Internet Engineering Task Force                               B. Foster
Internet Draft                                                 R. Kumar
Document: <draft-foster-mmusic-vbdformat-00.txt            F. Andreasen
Category: Informational                                   Cisco Systems
Expires: December 2001                                        June 2001


                      Voice-Band Data Media Format

Status of this Document

  This document is an Internet-Draft and is in full conformance with
  all provisions of Section 10 of RFC2026

  Internet-Drafts are working documents of the Internet Engineering
  Task Force (IETF), its areas, and its working groups. Note that other
  groups may also distribute working documents as Internet- Drafts.

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  The list of current Internet-Drafts can be accessed at
  http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
  http://www.ietf.org/shadow.html.

. Abstract

  Voice-band data (fax and modem) traffic can often require different
  processing and as such, the ability to specify a different payload
  type when passing this type of traffic is important. This document
  defines a specific "fmtp" parameter for this purpose.


. Conventions used in this document


  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
  document are to be interpreted as described in RFC-2119.


. Introduction

  There are a number of ways of passing modem and fax traffic over an
  IP network. One approach is to simply pass it in-band. Other
  approaches involve terminating the fax/modem at each end and relaying
  the data in some fashion. Either approach may be valid depending on
  the processing capability of the gateway, characteristics of the
  network etc.


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  This document is specifically concerned with the approach of passing
  modem and fax traffic in-band, and because voice-band data has
  distinctly different characteristics from voice, it is often
  important to be able to distinguish this difference by indicating an
  associated media format. This allows the receiver of the media to
  process the packets differently.


. Voice-Band Data Format

  Fax and modem traffic is less sensitive to delay but is more
  sensitive to packet loss than voice traffic. It is possible to take
  advantage of the first characteristic to compensate for the later
  when the loss is due to lack of clock-synch between gateways
  communicating over an IP network. One possible approach is to change
  the jitter adaptation scheme from an adaptive mechanism that is
  optimized to reduce end-to-end delay due to jitter (for voice) to a
  scheme that is optimized to reduce or eliminate modem retraining due
  to packet loss caused by a lack of clock synchronization.

  However, in order to take advantage of the different characteristics
  between voice-band data and voice, the receiver needs to be able to
  know that the media traffic that it is receiving is voice-band
  traffic rather than voice. This document introduced a voice-band data
  media format value for the "fmtp" SDP attribute in order to do this.

  More specifically, the syntax for use in SDP is:

     a=fmtp:<payload-type> vbd

  An example media description in SDP might be:

     m=audio 3456 RTP/AVP 15 98
     a=rtpmap:98 PCMU/8000
     a=fmtp:98 vbd

  In this example, two payload types are defined:

     * A fixed payload type of 15 (G.728 as defined in RFC 1890) and
     * A dynamic payload type of 98 for voice-band data over G.711 u-
       law (PCMU).

  Suppose that two gateways have previously exchanged the above media
  description parameters in session descriptions, as part of session
  initiation. A voice session is normally assumed at the beginning of
  the session (G.728 and payload type 15 in this case). However, if one
  gateway receives a modem tone (ANS or ANSAM as defined in V.25 [1]
  and V.8 [2]), it switches to G.711 with payload type 98. When the
  receiving gateway sees the new payload type, it recognizes this as
  voice-band data over G.711 and switches its codec and its jitter
  buffer adaptation scheme accordingly.




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. References

  [] ITU-T, V.25 specification.

  [] ITU-T, V.8 Specification.

  [] M. Handley, V. Jacobson, SDP: Session Description Protocol, RFC
  2327


. Author's Addresses

  Flemming Andreasen
  Cisco Systems
  499 Thornall Street, 8th Floor
  Edison, NJ 08837
  Phone: +1 732 452 1667
  EMail: fandreas@cisco.com


  Bill Foster
  Cisco Systems
  Phone: +1 250 758-9418
  EMail: bfoster@cisco.com


  Rajesh Kumar
  Cisco Systems
  170 West Tasman Dr
  San Jose, CA
  Phone: +1 408 527 0811
  Email: rkumar@cisco.com


. Full Copyright Statement

  Copyright (C) The Internet Society (2001).  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
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  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
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  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be
  followed, or as required to translate it into languages other than
  English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

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  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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  HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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  Acknowledgement

  Funding for the RFC Editor function is currently provided by the
  Internet Society.











































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