Internet DRAFT - draft-foster-mmusic-vbdformat
draft-foster-mmusic-vbdformat
Internet Engineering Task Force B. Foster
Internet Draft R. Kumar
Document: <draft-foster-mmusic-vbdformat-01.txt> F. Andreasen
Category: Informational Cisco Systems
Expires: September 1, 2002 March 1 2002
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
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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 MIME type, audio/vbd for
voiceband data media, and a specific "fmtp" parameter for specifying the
underlying encoding.
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
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the processing capability of the gateway, characteristics of the
network etc.
This document is specifically concerned with the approach of passing
modem and fax traffic in-band. 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. Rationale for distinct Voiceband Data payload types
The rationale for distinguishing between a payload type associated with voice
and a payload type associated with voiceband data is twofold:
* At the receiver, voiceband data traffic is found to work best with fixed-size
jitter buffers, while adaptive jitter buffers are optimal for voice.
* Packet loss concealment algorithms are the receiver are suitable for voice,
but not for voiceband data.
For discrimination between voice and voiceband data and to allow different
processing at a receiver, separate payload types must be used even if the
underlying encoding is the same e.g. PCMU for voice and voiceband data. To this
end, a new RTP audio encoding name, to be registered as the MIME type audio/vbd
is defined. For a session, this encoding name could be dynamically mapped into
one or more payload types; this is true for any encoding. Each payload type
associated with the encoding "vbd" can have a separate format, specified through
a 'fmtp' attribute, indicating a different underlying base encoding (e.g. PCMU,
PCMA, G726-32, G726-40).
This document proposes the use of dynamic payload types for voiceband data that
are distinct from the payload types, static or dynamic, for voice even if the
underlying encoding algorithms are the same. This is to enable different,
voiceband data-specific receiver processing. For a given encoding algorithm, a
receiver may include both in the media (m=) line in SDP. If it intends to
support the encoding algorithm for voiceband data but not for voice, it should
not include the applicable voice payload type in the 'm=' line.
5. Proposed representation in SDP
The encoding name, "vbd", may be dynamically associated with one or more RTP
payload types. Using the "fmtp" SDP attribute, each "vbd" payload type is
associated with an underlying encoding. Thus,
a=rtpmap:<vbd dynamic payload type> vbd/<clock rate>
a=fmtp:<vbd dynamic payload type> <non-vbd audio payload type>
indicates a dynamic payload type to be associated with the codec "vbd". The fmtp
attribute indicates the underlying audio encoding associated with the "vbd"
codec. The audio encoding used by the "vbd" codec may be represented by either a
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static or dynamic payload type. Note that it is possible to specify multiple
"vbd" payload types, each with a different "fmtp" value and, therefore, a
different audio encoding.
An example media description in SDP might be:
m=audio 3456 RTP/AVP 15 98 99
a=rtpmap:98 vbd/8000
a=fmtp:98 0
a=rtpmap:99 vbd/8000
a=fmtp:99 8
This specifies dynamic RTP payload types 98 and 99 as being "vbd" codecs.
Further, it specifies that the vbd codec associated with payload type 98 uses an
underlying PCMU codec format (indicated by the static payload type 0). It also
specifies that payload type 99 has an underlying format of PCMA, (indicated by
the static payload type 8).
Note that the payload types 0 (PCMU) and 8 (PCMA) do not appear in the media
line in this case. The only permitted voice encoding is G728 (payload type 15).
The audio encoding underlying the voiceband data might also be represented by a
dynamic payload type, as in the following segment:
m=audio 3456 RTP/AVP 15 98
a=rtpmap:96 G726-40/8000
a=rtpmap:98 vbd/8000
a=fmtp:98 96
Again, the dynamic payload type of 96 does not appear in the media line in this
case. However, it is used to bind G726-40 as the underlying encoding algorithm
for the payload type of 98, used in voiceband data packets.
