draft-ietf-avt-rtp-evrc-nw-08.txt   draft-ietf-avt-rtp-evrc-nw-09.txt 
Network Working Group Z. Fang Network Working Group Z. Fang
Internet-Draft Qualcomm Incorporated Internet-Draft Qualcomm Incorporated
Intended status: Standards Track November 15, 2012 Intended status: Standards Track December 13, 2012
Expires: May 19, 2013 Expires: June 16, 2013
RTP payload format for Enhanced Variable Rate Narrowband-Wideband Codec RTP payload format for Enhanced Variable Rate Narrowband-Wideband Codec
(EVRC-NW) (EVRC-NW)
draft-ietf-avt-rtp-evrc-nw-08 draft-ietf-avt-rtp-evrc-nw-09
Abstract Abstract
This document specifies real-time transport protocol (RTP) payload This document specifies real-time transport protocol (RTP) payload
formats to be used for the Enhanced Variable Rate Narrowband-Wideband formats to be used for the Enhanced Variable Rate Narrowband-Wideband
Codec (EVRC-NW). Three media type registrations are included for Codec (EVRC-NW). Three media type registrations are included for
EVRC-NW RTP payload formats. In addition, a file format is specified EVRC-NW RTP payload formats. In addition, a file format is specified
for transport of EVRC-NW speech data in storage mode applications for transport of EVRC-NW speech data in storage mode applications
such as e-mail. such as e-mail.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 19, 2013. This Internet-Draft will expire on June 16, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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16. Security Considerations . . . . . . . . . . . . . . . . . . . 29 16. Security Considerations . . . . . . . . . . . . . . . . . . . 29
17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30 17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
17.1. Normative References . . . . . . . . . . . . . . . . . . . 30 17.1. Normative References . . . . . . . . . . . . . . . . . . . 30
17.2. Informative References . . . . . . . . . . . . . . . . . . 31 17.2. Informative References . . . . . . . . . . . . . . . . . . 31
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 32 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 32
1. Introduction 1. Introduction
This document specifies the payload formats for packetization of This document specifies the payload formats for packetization of
EVRC-NW encoded speech signals into the real-time transport protocol EVRC-NW encoded speech signals into the real-time transport protocol
(RTP). It defines support for the header-free, interleaved/bundled (RTP). It defines support for the header-free, interleaved/bundled,
and compact bundle packet formats for the EVRC-NW codec as well as and compact bundle packet formats for the EVRC-NW codec as well as
discontinuous transmission (DTX) support for EVRC-NW encoded speech discontinuous transmission (DTX) support for EVRC-NW encoded speech
transported via RTP. The EVRC-NW codec offers better speech quality transported via RTP. The EVRC-NW codec offers better speech quality
than the EVRC and EVRC-B codecs and better capacity than EVRC-WB than the EVRC and EVRC-B codecs and better capacity than EVRC-WB
codec. EVRC-NW belongs to the EVRC family of codecs. codec. EVRC-NW belongs to the EVRC family of codecs.
2. Conventions 2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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3. Background 3. Background
EVRC-NW is an extension of both the EVRC-B [2] and EVRC-WB [3] speech EVRC-NW is an extension of both the EVRC-B [2] and EVRC-WB [3] speech
codecs developed in 3GPP2 with support for discontinuous transmission codecs developed in 3GPP2 with support for discontinuous transmission
(DTX). It provides enhanced voice quality and high spectral (DTX). It provides enhanced voice quality and high spectral
efficiency. efficiency.
The EVRC-NW codec operates on 20 ms frames, and the default sampling The EVRC-NW codec operates on 20 ms frames, and the default sampling
rate is 16 kHz. Input and output at 8 kHz sampling rate is also rate is 16 kHz. Input and output at 8 kHz sampling rate is also
supported. The EVRC-NW codec can operate in eight modes (0 to 7) supported. The EVRC-NW codec can operate in eight modes (0 to 7)
defined in [4]. EVRC-NW modes 0, 1 and 7 are interoperable with defined in [4]. EVRC-NW modes 0, 1, and 7 are interoperable with
EVRC-WB. EVRC-NW modes 1 to 7 are interoperable with EVRC-B. EVRC-WB. EVRC-NW modes 1 to 7 are interoperable with EVRC-B.
