[Asterisk-Users] Will Echo problems EVER be solved, I'm scared

[Bruce Ferrell]

and just for REALLY bad form, responding to my own post with a postscript:

I had to have a lot of pictures drawn for me when I was learning this 20 
years ago :)

Bruce Ferrell wrote:
> I'll do my comments in line and hope I don't offend.
> 
> Rich Adamson wrote:
> 
>>> First off, thank you *very* much for this unbelievably informative 
>>> post!  I've got it saved away now along with Kris Boutilier's 
>>> adjusting rxgain/txgain post.
>>>
>>> On Wednesday 24 August 2005 17:14, Bruce Ferrell wrote:
>>>
>>>> At the point where the phone line get's to your demarc the is supposed
>>>> to ba a -2 to 3db reference point, sometimes called a -2 or -3 test
>>>> level point (TLP).  So that milliwatt tone at that point should read in
>>>> the range of -2 to -3 dbm.
>>
>>
>>
>> If I read the above words exactly as written, the above is not true. 
>> Maybe
>> there was a different intent that I'm missing, or, maybe words left out?
> 
> 
> I'm a lousy typist :)
> 
>> I'm reading the words to say "if I put a transmission test set on the 
>> cable pair just before the pair leaves the central office, the reading
>> should be in the -2 to -3 dbm range." If that is what you meant, then 
>> its incorrect. Even the old analog step-by-step switch specs called 
>> for no more then .5db loss from the milliwatt generator to the cable 
>> pair (CO distribution frame).
> 
> 
>> If you mean placing a transmission test set at the customer's demarc (at
>> the customer's site), the -2 to -3 db is still incorrect for "analog"
>> pstn circuits. That level _will be_ the 0db generator tone minus the 
>> cable loss from the CO to the customer's demarc. That cable loss is 
>> 100% predictable if you know the length and gauge of the copper wires 
>> between
>> the central office and the customer's site. (That "is" exactly how the
>> engineering spec is set for the less technical telephone installers
>> to measure after installing a new pstn facility to a customer site.)
> 
> 
> at the last point leaving the CO, the tone level should be a nominal 
> 0dbm.  By the time it get's to the customer demarc, -2 to -3 dbm.  The 
> loops are "suppposed" to be engineered that way.  On a brand spanky new 
> loop, yes 100% predictable.  Over time, all sorts of oddities 
> (corrosion, half taps, loading coils, and just general funkieness) are 
> introduced in the real world.
> 
>> If you are referring to a Remote Line Module (where CO lines are extended
>> into a neighborhood using fiber, T1's, or whatever) and placing a 
>> transmission
>> test set on the customer's cable pair as it leaves that remote module,
>> then the -2 to -3 dbm range is correct "at the remote module". It is not
>> correct at the customer's demarc as you still have the physical copper
>> loss from the module to the customer's site. That loss is still 100%
>> dependent on the length and gauge of the copper cable to the customer's
>> demarc.
> 
>  >
> 
>> If you are referring to a customer site that is 100% digital (eg, T1, 
>> fiber) from the CO to the customer site demarc, then your words
>> are correct. But, the customer probably wouldn't be using an analog
>> asterisk interface if they had a clue.
> 
> 
> Can't argue that.
> 
> 
>> FWIW, the majority of the larger telco's now store a variable in the
>> customer's online record that represents the length of copper cable
>> from the customer's physical address to the CO (or remote line module).
>> That length is used by less technical types to predict whether DSL can
>> be supported, and, is translated into an "expected loss" value that is
>> given to the installer (on their service order). That expected loss
>> value becomes the boggy for the installer to determine whether the
>> completed installation meets company standards. Any deviation of that
>> value is suppose to be reported as trouble and resolved before the
>> service order is completed.
> 
> 
> We were supposed to too back when I did this, but if it meant delaying 
> an order... we tended to just take the pair and note it.
> 
>>> Ok so since -3dB is 1/2 power we should be expecting a reading of 
>>> 7422 in ztmonitor if it's a linear reading of the signal.
>>>
>>>
>>>> If the milliwatt is arriving at the demarc at the nominal -2 to -3dbm
>>>> and getting into the asterisk to be measured at 8dBm (+8dbm0), I'd say
