Media5 Mediatrix 4104, Reference Manual, Page: 108 / 466

Chapter 5 - Web Interface – Telephony Voice & Fax Codecs
86 Mediatrix 4104
Clear Channel Fax
The Mediatrix 4104 can send faxes in clear channel. The following is a clear channel fax call flow:
Figure 43: Clear Channel Fax Call Flow
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:
Standards Supported • Based on draft-ietf-sipping-realtimefax-01.txt
• Recommendation ITU T.38 version 0
INVITE
Ringing/Trying
ACK
[…]
m=audio 5006 RTP/AVP 18 0 13
a=rtpmap:18 G729/8000
a=rtpmap:0 PCMU/8000
[…]
m=audio 5004 RTP/AVP 18 0 13
a=rtpmap:18 G729/8000
a=rtpmap:0 PCMU/8000
BYE
200 OK
200 OK
RTP=G.729 (Voice Call)
Fax Tone Detected
RTP=PCMU (Echo Cancel Silence Suppression = disabled)
No re-INVITE!!

There is no need for a re-INVITE since the far end already supports the
data codec (PCMU).

When your SDP capabilities are inserted in a SIP packet, it implies that
you can receive any of these capabilities at any given time without notice.

In this case, both ends should switch to clear channel automatically upon
detection of the fax transmission.
Fax is terminated
User
Agent
#1
User
Agent
#2
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.