12-channel carrier system

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12 channel carrier system is a type of Frequency-division multiplexing system. In this system twelve voice channels are multiplexed in a High Frequency Carrier and passed through balanced pair trunk lines similar to those used for Voice Frequency connections.

The Bell System N1 Carrier of the 1950s was the most used twelve channel carrier system, using double sideband operation. N2 was similar but transistorized, while N3 used the same frequency plan but single sideband doubled the channels to 24. Repeaters were spaced approximately 6 miles (10 km) apart, depending on wire gauge. Each repeater either received from both directions at a low frequency band and sent in both directions at a higher band, or vice versa. This frequency frogging, inverting the whole spectrum, minimized delay distortion.