Re: [hylafax-users] [hylafax-devel] Fax over VoDSL PSTN emulators

["Luke McKee" <hojuruku@xxxxxxxxx>]

Andrew, Lee & Hylafax users,

Before I say too much jargon. This is about Voice over DSL hardware
and how it *could* cause problems with hylafax. Some sites have boxes
on the wall that provide PSTN lines - but these PSTN lines could have
issues depending how they are provisioned. It's not one size fits all
like ISDN.

I've done some more checking and well someone I don't know how let me
log into the IAD without a password. The device is clearly listening
for fax calling tones and doing something at the AAL2 protocol layer
when it sees it!

It's running AAL2 (Bundled Voice over ATM PVC - CBR in this case) but
the device, at least from how I'm much interpreting so far doesn't
clearly confirm the codecs. The best case scenario is G.711 and GOOD
QoS. I'm seeing there are some "under runs" in normal use - but
haven't ascertained if these equal gaps.

On the Whirlpool forums in Australia i ran into STRONG opposition that
it was different to a normal PSTN (analogue in anyway). Since opening
the thread modem data rates now work for PCM protocols (i.e. they may
have changed something). The interface / device was reset, and I can
confirm that I've never unplugged it from it's UPS which implies a
reset to get AAL2 configuration changes.

In countries where there is a former government phone companies who
charges other telco's to use their copper network (ULL), those other
telco's will naturally want to rent less copper lines and use ADSL to
provide PSTN like services.

With AAL2 now the most popular way of providing voice services over
ATM and it's capabilities being similar than SIP (but QoS better - one
would hope) this is going to become a big issue. AAL2 can also provide
64k PCM(G.711) channels that should be of a familiar grade to ISDN
NTU's with POTS ports in the right conditions. So far some telcos
don't want to disclose anything more than "VoDSL", which I find will
be a detriment to them and the technology. With AAL2 you can have low
bitrate codecs, silence suppression, fax proxy, jitter buffer, and all
this SIP stuff on a "normal" phone line. What's worse is you can not
always see what's really going on behind the scenes, so it's good to
ask your telco what you get.

It affects hylafax users, because if they use the AAL2 fax relay
protocol, data sheets of most of the IADs seem to only support max fax
speed of 14.4k (no V.34). It's always good to know what's under the
bonnet when you start tinkering with a problem.

Optus in Australia in their documentation say fax calls up to 14.4k is
supported. This could be due to less V.34 fax machines in the market,
leaving themselves open for AAL2 fax proxy or maybe they are using
it... V.34 modems are supported according to the same product
documentation.

Also line faults (or worse - mismanagement) can manifest themselves in
problems for hylafax users (hangups/not connecting #s). I just thought
I'd put this out their FYI in case you ever bump into this stuff as I
did.

Low bit rate voice codecs and relay protocols allow the Telco to have
less traffic on their backbone when a call is made.

Andrew: with ATM CBR/VBR-rt as the transport do you still think this
on the record as unreliable? Please check again with your CTO, I don't
want you to say anything your not sure of.

Hope this helps someone one day :)

Cheers,

Luke

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