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. 
   
  Internet-Drafts are draft documents valid for a maximum of six months 
  and may be updated, replaced, or obsoleted by other documents at any 
  time. It is inappropriate to use Internet- Drafts as reference 
  material or to cite them other than as work in progress." 
   
  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. 
 
1. 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. 
   
    
2. 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. 
   
   
3. 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. 
   
 
4. 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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5. References 
   
  [1] ITU-T, V.25 specification. 
   
  [2] ITU-T, V.8 Specification. 
   
  [2] M. Handley, V. Jacobson, SDP: Session Description Protocol, RFC 
  2327 
   
   
6. 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 
   
   
7. Full Copyright Statement 
   
  Copyright (C) The Internet Society (2001).  All Rights Reserved. 
   
  This document and translations of it may be copied and furnished to 
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  document itself may not be modified in any way, such as by removing 
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  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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  Acknowledgement 
   
  Funding for the RFC Editor function is currently provided by the 
  Internet Society. 
   
   
   
   






































 
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