RFC 4788 PROPOSED STANDARD VOICE · RTP · CODECS
EVRC RTP updates: Enhancements to RTP Payload Formats for EVRC Family Codecs
New ways to pack EVRC and EVRC-B voice frames into RTP packets, plus silence suppression settings.
RFC EDITOR
Xie, Q. and R. Kapoor, "Enhancements to RTP Payload Formats for EVRC Family Codecs", RFC 4788, DOI 10.17487/RFC4788, January 2007, <https://www.rfc-editor.org/info/rfc4788>.
BIBTEX
@misc{rfc4788,
series = {Request for Comments},
number = 4788,
howpublished = {RFC 4788},
publisher = {RFC Editor},
doi = {10.17487/RFC4788},
url = {https://www.rfc-editor.org/info/rfc4788},
author = {Q. Xie and R. Kapoor},
title = {{Enhancements to RTP Payload Formats for EVRC Family Codecs}},
pagetotal = 22,
year = 2007,
month = jan,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Network Working Group Q. Xie
Request for Comments: 4788 Motorola
Updates: 3558 R. Kapoor
Category: Standards Track Qualcomm
January 2007
Enhancements to RTP Payload Formats for EVRC Family Codecs
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document updates the Enhanced Variable Rate Codec (EVRC) RTP
payload formats defined in RFC 3558 with several enhancements and
extensions. In particular, it defines support for the header-free
and interleaved/bundled packet formats for the EVRC-B codec, a new
compact bundled format for the EVRC and EVRC-B codecs, as well as
discontinuous transmission (DTX) support for EVRC and EVRC-B-encoded
speech transported via RTP. Voice over IP (VoIP) applications
operating over low bandwidth dial-up and wireless networks require
such enhancements for efficient use of the bandwidth.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Support of EVRC-B Codec . . . . . . . . . . . . . . . . . 3
1.2. Compact (Header-free) Bundled Format . . . . . . . . . . . 3
1.3. Discontinuous Transmission (DTX) . . . . . . . . . . . . . 4
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. EVRC-B Codec . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Compact Bundled Format . . . . . . . . . . . . . . . . . . . . 5
4.1. Single-Rate Operation . . . . . . . . . . . . . . . . . . 5
5. Storage Format for EVRC-B Codec . . . . . . . . . . . . . . . 6
6. Media Type Definitions . . . . . . . . . . . . . . . . . . . . 6
6.1. Registration of Media Type EVRC1 . . . . . . . . . . . . . 6
6.2. Registration of Media Type EVRCB . . . . . . . . . . . . . 9
6.3. Registration of Media Type EVRCB0 . . . . . . . . . . . . 11
6.4. Registration of Media Type EVRCB1 . . . . . . . . . . . . 12
6.5. Updated Registration of Media Type EVRC . . . . . . . . . 13
6.6. Updated Registration of Media Type EVRC0 . . . . . . . . . 15
6.7. Mapping MIME Parameters into SDP . . . . . . . . . . . . . 17
6.8. Usage in Offer/Answer . . . . . . . . . . . . . . . . . . 18
7. Backward Compatibility with RFC 3558 . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
11.1. Normative References . . . . . . . . . . . . . . . . . . . 20
11.2. Informative References . . . . . . . . . . . . . . . . . . 20
1. Introduction
This document defines support for the header-free and interleaved/
bundled packet formats for the EVRC-B codec, a new compact bundled
format for the EVRC and EVRC-B codecs, as well as discontinuous
transmission (DTX) support for EVRC and EVRC-B-encoded speech
transported via RTP. Voice over IP (VoIP) applications operating
over low bandwidth dial-up and wireless networks require such EVRC
RTP payload capabilities for efficient use of the bandwidth.
1.1. Support of EVRC-B Codec
EVRC-B [3] is an extension to EVRC [2] developed in the Third
Generation Partnership Project 2 (3GPP2). EVRC-B [3] compresses each
20 milliseconds of 8000Hz, 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 [2]. One significant enhancement in
EVRC-B is the use of 1/4-rate frames that were not used in EVRC.
This provides lower average data rates (ADRs) compared to EVRC, for a
given voice quality.
Since speech frames encoded by EVRC-B are different from those
encoded by EVRC, EVRC-B and EVRC codecs do not interoperate with each
other. At the initiation of an 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.
