Network Working Group Q. Xie
Internet-Draft Motorola
Expires: July 8, 2006 R. Kapoor
Qualcomm
January 4, 2006
Enhancements to RTP Format for EVRC Family Codecs
draft-ietf-avt-compact-bundled-evrc-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines several enhancements and extensions to RFC3558
EVRC RTP payload format. In particular, it defines the support for a
compact bundled format and EVRC-B codec, as well as discontinuous
transmission (DTX) support for EVRC and EVRC-B encoded speech
transported via RTP sessions. Some VoIP applications, such as Push-
to-Talk and VoIP over low bandwidth dial-up and wireless networks,
require such enhancements for efficient use of the bandwidth.
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Table of Contents
1. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Support of EVRC-B Codec . . . . . . . . . . . . . . . . . 3
2.2 Compact (Header-free) Bundled Format . . . . . . . . . . . 3
2.3 Discontinuous Transmission (DTX) . . . . . . . . . . . . . 4
3. EVRC-B Codec . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Compact Bundled Format . . . . . . . . . . . . . . . . . . . . 5
4.1 Single Rate Operation . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5.1 Registration of Media Type EVRCB . . . . . . . . . . . . . 6
5.2 Registration of Media Type EVRC1 . . . . . . . . . . . . . 7
5.3 Registration of Media Type EVRCB1 . . . . . . . . . . . . 8
5.4 Payload Format Parameters for DTX . . . . . . . . . . . . 9
5.5 Mapping MIME Parameters into SDP . . . . . . . . . . . . . 11
5.6 Usage in Offer/Answer . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. Changes from Last Revision . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1 Normative References . . . . . . . . . . . . . . . . . . . 13
8.2 Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . 15
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1. Conventions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
they appear in this document, are to be interpreted as described in
[2].
2. Introduction
This document defines the RTP payload format for EVRC-B, a
complementing compact bundled format for EVRC and EVRC-B, and updates
to RFC3558 [5] formats with discontinuous transmission (DTX) support.
Some VoIP applications, such as Push-to-Talk and VoIP over low
bandwidth dial-up and wireless networks, require such EVRC RTP
payload capabilities for efficient use of the bandwidth.
2.1 Support of EVRC-B Codec
EVRC-B [4] is an extension to [3] developed in 3GPP2. EVRC-B [4]
compresses each 20 milliseconds of 16-bit sampled speech input into
output frames of one of the four different sizes: Rate 1 (171 bits),
Rate 1/2 (80 bits), Rate 1/4 (40 bits), or Rate 1/8 (16 bits). In
addition, there are two zero bit codec frame types: null frames and
erasure frames, similar to EVRC [3]. One significant enhancement in
EVRC-B is that there are Rate 1/4 frames that were not used in EVRC.
This provides lower average data rates (ADRs) compared to EVRC, for a
given voice quality.
Since EVRC-B encoded speech frames are different from those of EVRC,
EVRC-B and EVRC codecs do not interoperate with each other. At the
initiation of a RTP session, the RTP sender and receiver need to
indicate (e.g., using MIME subtypes that are separate from those of
EVRC) that EVRC-B is to be used for the ensuing session.
2.2 Compact (Header-free) Bundled Format
The current interleaved/bundled packet format defined in RFC 3558
allows bundling of multiple speech frames of different rate in a
single RTP packet, sending rate change request, and interleaving. To
support these functions, a Table of Content (ToC) is used in each RTP
packet in addition to the standard RTP header. The size of the ToC
is variable, depending on the number of EVRC frames carried in the
packet [5].
The current header-free packet format defined in RFC 3558 is more
compact and optimized for use over wireless links. It eliminates the
need for a ToC by requiring that each RTP packet contains only one
speech frame (of any allowable rate), i.e., bundling is not allowed.
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Moreover, interleaving and rate change request are not supported in
the header-free format [5].
