In message <9708102009.AA01560@reliant.uswc.uswest.com>, Kyle Platts writes:
[...]
> There are three ways to ensure ones density. First, there is MU-255
> PCM encoding (used in voice).
>
> The second is ZCS (Zero Code Suppression). In ZCS, the least
> significant (eighth) bit of every time slot is set to a 1 (this only
> allows 56k for data).
>
> The third is B8ZS (Bipolar with 8 Zero Substitution). B8ZS allows
> all 64k of a timeslot to be used by data without regard to the
> number of zeros in the data. This is acheived by substituting a
> special pattern for an all zero word. The special pattern is
> 00011011. How can the recevier determine whether the transmitter
> actually sent 00011011 or the special B8ZS pattern representing
> 00000000? The solution is to use deliberate violations in the
> alternating polarity scheme to mark the B8ZS pattern. These bipolar
> violations occur in bits 4 and 7 of the B8ZS word.
[...]
There is a fourth method, which is used for supporting the ISDN D channel
and SS7 A-links over AMI lines. It's called "inverted HDLC." The idea is
that since normal HDLC data cannot have a run of more than 6 ones in a row
(the frame mark is 01111110, and no legal code has more ones), if you invert
each bit you get a stream which has no more than 6 zero bits in a row, which
is sufficient ones-density for an AMI line.
This allows you to carry 64K sync data on a timeslot of a line without
requiring B8ZS support.
I don't know if anyone is using this for transport of user-level data, though.
There's no technical reason it couldn't be used this way, but it is probably
not worthwhile given the availability of B8ZS lines.
(And, of course, it should be noted that T1 lines are full-duplex. Always.)
--- James Carlson <carlson@xylogics.com>, Prin Engr Tel: +1 508 916 4351 Bay Networks - Annex I/F Develop. / 8 Federal ST +1 800 225 3317 Mail Stop BL08-05 / Billerica MA 01821-3548 Fax: +1 508 916 4789