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Versions: (draft-zfang-avt-rtp-evrc-nw) 00 01 02 03 04 05 06 07 08 09 10 RFC 6884

Network Working Group                                            Z. Fang
Internet-Draft                                     Qualcomm Incorporated
Intended status: Standards Track                        January 20, 2013
Expires: July 24, 2013


RTP payload format for Enhanced Variable Rate Narrowband-Wideband Codec
                               (EVRC-NW)
                     draft-ietf-avt-rtp-evrc-nw-10

Abstract

   This document specifies real-time transport protocol (RTP) payload
   formats to be used for the Enhanced Variable Rate Narrowband-Wideband
   Codec (EVRC-NW).  Three media type registrations are included for
   EVRC-NW RTP payload formats.  In addition, a file format is specified
   for transport of EVRC-NW speech data in storage mode applications
   such as e-mail.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 24, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  EVRC-NW codec  . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  RTP header usage . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Payload format . . . . . . . . . . . . . . . . . . . . . . . .  8
     6.1.  Encoding capability identification in EVRC-NW
           interleaved/bundled format . . . . . . . . . . . . . . . .  8
   7.  Congestion Control Considerations  . . . . . . . . . . . . . . 11
   8.  Storage format for the EVRC-NW Codec . . . . . . . . . . . . . 12
   9.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 13
     9.1.  Media Type Registrations . . . . . . . . . . . . . . . . . 13
       9.1.1.  Registration of Media Type audio/EVRCNW  . . . . . . . 13
       9.1.2.  Registration of Media Type audio/EVRCNW0 . . . . . . . 15
       9.1.3.  Registration of Media Type audio/EVRCNW1 . . . . . . . 16
   10. SDP mode attributes for EVRC-NW  . . . . . . . . . . . . . . . 19
   11. Mode Change Request/Response Considerations  . . . . . . . . . 20
   12. Mapping EVRC-NW media type parameters into SDP . . . . . . . . 22
   13. Offer-Answer Model Considerations for EVRC-NW  . . . . . . . . 23
   14. Declarative SDP Considerations . . . . . . . . . . . . . . . . 25
   15. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   16. Security Considerations  . . . . . . . . . . . . . . . . . . . 29
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 30
     17.2. Informative References . . . . . . . . . . . . . . . . . . 31
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 32



















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1.  Introduction

   This document specifies the payload formats for packetization of
   EVRC-NW encoded speech signals into the real-time transport protocol
   (RTP).  It defines support for the header-free, interleaved/bundled,
   and compact bundle packet formats for the EVRC-NW codec as well as
   discontinuous transmission (DTX) support for EVRC-NW encoded speech
   transported via RTP.  The EVRC-NW codec offers better speech quality
   than the EVRC and EVRC-B codecs and better capacity than EVRC-WB
   codec.  EVRC-NW belongs to the EVRC family of codecs.









































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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].














































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3.  Background

   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
   (DTX).  It provides enhanced voice quality and high spectral
   efficiency.

   The EVRC-NW codec operates on 20 ms frames, and the default sampling
   rate is 16 kHz (wideband).  Input and output at 8 kHz sampling rate
   (narrowband) is also 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 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 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 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 7 if the receiver supports wideband mode) results in codec
   output bit-rate change but does not cause any decoding problems at
   the receiver.  EVRC-NW provides a standardized solution for
   packetized voice applications that allow transitions between enhanced
   quality and increased capacity.  The most important service addressed
   is IP telephony.  Target devices can be IP phones or VoIP handsets,
   media gateways, voice messaging servers, etc.



























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4.  EVRC-NW codec

   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
   (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
   erasure frames.  The default sampling rate is 16 kHz.  Input and
   output at 8 kHz sampling rate is also supported.

