The audio/t140c payload specification is intended to allow gateways
interconnecting two PSTN networks to interleave, through a
single RTP session, audio and text data received on the PSTN
circuit. The T.140 text data is interleaved with audio codec data
using the same UDP port, but this can only be done (in the
design preserved by this specification) if there is a shared top-level
type to the registered media type. The same issue affected the
audio/clearmode media type for the PSTN applications in RFC 4040,
but in that case the interleaved streams were both more like audio.
Like RFC 4040, this specification offers a secure, well-engineered
payload and media registration for a specific, PSTN application
that needs support. This specification will not apply beyond that
application, in strong contrast to RFC 4103, which registred text/t140,
and offered an open field for uses. The present specification states
normatively that it must be have no further applications beyond the
PSTN gateway applications.
Working Group Summary
There was a lengthy discussion of this material. It was originally
included in the document that became RFC 4103 and was
split out to progress independently. There was consensus for the
work to be published.
In order not to set further precedent for using the audio
top level type for non-audio data, but in order to allow the
usage which is needed and deployed to be registered, the document was
Last Called as a Historic document. The editor and the IESG discussed
the designation, but there were no further community comments on this.
This protocol is implemented in products. The specification
represents engineering and security review by the AVT working
group of significant standards and deployment past efforts by
the ITU-T Study Group 16.
Notes to RFC Editor
IETF avt WG
IETF avt WG delegated from the IESG