1.2. Compact (Header-free) Bundled Format
The current interleaved/bundled packet format defined in RFC 3558
allows bundling of multiple speech frames of different rates in a
single RTP packet, sending mode change requests, and interleaving.
To support these functions, a Table of Contents (ToC) is used in each
RTP packet, in addition to the standard RTP header. The size of the
ToC varies depending on the number of EVRC frames carried in the
packet [4].
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 contain only one
speech frame (of any allowable rate), i.e., bundling is not allowed.
Moreover, interleaving and mode change requests are not supported in
the header-free format [4].
The compact bundled format described in this document presents the
user an alternative to the header-free format defined in RFC 3558.
This format allows bundling of multiple EVRC or EVRC-B frames without
the addition of extra headers, as would be in the case of the
interleaved/bundled format. However, in order to use this compact
bundled format, 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 mode change requests are not supported in
the compact bundled format.
1.3. Discontinuous Transmission (DTX)
Information carried in frames of EVRC and EVRC-B codecs varies little
during periods of silence. The transmission of these frames across
the radio interface in a wireless system is expensive, in terms of
capacity; therefore, suppression of these frames is desirable. Such
an operation is called DTX, also known as silence suppression.
In general, when DTX/silence suppression is applied, the first few
frames of silence may be transmitted at the beginning of the period
of silence to establish background noise. Then, a portion of the
stream of subsequent silence frames is not transmitted, and is
discarded at the sender. At the receiver, background or comfort
noise may be generated by using the previously received silence
frames.
The full detail of DTX/silence suppression operation can be found in
DTX [8] as well as in RFC 3551 [9], and in RFC 3558 [4]. This
document only defines the additional optional MIME parameters
(silencesupp, dtxmax, dtxmin, and hangover) for setting up a DTX/
silence suppression session, where "silencesupp" is for indicating
the capability and willingness of using DTX/silence suppression;
"dtxmax" and "dtxmin", for indicating the desired range of DTX update
interval; and "hangover", for indicating the desired number of
silence frames at the beginning of each silence period to establish
background noise at the receiver (see Section 6.1 for detailed
definition).
The EVRC and EVRC-B codecs, in variable-rate operation mode, send
1/8-rate frames during periods of silence, while in single-rate
operation mode (see Section 4), silence is encoded and sent in frames
of the same rate as that of speech frames. The DTX parameters
defined in this document apply to 1/8-rate frames in the variable-
rate mode and to silence frames in the single-rate operation mode.
For simplicity, in the rest of this document the term "silence frame"
refers either to an 1/8-rate frame in variable-rate operation or a
frame that contains only silence in the signal-rate operation.
2. Conventions
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 [1].
3. EVRC-B Codec
Three RTP packet formats are supported for the EVRC-B codec: the
interleaved/bundled packet format, the header-free packet format, and
the compact bundled packet format. For the interleaved/bundled and
header-free packet formats, 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 RFC 3558 [4], except that the
mode change request field in the ToC MUST be interpreted according to
the definition of the RATE_REDUC parameter in EVRC-B [3]. 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 codec data frames MUST be generated from the output of the codec
following the procedure described in Section 5.2 in RFC 3558 [4], and
all MUST be of the same rate and size.
4.1. Single-Rate Operation
As mentioned earlier, in order to use the compact bundled format, all
the EVRC/EVRC-B data frames in the session MUST be of the same rate.
This packet format may carry only full or half-rate frames.
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 Session Description Protocol (SDP) exchanges. See
Section 6 below for more details.
5. Storage Format for EVRC-B Codec
The storage format is used for storing EVRC-B-encoded speech frames,
e.g., as a file or e-mail attachment.
The file begins with a magic number to identify the vocoder that is
used. The magic number for EVRC-B corresponds to the ASCII character
string:
"#!EVRC-B\n"
(or 0x2321 0x4556 0x5243 0x2d42 0x0a in hexadecimal).
Note that the "\n" is an important part of both this magic number and
the "#!EVRC\n" magic number defined in Section 11 of RFC 3558, and
the "\n" MUST be included in any comparison of either magic number,
since, otherwise, a prefix of the EVRC-B magic number could be
mistaken for the EVRC magic number.
