Media5 Mediatrix 4104, Reference Manual, Page: 271 / 466

Clear Channel Fax Reference Manual (SIP Version)
Mediatrix 4104 249
Clear Channel Fax
The Mediatrix 4104 can send faxes in clear channel. The following is a clear channel fax call flow:
Figure 80: Clear Channel Fax Call Flow

To set a preferred clear channel fax transmission codec:
1. Set the clear channel codec to use upon detecting a fax tone in the
dataIfClearChannelCodecPreferred variable.
This variable is used to decide which of the following codecs is preferred, even for voice
transmissions:
• PCMU
• PCMA
• G.726 at 32 kbs
• G.726 at 40 kbs
• noPreferredCodec
When noPreferredCodec is selected and no data-capable codecs are negociated, data
transmission may fail.
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
Note:
In clear channel, G.726 at 16 kbs and 24 kbs are not available for fax transmission.
Note:
If you want to set the G.726 codec at 32 kbs or at 40 kbs as the preferred clear channel codec, you
must also select the corresponding G.726 codec as the preferred voice codec as described in “Preferred
Codec” on page 231 . Otherwise, the Mediatrix 4104 will fail to switch to the G.726 codec for clear channel
faxes because G.726 is not negotiated.