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VARIABLE RATE BLOCK ENCODING AND ADAPTIVE LATENCY CONTROL FOR PACKET VOICE COMMUNICATIONS

IP.com Disclosure Number: IPCOM000007283D
Original Publication Date: 1994-Oct-01
Included in the Prior Art Database: 2002-Mar-11
Document File: 8 page(s) / 388K

Publishing Venue

Motorola

Related People

Paul Odlyzko: AUTHOR [+3]

Abstract

Widespread use of networked computers in the workplace creates economic incentives for integra- tion of voice and data communications, at least with respect to connectivity (which in our view includes wiring, maintenance and administration). Further- more, technology allows merging of voice and data messaging: digital processing of voice and video com- pression make stored video and audio (CD-ROM or server on the network) available, increasing com- puting power of the PCs, new bus architectures and new software ensure multimedia capabilities at the desktop. What remains is to reduce latencies and thus making communications real-time. Integration of the telephone and the PC becomes then next to trivial, with the computer subsuming the telephone hmction.

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MOTOROLA Technical Developments Volume 23 October 1994

VARIABLE RATE BLOCK ENCODING AND ADAPjVE LATENCY CONTROL FOR PACKET VOICE COMMUNIqATIONS

by Paul Odlyzko, Hugh Wang and John Ley

1. BACKGROUND AND MOTIVATION

  Widespread use of networked computers in the workplace creates economic incentives for integra- tion of voice and data communications, at least with respect to connectivity (which in our view includes wiring, maintenance and administration). Further- more, technology allows merging of voice and data messaging: digital processing of voice and video com- pression make stored video and audio (CD-ROM or server on the network) available, increasing com- puting power of the PCs, new bus architectures and new software ensure multimedia capabilities at the desktop. What remains is to reduce latencies and thus making communications real-time. Integration of the telephone and the PC becomes then next to trivial, with the computer subsuming the telephone hmction.

  The question is only whether this computer must be connected to separate voice and data networks that extend all the way to the desktop or whether it can use a common LAN for all the communication services.

  Mechanisms for capacity reservation (real-time voice included in "isochronous" traffic) are com- monly considered indispensable for voice transport. The local area networks are considered inadequate because they lack such mechanisms and also suffer from apparently low efficiency due to packet over- head, especially for short packets deemed necessary for voice.

  This paper presents the methods to hold down the delay and avoid or lessen the bandwidth inefh- ciency associated with the use of asynchronous packet data networks for high quality real-time voice com- munications. Specific illustrations and estimates are provided for Ethernet networks, but the concepts are applicable to any other packet networks of com- parable or higher bandwidth.

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  The overall scheme, combines a method for compressing PCM encoded voice and a method for adaptive latency control ~ and elastic buffering that permit effective use of a packet network for real- time transport of 64kbps PCM voice, or audio encoded by some other method. It lowers the trade- off curve between delay and channel utilization toward lower delay and lower loading.

  Variable-rate block e~ncoding is a time-domain lossless method for compressing PCM encoded voice samples, so that the same amount of information tits in shorter variable-length packets. This is a low complexity method, exploiting pseudo-periodicity of voiced speech and low peak amplitudes during peri- ods of silence or unvoiced speech. When used over fixed-bandwidth digital channels, such as the unrestricted 64 kbps ISDN B-channel, this method frees on the average 20 to 30 kbps for packet data in each direction.

  Adaptive latency control permits use of variable- duration audio packetization intervals and variable buffer size to remove jitter at the receiver. Long in...