[Asterisk-Users] Fun with Echo

[Rich Adamson]

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.