   The frame type values and sizes of the associated codec data frames
   are listed in the table below:

   Value  Rate        Total codec data frame size in bytes (and in bits)
   --------------------------------------------------------------------
     0    Blank (Null)   0    (0 bits)
     1    1/8            2    (16 bits)
     2    1/4            5    (40 bits)
     3    1/2           10    (80 bits)
     4    1             22    (171 bits; 5 bits padded at the end)
     5    Erasure        0    (SHOULD NOT be transmitted by sender)































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5.  RTP header usage

   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
   as specified in RFC 3550 [5].

   EVRC-NW has also the capability to operate with 8 kHz sampled input/
   output signals.  The decoder does not require a priori knowledge
   about the sampling rate of the original signal at the input of the
   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
   implementation and the electroacoustic audio capabilities of the
   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
   always be used.  The RTP timestamp is increased by 320 for each 20
   milliseconds.

   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
   talkspurt.  For all other packets the marker bit SHALL be set to zero
   (M=0).






























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6.  Payload format

   Three RTP packet formats are supported for the EVRC-NW codec - the
   interleaved/bundled packet format, the header-free packet format, and
   the compact bundled packet format.  For all these formats, the
   operational details and capabilities, such as ToC, interleaving, DTX,
   and bundling, of EVRC-NW are exactly the same as those defined in
   EVRC [6], EVRC-B [2] and EVRC-WB [3], except that

   1.  the mode change request field in the interleaved/bundled packet
       format MUST be interpreted according to the definition of the
       RATE_REDUC parameter as defined in EVRC-NW [4].

   2.  the mode change request field in the interleaved/bundled packet
       format SHOULD be honored by an EVRCNW encoding end point in an
       one-to-one session with a dedicated EVRCNW decoding end point
       such as in a two-party call or in a conference leg.

   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
       EVRC-NW wideband/narrowband encoding capability identification
       flag.

   The media type audio/EVRCNW maps to the interleaved/bundled packet
   format, audio/EVRCNW0 maps to the header-free packet format, and
   audio/EVRCNW1 maps to the compact bundled packet format.

6.1.  Encoding capability identification in EVRC-NW interleaved/bundled
      format

   The EVRC-NW interleaved/bundled format defines an encoding capability
   identification flag, which is used to signal the local EVRC-NW
   wideband/narrowband encoding capability at the time of construction
   of an RTP packet to the far end of a communication session.  This
   capability identification flag allows the far end to use the MMM
   field in its out-going (returning) EVRC-NW interleaved/bundled format
   packets to request the desired EVRC-NW wideband or narrowband
   encoding mode in accordance with the dynamic/instantaneous encoding
   capability information.  See RFC 3558 [6] for the definition of MMM
   field.  The following examples illustrate a few scenarios where the
   encoding capability information is used:

   o  An end-to-end wideband communication is established first between
      two communication end points using EVRC-NW interleaved/bundled
      format.  The called end point becomes wideband encoding incapable
      during the call and makes the other end aware of this change using
      the encoding capability identification flag.  Based on the new
      information the calling end point could change the MMM value in



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      its outgoing EVRC-NW packets from Mode-0 to Mode-4 to request
      narrowband encoded traffic for bandwidth efficiency or from Mode-0
      to Mode-1 for best perceptual quality.

   o  An end-to-end narrowband communication is established between an
      EVRC-NW wideband encoding capable calling end point and an EVRC-NW
      wideband encoding incapable called end point.  The called end
      point becomes EVRC-NW wideband encoding capable during the call
      and makes the other end aware of this change using the encoding
      capability identification flag.  Based on the new information the
      calling end point could change the MMM value in its outgoing
      EVRC-NW packets from non-Mode-0 to Mode-0 to request wideband
      traffic.

   EVRC-NW interleaved/bundled format defines the encoding capability
   identification flag in bit 1 of the first octet, as illustrated in
   the figure below.  The flag shall be set to zero (C=0) when the local
   EVRC-NW encoder is capable of Mode-0 wideband encoding.  The flag
   shall be set to one (C=1) when the local EVRC-NW encoder is capable
   of non-Mode-0 narrowband encoding only.  See RFC 3558 [6] for
   original definitions of other fields in the interleaved/bundled
   format.

