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From: asuvax!mothra!bakerj@ncar.ucar.edu (Jon Baker)
Newsgroups: comp.dcom.telecom
Subject: Re: AMI on T1 Lines
Message-ID: <12592@accuvax.nwu.edu>
Date: 24 Sep 90 22:34:17 GMT
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In article <12480@accuvax.nwu.edu>, RAF@cu.nih.gov (Roger Fajman)
writes:

>I would appreciate a short definition of exactly how AMI works on T1
>lines.  Now I would like
>to compare AMI to B8ZS for meeting one's density requirements on our
>clear channel T1s, but can find only the definition of B8ZS in the
>course notes.  AMI is mentioned as being less preferable, but is not
>defined.

 From ANSI T1.403-1989, Carrier-To-Customer Installation - 
 DS1 Metallic Interface :

    AMI, Alternate Mark Inversion:  A pseudoternary signal, conveying
    bindary digits, in which successive "ones" (marks, pulses) are of
    alternating, positive (+) and negative (-) polarity, equal in
    amplitude, and in which a "zero" (space, no pulse) is of zero
    amplitude.

In AMI, one's density is maintained by converting a zero sample (i.e.
eight consecutive zero-bits) into a '2'.  Thus, clear-channel is not
possible.  This method of maintaining one's density is called Zero
Code Suppression (ZCS).

    B8ZS: Bipolar with 8-Zero Substitution.
    A code in which eight consecutive "zeros" are replaced with the
    sequence 000+-0-+ if the preceding pulse was +, and with the
    sequence 000-+0+- if the preceding pulse was -, where +
    represents a positive pulse, - represents a negative pulse,
    and 0 represents no pulse.

B8ZS basically inserts deliberate bi-polar violations when
transmitting a zero; the far-end is expected to detect the deliberate
BPV's and unencode them into the zero-sample without reporting an
actual BPV.


JB