Steve Underwood wrote:
Rich Adamson wrote:
Eric "ManxPower" Wieling wrote:
The number of taps the EC has to deal with is the delay on the PSTN
side. I can't imagine echo is more than a few ms in modern TDM
networks. This latency has NOTHING to do with VoIP latency, since
the echo must be canceled BEFORE it gets to the VoIP side of things.
Not so sure about that.
We've got four analog pstn lines from Alltel; three from one CO and a
fourth from a second CO (Centrex). Both CO's are relatively new
switches (last ten years), and both use some form of remote line
concentrator connected via fiber.
When using the X100P (and later the TDM04b), Mark had to implement
echotraining=800 as the pstn line had not settled down in the first
200 milliseconds to allow the EC to train. Seemed rather unusual at
the time, but he was actually logged into our system and played with
that delay (before it became a part of the code).
After the echotraining=800, things were significantly better and I
played with each of the ECs (KB, MG2, Aggressive, etc) to find
something that would train up in a reasonable fashion.
Now, when using a A200D and sangamo's utilities, I see the echo tail
is roughly 38 milliseconds. All four lines (from two COs) appear the
same.
I'm not sure what Alltel is using for a remote line concentrator, but
it almost appears they are doing multiple analog-digital conversions
before hitting the actual CO.
They wouldn't do multiple analogue-digital conversions - it would be
expensive, pointless, and hurt voice quality. What they may have is a
DSL type of link introducing more than 20ms of latency. Remember,
although the various forms of DSL are mostly used for IP, they are
actually ATM channels. If you have that much latency, and then some echo
from the analogue stage at the phone you call, you would see tails of
maybe 40ms for local calls. Obviously long distance/international could
be a lot longer for the remote end part of the echo.
Yup, don't know what they are doing internally, but would love to sit
down with someone that actually knows. Most of their engineering
functions have moved to Little Rock Arkansas (about 500 miles away). All
I know for sure is what I can "see" from here, and what I see is not
cool from a s/w ec perspective.
R.
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