>>>> something is grossly mal-adjusted.  You're seeing 8db of gain!
>>
>>
>>
>> Agreed, regardless of what type of facility exists between the CO and
>> the customer's site.
>>  
>>
>>> No, he was saying he had to set his rxgain to 8 in order to get a 
>>> level of 14844 (0dBm) in ztmonitor.
> 
> 
> Ahhh.... OK a dimensionless number 8. not a reading of 8dbm.
> 
>> Right, and if we knew the gauge of copper used to that customer's site,
>> we could calculate exactly how many feet of copper exists between the
>> customer and the CO (or remote module). An 8db loss is very very common,
>> and even 12 db of loss is acceptable by some telco's to distant 
>> customers.
>>
>> FWIW, those individuals that are responsible for planning the location
>> of a new central office or remote line module use programs that calculate
>> the copper loss from several potential sites to each potential customer.
>> Sort of like drawing a circle around a potential module location and
>> asking the question "how many customer sites can be reached that are
>> within 12 dbm of that module location?" (Substitute some realistic 
>> engineering value for the 12db.) Part of that planning process actually
>> involves playing what-if games with things like: 1) if I use 26 gauge
>> cable to every customer from that potential site, what is the total
>> cost of deploying a remote module? 2) if I use 19 gauge copper, what's
>> the total cost?  (19 gauge copper has a much lower transmission
>> loss then does 26 gauge, and can reach more customers. However, the
>> cost of 19 gauge copper is much greater then 26 gauge so some sort of
>> economic trade off is obvious.)
>>
>> I fully understand the TLP point referenced in the post, but the words
>> that were used is going to lead other readers to an incorrect conclusion.
> 
> 
> Again, I'm a lousy typist.  I do my best... and know I'll get soundly 
> flamed if it get's too bad :)
> 
>> The echo cancellation problem with the x100p and TDM cards are very
>> much related to the narrow operating range that the existing echo 
>> cancellation software can operate within.  In the above 8 db loss
>> example (assuming there is an actual 8 db of copper loss between the
>> CO and the customer's site), the _correct_ rxgain and txgain settings
>> would be to provide approximately +6 dbm of gain within asterisk/zaptel.
>> Those gain settings would fall within the stated -2 to -3 db TLP range.
>> However, that 6db of gain (in both directions) will fall outside the
>> operating range of the existing echo canceller, therefore smaller
>> gain values are almost always required in asterisk. It doesn't make
>> any difference whether the transmission level is measured with 
>> ztmonitor or a $10k transmission test set. Neither will make it work
>> right.
> 
> 
> Actually, the test sets I used to use can be had these days for about 
> $400.  $10K? please!
> 
>> I think it was Steve Underwood that commented on the echo cancel
>> routines some time ago (and if you look in zaptel code you'll see
>> where he attempted to provide another alternative). The issue of
>> recognizing reflected energy (eg, echo) and providing some sort of
>> subtractive routine to remove that energy is very complex. Its very
>> easy if the routine only has to deal with a single tone (eg 1000 hz),
>> but is very difficult and processor intensive to deal with harmonics,
>> male vs female voices, soft vs loud speakers, local vs distant sources
>> of reflected energy (latency), etc. The companies that engineer dedicated
>> echo cancellers can throw processing power at the problem or use
>> dedicated chips (DSP's) to do that function. Not likely asterisk's
>> canceller will ever be equivalent to dedicated hardware cancellers.
>> (And, why does the new digium T1 card have it on board?)
>>
>> One other point that may not be obvious from previous echo postings.
>> Those asterisk users that are physically located a greater distance
>> from the CO _always_ have greater echo issues. Those that are relatively
>> close don't have as big an issue. That _is_ due to the small operating 
>> range of asterisk's echo canceller. And, that is _one_ of the reasons 
>> why one implementor's settings don't work at another implementor's 
>> location. (Not to mention differences in motherboards, etc, etc.)
>>
>> Rich
> 
> 
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