| 摘要 |
An enhanced H.321-based multipoint, multimedia conferencing service is provided over the ATM WAN interconnecting geographically dispersed H.321 terminals with a centralized H.321 MCU located within the WAN. The performan ce of H.321 multimedia conferencing services over the ATM WAN is improved by providing a modified WAN-based H.321 MCU which de-jiterizes cell delay variation and detects cell losses. The MCU supplies the reference clock sign al that will be used by all communicating entities for synchronization, smoothe s ATM cell delay variation and creates dummy bits in the case of cell losses, to create a more accurate H.221 frame. The information about the dummy bits of the H.221 frame is sent to upper layer entities to facilitate compensation a t the appropriate positions of the cell/bit losses for audio and/or video traffic. The higher layer audio and video entities compensate for audio and/ or video cell/bit losses with the information provided by the lower entities. I n addition, intramedia and intermedia synchronization are provided for both audio and video traffic. Finally, transport protocol suits are incorporated on the top of H.221 layer to recover data cell losses or errors. Customer premises-based H.321 terminals may also employ the modified H.321 services noted above in the H.321MCU to compensate for audio and/or video cell losses or errors for improvement of performance.
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