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

[Andrew Kohlsmith]

On November 3, 2004 10:13 am, mgraves at mstvp.com wrote:
> 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?

VOIP introduces latencies that Just Do Not Exist in non-VOIP scenarios.  I 
would have thought that after a year with Asterisk you would have come to 
that conclusion.

> 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.

You're tuning the hybrid; the problem lies in the additional latency incurred 
with VOIP -- If you took the Panasonic system hardware and VOIP'd it you 
would likely see similar issues that you see with the X100P, TDM400P and even 
channel banks.  You need a good stable and performance system to get good 
VOIP quality.  There is no substitute.

Case in point:  I was running an X100P and a TDM410P in a P90MMX -- VOIP calls 
were pretty good but anything over the POTS was kind of shitty... put the 
exact same hardware into a P3-700 and the POTS problems went away for the 
most part.

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

Personal preference: T100P and a channel bank.  Excellent-quality hybrids and 
a good, solid computer interface.  It's about double the cost of your 
Panasonic system, though, but can handle 24 lines instead of 4.

-A.