John A. Shriver (firstname.lastname@example.org)
Thu, 24 Sep 87 11:18:16 EDT
Now that I'm at my office, I see I goofed a little on B8ZS. There are
two complex patterns that replace the 8 "zero"'s, depending on the
polarity of the last "one". If the last "one" was +1, then you send
0,0,0,+1,-1,0,-1,+1. If the last "one" was -1, you send
0,0,0,-1,+1,0,+1,-1. The fourth and seventh positions are bipolar
violations. This is better than what I described before, as it gives
you a lot more flux changes (4 vs. 1).
I beleive the eigth bit in voice use is used for signalling
(on-hook/off-hook), but not to provide one's density. It appears that
the one's density is provided by having the PCM voice not have any
voltage that comes out as seven "zero"s. (Or at least making this
However, the 56KB of DDS service definitely comes from stuffing that
eighth bit as a "one". This is also what the cheaper DSU/CSU's do.
For further reference, see AT&T PUB 62411.
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