The codec data frames are stored in consecutive order, with a single
ToC entry field, extended to one octet, prefixing each codec data
frame. The ToC field, as defined in Section 5.1 of [4], is extended
to one octet by setting the four most significant bits of the octet
to zero. For example, a ToC value of 4 (a full-rate frame) is stored
as 0x04.
Speech frames lost in transmission and non-received frames MUST be
stored as erasure frames to maintain synchronization with the
original media.
6. Media Type Definitions
6.1. Registration of Media Type EVRC1
Type name: audio
Subtype names: EVRC1
Required parameters: none
Optional parameters:
ptime: See RFC 4566 [7].
maxptime: The maximum amount of media that can be encapsulated in
each packet, expressed as time in milliseconds. The time MUST
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 MUST be 200 milliseconds.
fixedrate: 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.
silencesupp: Permissible values are 0 and 1. A value of 1
indicates that the sender of this parameter: a) is capable of
receiving silence-suppressed speech using DTX, AND b) is
capable of and will send out silence-suppressed speech using
DTX, unless the other end indicates that it does not want to
receive silence-suppressed speech using DTX.
A value of 0 indicates that the sender of this parameter: a)
does NOT want to receive silence-suppressed speech using DTX,
AND b) will NOT send out silence-suppressed speech using DTX.
If this parameter is not present, the default value 1 MUST be
assumed. If the RTP receiver indicates through the use of SIP
signaling or other means that it is incapable of or unwilling
to use silence suppression using DTX, silence suppression using
DTX as specified in this document MUST NOT be used for the
session.
dtxmax: Permissible values are from 0 to 255. 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 silence frame 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 32, as specified in [8], MUST be assumed.
This parameter MUST be ignored if silence suppression using DTX
is not used for the session.
Note also that if the RTP receiver elects to detect DTX using
dtxmax, the dtxmax parameter will affect the amount of delay
the RTP receiver sees before detecting DTX in the stream.
dtxmin: Permissible values are from 0 to 255. 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 12, as
specified in [8] MUST be assumed. This parameter MUST be
ignored if silence suppression using DTX is not used for the
session.
hangover: Permissible values are from 0 to 255. Indicates the
number of consecutive silence frames transmitted at the end of
an active speech interval but before the DTX interval begins.
When setting up an RTP session that uses DTX, an RTP receiver
can use this parameter to signal the number of silence frames
it expects to receive before the beginning of DTX. While
hangover=0 is allowed, it is RECOMMENDED that hangover be set
to 1 or greater since the presence of silence frames at the end
of an 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 1, as specified in [8] MUST be assumed. This
parameter MUST be ignored if silence suppression using DTX is
not used for the session.
Encoding considerations:
This media type is framed binary data (see RFC 4288, Section 4.8)
and is defined for transfer of EVRC-encoded data via RTP, using
the compact bundled format as described in RFC 4788.
Security considerations: See Section 9 of RFC 4788.
Interoperability considerations: none
Published specification:
The EVRC vocoder is specified in 3GPP2 C.S0014 [2]. Transfer
method with compact bundled RTP format is specified in RFC 4788.
Applications that 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 <Qiaobing.Xie@motorola.com>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing; hence, it is only defined
for transfer via RTP (RFC 3550 [5]). Transfer within other
framing protocols is not defined at this time.
Author:
Qiaobing Xie
Change controller:
IETF Audio/Video Transport working group delegated from the IESG.
6.2. Registration of Media Type EVRCB
Type name: audio
Subtype names: EVRCB
Required parameters: none
Optional parameters:
ptime: see RFC 4566 [7].
maxptime: The maximum amount of media that can be encapsulated in
each packet, expressed as time in milliseconds. The time MUST
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 MUST be 200 milliseconds.
maxinterleave: Maximum number for interleaving length (field LLL
in the Interleaving Octet). The interleaving lengths used in
the entire session MUST NOT exceed this maximum value. If not
signaled, the maxinterleave length MUST be 5.
silencesupp: see Section 6.1 for definition. If this parameter
is not present, the default value 1 MUST be assumed.
dtxmax: see Section 6.1
dtxmin: see Section 6.1
hangover: see Section 6.1
Encoding considerations:
This media type is framed binary data (see RFC 4288, Section 4.8)
and is defined for transfer of EVRC-B-encoded data via RTP using
the Interleaved/Bundled packet format specified in RFC 3558 [4].
Security considerations: See Section 9 of RFC 4788.
