Subject: Protocol Action: RTP payload format and file storage format for AMR and AMR-WB audio to Proposed Standard Date: Thu, 07 Mar 2002 16:02:05 -0500 From: The IESG To: IETF-Announce: ;;:@cs.columbia.edu; CC: RFC Editor , Internet Architecture Board , avt@ietf.org 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 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.