This hypothetical reference circuit is 5000 km long and applies to various types of carrier systems on coaxial cable and radio-relay systems, specially designed for very long international circuits. It has, for each direction of transmission, a total of: one pair of channel modulators which includes translation from the audio-frequency band to the basic group and vice versa; three pairs of group modulators, each pair including translation from the basic group to the basic supergroup and vice versa; six pairs of supergroup modulators, each pair including translation from the basic supergroup to a higher order modem and vice versa; twelve pairs of higher order modulators, each pair providing the necessary modulation stages to and from the line frequency.