Fax over T.38 or G.711

[Tomas Raska]

Hello,
how do I find out, if the 3CX fax server has received a fax via T.38 or G.711?
Thanks

 

[cobaltit]

Not really sure why it matters. Most people just care if they got the fax. But if it went to a fax extension it was T.38 unless you enabled G.711 fallback.

 

[eddv123]

Very simple really, if you were to run a Wireshark capture you can see it in the trace.

You will see T.38 packets under the protocol field.
Also look at the T.38 INVITE also and you will see T38 in the SDP body (see below).

T.38 also uses UDPTL (not UDP) for transport so you may see these in the trace as well.

G.711 Pass-through is like a standard VoIP Call and uses RTP and RTCP. G.711 is used as it is an un-compressed codec - you can see these in the trace as well.

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Also your ITSP must support T.38 if this is what you want to use also so check with them also.

 

[StefanW]

general for all supported provider, we state the fax Transmission mode. T.38 is way superior then audio, in audio it is to be expected that some lines of the fax are broken or the page cuts as there is no CRC possible.

 

[cobaltit]

Good notes. I took it as looking historically due to the past tense but absolutely wireshark will work if you are running a capture. Activity log will work as well as long as you are set to verbose logging. You'll see something like this:

Call-ID: O000V1Yo8Nzh5gL0J3goFQ..
CSeq: 2 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REGISTER, SUBSCRIBE, NOTIFY, REFER, INFO, MESSAGE
Content-Type: application/sdp
Supported: replaces
User-Agent: 3CX Fax
Content-Length: 249

v=0
o=3cxVCE 775649158 18446744073671076726 IN IP4 127.0.0.1
s=3cxVCE Audio Call
c=IN IP4 127.0.0.1
t=0 0
m=image 12440 udptl t38
a=T38FaxVersion:0
a=T38MaxBitRate:14400
a=T38FaxRateManagement:transferredTCF
a=T38FaxUdpEC:t38UDPRedundancy

 

[eddv123]

@StefanW I agree completely. I have highlighted some general comparison points below for the sake of the ticket:

T.38

• The standard for real time.
• Better than G.711 for packet loss issues.
• Positive receipt confirmation.
• Can failover to another endpoint.
• Less bandwidth hungry than G.711.
• T.38 uses UDPTL (not UDP) for transport.

G711 Pass-through:

• Simple protocol.
• Real time.
• G711 is like a VoIP call.
• Uses RTP and RTCP.
• Possibility of being faster than T.38 if used with SG3.
• Sensitive to delay, jitter and packet loss.
• Likes to eat bandwidth.
• Only codec option - cannot use a compressed codec.
• Can secure with SRTP.

I would try and encourage clients now to use an online FAX service over FoIP unless absolutely necessary - but some businesses still have an obligation to send documents as hard copies over electronic means so they still need FoIP obviously.