Since the biggest EVRC/EVRC-B speech frame is 171 bits (full rate),
it is therefore very inefficient in terms of bandwidth utilization to
send EVRC/EVRC-B speech frames one at a time over RTP/UDP/IP, as
being required by the header-free format. It is assumed that this
shortcoming of the header-free format will be overcome or at least
greatly alleviated in practice by the use of RTP/UDP/IP header
compression or removal techniques such as ROHC [9].
However, in many wireless systems such as 3GPP and 3GPP2, header
compression is only applied between the mobile station and the radio
access network. ROHC is not an end-to-end compression scheme, there
will be many hops between the EVRC/EVRC-B sender and the EVRC/EVRC-B
receiver where such overheads may be an issue. As speech will be
carried one speech frame per RTP packet with full RTP/UDP/IP headers,
it would result in very poor bandwidth utilization in that part of
the network.
The compact bundled format described in this document presents the
user an alternative to the header-free format defined in RFC 3558.
The compact bundled format is wireless-friendly since it does not use
a ToC. It also allows bundling of multiple EVRC or EVRC-B frames and
will not create bandwidth inefficiency problems in the network.
However, to use this compact bundled format the compromise one has to
make is that only one EVRC/EVRC-B rate (full rate or 1/2 rate) can be
used in the session. Similar to the header-free format defined in
RFC 3558, interleaving and rate change request are not supported in
the compact bundled format.
[Editor's note: it may be worthwhile to consider an alternate to the
above single rate operation - defining 2 payload formats (EVRC1-Full
and EVRC1-Half) and allow rate switching on a packet-to-packet basis.
If rate switching is undesirable, the endpoints could always limit
the session to one rate at the setup via SDP signaling.]
2.3 Discontinuous Transmission (DTX)
Originally, EVRC codec did not provide DTX support; in variable rate
operation mode the encoder sends 1/8 rate frames during periods of
silence, while in single rate operation mode (see 4.1 ???), silence
is encoded and sent in silence frames of the same rate as that of
speech frames.
Silence or 1/8 rate frames of EVRC and EVRC-B codecs vary little
during periods of silence. The transmission of these 1/8 rate frames
(or silence frames in the single rate operation mode) across the
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radio interface in a wireless system is expensive in terms of
capacity, and therefore, suppression of these frames is desirable.
Suppression of silence frames is accomplished using DTX. When DTX is
applied, the first few 1/8 rate frames (or silence frames in the
single rate operation mode) are transmitted to establish background
noise during the period of silence. Then a portion of the stream of
subsequent 1/8 rate frames (or silence frames in the single rate
operation mode) are not transmitted and discarded at the encoder. At
the receiver, background or comfort noise should be created by using
the previously received 1/8th rate frames (or silence frames in the
single rate operation mode).
3. EVRC-B Codec
Two RTP modes are supported for EVRC-B codec - the interleaved/
bundled packet format and compact bundled packet format. For the
interleaved/bundled packet format, the operational details and
capabilities, such as ToC, interleaving, and bundling, of EVRC-B are
exactly the same as those of EVRC, as defined in RFC3558 [5]. The
compact bundled packet format for EVRC-B is defined in Section 4 of
this document.
4. Compact Bundled Format
A packet in the compact bundled format consists of an RTP header
followed by a sequence of one or more consecutive EVRC/EVRC-B codec
data frames of the same rate, as shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header [4] |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
| One or more EVRC/EVRC-B data frames of same rate |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The vocoder date frames MUST be generated from the output of the
vocoder following the procedure described in 5.2 in RFC 3558 [5] and
they all MUST be of the same rate and size.
4.1 Single Rate Operation
As mentioned earlier, to use the compact bundled format, all the
EVRC/EVRC-B data frames in the session MUST be of the same rate
(either 1 or 1/2 rate). To some VoIP applications, such as Push-to-
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Talk over cellular, this is not viewed as much of a limitation due to
their unique communication style where rate changes in a session are
often difficult and sometimes even undesirable.
