Jitter

Consecutive packets that experience different amounts of delay have experienced jitter. In a packet network, with variable delay components, jitter always occurs—the question is whether the jitter impacts the application enough to degrade the service. Typically, data applications expect some jitter, and do not degrade. However, some traffic, such as digitized voice, requires that the packets be transmitted in a consistent, uniform manner (for instance, every 20 ms). The packets should also arrive at the destination with the same spacing between them. (This type of traffic is called isochronous traffic.)

Jitter is defined as a variation in the arrival rate (that is, variation in delay through the network) of packets that were transmitted in a uniform manner. Figure 1-14, for example, shows three packets as part of a voice call between phones at extension 301 and 201.

Figure 1-14 Jitter Example

Server 1

Hannah

Hannah

20 20

Server 1

301

RTP

RTP

The phone sends the packets every 20 ms. Notice that the second packet arrived 20 ms after the first, so no jitter occurred. However, the third packet arrived 30 ms after the second packet, so 10 ms of jitter occurred.

Voice and video degrade quickly when jitter occurs. Data applications tend to be much more tolerant of jitter, although large variations in jitter affect interactive applications.

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