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Network Working Group R. Zopf Request for Comments: 3389 Lucent Technologies Category: Standards Track September 2002

 Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN)

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 Internet Society (2002).  All Rights Reserved.


 This document describes a Real-time Transport Protocol (RTP) payload
 format for transporting comfort noise (CN).  The CN payload type is
 primarily for use with audio codecs that do not support comfort noise
 as part of the codec itself such as ITU-T Recommendations G.711,
 G.726, G.727, G.728, and G.722.

1. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in RFC-2119 [7].

2. Introduction

 This document describes a RTP [1] payload format for transporting
 comfort noise.  The payload format is based on Appendix II of ITU-T
 Recommendation G.711 [8] which defines a comfort noise payload format
 (or bit-stream) for ITU-T G.711 [2] use in packet-based multimedia
 communication systems.  The payload format is generic and may also be
 used with other audio codecs without built-in Discontinuous
 Transmission (DTX) capability such as ITU-T Recommendations G.726
 [3], G.727 [4], G.728 [5], and G.722 [6].  The payload format
 provides a minimum interoperability specification for communication
 of comfort noise parameters.  The comfort noise analysis and
 synthesis as well as the Voice Activity Detection (VAD) and DTX
 algorithms are unspecified and left implementation-specific.

Zopf Standards Track [Page 1] RFC 3389 RTP Payload for Comfort Noise September 2002

 However, an example solution for G.711 has been tested and is
 described in the Appendix [8].  It uses the VAD and DTX of G.729
 Annex B [9] and a comfort noise generation algorithm (CNG) which is
 provided in the Appendix for information.
 The comfort noise payload, which is also known as a Silence Insertion
 Descriptor (SID) frame, consists of a single octet description of the
 noise level and MAY contain spectral information in subsequent
 octets.  An earlier version of the CN payload format consisting only
 of the noise level byte was defined in draft revisions of the RFC
 1890.  The extended payload format defined in this document should be
 backward compatible with implementations of the earlier version
 assuming that only the first byte is interpreted and any additional
 spectral information bytes are ignored.

3. CN Payload Definition

 The comfort noise payload consists of a description of the noise
 level and spectral information in the form of reflection coefficients
 for an all-pole model of the noise.  The inclusion of spectral
 information is OPTIONAL and the model order (number of coefficients)
 is left unspecified.  The encoder may choose an appropriate model
 order based on such considerations as quality, complexity, expected
 environmental noise, and signal bandwidth.  The model order is not
 explicitly transmitted since the number of coefficients can be
 derived from the length of the payload at the receiver.  The decoder
 may reduce the model order by setting higher order reflection
 coefficients to zero if desired to reduce complexity or for other

3.1 Noise Level

 The magnitude of the noise level is packed into the least significant
 bits of the noise-level byte with the most significant bit unused and
 always set to 0 as shown below in Figure 1.  The least significant
 bit of the noise level magnitude is packed into the least significant
 bit of the byte.
 The noise level is expressed in -dBov, with values from 0 to 127
 representing 0 to -127 dBov.  dBov is the level relative to the
 overload of the system.  (Note: Representation relative to the
 overload point of a system is particularly useful for digital
 implementations, since one does not need to know the relative
 calibration of the analog circuitry.)  For example, in the case of a
 u-law system, the reference would be a square wave with values +/-
 8031, and this square wave represents 0dBov.  This translates into

Zopf Standards Track [Page 2] RFC 3389 RTP Payload for Comfort Noise September 2002

                      0 1 2 3 4 5 6 7
                     |0|   level     |
               Figure 1: Noise Level Packing

3.2 Spectral Information

 The spectral information is transmitted using reflection coefficients
 [8].  Each reflection coefficient can have values between -1 and 1
 and is quantized uniformly using 8 bits.  The quantized value is
 represented by the 8 bit index N, where N=0..,254, and index N=255 is
 reserved for future use.  Each index N is packed into a separate byte
 with the MSB first.  The quantized value of each reflection
 coefficient, k_i, can be obtained from its corresponding index using:
      k_i(N_i) = 258*(N_i-127)     for N_i = 0...254; -1 < k_i < 1