Interoperability considerations: none
Published specification:
The EVRC-B vocoder is specified in 3GPP2 C.S0014-B [3]. Transfer
method with Interleaved/Bundled packet format via RTP is specified
in RFC 3558.
Applications that use this media type:
It is expected that many VoIP applications (as well as mobile
applications) will use this type.
Additional information:
The following information applies for storage format only.
Magic number: #!EVRC-B\n (see Section 5 of RFC 4788)
File extensions: evb, EVB
Macintosh file type code: None
Object identifier or OID: None
Person & email address to contact for further information:
Qiaobing Xie <Qiaobing.Xie@motorola.com>
Intended usage: COMMON
Restrictions on usage:
This media type may be used with RTP framing (RFC 3550 [5]) and as
a storage format. When used with RTP, the procedures in Section 3
MUST be followed. In all other contexts, the storage format
defined in Section 5 MUST be used.
Author:
Qiaobing Xie
Change controller:
IETF Audio/Video Transport working group delegated from the IESG.
6.3. Registration of Media Type EVRCB0
Type name: audio
Subtype names: EVRCB0
Required parameters: none
Optional parameters:
silencesupp: see Section 6.1 for definition. If this parameter
is not present, the default value 1 MUST be assumed.
dtxmax: see Section 6.1
dtxmin: see Section 6.1
hangover: see Section 6.1
Encoding considerations:
This media type is framed binary data (see RFC 4288, Section 4.8)
and is defined for transfer of EVRC-B-encoded data via RTP using
the Header-Free packet format specified in RFC 3558 [4].
Security considerations: See Section 9 of RFC 4788.
Interoperability considerations: none
Published specification:
The EVRC-B vocoder is specified in 3GPP2 C.S0014-B [3]. Transfer
method with Header-Free packet format via RTP is specified in RFC
3558 and RFC 4788.
Applications that 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 <Qiaobing.Xie@motorola.com>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing; hence, it is only defined
for transfer via RTP (RFC 3550 [5]). Transfer within other
framing protocols is not defined at this time.
Author:
Qiaobing Xie
Change controller:
IETF Audio/Video Transport working group delegated from the IESG.
6.4. Registration of Media Type EVRCB1
Type name: audio
Subtype names: EVRCB1
Required parameters: none
Optional parameters:
ptime: see RFC 4566 [7].
maxptime: The maximum amount of media that can be encapsulated in
each packet, expressed as time in milliseconds. The time MUST
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 MUST be 200 milliseconds.
fixedrate: 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.
silencesupp: see Section 6.1 for definition. If this parameter
is not present, the default value 1 MUST be assumed.
dtxmax: see Section 6.1
dtxmin: see Section 6.1
hangover: see Section 6.1
Encoding considerations:
This media type is framed binary data (see RFC 4288, Section 4.8)
and is defined for transfer of EVRC-B-encoded data via RTP using
the compact bundled format as described in RFC 4788.
Security considerations: See Section 9 of RFC 4788.
Interoperability considerations: none.
Published specification:
The EVRC-B vocoder is specified in 3GPP2 C.S0014-B [3]. Transfer
method with compact bundled RTP format is specified in RFC 4788.
Applications that 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 <Qiaobing.Xie@motorola.com>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing; hence, it is only defined
for transfer via RTP (RFC 3550 [5]). Transfer within other
framing protocols is not defined at this time.
Author:
Qiaobing Xie
Change controller:
IETF Audio/Video Transport working group delegated from the IESG.
6.5. Updated Registration of Media Type EVRC
(The definition is from RFC 3558, added with the optional DTX
parameters, and updated with the new template specified in [10].)
Type name: audio
Subtype names: EVRC
Required parameters: none
Optional parameters:
ptime: Defined as usual for RTP audio (see RFC 4566).
maxptime: The maximum amount of media that 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.
maxinterleave: Maximum number for interleaving length (field LLL
in the Interleaving Octet). The interleaving lengths used in
the entire session MUST NOT exceed this maximum value. If not
signaled, the maxinterleave length SHALL be 5.
silencesupp: see Section 6.1 for definition. If this parameter
is not present, the default value 1 MUST be assumed.
dtxmax: see Section 6.1
dtxmin: see Section 6.1
hangover: see Section 6.1
Encoding considerations:
This media type is framed binary data (see RFC 4288, Section 4.8),
and is defined for transfer of EVRC-encoded data via RTP using the
Interleaved/Bundled packet format specified in Sections 4.1, 6,
and 7 of RFC 3558. It is also defined for other transfer methods
using the storage format specified in Section 11 of RFC 3558.
