[Asterisk-Users] PSTN line quality

[Rich Adamson]

> I'm looking for some input from someone with real experience of 
> telephony. I am having problems with the sound quality on our PSTN line 
> calls. Our channel banks are Adtran 600 and 750 and I spent a lot of 
> time on the phone with Adtran trying to work out the problem.
> We are getting hum and noise and very low volume on calls. I can 
> increase the gain in zapata.conf but that increases the echo also.
> I acquired a Line Test Set and talked to the telco about access to a 
> MilliWatt generator. They did not have one active, but they worked on it 
> and activated the MilliWatt Generator on the exchange for me.
> I measured my lines and also the lines at another location I am having 
> the same problem. The line levels were -20dbm at my office and -21.4dbm 
> at the other location. My question is, how does this compare, what is 
> the norm, and what is the recourse? Do I have the root of the problem or 
> should the system be able to handle this loss of level?

I don't use the Adtran channel banks but do have 21+ years experience
in all technical areas of telephony engineering, including transmission
engineering.

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.)

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.

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).

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).

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.