3.3 Payload Packing

 The first byte of the payload MUST contain the noise level as shown
 in Figure 1.  Quantized reflection coefficients are packed in
 subsequent bytes in ascending order as in Figure 2 where M is the
 model order.  The total length of the payload is M+1 bytes.  Note
 that a 0th order model (i.e., no spectral envelope information)
 reduces to transmitting only the energy level.
            Byte        1      2    3    ...   M+1
                     |level|  N1 |  N2 | ... |  NM |
              Figure 2: CN Payload Packing Format

4. Usage of RTP

 The RTP header for the comfort noise packet SHOULD be constructed as
 if the comfort noise were an independent codec.  Thus, the RTP
 timestamp designates the beginning of the comfort noise period.  When
 this payload format is used under the RTP profile specified in RFC
 1890 [10], a static payload type of 13 is assigned for RTP timestamp
 clock rate of 8,000 Hz; if other rates are needed, they MUST be
 defined through dynamic payload types.  The RTP packet SHOULD NOT
 have the marker bit set.

Zopf Standards Track [Page 3] RFC 3389 RTP Payload for Comfort Noise September 2002

 Each RTP packet containing comfort noise MUST contain exactly one CN
 payload per channel.  This is required since the CN payload has a
 variable length.  If multiple audio channels are used, each channel
 MUST use the same spectral model order 'M'.

5. Guidelines for Use

 An audio codec with DTX capabilities generally includes VAD, DTX, and
 CNG algorithms.  The job of the VAD is to discriminate between active
 and inactive voice segments in the input signal.  During inactive
 voice segments, the role of the CNG is to sufficiently describe the
 ambient noise while minimizing the transmission rate.  A CN payload
 (or SID frame) containing a description of the noise is sent to the
 receiver to drive the CNG.  The DTX algorithm determines when a CN
 payload is transmitted.  During active voice segments, packets of the
 voice codec are transmitted and indicated in the RTP header by the
 static or dynamic payload type for that codec.  At the beginning of
 an inactive voice segment (silence period), a CN packet is
 transmitted in the same RTP stream and indicated by the CN payload
 type.  The CN packet update rate is left implementation specific. For
 example, the CN packet may be sent periodically or only when there is
 a significant change in the background noise characteristics.  The
 CNG algorithm at the receiver uses the information in the CN payload
 to update its noise generation model and then produce an appropriate
 amount of comfort noise.
 The CN payload format provides a minimum interoperability
 specification for communication of comfort noise parameters.  The
 comfort noise analysis and synthesis as well as the VAD and DTX
 algorithms are unspecified and left implementation-specific.
 However, an example solution for G.711 has been tested and is
 described in Appendix II of ITU-T Recommendation G.711 [8].  It uses
 the VAD and DTX of G.729 Annex B [9] and a comfort noise generation
 algorithm (CNG), which is provided in the Appendix for information.
 Additional guidelines for use such as the factors affecting system
 performance in the design of the VAD/DTX/CNG algorithms are described
 in the Appendix.

5.1 Usage of SDP

 When using the Session Description Protocol (SDP) [11] to specify RTP
 payload information, the use of comfort noise is indicated by the
 inclusion of a payload type for CN on the media description line.
 When using CN with the RTP/AVP profile [10] and a codec whose RTP
 timestamp clock rate is 8000 Hz, such as G.711 (PCMU, static payload
 type 0), the static payload type 13 for CN can be used:
       m=audio 49230 RTP/AVP 0 13

Zopf Standards Track [Page 4] RFC 3389 RTP Payload for Comfort Noise September 2002

 When using CN with a codec that has a different RTP timestamp clock
 rate, a dynamic payload type mapping (rtpmap attribute) is required.
 This example shows CN used with the G.722.1 codec (see RFC 3047
       m=audio 49230 RTP/AVP 101 102
       a=rtpmap:101 G7221/16000
       a=fmtp:121 bitrate=24000
       a=rtpmap:102 CN/16000
 Omission of a payload type for CN on the media description line
 implies that this comfort noise payload format will not be used, but
 it does not imply that silence will not be suppressed.  RTP allows
 discontinuous transmission (silence suppression) on any audio payload
 format.  The receiver can detect silence suppression on the first
 packet received after the silence by observing that the RTP timestamp
 is not contiguous with the end of the interval covered by the
 previous packet even though the RTP sequence number has incremented
 only by one.  The RTP marker bit is also normally set on such a

