[Asterisk-Users] An anniversary and a lament for FXOs

[Damon Estep]

POTS Lines are analog, and a key system is the best way to deal with
analog lines since you are simply switching the voice path with no
analog to digital to analog conversion in the process.

VoIP is digital, so starting with a digital signal is always the better
way to go.

Consider the conversion that must take place on a POTS interface

Caller speaks into phone - analog signal
Call reaches Telco CO - converted to digital for switching on the PSTN
Call routed to your POTS line - converted back to analog - no need on
ISDN
Call answered by * - back to digital - no need on ISDN
Call router to your hard phone - back to analog

5 A/D D/A conversions, with lossy compression along the way (VoIP)

Also, digital service provides rock solid and predictable signaling for
call starts, ends, CID, etc. POTS lines provide shady analog signaling
the is hard to interpret because of line quality variations.

So unless you can get really clean POTS lines you are far better off
with ISDN/PRI, and if you cant get that a key system with an analog
voice path will sound better, always.

I realize this does not answer your question and solve the problem, it
is just an attempt to explain why you are not getting a better answer. I
would assume MOST * deployments will be in environments where nothing
but T1/E1 would ever be considered, leaving the SOHO user in the same
position they are in now, spend more or live with the limitations.

With all that being said, start by checking your POTS lines (or asking
the Telco to) for excessive resistance, impedance and noise. The fact
that you are not having good experiences with a variety of hardware
where others have been able to get good (acceptable) results indicates
there may be a line issue.

Damon (new to * but not new to Telecom and VoIP)
 

-----Original Message-----
From: asterisk-users-bounces at lists.digium.com
[mailto:asterisk-users-bounces at lists.digium.com] On Behalf Of
mgraves at mstvp.com
Sent: Wednesday, November 03, 2004 8:14 AM
To: Asterisk Group
Subject: [Asterisk-Users] An anniversary and a lament for FXOs

This week marks one year since I first setup an Asterisk server in the
hopes of transitioning my home office to a total VoIP system. The
process has been an incredible learning experience. I've tried numerous
IP hard phones, eventually settling upon the Polycom IP600 as my choice.
I've also used multiple ATAs including all the Sipura products.
Using Asterisk has been a challenge, a thrill and (when its working) a
joy. However, the one thing that I am not satisfied with is the
performance of the FXO interfaces that bring in my PSTN lines.

I've tried X100p cards but found them horribly unreliable. I presently
use Sipura SPA-3000s but they're only marginally better. How is it that
my Panasonic 4 line SOHO phone system (KX-TG4000B) can have four stable,
reliable FXOs with no echo at all in a device with a total cost of
<$500? It seems to me that there ought to be hardware available that
behaves just as well, but bridges the PSTN to the SIP/IAX domain?

I've read a lot on the list about how difficult designing FXOs can be,
but that flies in the face of the fact that every small multi-line phone
system has them...and without expection those behave better than the
devices I've been able to try with Asterisk. The Sipura SPA-3000 has
several settings to adjust for line impedance and inductive/capacitive
line loading....lots of settings, but it provides nowhere near the basic
performance of one of the lines on the Panasonic KSU. It's simply mind
boggling.

So, while I've posted with respect to FXOs previously, I must ask
again....what FXO interface device can anyone recommend from real
experience?

Michael

P.S.  -  I even investigated switching my lines to ISDN to get around
the need for FXOs, but SBC won't do it where I live.
--
Michael Graves                           mgraves at pixelpower.com
Sr. Product Specialist                          www.pixelpower.com
Pixel Power Inc.                                 mgraves at mstvp.com

o713-861-4005
o800-905-6412
c713-201-1262




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