Re: (PM) AMI vs B8ZS

Ed Schulz (edschulz@lucent.com)
Tue, 30 Mar 1999 15:20:53 -0500

Jake Messinger wrote:
>
> On Tue, 30 Mar 1999, Ed Schulz wrote:
>
> :See http://www.ttc.com/library/pdfs/t1_tn.pdf , page 15.
>
> Thanx for the info. I suppose I should say in either signalling, you are
> being "robbed" of bits because of the requirement of seeing at least 1 1 in
> any 8 bit sequence. B8ZS guarantees that you meet the 1's density
> requirement of the T1 standard.

You're right about B8ZS, but I'm still not sure what you're trying to say
about being "robbed of bits." Even with AMI line coding for mu law voice,
the ones density requirement is satisfied simply by omitting the all-zeros
code. The mu law coding doesn't change when the line uses B8ZS. In either
case, 255 of the 256 possible octets may be used, provided that the FCC
power limits aren't exceeded.

The term "robbed bit signaling" has a specific meaning that is independent
of whether the line uses AMI/B8ZS or D4(SF)/ESF. Using "robbed bit" in
the context of T1 to mean anything other than RBS is quite confusing (to
me, anyway.)

If you use ISDN PRI access, then you must use B8ZS for clear-channel data,
and that access link will not have the RBS impairment. But even then,
V.90 calls to your PortMaster might traverse multiple hops with RBS.

-- 
Ed Schulz
edschulz@lucent.com
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