EVRC-NW modes 0 to 6 use the full set or a subset of full rate, 1/2 EVRC-NW modes 0 to 6 use the full set or a subset of full rate, 1/2
rate, 1/4 rate and 1/8 rate frames. EVRC-NW mode 7 uses only 1/2 rate, 1/4 rate and 1/8 rate frames. EVRC-NW mode 7 uses only 1/2
rate and 1/8 rate frames. By default, EVRC-NW supports all rate and 1/8 rate frames. By default, EVRC-NW supports all
narrowband modes (modes 1 to 7). The support of wideband mode (mode narrowband modes (modes 1 to 7). The support of wideband mode (mode
0) is optional. Mode change among modes 1 to 7 (or among modes 0 to 0) is optional. Mode change among modes 1 to 7 (or among modes 0 to
7 if the receiver supports wideband mode) results in codec output 7 if the receiver supports wideband mode) results in codec output
bit-rate change but does not cause any decoding problems at the bit-rate change but does not cause any decoding problems at the
receiver. EVRC-NW provides a standardized solution for packetized receiver. EVRC-NW provides a standardized solution for packetized
voice applications that allow transitions between enhanced quality voice applications that allow transitions between enhanced quality
and increased capacity. The most important service addressed is IP and increased capacity. The most important service addressed is IP
telephony. Target devices can be IP phones or VoIP handsets, media telephony. Target devices can be IP phones or VoIP handsets, media
gateways, voice messaging servers, etc. gateways, voice messaging servers, etc.
4. EVRC-NW codec 4. EVRC-NW codec
The EVRC-NW codec operates on 20 ms frames. It produces output The EVRC-NW codec operates on 20 ms frames. It produces output
frames of one of the four different sizes: 171 bits (Rate 1), 80 bits frames of one of the four different sizes: 171 bits (Rate 1), 80 bits
(Rate 1/2), 40 bits (Rate 1/4) or 16 bits (Rate 1/8). In addition, (Rate 1/2), 40 bits (Rate 1/4), or 16 bits (Rate 1/8). In addition,
there are two zero bit codec frame types: blank (null) frames and there are two zero-bit codec frame types: blank (null) frames and
erasure frames. The default sampling rate is 16 kHz. Input and erasure frames. The default sampling rate is 16 kHz. Input and
output at 8 kHz sampling rate is also supported. output at 8 kHz sampling rate is also supported.
The frame type values and sizes of the associated codec data frames The frame type values and sizes of the associated codec data frames
are listed in the table below: are listed in the table below:
Value Rate Total codec data frame size in bytes (and in bits) Value Rate Total codec data frame size in bytes (and in bits)
-------------------------------------------------------------------- --------------------------------------------------------------------
0 Blank(Null) 0 (0 bit) 0 Blank (Null) 0 (0 bits)
1 1/8 2 (16 bits) 1 1/8 2 (16 bits)
2 1/4 5 (40 bits) 2 1/4 5 (40 bits)
3 1/2 10 (80 bits) 3 1/2 10 (80 bits)
4 1 22 (171 bits; 5 bits padded at the end) 4 1 22 (171 bits; 5 bits padded at the end)
5 Erasure 0 (SHOULD NOT be transmitted by sender) 5 Erasure 0 (SHOULD NOT be transmitted by sender)
5. RTP header usage 5. RTP header usage
The format of the RTP header is specified in RFC 3550 [5]. The The format of the RTP header is specified in RFC 3550 [5]. The
EVRC-NW payload formats (Section 6) use the fields of the RTP header EVRC-NW payload formats (Section 6) use the fields of the RTP header
as specified in RFC 3550 [5]. as specified in RFC 3550 [5].
EVRC-NW has also the capability to operate with 8 kHz sampled input/ EVRC-NW has also the capability to operate with 8 kHz sampled input/
output signals. The decoder does not require a priori knowledge output signals. The decoder does not require a priori knowledge
about the sampling rate of the original signal at the input of the about the sampling rate of the original signal at the input of the
encoder. The decoder output can be at 8kHz or 16kHz regardless of encoder. The decoder output can be at 8 kHz or 16 kHz regardless of
the sampling rate used at the encoder. Therefore, depending on the the sampling rate used at the encoder. Therefore, depending on the
implementation and the electroacoustic audio capabilities of the implementation and the electroacoustic audio capabilities of the
devices, the input of the encoder and/or the output of the decoder devices, the input of the encoder and/or the output of the decoder
can be configured at 8 kHz; however, a 16 kHz RTP clock rate MUST can be configured at 8 kHz; however, a 16 kHz RTP clock rate MUST
always be used. The RTP timestamp is increased by 320 for each 20 always be used. The RTP timestamp is increased by 320 for each 20
milliseconds. milliseconds.
