequency associated with each channel. Lower sidebands are then
selected, and the carrier frequencies and the upper sidebands are suppressed. Figure 4.19
shows the preferred approach to the formation of the standard CCITT group. It should
be noted that in the 60-kHz to 108-kHz band, voice channel 1 occupies the highest
frequency segment by convention, between 104 kHz and 108 kHz. The layout of the
standard group is illustrated in Figure 4.19. Single sideband suppressed carrier (SSBSC)
modulation techniques are utilized universally.
Formation of the Standard CCITT Supergroup
. A supergroup contains five
standard CCITT groups, equivalent to 60 voice channels. The standard supergroup, before
further translation, occupies the frequency band of 312 kHz to 552 kHz. Each of the five
groups making up the supergroup 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.
. 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.
An example of line frequency formation is illustrated in
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.