     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                             |
    +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    |R|C| LLL | NNN | MMM |  Count  |  TOC  |  ...  |  TOC  |padding|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        one or more codec data frames, one per TOC entry       |
    |                             ....                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Reserved (R): 1 bit

      Reserved bit.  MUST be set to zero by sender, SHOULD be ignored by
      receiver.

   Encoding capability identification (C): 1 bit

      Must be set to zero by sender to indicate wideband encoding
      capable or set to one to indicate narrowband encoding capable
      only.






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      C = 0 :  Mode-0 wideband encoding capable

        = 1 :  Mode-0 wideband encoding incapable, i.e. narrowband
         encoding only.















































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7.  Congestion Control Considerations

   Congestion control for RTP is discussed in RFC 3550 [5], and in
   applicable RTP profiles, e.g., RFC3551 [7].  This document does not
   change those considerations.

   Due to the header overhead, the number of frames encapsulated in each
   RTP packet influences the overall bandwidth of the RTP stream.
   Packing more frames in each RTP packet can reduce the number of
   packets sent and hence the header overhead, at the expense of
   increased delay and reduced error robustness.








































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8.  Storage format for the EVRC-NW Codec

   The storage format is used for storing EVRC-NW 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-NW corresponds to the ASCII
   character string "#!EVRCNW\n", i.e., "0x23 0x21 0x45 0x56 0x52 0x43
   0x4E 0x57 0x0A".

   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 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.  The Value column in the
   table in Section 4 provides the TOC values for corresponding frame
   types.

   Speech frames lost in transmission and non-received frames MUST be
   stored as erasure frames (ToC value of 5) to maintain synchronization
   with the original media.






























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9.  IANA considerations

   This document introduces a new EVRC-NW 'audio' media subtype.

9.1.  Media Type Registrations

   Following the guidelines in RFC 4855 [8] and RFC 4288 [9], this
   section registers new 'audio' media subtypes for EVRC-NW.

9.1.1.  Registration of Media Type audio/EVRCNW

   Type name: audio

   Subtype names: EVRCNW

   Required parameters: None

   Optional parameters:

   These parameters apply to RTP transfer only.

   mode-set-recv: A subset of EVRC-NW modes.  Possible values are a
   comma separated list of modes from the set {0,1,2,3,4,5,6,7} (see
   Table 2.6.1.2-4 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.  Absence of this parameter signals the
   mode set {1,2,3,4,5,6,7}.

   ptime: see RFC 4566 [10].

   maxptime: see RFC 4566.

   maxinterleave: Maximum number for interleaving length (field LLL in
   the Interleaving Octet)[0..7].  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 in RFC 4788.

   dtxmax: see Section 6.1 in RFC 4788.

   dtxmin: see Section 6.1 in RFC 4788.

   hangover: see Section 6.1 in RFC 4788.

   Encoding considerations:

   This media type is framed binary data (see RFC 4288, Section 4.8) and



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   is defined for transfer of EVRC-NW encoded data via RTP using the
   interleaved/bundled packet format specified in RFC 3558 [6].

   Security considerations: See Section 16.

   Interoperability considerations: None

   Published specification:

   The EVRC-NW vocoder is specified in 3GPP2 C.S0014-D.  The transfer
   method with the interleaved/bundled packet format via RTP is
   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
   number of this document.]

   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 applies to stored-file transfer methods:

   Magic number: #!EVRCNW\n (see Section 8)

   File extensions: enw, ENW

   Macintosh file type code: None

   Object identifier or OID: None

   EVRC-NW speech frames may also be stored in the file format "3g2"
   defined in 3GPP2 C.S0050-B, which is identified using the media types
   "audio/3gpp2" or "video/3gpp2" registered by RFC 4393 [11].