The RTP header marker bit (M) SHALL be set to 1 if the first frame The RTP header marker bit (M) SHALL be set to 1 if the first frame
carried in the packet contains a speech frame which is the first in a carried in the packet contains a speech frame which is the first in a
talkspurt. For all other packets the marker bit SHALL be set to zero talkspurt. For all other packets the marker bit SHALL be set to zero
(M=0). (M=0).
6. Payload format 6. Payload format
Three RTP packet formats are supported for the EVRC-NW codec - the Three RTP packet formats are supported for the EVRC-NW codec - the
interleaved/bundled packet format, the header-free packet format and interleaved/bundled packet format, the header-free packet format, and
the compact bundled packet format. For all these formats, the the compact bundled packet format. For all these formats, the
operational details and capabilities, such as ToC, interleaving, DTX, operational details and capabilities, such as ToC, interleaving, DTX,
and bundling, of EVRC-NW are exactly the same as those defined in and bundling, of EVRC-NW are exactly the same as those defined in
EVRC [6], EVRC-B [2] and EVRC-WB [3], except that EVRC [6], EVRC-B [2] and EVRC-WB [3], except that
1. the mode change request field in the interleaved/bundled packet 1. the mode change request field in the interleaved/bundled packet
format MUST be interpreted according to the definition of the format MUST be interpreted according to the definition of the
RATE_REDUC parameter as defined in EVRC-NW [4]. RATE_REDUC parameter as defined in EVRC-NW [4].
2. the mode change request field in the interleaved/bundled packet 2. the mode change request field in the interleaved/bundled packet
format SHOULD be honored by an EVRCNW encoding end point in an format SHOULD be honored by an EVRCNW encoding end point in an
one-to-one session with a dedicated EVRCNW decoding end point one-to-one session with a dedicated EVRCNW decoding end point
such as in a two-party call or in a conference leg. such as in a two-party call or in a conference leg.
3. the reserved bit field in the first octet of the interleaved/ 3. the reserved bit field in the first octet of the interleaved/
bundled format has only one bit. Bit 1 of the first octet is an bundled format has only one bit. Bit 1 of the first octet is an
EVRC-NW wideband/narrowband encoding capability identfication EVRC-NW wideband/narrowband encoding capability identification
flag. flag.
The media type audio/EVRCNW maps to the interleaved/bundled packet The media type audio/EVRCNW maps to the interleaved/bundled packet
format, audio/EVRCNW0 maps to the header-free packet format and format, audio/EVRCNW0 maps to the header-free packet format, and
audio/EVRCNW1 maps to the compact bundled packet format. audio/EVRCNW1 maps to the compact bundled packet format.
6.1. Encoding capability identification in EVRC-NW interleaved/bundled 6.1. Encoding capability identification in EVRC-NW interleaved/bundled
format format
The EVRC-NW interleaved/bundled format defines an encoding capability The EVRC-NW interleaved/bundled format defines an encoding capability
identification flag, which is used to signal the local EVRC-NW identification flag, which is used to signal the local EVRC-NW
wideband/narrowband encoding capability at the time of construction wideband/narrowband encoding capability at the time of construction
of an RTP packet to the far end of a communication session. This of an RTP packet to the far end of a communication session. This
capability identification flag allows the far end to use the MMM capability identification flag allows the far end to use the MMM
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only. only.
C = 0 : Mode-0 wideband encoding capable C = 0 : Mode-0 wideband encoding capable
= 1 : Mode-0 wideband encoding incapable, i.e. narrowband = 1 : Mode-0 wideband encoding incapable, i.e. narrowband
encoding only. encoding only.
7. Congestion Control Considerations 7. Congestion Control Considerations
Congestion control for RTP SHALL be used in accordance with RFC 3550 Congestion control for RTP SHALL be used in accordance with RFC 3550
[5], and with any applicable RTP profile; e.g., RFC 3551 [7]. [5], and with any applicable RTP profile, e.g., RFC 3551 [7].
Due to the header overhead, the number of frames encapsulated in each Due to the header overhead, the number of frames encapsulated in each
RTP packet influences the overall bandwidth of the RTP stream. RTP packet influences the overall bandwidth of the RTP stream.