For a session that uses the compact bundled format, the rate for the
session can be determined during the session setup signaling, for
example, via SDP exchanges. See Section 5 below for more details.
5. IANA Considerations
Three (3) new MIME subtype registration are required for the EVRC-B
support and compact bundled format payload types, and four (4)
optional parameters - "silencesupp", "dtxmax", "dtxmin", and
"hangover" - are defined and used in DTX, as described below.
5.1 Registration of Media Type EVRCB
MIME media type name: audio
MIME subtype names: EVRCB
Required parameters: none
Optional parameters: see RFC 3558 [5].
Encoding considerations: This media type is defined for transfer of
EVRC-B encoded data via RTP using the Interleaved/Bundled packet
format specified in RFC 3558 [5] and RFC XXXX.
Security considerations: See Section 6 of RFC XXXX.
Interoperability considerations: none
Published specification: The EVRC-B vocoder is specified in 3GPP2
C.S0014-B [4]. Transfer method with Interleaved/Bundled packet
format via RTP is specified in RFC 3558 and RFC XXXX.
Applications which use this media type: It is expected that many VoIP
applications (as well as mobile applications) will use this type.
Additional information: none
Person & email address to contact for further
information: Qiaobing.Xie@motorola.com
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Intended usage: COMMON
Restrictions on usage: This type is only defined for transfer via RTP
[6].
Author: Qiaobing Xie
Change controller: IETF Audio/Video Transport working group delegated
from the IESG.
5.2 Registration of Media Type EVRC1
MIME media type name: audio
MIME subtype names: EVRC1
Required parameters: none
Optional parameters:
ptime: see RFC 2327 [8].
maxptime: The maximum amount of media which can be encapsulated in
each packet, expressed as time in milliseconds. The time SHALL be
calculated as the sum of the time the media present in the packet
represents. The time SHOULD be a multiple of the duration of a
single codec data frame (20 msec). If not signaled, the default
maxptime value SHALL be 200 milliseconds.
evrcfixedrate: Indicates the EVRC rate of the session while in single
rate operation. Valid values include: 0.5 and 1, where a value of
0.5 indicates the 1/2 rate while a value of 1 indicates the full
rate. If this parameter is not present, 1/2 rate is assumed.
Encoding considerations: These types are defined for transfer of EVRC
encoded data via RTP using the compact bundled format as described
in RFC XXXX.
Security considerations: See Section 6 of RFC XXXX.
Published specification: The EVRC vocoder is specified in 3GPP2
C.S0014. Transfer method with compact bundled RTP format is
specified in RFC XXXX.
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Applications which use this media type: It is expected that many VoIP
applications (as well as mobile applications) will use this type.
Additional information: none
Person & email address to contact for further
information: Qiaobing.Xie@motorola.com
Intended usage: COMMON
Restrictions on usage: This type is only defined for transfer via RTP
[6].
Author: Qiaobing Xie
Change controller: IETF Audio/Video Transport working group delegated
from the IESG.
5.3 Registration of Media Type EVRCB1
MIME media type name: audio
MIME subtype names: EVRCB1
Required parameters: none
Optional parameters:
ptime: see RFC 2327 [8].
maxptime: The maximum amount of media which can be encapsulated in
each packet, expressed as time in milliseconds. The time SHALL be
calculated as the sum of the time the media present in the packet
represents. The time SHOULD be a multiple of the duration of a
single codec data frame (20 msec). If not signaled, the default
maxptime value SHALL be 200 milliseconds.
evrcfixedrate: Indicates the EVRC-B rate of the session while in
single rate operation. Valid values include: 0.5 and 1, where a
value of 0.5 indicates the 1/2 rate while a value of 1 indicates
the full rate. If this parameter is not present, 1/2 rate is
assumed.
Encoding considerations: These types are defined for transfer of
EVRC-B encoded data via RTP using the compact bundled format as
described in RFC XXXX.
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Security considerations: See Section 6 of RFC XXXX.
