[Asterisk-Users] Cisco's description of echo

[Rich Adamson]

> >Another common problem that causes echo in networks is not setting your
> loss plan correctly.    You need to be sure that you aren't coming in too
> hot at any of your analog interfaces.   In general you should see a signal
> between -20dbm and -12dbm when someone is talking on the line.   If it is
> significantly hotter then you run the chance of having a larger reflected
> signal resulting in echo.   I typically try padding down analog levels by
> 3dB at a time to see if echo is reduced.   
> 
> 
> How do you measure the amplitude of a pstn line? As an audio engineer in a
> previous life, I would love to be able to send standard level tones down a
> pstn line and measure the amplitude at my end, then adjust the input gain
> accurately. Is there a way to do this?

One way is to buy a relatively inexpensive analog transmission test 
set ($400 US). Most have a tone generator and level meter built in.
You didn't mention which country you're located in, but ensure whatever
test set you purchase, that it supports the line impedance in use by
your telco.

The inexpensive test sets won't function with digital circuits, however
by using something like a cisco ata186 (with a known rx & tx loss),
one can use the analog test set to measure almost anything going on
with asterisk and the pstn.

Most US telco's have a milliwatt generator and quiet termination box
attached to some local telephone number. Many of the US telco's 
encourage their installation people to use the test sets to access those
resources for new pstn line installations to verify end to end functions.
Its usually fairly easy to obtain the telephone numbers assigned to
those boxes and use them to measure pstn loss, noise, etc.

The Triplett Model 4 is one model. There are many others.