Packing more frames in each RTP packet can reduce the number of Packing more frames in each RTP packet can reduce the number of
packets sent and hence the header overhead, at the expense of packets sent and hence the header overhead, at the expense of
increased delay and reduced error robustness. increased delay and reduced error robustness.
8. Storage format for the EVRC-NW Codec 8. Storage format for the EVRC-NW Codec
The storage format is used for storing EVRC-NW encoded speech frames, The storage format is used for storing EVRC-NW encoded speech frames,
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dtxmax: see Section 6.1 in RFC 4788. dtxmax: see Section 6.1 in RFC 4788.
dtxmin: see Section 6.1 in RFC 4788. dtxmin: see Section 6.1 in RFC 4788.
hangover: see Section 6.1 in RFC 4788. hangover: see Section 6.1 in RFC 4788.
Encoding considerations: Encoding considerations:
This media type is framed binary data (see RFC 4288, Section 4.8) and This media type is framed binary data (see RFC 4288, Section 4.8) and
is defined for transfer of EVRC-NW encoded data via RTP using the is defined for transfer of EVRC-NW encoded data via RTP using the
Interleaved/Bundled packet format specified in RFC 3558 [6]. interleaved/bundled packet format specified in RFC 3558 [6].
Security considerations: See Section 16. Security considerations: See Section 16.
Interoperability considerations: None Interoperability considerations: None
Published specification: Published specification:
The EVRC-NW vocoder is specified in 3GPP2 C.S0014-D. The transfer The EVRC-NW vocoder is specified in 3GPP2 C.S0014-D. The transfer
method with the Interleaved/Bundled packet format via RTP is method with the interleaved/bundled packet format via RTP is
specified in RFC 3558 [6]. See Section 6 of RFC XXXX for details for specified in RFC 3558 [6]. See Section 6 of RFC XXXX for details for
EVRC-NW. [Note to the RFC editor: please replace XXXX with the RFC EVRC-NW. [Note to the RFC editor: please replace XXXX with the RFC
number of this document.] number of this document.]
Applications that use this media type: Applications that use this media type:
It is expected that many VoIP applications (as well as mobile It is expected that many VoIP applications (as well as mobile
applications) will use this type. applications) will use this type.
Additional information: Additional information:
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dtxmax: see Section 6.1 in RFC 4788. dtxmax: see Section 6.1 in RFC 4788.
dtxmin: see Section 6.1 in RFC 4788. dtxmin: see Section 6.1 in RFC 4788.
hangover: see Section 6.1 in RFC 4788. hangover: see Section 6.1 in RFC 4788.
Encoding considerations: Encoding considerations:
This media type is framed binary data (see RFC 4288, Section 4.8) and This media type is framed binary data (see RFC 4288, Section 4.8) and
is defined for transfer of EVRC-NW encoded data via RTP using the is defined for transfer of EVRC-NW encoded data via RTP using the
Header-Free packet format specified in RFC 3558 [6]. header-free packet format specified in RFC 3558 [6].
Security considerations: See Section 16. Security considerations: See Section 16.
Interoperability considertaions: None Interoperability considerations: None
Published specification: Published specification:
The EVRC-NW vocoder is specified in 3GPP2 C.S0014-D. The transfer The EVRC-NW vocoder is specified in 3GPP2 C.S0014-D. The transfer
method with the Header-Free packet format via RTP is specified in RFC method with the header-free packet format via RTP is specified in RFC
3558 [6]. 3558 [6].
Applications that use this media type: Applications that use this media type:
It is expected that many VoIP applications (as well as mobile It is expected that many VoIP applications (as well as mobile
applications) will use this type. applications) will use this type.
Additional information: None Additional information: None
Person & email address to contact for further information: Person & email address to contact for further information:
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Optional parameters: Optional parameters:
These parameters apply to RTP transfer only. These parameters apply to RTP transfer only.
mode-set-recv: A subset of EVRC-NW modes. Possible values are a mode-set-recv: A subset of EVRC-NW modes. Possible values are a
comma separated list of modes from the set {0,1} (see Table 2.6.1.2-4 comma separated list of modes from the set {0,1} (see Table 2.6.1.2-4
in 3GPP2 C.S0014-D). A decoder can use this attribute to inform an in 3GPP2 C.S0014-D). A decoder can use this attribute to inform an
encoder of its preference to operate in a specified subset of modes. encoder of its preference to operate in a specified subset of modes.
