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Jitter is a measure of the variability of delay; it is the average variation in the delivery time between packets. If the variation is greater than 20 milliseconds, jitter creates audible voice-quality problems similar to those created by high latency.
To compensate for jitter, many H.323 voice applications incorporate a jitter buffer that holds incoming packets for a specified period of time before decompression. Jitter buffers smooth packet flow, but they also add to packet delay. For best quality, jitter buffers should be dynamic or, if static, sized to twice the greatest statistical variance between packets. Buffers must also have the appropriate queue-unloading algorithm.
G700 Media Gateways, as well as DEFINITY� ECS, Avaya S8100 Media Server, IP SoftPhone software, and IP telephones, all incorporate dynamic jitter buffers to minimize delay by reducing the jitter buffer size, as the network allows. This feature can exacerbate problems in an uncontrolled network. Delay added by jitter buffers must remain within the network jitter maximum limit.
Network topology also affects jitter. Because there are fewer collisions, a hierarchical data-switched network has less jitter than a flat hub-based network.
Between endpoints, jitter should be under 20 milliseconds. The maximum acceptable jitter value may vary depending on the type of service the jitter buffer has in relationship to other router buffers.
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