A New Internet-Draft is available from the on-line Internet-Drafts directories.
This draft is a work item of the Session Initiation Proposal Investigation Working Group of the IETF.
Title : SIP Support for Real-time Fax: Call Flow Examples And
Best Current Practices
Author(s) : J. Mule, J. Li
Filename : draft-ietf-sipping-realtimefax-01.txt
Pages : 56
Date : 2003-3-5
The Session Initiation Protocol (SIP) allows the establishment of
real-time Internet fax communications. Real-time facsimile
communications over IP may follow 2 modes of operation: T.38 fax
relay as defined by the ITU-T T.38 recommendation or fax pass-
through.
This document clarifies the options available to Internet telephony
gateway vendors to handle real-time fax calls using SIP. While our
primary focus is to address the more reliable real-time T.38 Group 3
fax mode, we briefly cover the fax pass-through mode to enable
fallback operations and super G3 fax communications using SIP. We
also give examples of SIP call flows for real-time Internet fax
gateways or SIP proxy redirect servers. Elements in these call
flows include SIP User Agents, SIP Proxy Servers, and Gateways to
the PSTN (Public Switch Telephone Network).
This document introduces best current practices for SIP T.38 fax and
SIP fax pass-through sessions. A session starts with audio
capabilities, and, upon fax tone detection, T.38 fax capabilities
are negotiated; upon successful negotiation, the session continues
with fax capabilities and the media termination hosts exchange T.38
Internet fax packets. The T.38 fax call scenarios include various
aspects of the call sequence: the detection of fax transmission, the
usage of the T.38 session description attributes, the optional
fallback into fax pass-through mode and the session termination. The
fax pass-through call scenarios involve some specific SDP media
attributes to enable proper fax transmission.
Fax transmission can be detected by the receiving side, the emitting
side or both (in the latter case, a 'glare' effect may appear).
This document only covers the case when the fax transmission is
detected by the receiving side: it is the most common practice and
the other cases do not represent any particular challenges and are
therefore left for future discussions). Call flow diagrams and
message details are shown. A list of IANA defined SDP attribute
names for T.38 is summarized in section 7.
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