   Person & email address to contact for further information:

   Zheng Fang <zfang@qualcomm.com>

   Intended usage: COMMON

   Restrictions on usage:

   This media type can be used with the file format defined in Section 8
   of RFC XXXX in contexts other than RTP.  In context of transfers over
   RTP, the RTP payload format specified in Section 4.1 of RFC 3558 [6]
   is used for this media type.  [Note to the RFC editor: please replace



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   XXXX with the RFC number of this document.]

   Author:

   Zheng Fang <zfang@qualcomm.com>

   Change controller:

   IETF Payload working group delegated from the IESG.

9.1.2.  Registration of Media Type audio/EVRCNW0

   Type name: audio

   Subtype names: EVRCNW0

   Required parameters: None

   Optional parameters:

   These parameters apply to RTP transfer only.

   mode-set-recv: A subset of EVRC-NW modes.  Possible values are a
   comma separated list of modes from the set {0,1,2,3,4,5,6,7} (see
   Table 2.6.1.2-4 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.  Absence of this parameter signals the
   mode set {1,2,3,4,5,6,7}.

   ptime: see RFC 4566.

   silencesupp: see Section 6.1 in RFC 4788.

   dtxmax: see Section 6.1 in RFC 4788.

   dtxmin: see Section 6.1 in RFC 4788.

   hangover: see Section 6.1 in RFC 4788.

   Encoding considerations:

   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
   header-free packet format specified in RFC 3558 [6].

   Security considerations: See Section 16.

   Interoperability considerations: None



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   Published specification:

   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
   3558 [6].

   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:

   Zheng Fang <zfang@qualcomm.com>

   Intended usage: COMMON

   Restrictions on usage:

   This media type depends on RTP framing, and hence is only defined for
   transfer via RTP [5], the RTP payload format specified in Section 4.2
   of RFC 3558 [6] SHALL be used.  This media type SHALL NOT be used for
   storage or file transfer, instead audio/EVRCNW SHALL be used.

   Author:

   Zheng Fang <zfang@qualcomm.com>

   Change controller:

   IETF Payload working group delegated from the IESG.

9.1.3.  Registration of Media Type audio/EVRCNW1

   Type name: audio

   Subtype names: EVRCNW1

   Required parameters: None

   Optional parameters:

   These parameters apply to RTP transfer only.

   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



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   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.
   A value of 0 signals the support for wideband fixed rate (full or
   half rate, depending on the value of 'fixedrate' parameter).  A value
   of 1 signals narrowband fixed rate (full or half rate, depending on
   the value of 'fixedrate' parameter).  Absence of this parameter
   signals the mode 1.

   ptime: see RFC 4566.

   maxptime: see RFC 4566.

   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
   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 in RFC 4788.

   dtxmax: see Section 6.1 in RFC 4788.

   dtxmin: see Section 6.1 in RFC 4788.

   hangover: see Section 6.1 in RFC 4788.

   Encoding considerations:

   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
   compact bundled packet format specified in RFC 4788.

   Security considerations: See Section 16

   Interoperability considerations: None

   Published specification:

   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
   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




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   Person & email address to contact for further information:

   Zheng Fang <zfang@qualcomm.com>

   Intended usage: COMMON

   Restrictions on usage:

   This media type depends on RTP framing, and hence is only defined for
   transfer via RTP [5], the RTP payload format specified in Section 4
   of RFC 4788 SHALL be used.  This media type SHALL NOT be used for
   storage or file transfer, instead audio/EVRCNW SHALL be used.

   Author:

   Zheng Fang <zfang@qualcomm.com>

   Change controller:

   IETF Payload working group delegated from the IESG.































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10.  SDP mode attributes for EVRC-NW

   '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
   indicating a preference implicitly indicates support for that
   capability.  If mode 0 is not preferred for media type EVRCNW0 or
   EVRCNW1, then there is no indication that mode 0 is supported.
   However absence of this parameter or absence of mode 0 in this
   parameter for media type EVRCNW shall not preclude mode 0 support
   during a call where mode 0 may be requested via the MMM field.

