Cisco receiver what is it series#
The series of audio processing units-including the input buffer, decoder, and playout device-can be considered a data pipeline, each with its own delay. One common form of PLC is to just replay the most recent packet received from the network. Packet loss concealment (PLC) is the process of mitigating the loss of quality resulting from a lost packet. This replacement packet may contain audio silence, or it may contain audio that attempts to conceal the lost packet. When the jitter buffer underflows, the audio decoder must come to the rescue and supply a replacement for the missing audio packet. In fact, if the jitter buffer underflows and results in a dropped packet, the receiver immediately reestablishes a new jitter buffer with a higher average level to accommodate greater variance. The endpoint typically adapts the level of the jitter buffer over time by observing the recent history of jitter and increasing the average buffer level if necessary. Therefore, the goal of the receive endpoint is to establish a jitter buffer with the smallest average latency, which can minimize the probability of an audio packet dropout. This delay is equal to the average level of the jitter buffer, measured in milliseconds. The jitter buffer absorbs these arrival-time variations however, the jitter buffer imposes an additional delay in the end-to-end audio pipeline. Conversely, if the network then transfers these delayed packets in quick succession, the burst of packets causes the jitter buffer to rise back to its normal level quickly. This scenario, in which the jitter buffer runs out of data for the audio playout device, is called audio starvation. If the jitter buffer underflows, the audio device has no data to play out the audio speakers, and the user hears a glitch. The jitter buffer must be large enough to prevent the buffer from dropping to the point where it underflows.
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If the network imposes a temporary delay on a sequence of several packets, those packets arrive late, causing the jitter buffer on the receive side to decrease. However, jitter in the network due to transient delays causes nonuniform spacing between packet arrival times at the receiver.
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The sending endpoint typically sends fixed-sized RTP packets onto the network at uniform intervals, generating a stream with a constant audio bit rate. The receiver requires the jitter buffer in the audio path because packets arriving at the receiver do not have uniform arrival times. Figure 7-10 Receiver-Side Processing Audio Receiver Path