When both voice and voiceband data payload types are distinctly earmarked for a
session at session establishment, a transmitter may switch from a voice payload
type (15 in the example above) to a voiceband data payload type (98 in the
example above) when it detects an appropriate event such as an ANS or ANSAM as
defined in V.25 [1] and V.8 [2] respectively. When the receiving gateway or
endpoint sees a voiceband data payload type (98 in the example above), it
recognizes this as a voiceband data codec (with G726-40 encoding) and adjusts
the jitter buffer accordingly.
The packet format defined in RFC 2198 can be used with a voiceband data codec
for greater reliability by virtue of redundant transmission. A dynamic payload
type is defined for the encoding name "red". The encapsulated voiceband data
packets are, in this case, staggered in time (earlier and later packets combined
in an RFC 2198 composite packet). In the following example media description:
m=audio 3456 RTP/AVP 15 98 100
a=rtpmap:98 vbd/8000
a=fmtp:98 0
a=rtpmap:100 red/8000
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a=fmtp:100 98/98
a dynamic payload type of 100 is associated with RFC 2198 packets. A 'fmtp' line
indicates that these RFC 2198 packets encapsulate two voiceband data payloads,
each with payload type 98. The encapsulated packets are staggered in time
(i.e. earlier and later packets combined in an RFC 2198 composite packet).
A "vbd" payload type is negotiated like any other codec type. For symmetric
connections that can be transitioned to a specific voiceband data payload type,
both ends must declare support for that payload type. For backward
compatibility, if this codec type ("vbd") is not bound to a connection, then
suitable voice payload types may be used for voiceband data.
6. Other Characteristics of Voiceband Data Sessions
This section is informational and is intended to elaborate on other differences
between voice and voiceband data traffic.
* Silence suppression can be used with voice, but not with voiceband data
which requires a continuous carrier signal.
* Since voiceband data has a much lower distortion tolerance, it requires an
audio encoding algorithm in which DC removal filters are absent. Examples
of suitable schemes are PCM (ITU G.711) and 32 kbps/40 kbps ADPCM (ITU
G.726). By contrast, many more encoding algorithms are available for voice
traffic. Note: this document does not intend to list all encoding
algorithms suitable for voiceband data.
7. Proposed Registration of MIME media type audio/vbd
MIME media type name: audio
MIME subtype name: vbd
Required parameters:
rate: The RTP timestamp clock rate, which is equal to the sampling rate. The
typical rate is 8000, but other rates may be specified.
baseAlgorithm: The encoding scheme, such as PCMU, PCMA, G.726-32, G726-40 etc.,
used. No MIME parameters are inherited.
Optional parameters: channels, ptime, maxptime (Refer to Ref. 7).
Encoding considerations:
This type is only defined for transfer via RTP.
Security considerations: See Section 5 of Ref. 7.
Interoperability considerations: none
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Voice-Band Data Media Format March 2002
Published specification: The RFC that will evolve out of this document.
Applications which use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Intended usage: Modulated facsimile and modem signals that benefit from special
handling e.g. jitter buffer adjustment at a receiver.
Additional information:
1. Magic number(s): N/A
2. File extension(s): N/A
3. Macintosh file type code: N/A
Author/Change controller:
Bill Foster, Rajesh Kumar and Flemming Andreasen
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134-1706
bfoster@cisco.com, rkumar@cisco.com, fandreas@cisco.com
8. References
[1] ITU-T, V.25 specification.
[2] ITU-T, V.8 Specification.
[3] M. Handley, V. Jacobson, SDP: Session Description Protocol, RFC
2327.
[4] H. Schulzrinne, RTP Profile for Audio and Video Conferences with
Minimal Control, RFC 1890.
[5] http://www.iana.org/assignments/rtp-parameters.
[6] C. Perkins et al, RTP payload for redundant audio data, RFC 2198.
[7] The RFC that will come out of draft-ietf-avt-rtp-mime-06.txt, Casner,
S. and Hoschka, P.
9. Author's Addresses
Flemming Andreasen
Cisco Systems
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Voice-Band Data Media Format March 2002
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.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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