Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN) (RFC3389)
Original Publication Date: 2002-Sep-01
Included in the Prior Art Database: 2002-Oct-01
Internet Society Requests For Comment (RFCs)
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.
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 (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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 .
This document describes a RTP  payload format for transporting
comfort noise. The payload format is based on Appendix II of ITU-T
Recommendation G.711  which defines a comfort noise payload format
(or bit-stream) for ITU-T G.711  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
, G.727 , G.728 , and G.722 . 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 . It uses the VAD and DTX of G.729
Annex B  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 payl...