Problems sending Fax over IP with ATA controlled by CUCM (MGCP)

[daniel.espi]

Hello,

I have some faxes with problems to send some faxes to particular destinations. My configuration is:

FAX-ATA187-CUCM(MGCP)-RT2901(MGCP)-PRI-PSTN

The configuration for ATA187 is FAX mode NSE pass-through g711alaw, ECM Off and maximum Fax Rate 14400.

The configuration in router 2901 relative for faxes is:

no ccm-manager fax protocol cisco

mgcp modem passthrough voip mode nse
mgcp modem passthrough voip codec g711alaw

I'd like to know if:

1) Is there any configuration more compatible for sending and receiving faxes over PSTN?

2) Have I missed some important configuration?

Thanks.

[Gordon Ross · Accepted solution]

That's roughly correct. Faxes can be a bit of a pain.

First, just double-check your ISDN configuration - especially the clocking aspect. Faxes (and such) are very good at highlighting clocking issues on ISDN.

Next, check the fax machine. Fax speeds above 14.4K don't work on VoIP but some fax machines are rubbish at down-speeding from 33.3K to 14.4K (or 9.6K) See if you can force the fax machine not to use the higher-speeds from the outset. (If you get the option to disable Super G3 do so)

Finally, make sure you're on the latest version of ATA firmware. If you have a Cisco support contract, ask them for it. It's not public but has a bucket load of bug fixes in it. (I have 9-2-3-1-es9 but even that could be out of date)

GTG

[Francisco Granados · Accepted solution]

All good tips. 

Most importantly, since you are using pass-through, you MUST make sure that your region relationship between your PRI gateway and your fax is 64kbps. To avoid this, you could look into using T.38 instead, it's about 99% reliable when configured properly.

Thanks,

FG

[Gordon Ross]

The problem with T.38 is that it's not the panacea it's made out to be.

We looked at implementing T.38 at our site. Due to the way different gateways (MGCP, ATA, VG, etc.) handle T.38 we gave up.

GTG

[Francisco Granados]

Perhaps, the design has a lot to do with it. I have a client with 70 sites, they run ATAs 186s (SCCP) and 187 (SIP) along with MGCP controlled FXS ports. When I took over the project it was a mess, I switched everything to T.38 with fallback to pass-through and never looked back. 

Fax machines with pass-through can fail with even 1% packet loss, if using a local gateway then it's probably not a big deal, but if you have to go over the WAN then T.38 is my preferred solution. Furthermore, you can negotiate T.38 with either G711X or G.729 codecs. There's definitely no wrong answer, it depends on the circumstances.

FG

[daniel.espi]

Hi,

I think I'm going to follow both recommendations. First, I'll review configuration on fax machines, and if that doesn's sort out the problem, I'll try to use T38 with passthrough fallback.

I'll keep you updated.

Thanks.

[Gordon Ross]

The problem we discovered with T.38 was the switch-over from fax-passthru to T.38.

Some products/protocols did it in-band, and others did it out-of-band.

GTG