   To inform the capability for wideband mode support, a decoder can
   always decode all the narrowband modes (modes 1 to 7).  Unless the
   decoder indicates the support of mode 0 (i.e., preference) in this
   parameter or in the MMM mode request field in interleaved/bundled
   payload format, an encoder at the other side shall not operate in
   mode 0.

   To indicate a preference to operate in a subset of modes, a set has
   been defined so that several modes can be expressed as a preference
   in one attempt.  For instance, the set {4,5,6,7} signals that the
   receiver prefers the sender to operate in bandwidth-efficient
   narrowband modes of EVRC-NW.

   Note, during an active call session using the interleaved/bundled
   packet format, the MMM mode request received from a communication
   partner can contain a mode request different than the values in the
   last mode-set-recv attribute.  The partner's EVRC-NW wideband
   decoding capability is determined by the latest mode-set-recv
   attribute or MMM mode request field.  For example, a mode request
   with MMM=0 from a communication partner is an implicit indication of
   the partner's EVRCNW wideband decoding capability and preference.  An
   EVRCNW wideband capable node receiving the request can operate in
   wideband mode.  A mode request with MMM=1, 2, ..., or 7 from a
   communication partner is an implicit indication of the partner's
   EVRCNW narrowband decoding preference.  The encoder of an EVRCNW node
   receiving the request shall honor the request and operate in
   narrowband mode.

   '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.










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11.  Mode Change Request/Response Considerations

   The interleaved/bundled packet format for the EVRC family of vocoders
   supports a 3-bit field (MMM) that a communication node can use to
   indicate its preferred compression mode to an opposite node.  The
   concept of the compression mode (also known as Capacity Operating
   Point) was introduced to allow a controlled trade-off between voice
   quality and channel capacity.  The notion makes it possible to
   exercise vocoders at the highest possible (average) bit-rate (hence,
   highest voice quality) when the network is lightly loaded.
   Conversely, once the network load increases the vocoders can be
   requested to operate at lower average bit-rates so as to absorb the
   additional network load without causing an undue increase in the
   frame-erasure rates; the underlying premise is that while a higher
   bit-rate improves the vocoder performance, it also increases the
   network loading, risking a sharp decline in voice quality should the
   frame-erasure rate be too high.  By contrast, a lower bit-rate mode
   of operation can result in accommodation of the additional network
   load without causing unduly high frame-erasure rates, resulting in
   better overall quality despite the inherently lower voice quality of
   the lower bit-rate mode of the vocoder.

   Accordingly, the MMM field should be used to request the far-end to
   transmit compressed-speech using a mode that provides the best
   balance between voice quality and capacity.  However, in the case of
   mobile-mobile calls, for example, there are two wireless sides
   involved, each with a potentially different network load level and
   hence a different preferred mode.  In such cases, achieving optimal
   end-to-end performance depends on coherent management of 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
   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
   far end network.  For similar reasons, in cases where a mode
   requested by the far end should not be supported, it might still be
   beneficial to consider switching to a supported vocoder mode
   corresponding to a lower average bit-rate than requested.  It is
   recommended that the next lower average bit-rate supported vocoder
   mode be used for encoding when a mode requested by the far end is not
   supported.

   A wideband-capable endpoint can use the information conveyed by the
   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
   (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
   capability (C-bit=0), then even if the local network load is not
   light, Mode0 can be requested knowing definitively that it will be



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   supported.  This will permit operators to treat wideband-capable
   mobiles preferentially, should they wish to adopt such policy.

















































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12.  Mapping EVRC-NW media type parameters into SDP

   Information carried in the media type specification has a specific
   mapping to fields in the Session Description Protocol (SDP) [10],
   which is commonly used to describe RTP sessions.  When SDP is used to
   specify sessions employing EVRC-NW encoded speech, the mapping is as
   follows.

   o  The media type ("audio") goes in SDP "m=" as the media name.

   o  The media subtype ("EVRCNW", "EVRCNW0" or "EVRCNW1") goes in SDP
      "a=rtpmap" as the encoding name.

   o  The optional parameters 'ptime and 'maxptime' (for subtypes
      EVRCNW, EVRCNW1) go in the SDP "a=ptime" and "a=maxptime"
      attributes, respectively.

   o  Any remaining parameters (for subtypes EVRCNW, EVRCNW0 and
      EVRCNW1) go in the SDP "a=fmtp" attribute by copying them from the
      media type string as a semicolon separated list of parameter=value
      pairs.






























