Fax protocol

[Cisco.Rookie]

Hello Experts

I am a newbie in the domain and doing baby steps now on this field. I need to know more about the fax protocols and I would be grateful if you can guide me.

What are the protocols that comes in when we talk about Cisco fax deployment ?

Which is the best protocol to run faxes (mgcp/sccp), Why?

Can mgcp work only with T38 ? If not what are the other options ?

Please enlighten me

Thanx

[Priyam Acharya]

Hi,

Please find few documents which has complete details on all protocols used for FAX, multiple call flows with fax modem, details about T.38 and how it can be used with other protocols other than MGCP and also configuration needed for FAX to work.

https://www.cisco.com/c/en/us/support/docs/voice-unified-communications/unified-communications-manager-callmanager/13949-fax-hairpin.html

https://www.cisco.com/c/en/us/support/docs/voice-unified-communications/unified-border-element/115742-fax-modem-call-flows-00.html  

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/fax/configuration/15-mt/vf-15-mt-book/vf-cfg-t38-fxrly.pdf

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/fax/configuration/15-mt/vf-15-mt-book/vf-cfg-fx-passthr.html

Call Control for Fax Passthrough

Fax pass-through is supported under the following call-control protocols:

• H.323
• Session Initiation Protocol (SIP)
• Media Gateway Control Protocol (MGCP)

Thanks,

Priyam Acharya

[Jaime Valencia]

Suggest you watch the fax session here:

https://learningnetwork.cisco.com/community/learning_center/collaboration-training-videos

[R0g22]

A couple of suggestions regarding faxes -

1. T.38 handles packet loss better and supports redundancy.
2. SCCP does not support protocol based switchover so you cannot use pass-through and T.38 with it. You can use T.38 NSE but that is Cisco proprietary.
3. MGCP supports T.38 protocol based switchover but not pass-through.
4. T.38 is the accepted-everywhere protocol. Nowadays typical deployments are CUBE with a SIP - SIP call flow. T.38 is best here since your ITSP will support this. Any 3rd party IP fax servers will support this as well.
5. Alternative to this is pass-through but remember both SCCP and MGCP don't support this. H.323/SIP can do this but if you have a CUCM with H.323 then there is a restriction where in CUCM does not support H.245 Request Mode for anything other than T.38 or data so potentially with pass-through you can only use SIP if going through CUCM and either SIP or H.323 if you don't have CUCM in the call flow.

Hope that is not too confusing. Get your hands on the David Hanes, Gonzalo Salgueiro fax book. These guys are Fax Veterans. You can find their Cisco Live videos as well. Very helpful.

[Cisco.Rookie]

Thanks Nipun for the clear answer. I need to check with our ITSP and see they support T38. I just wonder what should be the next step, if our ITSP do not support T38 and the VG is configured as MGCP ?

Thanx

[R0g22]

If you have an ITSP, you cannot have MGCP. MGCP cannot control an IP trunk. So, if you have a SIP trunk with ITSP, recommended to have SIP to your CUCM as well making your router CUBE.
Now, if your ITSP does not support T.38, then they would and should support fax pass-through which is a protocol based switchover. CUBE w/SIP does it no problem.

[gmgarrian]

One thing I'll add is that some providers will support t38 but they won't initiate the change over.  Normally it's the called party device that will initiate the changeover.  Most SIP based fax servers (Rightfax, Xmedius, etc.) will support initiating the changeover to t38 when it's the calling party.  However, Cisco hardware like voice gateways, VG3xx, and ATAs, cannot do this.  

Also, sometimes providers will have requirements to support t38.  I dealt with a case once where the provider would only change over if the initial call setup as g729.  I had to setup an LTI transcoder just for this one site since the rest of the organization's fax environment was g711 and I had to force fax calls for this one provider to use g729.  

Good times.