A DS level and framing specification for digital signals in the North American digital transmission hierarchy. A DS1C signal uses 48 PCM channels and has a transmission rate of 3.15 Megabits per second, twice that of DS1. DS1C uses two DS1 signals combined and sent on a 3.152 megabit per second carrier which allows 64 kilobits per second for synchronisation and framing using "pulse stuffing". The channel 2 signal is logically inverted, and a framing bit is stuffed in two out of three code words, resulting in 26-bit information units. The channels are interleaved and then scrambled by the addition modulo 2 of the signal with the previous bit. Finally the bit stream is combined with a control bit sequence that permits the demultiplexor to function by preceding each 52 bits with one DS1C framing bit. A series of 24 such 53-bit frames forms a 1272-bit "M-frame".