Security considerations: See Section 14, "Security Considerations",
of RFC 3558.
Interoperability considerations:
The DTX parameters are receiver options. Existing RFC 3558
implementations will not send any of the DTX parameters in their
SDP and will ignore any DTX parameters they receive. The adaptive
DTX behavior of DTX-capable EVRC codecs (as detailed in [8],
Section 4.3.5) ensures interoperability with non-DTX EVRC codecs.
Published specification:
The EVRC vocoder is specified in 3GPP2 C.S0014 [2]. Transfer
methods are specified in RFC 3558.
Applications that use this media type:
It is expected that many VoIP applications (as well as mobile
applications) will use this type.
Additional information:
The following information applies for storage format only.
Magic number: #!EVRC\n (see Section 11 of RFC 3558)
File extensions: evc, EVC
Macintosh file type code: none
Object identifier or OID: none
Person & email address to contact for further information:
Qiaobing Xie <Qiaobing.Xie@motorola.com>
Intended usage: COMMON
Restrictions on usage:
This media type may be used with RTP framing (RFC 3550 [5]) and as
a storage format. When used with RTP, the procedures in RFC 3558,
Section 4.1, MUST be followed. In all other contexts, the storage
format defined in RFC 3558, Section 11, MUST be used.
Author:
Adam Li/Qiaobing Xie
Change controller:
IETF Audio/Video Transport working group delegated from the IESG.
6.6. Updated Registration of Media Type EVRC0
(The definition is from RFC 3558, added with the optional DTX
parameters, and updated with the new template specified in [10].)
Type name: audio
Subtype names: EVRC0
Required parameters: none
Optional parameters:
silencesupp: see Section 6.1 for definition. If this parameter
is not present, the default value 1 MUST be assumed.
dtxmax: see Section 6.1
dtxmin: see Section 6.1
hangover: see Section 6.1
Encoding considerations:
This media type is framed binary data (see RFC 4288, Section 4.8)
and is only defined for transfer of EVRC-encoded data via RTP
using the Header-Free packet format specified in Section 4.2 of
RFC 3558.
Security considerations: See Section 14, "Security Considerations",
of RFC 3558.
Interoperability considerations:
The DTX parameters are receiver options. Existing RFC 3558
implementations will not send any of the DTX parameters in their
SDP and will ignore any DTX parameters they receive. The adaptive
DTX behavior of DTX-capable EVRC codecs (as detailed in [8],
Section 4.3.5) ensures interoperability with non-DTX EVRC codecs.
Published specification:
The EVRC vocoder is specified in 3GPP2 C.S0014 [2]. Transfer
methods are specified in RFC 3558.
Applications that 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 <Qiaobing.Xie@motorola.com>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing; hence, it is only defined
for transfer via RTP (RFC 3550 [5]). Transfer within other
framing protocols is not defined at this time.
Author:
Adam Li/Qiaobing Xie
Change controller:
IETF Audio/Video Transport working group delegated from the IESG.
6.7. 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)
[7], 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 ("EVRC", "EVRC0", "EVRC1", "EVRCB", EVRCB0", or
"EVRCB1") goes in SDP "a=rtpmap" as the encoding name.
o The optional parameters "ptime" and "maxptime" (for subtypes EVRC,
EVRC1, EVRCB, and EVRCB1) go in the SDP "a=ptime" and "a=maxptime"
attributes, respectively.
o The optional parameter "maxinterleave" (for subtypes EVRC and
EVRCB) goes in the SDP "a=fmtp" attribute by copying it directly
from the MIME media type string as "maxinterleave=value".
o The optional parameter "fixedrate" (for subtypes EVRC1 and EVRCB1)
goes in the "a=fmtp" attribute by copying it directly from the
MIME media type string as "fixedrate=value".
