Can not receive fax from PSTN over T.38 | T.37

[Fikri FIRAT]

Hi all,

We are facing an interesting yet harmful issue with recieving fax over T.38. In a location which uses T.37 and we deployed a T.38 as a backup, it appeared that they can not recieve fax from a specific sender X. X can send fax everywhere else but this location, and our location has no problem with anyone else. Just from sender X, the fax can not be transmitted. We tried fax over T.38 protocol but it didn't work either. What can be the cause / solution for this issue?

Edit : We have been informed that the problem occurs in several locations (was 3 yesterday, rose up to 5 today)

Here's some relevant configuration ;

!

voice service voip

allow-connections h323 to h323

allow-connections h323 to sip

allow-connections sip to h323

allow-connections sip to sip

fax protocol t38 version 0 ls-redundancy 5 hs-redundancy 2 fallback none

no fax-relay sg3-to-g3

sip

  bind control source-interface GigabitEthernet0/0

  bind media source-interface GigabitEthernet0/0

!

voice class codec 1

codec preference 1 g711ulaw

codec preference 2 g729r8

!

voice class codec 99

codec preference 1 g711ulaw

!

!

dial-peer voice 12 pots

trunkgroup PRI

translation-profile incoming PRI

destination-pattern 0T

progress_ind setup enable 3

progress_ind alert enable 8

incoming called-number .

direct-inward-dial

dial-peer voice 75 voip

translation-profile outgoing FAX

huntstop

destination-pattern 3352959

session protocol sipv2

session target ipv4:10.114.130.136

session transport udp

incoming called-number .

voice-class codec 99 

fax-relay ecm disable

fax protocol t38 version 0 ls-redundancy 2 hs-redundancy 0 fallback none

!

Regards,

Fikri

[Steven Griffin]

Fikri,

T.38 fax relay only works if both ends of the VoIP path support it. Also, unless your LAN/WAN is very noisy and lossy you don't need to turn up the redundancy numbers.   Another note is that T.38 version 0 is G3 only so you'll need to force SG3 to G3.  If you want SG3 faxes and both ends support it then change it to version 1 (at both ends).  Keeping ECM turned on is fine in my experience.  You don't need to disable it unless it becomes necessary and then you can probably create a one-off dial-peer for the one machine that doesn't like it.  Finally, you'll want to fallback to g711ulaw fax passthrough if things fail to setup.

So couple of things to change (assuming T.38 version 0):

Voice service voip

     fax protocol t38 version 0 ls-redundancy 0 hs-redundancy 0 fallback g711ulaw

     fax-relay sg3-to-g3

Then make dial-peer 75 look like:

dial-peer voice 75 voip

    translation-profile outgoing FAX

     huntstop

     destination-pattern 3352959

     session protocol sipv2

     session target ipv4:10.114.130.136

     session transport udp

     voice-class codec 99

When the call is in progress, login to the router and type: "show voice call status" you should see a codec like 14400 or 33600 if T.38 is working.  You can also turn on terminal monitoring type show voice call serial 0/0/0:23.5 where 0/0/0:23.5 is the channel on the PRI the T.38 fax relay call is running on.  You'll get a bunch of debug data about how T.38 is doing.

-Steven