Efficient variable rate coding of voice over asynchronous transfer mode

Abstract

The invention uses an ATM Adaptation Layer of type 2 (AAL2) standard mechanism to define efficient support for Variable Rate Coding (VRC). The VRC in this context typically refers to codecs, which adapt their rate to information content variations in speech and audio. Such VRC results in lower average rate than the constant rate codecs or use of constant rate codecs coupled with silence suppression (SS), currently deployed in voice over ATM schemes using AAL2. Possible ATM transport, trunking and access applications encompass both Circuit Emulation Services (CES) and Local Loop Emulation Services (LLES). A typical VRC profile encompasses options for all sub-rates within one or more VRC standard or proprietary variable rate codec. The output of a rate determination algorithm (RDA), commonly part of variable rate codec, is fed into present AAL2 inter-working function (IWF). The IWF in AAL2, which normally supports SS or multiple rates (as opposed to voice content VRC), is extended to accommodate VRC and thereafter automatically furnishes AAL2 capabilities with extensions to VRC. As AAL2 standard capability is extended to VRC, all other advantages of AAL2 and Voice over ATM, including channel multiplexing, idle channel suppression, and alternative signaling methods, are maintained and leveraged. A simple mapping of frame number in a typical VRC speech codec into AAL2 sequence number is employed. This allows for easy use of available error concealment techniques based on frame error, already part of VRC codec standards such as Enhanced VRC (EVRC). When using tandem-free-operation (TFO) of VRC-ATM, higher end-to-end voice quality and bandwidth efficiency can be provided. More advanced capabilities can provide TFO VRC across access and transport networks involving ATM.

Dependencies are inferred from the claim text.