Network Working Group Q. Xie
Internet-Draft Motorola
Expires: March 27, 2006 R. Kapoor
Qualcomm
September 23, 2005
Enhancements to RTP Format for EVRC Family Codecs
draft-xie-avt-compact-bundle-evrc-01.txt
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Copyright (C) The Internet Society (2005).
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 and EVRCB1 . . . . . . . 6
5.3 Payload Format Parameters for DTX . . . . . . . . . . . . 8
5.4 Mapping MIME Parameters into SDP . . . . . . . . . . . . . 9
5.5 Usage in Offer/Answer . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1 Normative References . . . . . . . . . . . . . . . . . . . 10
7.2 Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . 12
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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 several enhancements and extensions to RFC3558
EVRC RTP payload format [5]. 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.
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
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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.
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/removal 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.
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
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
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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-
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.
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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
Media Type name: audio
Media 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 7 of RFC XXXX.
Public 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.
Person & email address to contact for further
information: Qiaobing.Xie@motorola.com
Intended usage: COMMON. It is expected that many VoIP applications
(as well as mobile applications) will use this type.
Author/Change controller:
* Qiaobing.Xie@motorola.com
* IETF Audio/Video transport working group
5.2 Registration of Media Type EVRC1 and EVRCB1
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Media Type name: audio
Media subtype names: EVRC1, 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/EVRC-B rate of the session while in
single rate operation. Valid values for EVRC1 and EVRCB1 subtype
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
or EVRC-B encoded data via RTP using the compact bundled format as
described in RFC XXXX.
Security considerations: See Section 7 of RFC XXXX.
Public specification: The EVRC and EVRC-B vocoders are specified in
3GPP2 C.S0014 and 3GPP-2 C.S0014-B, respectively. Transfer method
with compact bundled RTP format is specified in RFC XXXX.
Person & email address to contact for further
information: Qiaobing.Xie@motorola.com
Intended usage: COMMON. It is expected that many VoIP applications
(as well as mobile applications) will use this type.
Author/Change controller:
* Qiaobing.Xie@motorola.com
* IETF Audio/Video transport working group
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5.3 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
the sender's ability and desire to support silence suppression
using DTX. A value of 0 or when not present, it indicates that
the sender is either incapable or unwilling to use silence
suppression using DTX. Unless indicated by both the RTP sender
and receiver, silence suppression using DTX MUST NOT be used for
the session.
dtxmax: indicates the maximum number of 1/8 rate frames (or silence
frames in the single rate operation mode) that may be continuously
suppressed and thus provides a "keep alive" function to the
bearer. When setting up an RTP session that uses DTX, an RTP
receiver can use this parameter to signal the maximum number of
1/8 rate frames (or silence frames in the single rate operation
mode) that may be continuously suppressed by the RTP sender. If
this parameter is not present for a session that uses DTX, the
default value MUST be assumed. This parameter MUST be ignored if
silence suppression using DTX is not used for the session.
[Editor's note: Default value is 50 and will be specified in a
3GPP2 TSG-C 1.1 document].
dtxmin: indicates the minimum number of contiguous 1/8 rate frames
(or silence frames in the single rate operation mode) that MUST be
suppressed when they occur in the output of the codec. When
setting up an RTP session that uses DTX, an RTP receiver can use
this parameter to signal the minimum number of contiguous 1/8 rate
frames (or silence frames in the single rate operation mode) that
SHOULD be suppressed by the RTP sender. If this parameter is not
present for a session that uses DTX, the default value MUST be
assumed. This parameter MUST be ignored if silence suppression
using DTX is not used for the session. [Editor's note: Default
value is 8 and will be specified in a 3GPP2 TSG-C 1.1 document.]
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. If this parameter is not
present for a session that uses DTX, the default value SHOULD be
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assumed. This parameter MUST be ignored if silence suppression
using DTX is not used for the session. [Editor's note: Default
value is 2 and will be specified in a 3GPP2 TSG-C 1.1 document.]
Additionally, the RTP marker (M) bit MUST be used by the RTP senders
to indicate that a DTX interval may begin. The marker bit may be
used by RTP receivers in order to assist in managing the play-out
buffers. In particular, the M bit MUST be set to '1' to indicate
that discontinuous transmission may begin with the next packet and
MUST be set to '0' otherwise. [Editor's note: This is still very
confusing. There is no specific behavior of the receiver being
requested here. Either this will be clarified in the following
revisions or this para will be removed.]
5.4 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:
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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:
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
5.5 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 the RTP
specification RFC 3550 [6] and any appropriate profile (for example
RFC3551 [10]). This payload does not specify any different security
services.
7. References
7.1 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,
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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.
7.2 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.
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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