Time Alignment Using RTCP Feedback
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This memo describes the use of RTCP-based feedback to reduce end-to- end delay through the use of time alignment between the source and receiver. Time alignment is useful, for example, in certain gateway-to-gateway scenarios and certain gateway-to-VoIP-terminal scenarios. It is the ability for a receiver to request that the sender shift the time reference for packetization by a specified amount. When a receiver with strict packet timing limitations, e.g., due to a 3G UMTS Iu interface, is connected to an sender at a peer endpoint capable of responding to time alignment requests, e.g., with an AMR encoder associated with a PCM interface to the PSTN, this allows the endpoints to reduce delay in the speech path by as much as the 20 ms frame-block duration, depending on the degree of misalignment in the time reference.
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