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13.  Offer-Answer Model Considerations for EVRC-NW

   The following considerations apply when using the SDP offer-answer
   procedures of RFC 3264 [12] to negotiate the use of EVRC-NW payload
   in RTP:

   o  Since EVRC-NW is an extension of both EVRC-B and EVRC-WB, the
      offerer SHOULD also announce EVRC-B and EVRC-WB support in its
      "m=audio" lines, with EVRC-NW as the preferred codec.  This will
      allow interoperability with an answerer which supports only EVRC-B
      and/or EVRC-WB.

   Below is an example of such an offer:

          m=audio 55954 RTP/AVP 98 99 100
          a=rtpmap:98 EVRCNW0/16000
          a=rtpmap:99 EVRCWB0/16000
          a=rtpmap:100 EVRCB0/8000
          a=fmtp:98 mode-set-recv=0,1,2,3,4,5,6
          a=fmtp:99 mode-set-recv=0,4
          a=fmtp:100 recvmode=0


   If the answerer supports EVRC-NW then the answerer can keep the
   payload type 98 in its answer and the conversation can be done using
   EVRC-NW.  Else, if the answerer supports only EVRC-WB and/or EVRC-B
   then the answerer will leave only the payload type 99 and/or 100
   respectively in its answer and the conversation will be done using
   EVRC-WB and/or EVRC-B respectively.

   An example answer for the above offer:

          m=audio 55954 RTP/AVP 98
          a=rtpmap:98 EVRCNW0/16000
          a=fmtp:98 mode-set-recv=4


   o  'mode-set-recv' is a uni-directional receive only parameter.

   o  An offerer can use 'mode-set-recv' to request that the remote
      sender's encoder be limited to the list of modes signaled in
      'mode-set-recv'.  A remote sender MAY ignore 'mode-set-recv'
      requests.  However, a remote sender shall not assume the other
      side can support mode 0, unless the offer includes mode 0
      explicitly in 'mode-set-recv' or the remote sender receives mode
      requests with MMM = 0 from the communication partner during an
      active call using EVRC-NW interleaved/bundled format.




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   o  The parameters 'maxptime' and 'ptime' will in most cases not
      affect interoperability, however the setting of the parameters can
      affect the performance of the application.  The SDP offer-answer
      handling of the 'ptime' parameter is described in RFC 3264 [12].
      The 'maxptime' parameter MUST be handled in the same way.

   o  For a sendonly stream, the 'mode-set-recv' parameter is not useful
      and SHOULD NOT be used.

   o  When using EVRCNW1, the entire session MUST use the same fixed
      rate and mode (0-Wideband or 1-Narrowband).

   o  For additional rules which MUST be followed while negotiating DTX
      parameters, see Section 6.8 in RFC 4788 [2].

   o  Any unknown parameter in an SDP offer MUST be ignored by the
      receiver and MUST NOT be included in the SDP answer.


































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14.  Declarative SDP Considerations

   For declarative use of SDP in SAP [14] and RTSP [15], the following
   considerations apply:

   o  Any 'maxptime' and 'ptime' values should be selected with care to
      ensure that the session's participants can achieve reasonable
      performance.

   o  The payload format configuration parameters are all declarative
      and a participant MUST use the configuration(s) that is provided
      for the session.  More than one configuration MAY be provided if
      necessary by declaring multiple RTP payload types, however the
      number of types SHOULD be kept small.  For declarative examples,
      see Section 15.

   o  The usage of unidirectional receive-only parameters, such as
      'mode-set-recv', should be excluded in any declarations, since
      these parameters are meaningless in one-way streaming
      applications.































