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".