Published specification: The EVRC-B vocoder is specified in 3GPP2
C.S0014-B. Transfer method with compact bundled RTP format is
specified in RFC XXXX.
Applications which use this media type: It is expected that many VoIP
applications (as well as mobile applications) will use this type.
Additional information: none
Person & email address to contact for further
information: Qiaobing.Xie@motorola.com
Intended usage: COMMON
Restrictions on usage: This type is only defined for transfer via RTP
[6].
Author: Qiaobing Xie
Change controller: IETF Audio/Video Transport working group delegated
from the IESG.
5.4 Payload Format Parameters for DTX
For all the EVRC and EVRC-B RTP payload formats defined in RFC 3558
[5] and RFC XXXX, four additional optional parameters -
"silencesupp", "dtxmax", "dtxmin", and "hangover" - are defined and
used in DTX, as described below.
silencesupp: Permissible values are 0 and 1. A value of 1 indicates
that the sender of this parameter is capable of and desires to
support silence suppression using DTX. A value of 0 indicates
that the sender of this parameter is either incapable or unwilling
to use silence suppression using DTX. An RTP receiver may use
this parameter to declare its ability and desire to support
silence suppression using DTX. If this parameter is not present,
the default value MUST be assumed.
[Editor's note: Default value is 1 and will be specified in a
3GPP2 TSG-C 1.1 document. Furthermore, the backward compatibility
issue with this default value is studied and understood by 3GPP2
TSG-C 1.1.]
If the RTP receiver indicates that it is incapable of or unwilling
to use silence suppression using DTX at the beginning of a
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session, silence suppression using DTX MUST NOT be used for the
session.
dtxmax: indicates the maximum DTX update interval in number of
frames. During DTX, the RTP sender occasionally updates the RTP
receiver about the change in background noise characteristics,
etc., by sending a new 1/8 rate frame (or silence frame in the
single rate operation mode) to the RTP receiver. The RTP receiver
may use 'dtxmax' to indicate to the RTP sender the maximum
interval (in number of frames) between any two DTX updates it
expects to receive from the RTP sender. If this parameter is not
present in a session that uses DTX, the default value MUST be
assumed.
[Editor's note: Default value is 50 and will be specified in a
3GPP2 TSG-C 1.1 document].
This parameter MUST be ignored if silence suppression using DTX is
not used for the session.
dtxmin: indicates the minimum DTX update interval in number of
frames. The RTP receiver may use 'dtxmin' to indicate to the RTP
sender the minimal interval (in number of frames) between any two
DTX updates it expects to receive from the RTP sender. If this
parameter is not present, the default value MUST be assumed.
[Editor's note: Default value is 8 and will be specified in a
3GPP2 TSG-C 1.1 document.]
This parameter MUST be ignored if silence suppression using DTX is
not used for the session.
hangover: indicates the number of 1/8 rate frames (or silence frames
in the single rate operation mode) transmitted at the end of
active speech but before DTX actions begin. When setting up an
RTP session that uses DTX, an RTP receiver can use this parameter
to signal the number of 1/8 rate frames (or silence frames in the
single rate operation mode) that SHOULD be transmitted by the RTP
sender prior to beginning DTX actions.
Note, the RTP sender SHOULD always follow the hangover specified
by the RTP receiver. While hangover=0 is allowed, it is
RECOMMENDED that hangover be set to 1 or greater since the
presence of 1/8th rate frames (or silence frames in the single
rate operation mode) at the end of active speech can help the RTP
receiver to identify the beginning of the DTX period. If this
parameter is not present for a session that uses DTX, the default
value MUST be assumed.
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[Editor's note: Default value is 1 and will be specified in a
3GPP2 TSG-C 1.1 document.]
This parameter MUST be ignored if silence suppression using DTX is
not used for the session.
