V.34 Fax Relay for SG3 Fax Machines
Super Group 3 (SG3) is a standard for fax machines that support speeds of up to 33.6 kbps through V.34 half duplex (HD) modulation. The following procedure describes how to configure V.34 (SG3) fax relay support based on ITU Specification T.38 Version 3.
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To enable support for V.34 fax relay (T.38) at SG3 speed: |
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1.
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In the IP Profiles table (see Configuring IP Profiles), add an IP Profile with the 'Fax Signaling Method' parameter configured to T.38 Relay. |
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2.
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In the Coders Groups table (see Configuring Coder Groups) set the coder used by the device to G.729 (or any other supported codec). |
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3.
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On the Fax/Modem/CID Settings page, do the following settings: |
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'SIP T.38 Version' to Version 3 (SIPT38Version = 3). |
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'Fax Relay Max Rate' (RelayMaxRate) to 33,600bps (default). |
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'CNG Detector Mode' (CNGDetectorMode) to Disable (default). |
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'V.21 Modem Transport Type' to Disable (V21ModemTransportType = 0). |
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'V.22 Modem Transport Type' to Disable (V22ModemTransportType = 0). |
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'V.23 Modem Transport Type' to Disable (V23ModemTransportType = 0). |
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'V.32 Modem Transport Type' to Disable (V32ModemTransportType = 0). |
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'V.34 Modem Transport Type' to Disable (V34ModemTransportType = 0). |
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'CED Transfer Mode' to Fax Relay or VBD (CEDTransferMode = 0). |
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4.
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Set the ini file parameter, V34FaxTransportType to 1 (i.e., relay). |
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5.
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Set the ini file parameter, T38MaxDatagramSize to 560 (default). |
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The T.38 negotiation should be completed at call start according to V.152 procedure (as shown in the INVITE example below). |
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T.38 mid-call Re-INVITEs are supported. |
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If the remote party supports only T.38 Version 0, the device "downgrades" the T.38 Version 3 to T.38 Version 0. |
For example, the device sends or receives the following INVITE message, negotiating both audio and image media:
INVITE sip:2001@10.8.211.250;user=phone SIP/2.0
Via: SIP/2.0/UDP 10.8.6.55;branch=z9hG4bKac1938966220
Max-Forwards: 70
From: <sip:318@10.8.6.55>;tag=1c1938956155
To: <sip:2001@10.8.211.250;user=phone>
Call-ID: 193895529241200022331@10.8.6.55
CSeq: 1 INVITE
Contact: <sip:318@10.8.6.55:5060>
Supported: em,100rel,timer,replaces,path,resource-priority,sdp-anat
Allow: REGISTER,OPTIONS,INVITE,ACK,CANCEL,BYE,NOTIFY,PRACK,REFER,INFO,SUBSCRIBE,UPDATE
Remote-Party-ID: <sip:318@10.8.211.250>;party=calling;privacy=off;screen=no;screen-ind=0;npi=1;ton=0
Remote-Party-ID: <sip:2001@10.8.211.250>;party=called;npi=1;ton=0
User-Agent: AudioCodes-Sip-Gateway-/7.40A.600.203
Content-Type: application/sdp
Content-Length: 433
v=0
o=AudioCodesGW 1938931006 1938930708 IN IP4 10.8.6.55
s=Phone-Call
c=IN IP4 10.8.6.55
t=0 0
m=audio 6010 RTP/AVP 18 97
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=no
a=rtpmap:97 telephone-event/8000
a=fmtp:97 0-15
a=ptime:20
a=sendrecv
m=image 6012 udptl t38
a=T38FaxVersion:3
a=T38MaxBitRate:33600
a=T38FaxMaxBuffer:1024
a=T38FaxMaxDatagram:122
a=T38FaxRateManagement:transferredTCF
a=T38FaxUdpEC:t38UDPRedundancy