A value of 0 signals the support for wideband fixed rate (full or A value of 0 signals the support for wideband fixed rate (full or
half rate, depending on the value of 'fixedrate' parameter). A value half rate, depending on the value of 'fixedrate' parameter). A value
of 1 signals narroband fixed rate (full or half rate, depending on of 1 signals narrowband fixed rate (full or half rate, depending on
the value of 'fixedrate' parameter). Absence of this parameter the value of 'fixedrate' parameter). Absence of this parameter
signals the mode 1. signals the mode 1.
ptime: see RFC 4566. ptime: see RFC 4566.
maxptime: see RFC 4566. maxptime: see RFC 4566.
fixedrate: Indicates the EVRC-NW rate of the session while in single fixedrate: Indicates the EVRC-NW rate of the session while in single
rate operation. Valid values include: 0.5 and 1, where a value of 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 0.5 indicates the 1/2 rate while a value of 1 indicates the full
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dtxmax: see Section 6.1 in RFC 4788. dtxmax: see Section 6.1 in RFC 4788.
dtxmin: see Section 6.1 in RFC 4788. dtxmin: see Section 6.1 in RFC 4788.
hangover: see Section 6.1 in RFC 4788. hangover: see Section 6.1 in RFC 4788.
Encoding considerations: Encoding considerations:
This media type is framed binary data (see RFC 4288, Section 4.8) and This media type is framed binary data (see RFC 4288, Section 4.8) and
is defined for transfer of EVRC-NW encoded data via RTP using the is defined for transfer of EVRC-NW encoded data via RTP using the
Compact Bundled packet format specified in RFC 4788. compact bundled packet format specified in RFC 4788.
Security considerations: See Section 16 Security considerations: See Section 16
Interoperability considertaions: None Interoperability considerations: None
Published specification: Published specification:
The EVRC-NW vocoder is specified in 3GPP2 C.S0014-D. The transfer The EVRC-NW vocoder is specified in 3GPP2 C.S0014-D. The transfer
method with the Compact Bundled packet format via RTP is specified in method with the compact bundled packet format via RTP is specified in
RFC 4788. RFC 4788.
Applications that use this media type: Applications that use this media type:
It is expected that many VoIP applications (as well as mobile It is expected that many VoIP applications (as well as mobile
applications) will use this type. applications) will use this type.
Additional information: None Additional information: None
Person & email address to contact for further information: Person & email address to contact for further information:
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Zheng Fang <zfang@qualcomm.com> Zheng Fang <zfang@qualcomm.com>
Change controller: Change controller:
IETF Payload working group delegated from the IESG. IETF Payload working group delegated from the IESG.
10. SDP mode attributes for EVRC-NW 10. SDP mode attributes for EVRC-NW
'mode-set-recv' can be used by a decoder to inform an encoder of its 'mode-set-recv' can be used by a decoder to inform an encoder of its
preference to operate in a specified subset of modes. Note that preference to operate in a specified subset of modes. Note that
indicating a preference, implicitly indicates support for that indicating a preference implicitly indicates support for that
capability. If mode 0 is not preferred for media type EVRCNW0 or capability. If mode 0 is not preferred for media type EVRCNW0 or
EVRCNW1, then there is no indication that mode 0 is supported. EVRCNW1, then there is no indication that mode 0 is supported.
However absence of this parameter or absence of mode 0 in this However absence of this parameter or absence of mode 0 in this
parameter for media type EVRCNW shall not preclude mode 0 support parameter for media type EVRCNW shall not preclude mode 0 support
during a call where mode 0 may be requested via the MMM field. during a call where mode 0 may be requested via the MMM field.
1. To inform the capability of wideband mode support: A decoder can To inform the capability for wideband mode support, a decoder can
always decode all the narrowband modes (modes 1 to 7). Unless always decode all the narrowband modes (modes 1 to 7). Unless the
the decoder indicates the support of mode 0 (i.e., preference) in decoder indicates the support of mode 0 (i.e., preference) in this
this parameter or in the MMM mode request field in interleaved/ parameter or in the MMM mode request field in interleaved/bundled
bundled payload format, an encoder at the other side shall not payload format, an encoder at the other side shall not operate in
operate in mode 0. mode 0.
