[Asterisk-Users] PSTN line quality

[Chris Mason (Lists)]

Rich Adamson wrote:
> I'm assuming from the above description that you're using a T1 card in
> the asterisk box with the Adtran channel banks connected to that T1,
> and analog pstn lines attached to your channel banks. (Can't tell for
> sure if that assumption is correct or not from the above.)
>   
That is correct. We are connecting pstn lines to Asterisk via the 
channel bank and a T1 card.
> If that's correct, first ensure your fxo ports on the Adtrans are set
> to match the impedence of the pstn lines (600 ohms in the US). If that
> is not set correctly, you will almost always have issues with imbalance
> resulting in hum, noise, etc.
>   
They are set to 600 ohms and the telco confirms that is the correct 
impedance.
> Forgeting about echo cancellation for the moment, your objective in
> measuring the milliwatt generator is to get as close to 0 db of end-to-
> end loss as possible. If the above config assumption is correct, then 
> adjust the transmit and receive gains on the Adtran fxo ports.
>
> To pick a starting point, simply use your new transmission test set to
> measure the loss on an ordinary analog pstn line to the milliwatt gen
> (no asterisk involvement). If that value really is -21 db, that seems 
> like an awful lot of loss. I would expect that loss to be no more than 
> about 10 db or so. Most telco's would find -21 db of loss unacceptable
> for any use, so if that value is correct, I'd suggest you have a telco
> problem (or we're not talking about the right config, above).
>   
Is there a way to find out what the acceptable standards are? I would 
like to have facts and figures to confront the telco with.
> The asterisk echo canceller will not function correctly with anything
> less then about a 5db to 7 db loss for long loops, therefore if your 
> measured pstn loss is really -21 db, then start by setting your fxo 
> ports to 21 - 7 = 14 db (of gain).
>   
RX gain or TX gain. I am guesing rx gain, would I then need additional 
TX gain also?
> Once you have something of reasonable volume and small (or no) echo, then
> try increasing the gains in 1 or 2 db steps to balance audio levels 
> against minimal echo.
>   
Will try that.

-- 
Chris Mason
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