N AND SWITCHING: CORNERSTONES OF A NETWORK medium). The modulation plan usually is a diagram showing the necessary mixing, local oscillator mixing frequencies, and the sidebands selected by means of the triangles described previously in a step-by-step process from voice channel input to line frequency output. The CCITT has recommended a standardized modulation plan with a common terminology. This allows large telephone networks, on both national and multinational systems, to interconnect. In the following paragraphs the reader is advised to be careful with terminology. 4.5.2.3.2 Formation of the Standard CCITT Group. The standard group, as defined by the CCITT, occupies the frequency band of 60 kHz to 108 kHz and contains 12 voice channels. Each voice channel is the nominal 4-kHz channel occupying the 300-Hz to Channel 1 108 kHz Channel 2 104 kHz Channel 3 100 kHz Channel 4 96 kHz Channel 5 92 kHz 108 kHz Channel 6 88 kHz Channel 7 84 kHz Channel 8 80 kHz Channel 9 76 kHz Channel 10 72 kHz Channel 11 68 kHz Channel 12 64 KHz Figure 4.19 Formation of the standard CCITT group. 4.5 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. 4.5.2.3.3 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 s