Subject: Protocol Action: RTP payload format and file storage format for AMR and AMR-WB audio to Proposed Standard

From: The IESG <[email-masked]>
Date: Thu, 07 Mar 2002 16:02:05 -0500

The IESG has approved RTP payload format and file storage format 
for the Adaptive Multi-Rate (AMR) Adaptive Multi-Rate Wideband (AMR-WB)
audio codecs <draft-ietf-avt-rtp-amr-13.txt> as a Proposed
Standard.  This document is the product of the Audio/Video Transport
Working Group.  The IESG contact persons are Allison Mankin and Scott
Bradner.

Technical Summary
 
   This document specifies a real-time transport protocol (RTP)
   payload format to be used for Adaptive Multi-Rate Codec (AMR) and
   Adaptive Multi-Rate Wideband Codec (AMR-WB) encoded speech
   signals.

   The AMR codec was originally developed and standardized by the
   European Telecommunications Standards Institute (ETSI) for GSM
   cellular systems. It has been chosen by the Third Generation
   Partnership Project (3GPP) as the mandatory codec for third
   generation (3G) cellular systems.  It supports 8 encoding modes
   with bit rates between 4.75 and 12.2 kbps.

   The Adaptive Multi-Rate Wideband (AMR-WB) speech codec was
   originally developed by 3GPP to be used in GSM and 3G cellular
   systems.   Like AMR, AMR-WB is also multi-mode.  It supports
   9 encoding modes with bit rates ranging from 6.6 to 23.85 kbps.

   In both codecs, mode changes are communicated from the receiver
   to the sender and may occur at any time, with inband signals
   called Codec Mode Requests (CMR).  These are typically piggy-
   backed because the speech flow is two-way.

   Besides the adaptation of rate, another unusual feature of the
   AMR codecs is Unequal Bit-error Detection and Protection (UED,
   UEP).  The UEP/UED mechanisms allow more speech over a lossy
   network by sorting the bits into perceptually more and less
   sensitive classes.  A frame is only declared damaged and not
   delivered if there are bit errors found in the most sensitive
   bits.  On the other hand, acceptable speech quality results if the
   speech frame is delivered with bit errors in the less sensitive bits,   
   based on human aural perception.  It is planned that AMR codec
   payloads will be used with UDP-lite, a transport protocol close
   to UDP, except that the checksum covers only a specified
   first n bytes of the datagram, thus permitting the delivery of
   partially errored payloads all the way to voice applications.

   The specification defines the payload format for carrying AMR
   and AMR-WB, as well as the Codec Mode Requests, in RTP.  Design
   goals included compatibility with AMR and AMR-WG transport formats
   in use on non-IP networks.  In addition to specifying the transport
   formats, the document also specifies the storage format for AMR
   and AMR-WB speec when carried in applications such as email.  MIME
   type registrations for AMR and AMR-WB are defined.

Working Group Summary

   The Working Group achieved a strong consensus for the specification,
   after energetic debate on simplicity of design given the the goals.
   The IESG requested a rewrite to improve readability and a second
   IETF Last Call was conducted on the revision.

Protocol Quality

   The specification was reviewed for the IESG by Allison Mankin.