6. IANA Considerations

 This section defines a new RTP payload name and associated MIME type,
 CN (audio/CN).  The payload format specified in this document is also
 assigned payload type 13 in the RTP Payload Types table of the RTP
 Parameters registry maintained by the Internet Assigned Numbers
 Authority (IANA).

6.1 Registration of MIME media type audio/CN

 MIME media type name: audio
 MIME subtype name: CN
 Required parameters: None
 Optional parameters:
 rate: specifies the RTP timestamp clock rate, which is usually (but
 not always) equal to the sampling rate.  This parameter should have
 the same value as the codec used in conjunction with comfort noise.
 The default value is 8000.

Zopf Standards Track [Page 5] RFC 3389 RTP Payload for Comfort Noise September 2002

 Encoding considerations:
 This type is only defined for transfer via RTP [RFC 1889].
 Security considerations: see Section 7 "Security Considerations".
 Interoperability considerations: none
 Published specification:
 This document and Appendix II of ITU-T Recommendation G.711
 Applications which use this media type:
 Audio and video streaming and conferencing tools.
 Additional information: none
 Person & email address to contact for further information:
 Robert Zopf
 Intended usage: COMMON
 Author/Change controller:
 Author: Robert Zopf
 Change controller: IETF AVT Working Group

7. Security Considerations

 RTP packets using the payload format defined in this specification
 are subject to the security considerations discussed in the RTP
 specification [1].  This implies that confidentiality of the media
 streams is achieved by encryption.  Because the payload format is
 arranged end-to-end, encryption MAY be performed after encapsulation
 so there is no conflict between the two operations.
 As this format transports background noise, there are no significant
 security, confidentiality, or authentication concerns.

8. References

 [1]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
      "RTP: A Transport Protocol for Real-Time Applications", RFC
      1889, January 1996.
 [2]  ITU Recommendation G.711 (11/88) - Pulse code modulation (PCM)
      of voice frequencies.

Zopf Standards Track [Page 6] RFC 3389 RTP Payload for Comfort Noise September 2002

 [3]  ITU Recommendation G.726 (12/90) - 40, 32, 24, 16 kbit/s
      Adaptive Differential Pulse Code Modulation (ADPCM).
 [4]  ITU Recommendation G.727 (12/90) - 5-, 4-, 3- and 2-bits sample
      embedded adaptive differential pulse code modulation (ADPCM).
 [5]  ITU Recommendation G.728 (09/92) - Coding of speech at 16
      kbits/s using low-delay code excited linear prediction.
 [6]  ITU Recommendation G.722 (11/88) - 7 kHz audio-coding within 64
 [7]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [8]  Appendix II to Recommendation G.711 (02/2000) - A comfort noise
      payload definition for ITU-T G.711 use in packet-based
      multimedia communication systems.
 [9]  Annex B (08/97) to Recommendation G.729 - C source code and test
      vectors for implementation verification of the algorithm of the
      G.729 silence compression scheme.
 [10] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
      with Minimal Control", RFC 1890, January 1996.
 [11] Handley, M. and V. Jacobson, "SDP: Session Description
      Protocol", RFC 2327, April 1998.
 [12] Luthi, P., "RTP Payload Format for ITU-T Recommendation
      G.722.1", RFC 3047, January 2001.

9. Author's Address

 Robert Zopf
 Lucent Technologies
 INS Access VoIP Networks
 101 Crawfords Corner Rd
 Holmdel, NJ  07733-3030  US
 Phone:   1-732-949-1667
 Fax:   1-732-949-7016

Zopf Standards Track [Page 7] RFC 3389 RTP Payload for Comfort Noise September 2002

10. Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Zopf Standards Track [Page 8]

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