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15.  Examples

   Some example SDP session descriptions utilizing EVRC-NW encodings
   follow.  In these examples, long a=fmtp lines are folded to meet the
   column width constraints of this document.  The backslash ("\") at
   the end of a line and the carriage return that follows it should be
   ignored.  Note that media subtype names are case-insensitive.
   Parameter names are case-insensitive both in media types and in the
   mapping to the SDP a=fmtp attribute.

   Example usage of EVRCNW if wideband mode is supported:

          m=audio 49120 RTP/AVP 97 98 99
          a=rtpmap:97 EVRCNW/16000
          a=rtpmap:98 EVRCWB/16000
          a=rtpmap:99 EVRCB/8000
          a=fmtp:97 mode-set-recv=0,1,2,3,4,5,6
          a=fmtp:98 mode-set-recv=0,4
          a=fmtp:99 recvmode=0
          a=maxptime:120

   Example usage of EVRCNW if wideband mode is not supported:

          m=audio 49120 RTP/AVP 97 98 99
          a=rtpmap:97 EVRCNW/16000
          a=rtpmap:98 EVRCWB/16000
          a=rtpmap:99 EVRCB/8000
          a=fmtp:97 mode-set-recv=1,2,3,4,5,6
          a=fmtp:98 mode-set-recv=4
          a=fmtp:99 recvmode=0
          a=maxptime:120

   Example usage of EVRCNW0:

          m=audio 49120 RTP/AVP 97 98 99
          a=rtpmap:97 EVRCNW0/16000
          a=rtpmap:98 EVRCWB0/16000
          a=rtpmap:99 EVRCB0/8000
          a=fmtp:97 mode-set-recv=0,1,2,3,4,5,6
          a=fmtp:98 mode-set-recv=0,4
          a=fmtp:99 recvmode=0

   Example SDP answer from a media gateway requesting a terminal to
   limit its encoder operation to EVRC-NW mode 4.

          m=audio 49120 RTP/AVP 97
          a=rtpmap:97 EVRCNW0/16000
          a=fmtp:97 mode-set-recv=4



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   Example usage of EVRCNW1:

          m=audio 49120 RTP/AVP 97 98 99
          a=rtpmap:97 EVRCNW1/16000
          a=rtpmap:98 EVRCWB1/16000
          a=rtpmap:99 EVRCB1/8000
          a=fmtp:97 fixedrate=0.5
          a=fmtp:98 fixedrate=0.5
          a=fmtp:99 fixedrate=0.5
          a=maxptime:100

   Example usage of EVRCNW with DTX with silencesupp=1:

          m=audio 49120 RTP/AVP 97 98 99
          a=rtpmap:97 EVRCNW/16000
          a=rtpmap:98 EVRCWB/16000
          a=rtpmap:99 EVRCB/8000
          a=fmtp:97 silencesupp=1;dtxmax=32;dtxmin=12;hangover=1; \
          mode-set-recv=0,1,2,3,4,5,6
          a=fmtp:98 silencesupp=1;dtxmax=32;dtxmin=12;hangover=1; \
          mode-set-recv=0,4
          a=fmtp:99 recvmode=0
          a=maxptime:120

   Examples usage of EVRCNW with DTX with silencesupp=0:

          m=audio 49120 RTP/AVP 97 98 99
          a=rtpmap:97 EVRCNW/16000
          a=rtpmap:98 EVRCWB/16000
          a=rtpmap:99 EVRCB/8000
          a=fmtp:97 silencesupp=0;dtxmax=32;dtxmin=12;hangover=1; \
          mode-set-recv=0,1,2,3,4,5,6
          a=fmtp:98 silencesupp=0;dtxmax=32;dtxmin=12;hangover=1; \
          mode-set-recv=0,4
          a=fmtp:99 recvmode=0
          a=maxptime:120

