Media5 Mediatrix 4104, Reference Manual

Page: 274 / 466

Chapter 16 - Fax Transmission T.38 Fax
252 Mediatrix 4104
T.38 Fax
T.38 fax relay is a real-time fax transmission; that is, two fax machines communicating with each other as if
there were a direct phone line between the two. T.38 is called a fax relay, which means that instead of sending
inband fax signals, which implies a loss of signal quality, it sends those fax signals out-of-band in a T.38
payload, so that the remote end can reproduce the signal locally.
The Mediatrix 4104 can send faxes in T.38 mode over UDP or TCP. T.38 is used for fax if both units are T.38
capable; otherwise, transmission in clear channel over G.711 as defined is used (if
G.711 µ-law and/or G.711
A-law are enabled). If no clear channel codecs are enabled and the other endpoint is not T.38 capable, the fax
transmission fails.
The following is a T.38 fax call flow:
Figure 82: T.38 Fax Call Flow

To set T.38 fax transmission:
1. Enable T.38 by setting the dataIfCodecT38Enable variable to enable .
Standards Supported • Based on draft-ietf-sipping-realtimefax-01.txt
• Recommendation ITU T.38 version 0
Caution:
The Mediatrix 4104 opens the T.38 channel only after receiving the “200 OK” message from the
peer. This means that the Mediatrix 4104 cannot receive T.38 packets before receiving the “200 OK”. Based
on RFC 3264, the T.38 channel should be opened as soon as the unit sends the “INVITE” message. See
“Fax Issues” on page 355 for more details.
In the Unit Manager Network Administration Manual , refer to chapter Ports Parameters , section Port
Configuration Window .
INVITE
Ringing/Trying
ACK
INVITE
Trying
ACK
INVITE
[…]
m=audio 5006 RTP/AVP 0 18 8
a=rtpmap:0 PCMU/8000
a=rtpmap:18 G729/8000
a=rtpmap:8 PCMA/8000 […]
m=audio 5004 RTP/AVP 0 18 8
a=rtpmap:0 PCMU/8000
a=rtpmap:18 G729/8000
a=rtpmap:8 PCMA/8000
User
Agen
t #1
User
Agent
#2
Trying
ACK
BYE
200 OK
200 OK
Fax Tone Detected
[…]
m=image 6006 udptl t38
a=T38MaxBitRate:14400
a=T38FaxRateManagement
:transferredTCF
a=T38FaxFillBitRemoval :0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxVersion :0
a=T38FaxUdpEC:t38UDPRedundancy
[…]
m=image 6006 udptl t38
a=T38MaxBitRate:14400
a=T38FaxRateManagement
:transferredTCF
a=T38FaxFillBitRemoval :0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxVersion :0
a=T38FaxUdpEC:t38UDPRedundancy
200 OK
Fax is terminated
[…]
m=audio 5006 RTP/AVP 0 18 8
a=rtpmap:0 PCMU/8000
a=rtpmap:18 G729/8000
a=rtpmap:8 PCMA/8000 […]
m=audio 5004 RTP/AVP 0 18 8
a=rtpmap:0 PCMU/8000
a=rtpmap:18 G729/8000
a=rtpmap:8 PCMA/8000
200 OK