| |
 |
Jitter is a specific VoIP Quality of Service issue that may affect the quality of the conversation if it goes out of control.
Unlike network delay, jitter does not occur because of the packet delay, but because of a variation of packet delays. As VoIP endpoints try to compensate for jitter by increasing the size of the packet buffer, jitter causes delays in the conversation. If the variation becomes too high and exceeds 150ms, callers notice the delay and often revert to a walkie-talkie style of conversation.
There are several steps to be taken to reduce jitter both on the network level and in the VoIP endpoints such as VoIP software, IP phones or dedicated VoIP adaptors. By definition, reducing the delays on the network helps keep the buffer under 150ms even if a significant variation is present. While the reduced delay does not necessarily remove the variation, it still effectively reduces the degree to which the effect is pronounced and brings it to the point where it's unnoticeable by the callers. Prioritizing VoIP traffic and implementing bandwidth shaping also helps reduce the variation of packet delay.
At the endpoint, it is essential to optimize jitter buffering. While greater buffers reduce and remove the jitter, anything over 150ms noticeably affects the perceived quality of the conversation. Adaptive algorithms to control buffer size depending on the current network conditions are often quite effective. Fiddling with packet size or using a different codec (e.g. G.711) often helps control jitter.
While jitter is caused by network delays more often than by endpoints, certain resource-struggling systems that are executed in concurrent environments, such as VoIP soft-phones, may introduce significant and unpredictable variations in packet delays. While developing VoIP endpoints or examining call quality problems within existing VoIP infrastructure, it is very important to isolate the cause of jitter. A network analyzing tool can be extremely handy in localizing the source of the problem quickly and efficiently. A good network analyzer is capable of calculating jitter for every RTP stream and building jitter and jitter deviation charts along the time axis.
|
