[Asterisk-Users] FX-200 VOIP PORT CONVERTER FXS to FXO

[Rich Adamson]

Having spent 21 years in a telephone company as an engineer, reversing
tip & ring will have zero impact on any 2-wire fx pstn line. The equipment 
in the central office (regardless of who the manufacturer happens to be) 
is balanced and supplies -48 volts that is fed through the outside plant
to your location. The x100p card (and equivalents) do not care whether 
the tip and ring are reversed. If the cards really cared, they would
totally fail and not just create an echo condition. (Polarity can be an 
issue with some other equipment, and with some types of central office
trunks, but not the stuff we're talking about here.)

Echo is frequently produced by external 2-wire imbalances or imperfections 
on the pstn line, and not by the x100p (etc) card itself. You can create 
an example of imbalance by placing a resister from ground to either tip 
or ring (not both). Some real world examples are:
 - wet or damaged pstn cable
 - bridge-taps (something done by the telco, seldom seen any more)
 - cheap analog phones attached to the pstn line
 - some expensive analog phones on the pstn line
 - use of lengthy untwisted inside-wire within your home/business
 - poor cabling techniques (inside-wire near ballast or other AC induction)
 - as well as many other problems external to the asterisk cards

In many cases, the imperfections may be bad enough that you might even
hear AC hum, noise, crosstalk, or other degradation by listening very 
carefully with an ordinary analog phone. (If you can hear any imperfections,
the problem is rather bad.)

The echo cancel function is attempting to compensate for those imperfections
and imbalances. One can either muck with the echo canceling software or
go find the "real" source of the problem. (Incidently, that's why some
people are having echo problems on this list and others don't.)

Given how many of our offices/labs look in terms of cabling, etc, one 
rather simple step to help find the source of problems is to simply run
shielded twisted pair cable between the asterisk interface and the telco
demarc (ensuring the shield is actually grounded), remove _all_ other
cabling and phones from the line, and verify the asterisk box is running 
on clean AC power with an appropriate ground. (If you find the echo is
gone or better, start adding those items back one at a time to see which
item(s) is causing the problem. Then post it to the list so the rest
of us can learn from it.)

Most of the US telco's have technicians with the equipment necessary to
help find the problem if the problem really is their outside plant.
However, getting to that person can be a real challenge.

Rich

------------------------
> Just to be sure again, I did a reversal on the tip and ring with no
> improvement.
> 
> -----Original Message-----
> The echo issues are line or PSTN.
> 
> make sure your tip and ring are correctly wired.  Polarity 
> does matter and teh X100P does not do polarity fixing like
> most consumer phones today.
> 
> On Sun, Nov 16, 2003 at 01:35:13PM -0500, Kevin wrote:
> > I do support Asterisk. I have a TDM40B and X100P from Digium, I can't
> > take the echo on the X100P.
> > 
> > -----Original Message-----
> > Kevin wrote:
> > 
> > >Has anyone seen an FXO converter for a Cisco ATA.  There is someone
> > >selling a device on Ebay that claims to convert a Cisco ATA FXS port
> to
> > >an FXO.
> > >
> > >FX-200 VOIP PORT CONVERTER FXS to FXO
> > >
> > >http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3057281388
> > >  
> > >
> > 
> > Don't bother. Support Asterisk and pick up a X100P from Digium. You
> will
> > 
> > have more hair and less stress.
> >