Packet delay variation

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Packet Delay Variation (PDV) is defined by RFC 3393 as the difference in end-to-end delay between selected packets in a flow with any lost packets being ignored. The means of selection is not specified in RFC 3393, but e.g. could be the packets which gave the biggest variation in delay in a selected time period. The delay is specified from the start of the packet being transmitted at the source to the end of the packet being received at the destination. N.B. a component of the delay which does not vary from packet to packet can be ignored, hence if the packet lengths are the same and packets always take the same time to be re-assembled at the destination then the packet arrival time at the destination could be used instead of the time the end of the packet is received.

In computer networking (although not in electronics), jitter is a poorly defined term and should not be used. From RFC 3393 (section 1.1) "The variation in packet delay is sometimes called "jitter". This term, however, causes confusion because it is used in different ways by different groups of people. ... In this document we will avoid the term "jitter" whenever possible and stick to delay variation which is more precise." Or indeed use Packet Delay Variation (PDV) as in the title of RFC 3393: "IP Packet Delay Variation Metric for IP Performance Metrics (IPPM)".

The Instantaneous Packet Delay Variation (IPDV) is the difference between successive packets (here RFC 3393 does specify the selection criteria), and this is usually what is loosely but incorrectly termed "jitter" (although jitter is also sometimes used for the variance of the different PDs). As an example, say packets are transmitted every 20 ms, thus we expect on average to receive a second packet 20 ms after the 1st. However:

If the 2nd packet is received 30 ms after the 1st packet, IPDV = -10 ms. This is referred to as dispersion.

If the 2nd packet is received 10 ms after the 1st packet, IPDV = +10 ms. This is referred to as clumping.

In multimedia streams PDV can be removed by the choice of a 'big-enough' play out buffer at the destination, which only causes an annoying delay before the media starts playing. For interactive real time applications however (e.g. VoIP) PDV can be a serious issue and hence VoIP flows need Quality of Service enabled networks to provide a high quality channel.