DDS-PRI is full duplex 56k link, No framing in loop data
so loop speed is also 56k. Data stream:
--d1 d2 d3 d4 d5 d6 d7--
DDS-SC is also full duplex 56k link, requires framing bits
to distinguish between primary and secondary channels.
--d1 d2 d3 d4 d5 d6 d7 F C/S--
where F is framing bit and C/S for secondary channel as
well as used for control bit.
With these two extra bits the loop speed is increased to
72k. primary channel rate is 56k.
DDS-CC is full duplex 64k link. Does not require framing
bit but uses a same format as SC, nineth bit position is
lumped with primary channel and not used for control.
Loop speed is 72k, Primary channel rate is 64k.
--d1 d2 d3 d4 d5 d6 d7 F d8--