[Asterisk-Users] How does Vonage support fax machines?

[Damon Estep]

> -----Original Message-----
> From: asterisk-users-bounces at lists.digium.com [mailto:asterisk-users-
> bounces at lists.digium.com] On Behalf Of Deon
> Sent: Tuesday, July 05, 2005 8:32 AM
> To: Asterisk Users
> Subject: [Asterisk-Users] How does Vonage support fax machines?
> 
> My boss is insisting we support fax, and I keep telling him that Fax
over
> IP is very unreliable and not recommended and his immediate come-back
is
> "Vonage does it." and it's very hard to figure out how.
> 
> I don't think Vonage does T.38, the Linksys/Sipura units they're using
> doesn't support T.38 to my knowledge.
> 
> That means they have to be using G.711Ulaw to send faxes. But how do
they
> compensate for packet loss/jitter/etc.
> 
> In our test lab, the best we could get was 90% success at sending
faxes.
> It seemes to screw up the longer the transmission, ie page 1 was
usually
> ok, but page 2 and 3 and 4 was at serious risk. So if I bought a
Vonage
> adapter, can I send a 30 page fax? My best guess is they have high
quality
> voice T1's, like from an ILEC, usually more expensive, and when they
sell
> a "Fax Line" I noticed it's more expensive. Maybe they route all their
fax
> calls specifically out these high quality T1's that they own, so that
they
> can do some type of quality control.
> 
> My test lab was a private network, a Cisco 3640 connected to a local
voice
> PRI T1, and converting to SIP. Asterisk would push the calls to the
Cisco
> 3640 and the Linksys PAP2 would register with Asterisk. All local. I
then
> tried several test faxes throughout the PSTN. Would it be better to
plug
> the voice T1 straight into Asterisk using one of Digium's cards?
> 
We relay faxes to/from a PRI connected to * to a fax machine using a
sipura spa-2000 and g.711u. The span between * and the spa-2000 is a
single LAN switch, 100mbit.

While t.38 would be better, it appears to us that g.711u works well on
low latency links, so "supporting fax over IP" would likely require QoS
implementation on the IP link if there are latency/jitter issues.