2. To inform the preference to operate in a subset of modes: A set To indicate a preference to operate in a subset of modes, a set has
has been defined so that several modes can be expressed as a been defined so that several modes can be expressed as a preference
preference in one attempt. For instance, the set {4,5,6,7} in one attempt. For instance, the set {4,5,6,7} signals that the
signals that the receiver prefers the sender to operate in receiver prefers the sender to operate in bandwidth-efficient
bandwidth-efficient narrowband modes of EVRC-NW. narrowband modes of EVRC-NW.
Note, during an active call session using the interleaved/bundled Note, during an active call session using the interleaved/bundled
packet format, the MMM mode request received from a communication packet format, the MMM mode request received from a communication
partner can contain a mode request different than the values in the partner can contain a mode request different than the values in the
last mode-set-recv attribute. The partner's EVRC-NW wideband last mode-set-recv attribute. The partner's EVRC-NW wideband
decoding capability is determined by the latest mode-set-recv decoding capability is determined by the latest mode-set-recv
attribute or MMM mode request field. For example, a mode request attribute or MMM mode request field. For example, a mode request
with MMM=0 from a communication partner is an implicit indication of with MMM=0 from a communication partner is an implicit indication of
the partner's EVRCNW wideband decoding capability and preference. An the partner's EVRCNW wideband decoding capability and preference. An
EVRCNW wideband capable node receiving the request can operate in EVRCNW wideband capable node receiving the request can operate in
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communication partner is an implicit indication of the partner's communication partner is an implicit indication of the partner's
EVRCNW narrowband decoding preference. The encoder of an EVRCNW node EVRCNW narrowband decoding preference. The encoder of an EVRCNW node
receiving the request shall honor the request and operate in receiving the request shall honor the request and operate in
narrowband mode. narrowband mode.
'sendmode' is used as a SDP mode attribute in EVRC [6], EVRC-B [2] 'sendmode' is used as a SDP mode attribute in EVRC [6], EVRC-B [2]
and EVRC-WB [3]. However it is deprecated in EVRC-NW. and EVRC-WB [3]. However it is deprecated in EVRC-NW.
11. Mode Change Request/Response Considerations 11. Mode Change Request/Response Considerations
The Interleaved/Bundled packet format for the EVRC family of vocoders The interleaved/bundled packet format for the EVRC family of vocoders
supports a 3-bit field (MMM) that a communication node can use to supports a 3-bit field (MMM) that a communication node can use to
indicate its preferred compression mode to an opposite node. The indicate its preferred compression mode to an opposite node. The
concept of the compression mode (also known as Capacity Operating concept of the compression mode (also known as Capacity Operating
Point) was introduced to allow a controlled trade-off between voice Point) was introduced to allow a controlled trade-off between voice
quality and channel capacity. The notion makes it possible to quality and channel capacity. The notion makes it possible to
exercise vocoders at the highest possible (average) bit-rate (hence, exercise vocoders at the highest possible (average) bit-rate (hence,
highest voice quality) when the network is lightly loaded. highest voice quality) when the network is lightly loaded.
Conversely, once the network load increases the vocoders can be Conversely, once the network load increases the vocoders can be
requested to operate at lower average bit-rates so as to absorb the requested to operate at lower average bit-rates so as to absorb the
additional network load without causing an undue increase in the additional network load without causing an undue increase in the
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Accordingly, the MMM field should be used to request the far-end to Accordingly, the MMM field should be used to request the far-end to
transmit compressed-speech using a mode that provides the best transmit compressed-speech using a mode that provides the best
balance between voice quality and capacity. However, in the case of balance between voice quality and capacity. However, in the case of
mobile-mobile calls, for example, there are two wireless sides mobile-mobile calls, for example, there are two wireless sides
involved, each with a potentially different network load level and involved, each with a potentially different network load level and
hence a different preferred mode. In such cases, achieving optimal hence a different preferred mode. In such cases, achieving optimal
end-to-end performance depends on coherent management of the end-to-end performance depends on coherent management of the
operative mode by the two sides. This requires that even if the operative mode by the two sides. This requires that even if the
local node prefers a higher bit-rate vocoder mode, it should adjust local node prefers a higher bit-rate vocoder mode, it should adjust
to a lower bit-rate mode if requested by the far end, in order to to a lower bit-rate mode if requested by the far end, in order to
avoid potentially-high frame erasure rates due to heavy load at the avoid potentially high frame erasure rates due to heavy load at the
far end network. For similar reasons, in cases where a mode far end network. For similar reasons, in cases where a mode
requested by the far end should not be supported, it might still be requested by the far end should not be supported, it might still be
beneficial to consider switching to a supported vocoder mode beneficial to consider switching to a supported vocoder mode
corresponding to a lower average bit-rate than requested. It is corresponding to a lower average bit-rate than requested. It is
recommended that the next lower average bit-rate supported vocoder recommended that the next lower average bit-rate supported vocoder
mode is used for encoding when a mode requested by the far end is not mode be used for encoding when a mode requested by the far end is not
supported. supported.