o The optional parameters "silencesupp", "dtxmax", "dtxmin", and
"hangover" go in the "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 EVRC1:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 EVRC1/8000
a=fmtp:97 fixedrate=0.5
a=maxptime:120
Example of usage of EVRCB:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 EVRCB/8000
a=maxptime:120
Example of usage of EVRCB0:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 EVRCB0/8000
Example of usage of EVRCB1:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 EVRCB1/8000
a=fmtp:97 fixedrate=0.5
a=maxptime:100
Example of usage of EVRC with DTX with silencesupp=1:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 EVRC/8000
a=fmtp:97 silencesupp=1 dtxmax=32 dtxmin=12 hangover=1
Example of usage of EVRC with DTX with silencesupp=0:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 EVRC/8000
a=fmtp:97 silencesupp=0
6.8. Usage in Offer/Answer
All SDP parameters in this payload format are declarative, and all
reasonable values are expected to be supported. In particular, when
DTX is supported, the RTP sender implementation SHOULD support
hangover, dtxmin, and dtxmax values from 0 to 255. Thus, the
standard usage of Offer/Answer, as described in RFC 3264 [6], SHOULD
be followed.
In addition, the following rules MUST be followed while negotiating
DTX parameters:
1. If any DTX parameter is not present in either offer and/or
answer, the default value of the DTX parameter MUST be assumed.
2. If silencesupp is present and set to 0 in either offer or answer,
the values of all received DTX parameters other than silencesupp
SHOULD be ignored.
3. In an offer or answer, the value of dtxmax SHOULD always be
larger than or equal to the value of dtxmin, regardless of
whether the values are indicated explicitly or implicitly by
default. Moreover, if the indicated value of dtxmin is larger
than that of dtxmax, an RTP sender MUST ignore the indicated
values and MUST fall back on using the default dtxmin and dtxmax
values.
7. Backward Compatibility with RFC 3558
This document adds new optional DTX parameters to the original EVRC
payload subtypes "EVRC" and "EVRC0" defined in RFC 3558. Since the
new DTX parameters are receiver options, we expect that the existing
RFC 3558 implementations will not send any of the DTX parameters in
their SDP and will ignore any DTX parameters they receive. The
adaptive DTX behavior of DTX-capable EVRC codecs (as detailed in [8],
Section 4.3.5) ensures the backward interoperability between the DTX-
capable EVRC codec and non-DTX EVRC codecs.
8. IANA Considerations
Four (4) new MIME subtype registrations - "EVRC1", "EVRCB", "EVRCB0",
and "EVRCB1" - are defined in this document (see Section 6.1 -
Section 6.4) for EVRC-B and compact bundled payload format support.
For all the EVRC and EVRC-B RTP payload formats defined in RFC 3558
[4] and RFC 4788, four additional optional parameters -
"silencesupp", "dtxmax", "dtxmin", and "hangover" - are defined and
used in DTX.
The MIME subtype registrations "EVRC" and "EVRC0", originally defined
in RFC 3558 [4], are updated with the optional DTX parameters (see
Sections 6.5 and 6.6).
9. Security Considerations
Implementations using the payload defined in this specification are
subject to the security considerations discussed in RFC 3558 [4], RFC
3550 [5], and any appropriate profile (for example, RFC 3551 [9]).
This payload does not specify any different security services.
10. Acknowledgements
The following people have made significant contributions to this
document (in alphabetical order): Parag Agashe, Jim Ashley,
Harikishan Desineni, Serafin Diaz, Harinath Garudadri, Gouri
Johanssen, Ananth Kandhadai, Waqar Mohsin, Ashok Roy, Gino Scribano,
and Gajinder Singh Vij.
Special thanks to Colin Perkins, Magnus Westerlund, and Adam Li for
their careful review and comments that significantly improved the
quality of this document.
11. References
11.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] "Enhanced Variable Rate Codec, Speech Service Option 3 for
Wideband Spread Spectrum Digital Systems", 3GPP2 C.S0014,
January 1997.
[3] "Enhanced Variable Rate Codec, Speech Service Option 3 and 68
for Wideband Spread Spectrum Digital Systems", 3GPP2 C.S0014-B
v1.0, May 2006.
[4] Li, A., "RTP Payload Format for Enhanced Variable Rate Codecs
(EVRC) and Selectable Mode Vocoders (SMV)", RFC 3558,
July 2003.
[5] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 3550, July 2003.
[6] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
the Session Description Protocol (SDP)", RFC 3264, June 2002.
