[Asterisk-Users] TDM0xB vs. SIP for FXO

[Chris Bagnall]

> I am planning to connect my Asterisk PBX to one or two POTS 
> lines, and am wondering if it is better to use a TDM card for 
> this, or one or two SIP devices with FXO ports on them (such 
> as an SPA-3000, Grandstream 488).

I think it largely depends on where you're located and how much work has
been done supporting phone lines in your area.

I've used all 3 of the devices you mentioned in the UK, and here are my
thoughts:

HT-488: Initially very promising - no noticable echo, seemed to detect
hangup cleanly, etc. when I tested them late evening at a client's place.
The following day, no end of complaints from the client - it seemed the 488s
were only picking up about one call in three, the others weren't picked up
at all. Tried upgrading the firmware, but this had the side-effect of making
the 488 answer the call before passing it into Asterisk (so the BT ringing
sound disappears, call answered by the 488, then it generated its own
ringing tone for the caller) which wasn't acceptable to the client. Found
that the units needed restarting a couple of times a day to get them to pick
up incoming calls again.

SPA-3000: I have one of these on my home phone line since it's some distance
away from the asterisk box. It's been there for about 4 months now and I've
had no reliability problems at all. It seems to detect hangups fine, doesn't
hold the line open for extended time periods, etc.
There's plenty of forum posts out there detailing what settings to change
for different countries. I found those very helpful when configuring the
thing for a BT line. Other thing to note - you will have to spend quite some
time configuring the PSTN gain levels - when I first used the thing callers
could barely hear me at all. There is still minor echo at some times during
calls from/to PSTN, but I think this is probably because I've had to adjust
up the gain level so much. Apparently this is fixable in later firmwares,
but I've failed miserably to update this one recently. Picks up UK caller ID
perfectly.

TDM: Definitely the neatest solution, especially if you need multiple PSTN
lines. Probably works perfectly for US-type phone lines, but I've had no end
of difficulties getting it to recognise UK CLID. Doesn't seem to detect UK
hangup in more than about 30% of cases and the lines don't drop until a good
minute or so after the caller has hung up.

If you have one line, I'd suggest the SPA3000. If you think you're likely to
expand it *and* you're going to be in a country where the TDMs are known to
work fine, then that's a neater solution than lots of ATAs around the place.

Regards,

Chris
-- 
C.M. Bagnall, Director, Minotaur I.T. Limited
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