RTP Payload Format and File Storage Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs
RFC 4867

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Network Working Group                                         J. Sjoberg
Request for Comments: 4867                                 M. Westerlund
Obsoletes: 3267                                                 Ericsson
Category: Standards Track                                   A. Lakaniemi
                                                                   Nokia
                                                                  Q. Xie
                                                                Motorola
                                                              April 2007

          RTP Payload Format and File Storage Format for the
  Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB)
                              Audio Codecs

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document specifies a Real-time Transport Protocol (RTP) payload
   format to be used for Adaptive Multi-Rate (AMR) and Adaptive Multi-
   Rate Wideband (AMR-WB) encoded speech signals.  The payload format is
   designed to be able to interoperate with existing AMR and AMR-WB
   transport formats on non-IP networks.  In addition, a file format is
   specified for transport of AMR and AMR-WB speech data in storage mode
   applications such as email.  Two separate media type registrations
   are included, one for AMR and one for AMR-WB, specifying use of both
   the RTP payload format and the storage format.  This document
   obsoletes RFC 3267.

Sjoberg, et al.             Standards Track                     [Page 1]
RFC 4867         RTP Payload Format for AMR and AMR-WB        April 2007

 Table of Contents

   1. Introduction ....................................................4
   2. Conventions and Acronyms ........................................4
   3. Background on AMR/AMR-WB and Design Principles ..................5
      3.1. The Adaptive Multi-Rate (AMR) Speech Codec .................5
      3.2. The Adaptive Multi-Rate Wideband (AMR-WB) Speech Codec .....6
      3.3. Multi-Rate Encoding and Mode Adaptation ....................6
      3.4. Voice Activity Detection and Discontinuous Transmission ....7
      3.5. Support for Multi-Channel Session ..........................7
      3.6. Unequal Bit-Error Detection and Protection .................8
           3.6.1. Applying UEP and UED in an IP Network ...............8
      3.7. Robustness against Packet Loss ............................10
           3.7.1. Use of Forward Error Correction (FEC) ..............10
           3.7.2. Use of Frame Interleaving ..........................12
      3.8. Bandwidth-Efficient or Octet-Aligned Mode .................12
      3.9. AMR or AMR-WB Speech over IP Scenarios ....................13
   4. AMR and AMR-WB RTP Payload Formats .............................15
      4.1. RTP Header Usage ..........................................15
      4.2. Payload Structure .........................................17
      4.3. Bandwidth-Efficient Mode ..................................17
           4.3.1. The Payload Header .................................17
           4.3.2. The Payload Table of Contents ......................18
           4.3.3. Speech Data ........................................20
           4.3.4. Algorithm for Forming the Payload ..................21
           4.3.5. Payload Examples ...................................21
                  4.3.5.1. Single-Channel Payload Carrying a
                           Single Frame ..............................21
                  4.3.5.2. Single-Channel Payload Carrying
                           Multiple Frames ...........................22
                  4.3.5.3. Multi-Channel Payload Carrying
                           Multiple Frames ...........................23
      4.4. Octet-Aligned Mode ........................................25
           4.4.1. The Payload Header .................................25
           4.4.2. The Payload Table of Contents and Frame CRCs .......26
                  4.4.2.1. Use of Frame CRC for UED over IP ..........28
           4.4.3. Speech Data ........................................30
           4.4.4. Methods for Forming the Payload ....................31
           4.4.5. Payload Examples ...................................32
                  4.4.5.1. Basic Single-Channel Payload
                           Carrying Multiple Frames ..................32
                  4.4.5.2. Two-Channel Payload with CRC,
                           Interleaving, and Robust Sorting ..........32
      4.5. Implementation Considerations .............................33