A wideband-capable endpoint can use the information conveyed by the A wideband-capable endpoint can use the information conveyed by the
C-bit of the RTP payload header to determine the optimal mode to C-bit of the RTP payload header to determine the optimal mode to
request of the far end. If the far end cannot provide Mode0 packets request of the far end. If the far end cannot provide Mode0 packets
(C-bit=1), then the choice of MMM can be based strictly on the local (C-bit=1), then the choice of MMM can be based strictly on the local
network load. If the C-bit indicates remote end's Mode0 encoding network load. If the C-bit indicates remote end's Mode0 encoding
capability (C-bit=0), then even if the local network load is not capability (C-bit=0), then even if the local network load is not
light, Mode0 can be requested knowing definitively that it will be light, Mode0 can be requested knowing definitively that it will be
supported. This will permit operators to treat wideband-capable supported. This will permit operators to treat wideband-capable
skipping to change at page 25, line 16 skipping to change at page 25, line 16
For declarative use of SDP in SAP [14] and RTSP [15], the following For declarative use of SDP in SAP [14] and RTSP [15], the following
considerations apply: considerations apply:
o Any 'maxptime' and 'ptime' values should be selected with care to o Any 'maxptime' and 'ptime' values should be selected with care to
ensure that the session's participants can achieve reasonable ensure that the session's participants can achieve reasonable
performance. performance.
o The payload format configuration parameters are all declarative o The payload format configuration parameters are all declarative
and a participant MUST use the configuration(s) that is provided and a participant MUST use the configuration(s) that is provided
for the session. More than one configuration may be provided if for the session. More than one configuration MAY be provided if
necessary by declaring multiple RTP payload types, however the necessary by declaring multiple RTP payload types, however the
number of types should be kept small. For declarative examples, number of types SHOULD be kept small. For declarative examples,
see Section 15. see Section 15.
o The usage of unidirectional receive only parameters, such as o The usage of unidirectional receive-only parameters, such as
'mode-set-recv', should be excluded in any declarations, since 'mode-set-recv', should be excluded in any declarations, since
these parameters are meaningless in one-way streaming these parameters are meaningless in one-way streaming
applications. applications.
15. Examples 15. Examples
Some example SDP session descriptions utilizing EVRC-NW encodings Some example SDP session descriptions utilizing EVRC-NW encodings
follow. In these examples, long a=fmtp lines are folded to meet the follow. In these examples, long a=fmtp lines are folded to meet the
column width constraints of this document. The backslash ("\") at column width constraints of this document. The backslash ("\") at
the end of a line and the carriage return that follows it should be the end of a line and the carriage return that follows it should be
skipping to change at page 29, line 12 skipping to change at page 29, line 12
m=audio 55954 RTP/AVP 98 99 m=audio 55954 RTP/AVP 98 99
a=rtpmap:98 EVRCWB0/16000 a=rtpmap:98 EVRCWB0/16000
16. Security Considerations 16. Security Considerations
Since compression is applied to the payload formats end-to-end, and Since compression is applied to the payload formats end-to-end, and
the encodings do not exhibit significant non-uniformity, the encodings do not exhibit significant non-uniformity,
implementations of this specification are subject to all the security implementations of this specification are subject to all the security
considerations specified in RFC 3558 [6]. Implementations using the considerations specified in RFC 3558 [6]. Implementations using the
payload defined in this specification are subject to the security payload defined in this specification are subject to the security
considerations discussed in RFC 3558 [6], RFC 3550 [5] and any considerations discussed in RFC 3558 [6], RFC 3550 [5], and any
appropriate profile (for example RFC 3551 [7]). Additional security appropriate profile (for example RFC 3551 [7]). Additional security
considerations are described in RFC 6562 [13]. considerations are described in RFC 6562 [13].
17. References 17. References
17.1. Normative References 17.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
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