I'm not sure why this came up on ISDN material, since it is most often used
in T1's.
AMI is "alternate mark inversion," and it is a way of varying a continuous
pattern of 1's or 0's so that the receiving device can separate them out.
When you're pulling clock signal from the line, you are relying on a
relatively steady "pulse" to break out your bits. If a bunch of 1's comes
across, represented by a long period of positive voltage,
++++++++++++++++++ , the device has a hard time figuring out where to break
the bits out of that signal. What AMI does is vary the polarity of every
other 1, for instance +-+-+-+-+-+-+-. That allows discrete 1's to be
picked out of the signal.
But what happens when you have a bunch of zeroes? Same problem. Enter
B8ZS, Binary 8 Zeroes Suppression.
It becomes necessary if you are sending a long string of 0's across the
wire, which is often done by a lack of signal. What B8ZS does, is replace
a string of eight zeroes with a known pattern of voltage ( +-0-+ ) The
other device recognizes this, and says, "Oh, you must be sending eight
zeroes." This technique is called "maintaining the 1's density."
This is it in a rough explanation, don't quote me on this ;)
But the good news is that this is as much as you'll likely ever have to
know about B8ZS and AMI, unless your job involves desinging CSU/DSU's or
something.
Kenneth Mays
Sr. Network Analyst
AutoZone
Date: Mon, 21 May 2001 20:51:54 -0400
From: "Lists Wizard"
Subject: ISDN line transition and the linecode command [7:5353]
Hi group.
I am using the CISCO CIM for ISDN. There is talk about line transition and
line-level changes and how the linecode command prvent the loss of
synchronization. Can some one explain this better.
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