Audio/Video Transport
Internet Draft R. Kreuter
Document: draft-kreuter-avt-rtp-clearmode-00.txt Siemens AG
Expires: February 2003 August 2002
RTP payload format for a 64 kbit/s voice band data 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].
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Abstract
This document describes how to carry 64 kbit/s data streams
transparently in RTP packets, using a pseudo-codec called
'CLEARMODE'. It also serves as registration for a related MIME type
called 'audio/CLEARMODE'.
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 [2].
Table of Contents
1. Introduction...................................................2
2. Data stream handling and RTP header parameters.................2
3. Registration of audio/CLEARMODE................................3
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4. Security Considerations........................................3
5. References.....................................................4
6. Author's Address...............................................4
1. Introduction
Voice over IP media gateways need to carry all data streams generated
by analog TDM or ISDN narrowband terminals via an IP network.
ISDN data terminals e.g. will produce data streams that are not
compatible with a non-linear encoding as is used for voice. For these
purposes, there exists a necessity for a transparent transport of 64
kbit/s data streams in RTP packets. This mode is often referred to as
ôclear-channel dataö. 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 can be used for plain data streams (e.g. ISDN data),
also referred to as ôclear-channel dataö. It is in accordance with
RFC 2736 [8], which provides a guideline for the specification of new
RTP payload formats.
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. Data stream handling and RTP header parameters
CLEARMODE does not use any encoding or decoding. It just provides
packetization.
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. By leaving one or more bits per octed unused, it
is possible to transport lower transmission rates (56 kbit/s, 48
kbit/s, à) too.
The Timestamp SHALL be set according [5]
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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
Applications which use this media type: Media Gateways, transferring
ISDN 64 kb/s data or other VoIP-related 64 kb/s data streams
via an RTP connection.
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
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64kbit/s voice band data call August 2002
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.
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, ôGuidelines for Writers of RTP Payload
Format Specificationsö, RFC 2736, December 1999
6. Author's Address
Ruediger Kreuter
Siemens AG
81359 Munich, Germany
Phone: +49 89 722 62553
Email: Ruediger.Kreuter@icn.siemens.de
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