Fax issue over IP trunk

[jneiberger]

Forgive me for not knowing the exact hardware involved. I'm not really an Avaya guy. But we're having a problem that I think is related to a phone system upgrade. We had some flavor of G3 on one end and some flavor of s8x00 at the other side. We recently upgraded the G3 to, I think, an s8700. Since then, we have a fax machine across an IP trunk (hanging off the s8x00 at the other side) that is acting weird.

When someone calls the fax machine, it answers but we only hear a fraction of a second of the fax tone then it cuts out. The call stays active, but there is no sound from the fax machine. We played around with the ip-codec-set on each side, but it's not making a difference.

What would cause the fax to answer but then cut out while leaving the call active?

Thanks!

 

[98Converter]

Does it actually rec've faxes..?

You may want to check the IP-CODEC-SET it's using, and on page 2 set it to pass-through mode...





Thanks,
98C

We are what we repeatedly do. Excellence, then, is not an act but a habit. ARISTOTLE 384-322 B.C.

 

[jneiberger]

We've tried relay, pass-through and T-38. It does not receive faxes. When it answers, we hear a fraction of a second of fax tone before the audio completely cuts out. It's very strange. We've rebooted the fax machine just in case, but that didn't help.

 

[98Converter]

Also ensure it's using G.711MU (if North America) or G.711A if somewhere else. Make that your first choice...

Audio Silence Frames Packet
Codec Suppression Per Pkt Size(ms)
1: G.711MU n 2 20





Thanks,
98C

We are what we repeatedly do. Excellence, then, is not an act but a habit. ARISTOTLE 384-322 B.C.

 

[jneiberger]

I checked just now. G.711MU is the first choice on both sides.

 

[98Converter]

Type list trace sta xxxx (xxx being the extension of the 'broken' fax machine) and call the fax machine from another fax machine... This should provide some insight to what's happening.



Thanks,
98C

We are what we repeatedly do. Excellence, then, is not an act but a habit. ARISTOTLE 384-322 B.C.

 

[dufus2506]

Fax across VoIP is generally bad to do.

VoIP trunks break faxes in many different ways. For example you could have everything set perfectly within the two PBXs, but if you have no QoS (Quality of Service) set on your network, you may have fax problems.

VoIP wasn't designed to carry faxes. (Or, for that matter, modem calls.) It assumes that if you have data connectivity between the two sites, you have a better method to move the information.

The reason you hear the call cut out, is the PBX is putting a modulator in between the two fax machines to allow fax over IP.

Have a look at this white paper. Maybe it will explain it a bit better.

http://www.avayausers.com/attachment.php?attachmentid=390&d=1195091246

Carpe dialem! (Seize the line!)

 

[jneiberger]

Faxes over this trunk have been working just fine until our upgrade, though. And we have gigabit ethernet over fiber between facilities, so not much need for QoS here.

Thanks for mentioning why the fax tones cutout. I thought that was a symptom of a problem, but it turns out that's just the way it's supposed to be working!