[Asterisk-Users] Static on TDM Zaptel FXO

[Rich Adamson]

> > Those numbers don't sound right. In the US, many of the telcos have 
> > the same type of static information available (at least within their
> > internal records), but the numbers are intended to be used by non-
> > technical telco types to determine whether a customer's location can
> > reasonably expect to support dsl.
> > 
> > If the 1355 meters is correct, the cable loss should be more like
> > about 4db (give or take a little based upon exactly what gauge of
> > copper is actually used to serve your location).
> > 
> > My guess is the 18db number is only there to suggest some sort of
> > upper 'limit' for adsl.
> > 
> > If that guess is correct, then I'd expect settings somewhere close
> > to rxgain=3 and txgain=0 might be a reasonable starting point for
> > zapata.conf parameters.
> > 
> 
> Interestingly I've found another site that I can get the data for any
> number in France, the data is supposed to come from France Telecom's own
> database. Even if you give an ISDN number it still reports the figures
> but, of course, says the line is not suitable for ADSL.

In very very general terms, adsl is limited to sites that are within
about 18,000 feet of the central office. (The real limit is expressed
in terms of loss, but the limit was restated in terms of something the
average non-technical telco employees can understand -- distance.)
Adsl would never function on a cable pair that had 18db of loss, so
something's wrong with that reported number, or, it was meant to
communicate something different then how we're reading it.

If asterisk (zaptel) would behave like commercial pbx's (and associated
hardware), a real pbx engineer would:
 - measure the transmission loss from the customer's site to the
   central office milliwatt generator (eg, -7db loss)
 - then set the rxgain and txgain parameters to something slightly
   less then the measured loss (eg, rxgain=5, txgain=5). Its not
   uncommon to see those settings around 2db below the measured
   loss, but that varys by each pbx/telco engineering staff.

However, the echo canceller in asterisk is not very good, and that
forces us to use gain values substantially lower (causing complaints 
about low volume in many cases).

Part of the echo canceller problems seem to be related to highly
variable general-purpose PC hardware (amoung other things). That
includes pci/interrupt latency issues, OS overhead, etc. As a 
result, gain settings that work for one asterisk system may not
even be close for another system, and generally will be very
different from those used in commercial pbxs. (That is exactly
why T1/E1 interfaces to asterisk are preferred over analog
interfaces.)

Those asterisk systems further away from the central office typically
have more issues with echo and audio levels then do those systems 
closer to the central office (distance measured in terms of pstn 
cable loss). That's also one of the reasons why colocated asterisk
boxes don't have as many echo & audio level issues.