Joined: Fri 29 of May, 2009


Posted:Fri 29 of May, 2009 (06:25 UTC)
Can someone confirm if T30_INDICATOR CED tone is mandatory to send from receiving side when codec is switched from G.723 (or any other) to T.38 after detecting CED 2100Hz tone from FAX equipment?

In our case our equipment detect this tone and send it transparently through voice after that we switch to T.38, so we didnt sent again T30_INDICATOR CED tone as signalization message. But someone says that this tone is mandatory to send.

Can anyone confirm this and where exactly in standard is written? I checked T.38 standard and there is written:

The T30_INDICATOR TYPE is used by the gateways to indicate the detection of signals such as
CED, HDLC preamble flags, and modem modulation training. It is sent by the receiving gateway to
the emitting gateway, and by the emitting gateway to the receiving gateway. The use of this
message is mandatory, except in the case where both G3FE devices are identified via
DIS/DCS exchange as Internet-aware fax devices. A peer may send this message in order to notify
its peer about upcoming messages.

But anyway I am not sure if exactly sending of T30_INDICATOR CED is mandatory or just other T30_INDICATOR messages that we are sending.