Delay Variation ("Jitter")

Delay variation or "jitter" is a metric that describes the level of disturbance of packet arrival times with respect to an "ideal" pattern, typically the pattern in which the packets were sent. Such disturbances can be caused by competing traffic (i.e. queueing), or by contention on processing resources in the network.

RFC 3393 defines an IP Delay Variation Metric (IPDV). This particular metric only compares the delays experienced by packets of equal size, on the grounds that delay is naturally dependent on packet size, because of serialization delay.

Delay variation is an issue for real-time applications such as audio/video conferencing systems. They usually employ a jitter buffer to eliminate the effects of delay variation.

Delay variation is related to packet reordering. But note that the RFC 3393 IPDV of a network can be arbitrarily low, even zero, even though that network reorders packets, because the IPDV metric only compares delays of equal-sized packets.

Decomposition

Jitter is usually introduced in network nodes (routers), as an effect of queueing or contention for forwarding resources, especially on CPU-based router architectures. Some types of links can also introduce jitter, for example through collision avoidance (shared Ethernet) or link-level retransmission (802.11 wireless LANs).

Measurement

Contrary to one-way delay, one-way delay variation can be measured without requiring precisely synchronized clocks at the measurement endpoints. Many tools that measure one-way delay also provide delay variation measurements.

References

The IETF IPPM (IP Performance Metrics) Working Group has formalized metrics for IPDV, and more recently started work on an "applicability statement" that explains how IPDV can be used in practice and what issues have to be considered.

  • RFC 3393, IP Packet Delay Variation Metric for IP Performance Metrics (IPPM), C. Demichelis, P. Chimento. November 2002.
  • draft-morton-ippm-delay-var-as-03.txt, Packet Delay Variation Applicability Statement, A. Morton, B. Claise, July 2007.

-- SimonLeinen - 28 Oct 2004 - 24 Jul 2007

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Topic revision: r6 - 2007-07-24 - SimonLeinen
 
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