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Versions: 00 01 02                                                      
   Audio/Video Transport
   Internet Draft                                            R. Kreuter
   Document: draft-kreuter-avt-rtp-clearmode-02.txt          Siemens AG
   Expires: August 2003                                        February

            RTP payload format for a 64 kbit/s transparent call

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

   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-

   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
   The list of Internet-Draft Shadow Directories can be accessed at


   This document describes how to carry 64 kbit/s data streams
   transparently in RTP packets, using a pseudo-codec called
   It also serves as registration for a related MIME type called

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [2].

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Table of Contents

   1. Introduction..................................................2
   2. 64 kbit/s data stream handling and RTP header parameters......2
   3. Registration of audio/CLEARMODE...............................3
   4. Security Considerations.......................................4
   5. References....................................................4
   6. Author's Address..............................................5

1. Introduction

   Voice over IP media gateways need to carry all data streams generated
   by analog TDM or ISDN terminals via an IP network.
   ISDN wideband speech terminals do not rely on a voice data processing
   (e.g. echo cancellation or DTMF detection) within a Voice over IP
   media gateway. And ISDN data terminals e.g. will produce data streams
   that are not compatible with a non-linear encoding as is used for
   For such applications, there exists a necessity for a transparent
   relay of 64 kbit/s data streams in RTP packets. This mode is often
   referred to as "clear-channel data" or "64 kbit/s unrestricted". No
   encoder/decoder is needed in that case, but a unique RTP [5] payload
   type is necessary and a related MIME type is to be registered for
   signaling purposes.

   This payload format document describes a pseudo-codec called
   "CLEARMODE", for sample-oriented 64 kbit/s data streams with 8 bits
   per sample. It is in accordance with RFC 2736 [8], which provides a
   guideline for the specification of new RTP payload formats.

   Examples for the use of CLEARMODE in current VoIP media gateways are
   the transfer of "ISDN 7 kHz voice" and "ISDN data".

   This document also serves as the MIME type registration according RFC
   2048 [4], which defines procedures for registration of new MIME types
   within the IETF tree.

2. 64 kbit/s data stream handling and RTP header parameters

   The profile specifies the use of RTP over unicast and multicast UDP
   as well as TCP.
   (This does not preclude the use of this definition when RTP is
   carried by other lower-layer protocols.)

   CLEARMODE does not use any encoding or decoding. It just provides

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   The payload handling for CLEARMODE is similar to the PCMU or PCMA
   handling described in [9]. Each CLEARMODE octet is octet-aligned in a
   RTP packet. The sign bit of each CLEARMODE octet corresponds to the
   most significant bit of the octet in the RTP packet (i.e., assuming
   the CLEARMODE samples are handled as octets on the host machine, the
   sign bit is the most significant bit of the octet as defined by the
   host machine format).

   A sample rate of 8000 Hz is used.

   Note, that the payload format described here assumes that the data to
   be handled is sample oriented with one octet (8bits) per sample.
   Together with the 8000 Hz sample rate this calculates to a 64 kbit/s
   transmission rate per channel.

   The Timestamp SHALL be set according [5]

   The marker bit is always zero. Silence suppression is not applicable
   for CLEARMODE data streams.

   The payload type is dynamically assigned by means outside the scope
   of this document.

3. Registration of audio/CLEARMODE

   To: ietf-types@iana.org

   Subject: Registration of MIME media type audio/CLEARMODE

   MIME media type name: audio

   MIME subtype name: CLEARMODE

   Required parameters: none

   Optional parameters: ptime
          This represents the packet length in milliseconds [7].

   Encoding considerations: This type is only defined for transfer via
          RTP [5].

   Security considerations: Implementations using the profile defined in
          this specification are subject to the security considerations
          discussed in the RTP specification [5].

   Interoperability considerations: none

   Published specification: This document

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   Applications which use this media type: Voice over IP Media Gateways,
          transferring "ISDN 64 kb/s data" or "ISDN 7 kHz voice" or
          other VoIP-related 64 kbit/s data streams via a RTP

   Additional information: none

   Intended usage: COMMON

   Author/Change controller: This registration is part of the IETF
   registration tree.

4. Security Considerations

   Implementations using the profile defined in this specification are
   subject to the security considerations discussed in the RTP
   specification [5]. This profile does not specify any different
   security services. The primary function of this profile is to add a
   transparent transport for a 64 kbit/s data stream.
   Confidentiality of the media streams is achieved by encryption. Since
   there is no processing of the data stream other than packetization
   and depacketization, there is no interference to an end-to-end
   encryption mechanism.

5. References

   1  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.

   2  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997

   3  Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet Mail
      Extensions (MIME) Part Four: Registration Procedures", RFC 2048,
      November 1996.

   4  Casner, S. and P. Hoschka, "MIME type registration of RTP payload
      formats", Work in Progress.

   5  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
      a transport protocol for real-time applications", RFC 1889,
      January 1996.

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   6  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
      transport protocol for real-time applications", Work in Progress.

   7  Handley, M. and V. Jacobson, "SDP: Session Description Protocol",
      RFC 2327, April 1998.

   8  Handley, M. and C. Perkins, "Guidelines for Writers of RTP Payload
      Format Specifications", RFC 2736, December 1999

   9  Schulzrinne, H., Casner, S., "RTP Profile for Audio and Video
      Conferences with Minimal Control", Work in Progress

6. Author's Address

   Ruediger Kreuter
   Siemens AG
   81359 Munich, Germany
   Phone: +49 89 722 62553
   Email: ruediger.kreuter@siemens.com

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