sentences:  N-2 was similar in heterodyning scheme, but in discrete transistorized "plug-in unit" architecture, while N3 used the same frequency plan but a scheme of using single sideband with a different voice channel on each side of the carrier, a technique first seen on the 16 channel Type "O" open wire short haul carrier of the 1950s.This doubled the capacity to 24 channels, the same as a basic digital Type T PCM carrier introduced in the late 1950s, which became the now-ubiquitous "T-1" of the digital world.Repeaters were spaced approximately 6 miles (10 km) apart, depending on wire gauge.