|“||[t]he short-term phase variations of the significant instants of a timing signal from their ideal position in time (where short-term implies here that these variations are of frequency greater than or equal to 10 Hz).||”|
- "refers to non-uniform packet delays that can cause packets to arrive and be processed out of sequence."
Jitter is often caused by low bandwidth situations in VoIP and can be exceptionally detrimental to the overall quality of service. Jitter can cause packets to arrive and be processed out of sequence. When jitter is high, packets arrive at their destination in spurts. This situation is analogous to uniform road traffic coming to a stoplight. As soon as the stoplight turns green (bandwidth opens up), traffic races through in a clump. The general prescription to control jitter at VoIP endpoints is the use of a buffer, but such a buffer has to release its voice packets at least every 150 ms (usually a lot sooner given the transport delay) so the variations in delay must be bounded. The buffer implementation issue is compounded by the uncertainty of whether a missing packet is simply delayed an anomalously long amount of time, or is actually lost. Jitter can also be controlled throughout the VoIP network by using routers, firewalls, and other network elements that support quality of service. These elements process and pass along time urgent traffic like VoIP packets sooner than less urgent data packets.
- NIST Special Publication 800-58, at 20.