[7] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[8] "Discontinuous Transmission (DTX) of Speech in cdma2000
Systems", 3GPP2 C.S0076-0, Version 1.0, December 2005.
11.2. Informative References
[9] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", RFC 3551, July 2003.
[10] Casner, S., "Media Type Registration of RTP Payload Formats",
Work in Progress, March 2006.
Authors' Addresses
Qiaobing Xie
Motorola, Inc.
1501 W. Shure Drive, 2-F9
Arlington Heights, IL 60004
US
Phone: +1-847-632-3028
EMail: Qiaobing.Xie@Motorola.com
Rohit Kapoor
Qualcomm Inc.
US
Phone: +1-858-845-1161
EMail: rkapoor@qualcomm.com
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On this page
PROBLEMVoIP over low-bandwidth dial-up and wireless networks needs to waste as little bandwidth as possible, but RFC 3558 only had header-free packets (one frame each) or interleaved/bundled packets with a table of contents, and it did not cover the EVRC-B codec.
MECHANISMRFC 4788 carries EVRC-B in the existing formats, adds a compact bundled format of same-rate frames with no extra header, adds a storage format for EVRC-B, and defines optional DTX parameters and four new media subtypes.
WHY IT MATTERSImplementations can pick the format that suits their link, and can negotiate silence suppression through SDP. It updates RFC 3558 and was itself later updated by RFC 5188.
§1What the Update Adds
RFC 4788 updates the RTP payload formats for the Enhanced Variable Rate Codec (EVRC) family that were defined in RFC 3558. It defines three things: support for the header-free and interleaved/bundled packet formats for the EVRC-B codec, a new compact bundled format for EVRC and EVRC-B, and discontinuous transmission (DTX) support for both codecs.
The motivation given is that VoIP applications over low-bandwidth dial-up and wireless networks need these capabilities to use bandwidth efficiently.
The three formats differ in overhead. The interleaved/bundled format from RFC 3558 allows several frames of different rates in one packet, mode change requests and interleaving, at the cost of a table of contents (ToC). The header-free format needs no ToC because each packet holds exactly one frame, with no bundling, interleaving or mode change requests. The new compact bundled format sits between them.
There were two ways to pack voice into a packet: a fancy box with a label, or one chunk per envelope. This RFC adds a third: several matching chunks in one envelope, with no label.
§3The EVRC-B Codec
EVRC-B is an extension to EVRC developed in 3GPP2. It compresses each 20 milliseconds of 8000 Hz, 16-bit sampled speech into a frame of one of four sizes: Rate 1 (171 bits), Rate 1/2 (80 bits), Rate 1/4 (40 bits) or Rate 1/8 (16 bits). Like EVRC, it also has two zero-bit frame types, null frames and erasure frames. The use of 1/4-rate frames is the key enhancement, giving lower average data rates for a given voice quality.
Because EVRC-B frames differ from EVRC frames, the two codecs do not interoperate. At session start the sender and receiver must indicate that EVRC-B is in use, for example with MIME subtypes separate from those of EVRC.
For the interleaved/bundled and header-free formats, EVRC-B works exactly as EVRC does in RFC 3558, except that the mode change request field in the ToC MUST be interpreted according to the RATE_REDUC parameter of EVRC-B.
EVRC-B is a newer way of squeezing speech that sometimes uses a smaller chunk. Old and new squeezers cannot read each other's chunks, so each side has to say which one it uses.
§4The Compact Bundled Format
A compact bundled packet is an RTP header followed by one or more consecutive EVRC or EVRC-B data frames, all of the same rate and size, with no ToC and no other header. The frames MUST be produced by the procedure in Section 5.2 of RFC 3558.
The price of the saving is that the whole session must use a single rate. The format may carry only full-rate or half-rate frames, and the rate is determined during session setup signaling, for example through SDP. As with the header-free format, interleaving and mode change requests are not supported.
It is a packet with the standard envelope and then a stack of identical-size voice chunks, with no table of contents because everyone already knows the chunk size.
§5Storage Format for EVRC-B
EVRC-B speech can also be stored, for example as a file or e-mail attachment. The file begins with a magic number, the ASCII string #!EVRC-B\n (0x2321 0x4556 0x5243 0x2d42 0x0a). The newline is part of the magic number and MUST be included in comparisons, otherwise a prefix of the EVRC-B number could be mistaken for the EVRC one, #!EVRC\n.
