DSU/CSU units


John A. Shriver (jas@monk.proteon.com)
Wed, 23 Sep 87 22:37:55 EDT


(This may be a little rusty, since I'm not near my PUB's, but I think
I've got the spirit, if not the letter, right.)

T1 coding is trilevel, with levels I will call -1, 0, and +1. A bit
of "zero" is always represented as 0. A bit "one" of will will be
represented as -1 if the last "one" was +1, and as +1 if the last
"one" was -1.

T1 uses regenerative reclocking repeaters along the line. They
require flux transitions to ge the clock out of. If they don't get
enough, they lose it. To get transitions, you've got to send some
"one"'s. This is why the rule of no more than 7 (?) "zero"'s in a
row.

B8ZS avoids this by putting a code violation in the bit cell for the
8th "zero". Thus, if the last "one" was -1, the 8th "zero" will be
-1, and if the last "one" was +1, the 8th "zero" will be +1.

This offends Central Offices, who constantly check for code
violations. This one way they decide the line is sick. The nation
may not be fully B8ZS safe for another decade.

Many CSU/DSU's offer to run without B8ZS. They use different schemes
to put your raw sync data on the T1 line without needing B8ZS to
provide data transparency. Their price is usually correlated with how
many bits they waste doing this, and how high a clock rate you get at
the sync (RS-449) interface.



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