5.5 Mapping MIME Parameters into SDP
The information carried in the MIME media type specification has a
specific mapping to fields in the Session Description Protocol (SDP)
[8], which is commonly used to describe RTP sessions. When SDP is
used to specify sessions employing the compact bundled format for
EVRC/EVRC-B encoded speech, the mapping is as follows:
o The MIME type ("audio") goes in SDP "m=" as the media name.
o The MIME subtype ("EVRCB", "EVRC1", or "EVRCB1") goes in SDP
"a=rtpmap" as the encoding name.
o The optional parameters "ptime" and "maxptime" go in the SDP
"a=ptime" and "a=maxptime" attributes, respectively.
o The optional parameter "evrcfixedrate" (for subtypes EVRC1 and
EVRCB1) goes in "a=fmtp" attribute by copying it directly from the
MIME media type string as "evrcfixedrate=value".
o The optional parameters "silencesupp", "dtxmax", "dtxmin", and
"hangover" go in "a=fmtp" attribute by copying it directly from
the MIME media type string as "silencesupp=value", "dtxmax=value",
"dtxmin=value", and "hangover=value", respectively.
Example of usage of EVRCB:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 EVRCB/8000
a=maxptime:120
Example of usage of EVRC1:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 EVRC1/8000
a=fmtp:97 evrcfixedrate=0.5
a=maxptime:120
Example of usage of EVRCB1:
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m=audio 49120 RTP/AVP 97
a=rtpmap:97 EVRCB1/8000
a=fmtp:97 evrcfixedrate=0.25
a=maxptime:100
Example of usage of EVRC1 with DTX:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 EVRC1/8000
a=fmtp:97 evrcfixedrate=0.5
a=fmtp:97 silencesupp=1 dtxmax=50 dtxmin=8 hangover=2
a=maxptime:120
5.6 Usage in Offer/Answer
All SDP parameters in this payload format are declarative, and all
reasonable values are expected to be supported. Thus, the standard
usage of Offer/Answer as described in RFC 3264 [7] should be
followed.
6. Security Considerations
Implementations using the payload defined in this specification are
subject to the security considerations discussed in RFC 3558 [5], RFC
3550 [6], and any appropriate profile (for example RFC3551 [10]).
This payload does not specify any different security services.
7. Changes from Last Revision
(This section is temporary and will be removed when the work on the
draft finishes)
- updated MIME definition text to follow new template/rules
- separate MIME subtypes EVRC1 and EVRCB1 into two subsections
- added ref to RFC3558 in security consideration sections
- clarification that silencesupp is an RTP receiver declaration
- some editorial changes per Magnus's comments
- re-defined 'dtxmax' and 'dtxmin' as "update intervals"
- revised the wording of DTX parameter definitions, including
clarification to the hangover usage
- removed the old RTP marker bit usage section.
8. References
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8.1 Normative 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] "Enhanced Variable Rate Codec, Speech Service Option 3 for
Wideband Spread Spectrum Digital Systems", 3GPP2 C.S0014,
January 1997.
[4] "Enhanced Variable Rate Codec, Speech Service Option XX for
Wideband Spread Spectrum Digital Systems", 3GPP2 C.S0014-B, in
progress.
[5] Li, A., "RTP Payload Format for Enhanced Variable Rate Codecs
(EVRC) and Selectable Mode Vocoders (SMV)", RFC 3558, July 2003.
[6] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 3550, July 2003.
[7] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
the Session Description Protocol (SDP)", RFC 3264, June 2002.
[8] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
8.2 Informative References
[9] Bormann, C., "RObust Header Compression (ROHC): Framework and
four profiles: RTP, UDP, ESP, and uncompressed", RFC 3095,
July 2001.
[10] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", RFC 3551, July 2003.
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Authors' Addresses
Qiaobing Xie
Motorola, Inc.
1501 W. Shure Drive, 2-F9
Arlington Heights, IL 60004
US
Phone: +1-847-632-3028
Email: qxie1@email.mot.com
Rohit Kapoor
Qualcomm Inc.
US
Phone: +1-858-845-1161
Email: rkapoor@qualcomm.com
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