Frames follow in consecutive order, each prefixed by a single ToC entry extended to one octet by setting the four most significant bits to zero. A full-rate ToC value of 4 is therefore stored as 0x04. Frames lost in transmission or not received MUST be stored as erasure frames to stay in sync with the original media.
When saved to a file, the voice starts with a secret handshake word so programs know which codec it is, and every chunk gets a small tag. Missing chunks are saved as blanks to keep the timing.
§6Media Types and DTX Parameters
Four new audio subtypes are registered, and two from RFC 3558 are updated.
EVRC1: EVRC in the compact bundled format.EVRCB: EVRC-B in the interleaved/bundled format.EVRCB0: EVRC-B in the header-free format.EVRCB1: EVRC-B in the compact bundled format.EVRCandEVRC0: the existing RFC 3558 subtypes, with the DTX parameters added.
DTX, also called silence suppression, saves capacity in wireless systems by not sending most silence frames. Four optional parameters control it. silencesupp (0 or 1) says whether the sender can and will use DTX, and defaults to 1. dtxmax and dtxmin (0 to 255) give the longest and shortest interval in frames between DTX updates, with defaults of 32 and 12. hangover (0 to 255) is the number of silence frames sent after speech before DTX begins, default 1, and the RFC recommends 1 or more.
Other parameters include maxptime (default 200 milliseconds, ideally a multiple of the 20 ms frame), maxinterleave (default 5) and fixedrate for the compact formats (0.5 or 1, with 1/2 rate assumed if absent). In variable-rate mode silence is sent as 1/8-rate frames; in single-rate mode it is sent in frames of the same rate as speech.
The RFC gives each way of packing its own name tag. It also adds settings for how often to send a quiet noise sample while nobody speaks, so the line does not carry empty chatter.
- EVRCInterleaved/bundled — EVRC, RFC 3558 format, DTX parameters added.
- EVRC0Header-free — EVRC, RFC 3558 format, DTX parameters added.
- EVRC1Compact bundled — EVRC, new in RFC 4788.
- EVRCBInterleaved/bundled — EVRC-B, new in RFC 4788.
- EVRCB0Header-free — EVRC-B, new in RFC 4788.
- EVRCB1Compact bundled — EVRC-B, new in RFC 4788.
§6.7SDP Mapping and Offer/Answer
The media type parameters map into the Session Description Protocol (RFC 4566). The type audio goes in the m= line, the subtype in a=rtpmap, ptime and maxptime in their own attributes, and the remaining parameters in a=fmtp as name=value.
All SDP parameters in this format are declarative, so the normal offer/answer usage of RFC 3264 SHOULD be followed. Three extra rules apply to DTX. A missing DTX parameter takes its default. If silencesupp is 0 in either the offer or the answer, other DTX parameters SHOULD be ignored. And dtxmax SHOULD never be smaller than dtxmin; if it is, the RTP sender MUST ignore the values and fall back on the defaults.
Both sides list their settings when they call each other. If one side says no silence tricks, the other ignores the silence settings. If the numbers make no sense, they use the usual ones.
EVRCB1 session description
m=audio 49120 RTP/AVP 97
a=rtpmap:97 EVRCB1/8000
a=fmtp:97 fixedrate=0.5
a=maxptime:100§7Compatibility and Security
The new DTX parameters are receiver options added to the existing EVRC and EVRC0 subtypes. Existing RFC 3558 implementations will not send them and will ignore them if received, and the adaptive DTX behavior of DTX-capable EVRC codecs keeps them interoperable with codecs that do not use DTX.
On security, the payload is subject to the considerations of RFC 3558, RFC 3550 and any suitable profile such as RFC 3551. It specifies no different security services.
Old phones simply do not know about the new settings and ignore them, so everything still works. Safety rules are the same as for the earlier voice packing.
§8References
[1]RFC 2119Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.March 1997[4]RFC 3558Li, A., "RTP Payload Format for Enhanced Variable Rate Codecs (EVRC) and Selectable Mode Vocoders (SMV)", RFC 3558, July 2003.July 2003[5]RFC 3550Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, July 2003.July 2003[6]RFC 3264Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with the Session Description Protocol (SDP)", RFC 3264, June 2002.July 2002[7]RFC 4566Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.July 2006[9]RFC 3551Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 3551, July 2003.July 2003