Specifications
T.38 Fax Reference Manual (MGCP/NCS Version)
Mediatrix 4100
149
If the selected codec is disabled, it is ignored.
To set a preferred clear channel fax transmission codec:
1. Set the dataIfCodecT38Enable variable to disable.
2. 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
There is no codec negotiation. The Mediatrix 4100 uses this codec if the remote unit suggests both
PCMU and PCMA for its fax. If the preferred codec is PCMU and the remote unit only suggests
PCMA, the call uses PCMA.
This variable increases the relative priority of the selected codec vs other data-capable codecs.
However, the priority of the preferred clear channel codec remains lower than the
voiceIfCodecPreferred variable (see “
Preferred Codec” on page 135).
Moreover, when no data-capable codec is part of the list of negotiated codecs, this variable
indicates which codec to use when fax or modem tones are detected. However, if the negotiated
voice codec is data-capable, the voice codec will be used for data instead of the preferred data
codec.
Media5 suggests to use pcma if you are located in Europe and pcmu anywhere else. However, you
should check first which codec is supported in your telephone network.
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.
pcmu-pcma The image media type is present in the SDP for PCMU and PCMA.
g726 The image media type is present in the SDP for G.726 only.
pcmu-g726 The image media type is present in the SDP for PCMU and G.726.
pcma-g726 The image media type is present in the SDP for PCMA and G.726.
pcmu-pcma-g726 The image media type is present in the SDP for PCMU, PCMA and G.726.
Table 109: Codec Negotiation Parameters (Continued)
Parameter Description
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 135. Otherwise, the Mediatrix 4100 will fail to switch to the G.726 codec for clear channel
faxes because G.726 is not negotiated.
Standards Supported • Recommendation ITU T.38 version 0