   Example offer answer exchange between EVRC-NW and legacy EVRC-B (RFC
   4788):












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         Offer:

           m=audio 55954 RTP/AVP 97 98 99
           a=rtpmap:97 EVRCNW0/16000
           a=rtpmap:98 EVRCWB0/16000
           a=rtpmap:99 EVRCB0/8000
           a=rtpmap:97 mode-set-recv=0,1,2,3,4,5,6
           a=fmtp:98 mode-set-recv=0,4
           a=fmtp:99 recvmode=0

         Answer:

           m=audio 55954 RTP/AVP 99
           a=rtpmap:99 EVRCB0/8000

   Example offer answer exchange between EVRC-NW and legacy EVRC-WB (RFC
   5188):

         Offer:

           m=audio 55954 RTP/AVP 97 98 99
           a=rtpmap:97 EVRCNW0/16000
           a=rtpmap:98 EVRCWB0/16000
           a=rtpmap:99 EVRCB0/8000
           a=rtpmap:97 mode-set-recv=0,1,2,3,4,5,6
           a=fmtp:98 mode-set-recv=0,4
           a=fmtp:99 recvmode=0

         Answer:

           m=audio 55954 RTP/AVP 98 99
           a=rtpmap:98 EVRCWB0/16000



















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16.  Security Considerations

   Since compression is applied to the payload formats end-to-end, and
   the encodings do not exhibit significant non-uniformity,
   implementations of this specification are subject to all the security
   considerations specified in RFC 3558 [6].  Implementations using the
   payload defined in this specification are subject to the security
   considerations discussed in RFC 3558 [6], RFC 3550 [5], and any
   appropriate profile (for example RFC 3551 [7]).  Additional security
   considerations are described in RFC 6562 [13].









































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17.  References

17.1.  Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [2]   Xie, Q. and R. Kapoor, "Enhancements to RTP Payload Formats for
         EVRC Family Codecs", RFC 4788, January 2007.

   [3]   Desineni, H. and Q. Xie, "RTP Payload Format for the Enhanced
         Variable Rate Wideband Codec (EVRC-WB) and the Media Subtype
         Updates for EVRC-B Codec", RFC 5188, February 2008.

   [4]   "Enhanced Variable Rate Codec, Speech Service Options 3, 68,
         70, and 73 for Wideband Spread Spectrum Digital Systems",
         3GPP2 C.S0014-D v3.0, October 2010.

   [5]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", STD 64,
         RFC 3550, July 2003.

   [6]   Li, A., "RTP Payload Format for Enhanced Variable Rate Codecs
         (EVRC) and Selectable Mode Vocoders (SMV)", RFC 3558,
         July 2003.

   [7]   Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
         Conferences with Minimal Control", STD 65, RFC 3551, July 2003.

   [8]   Casner, S., "Media Type Registration of RTP Payload Formats",
         RFC 4855, February 2007.

   [9]   Freed, N. and J. Klensin, "Media Type Specifications and
         Registration Procedures", BCP 13, RFC 4288, December 2005.

   [10]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
         Description Protocol", RFC 4566, July 2006.

   [11]  Garudadri, H., "MIME Type Registrations for 3GPP2 Multimedia
         Files", RFC 4393, March 2006.

   [12]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264, June 2002.

   [13]  Perkins, C. and JM. Valin, "Guidelines for the Use of Variable
         Bit Rate Audio with Secure RTP", RFC 6562, March 2012.





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17.2.  Informative References

   [14]  Handley, M., Perkins, C., and E. Whelan, "Session Announcement
         Protocol", RFC 2974, October 2000.

   [15]  Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
         Protocol (RTSP)", RFC 2326, April 1998.












































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Author's Address

   Zheng Fang
   Qualcomm Incorporated
   5775 Morehouse Drive
   San Diego, CA  92126
   USA

   Phone: +1 858 651 9484
   Email: zfang@qualcomm.com
   URI:   http://www.qualcomm.com








































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