- Packet delay variation
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In computer networking, packet delay variation (PDV) is the difference in end-to-end delay between selected packets in a flow with any lost packets being ignored (RFC 3393).[1] The effect is sometimes referred to as jitter, although the definition is an imprecise fit.
Contents
Terminology
The term PDV is defined in ITU-T Recommendation Y.1540, Internet protocol data communication service - IP packet transfer and availability performance parameters, section 6.2.
In computer networking, although not in electronics, usage of the term jitter may cause confusion. 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.
Measurement of packet delay variation
The means of packet selection for measurement is not specified in RFC 3393, but could, for example, be the packets which had the largest 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. A component of the delay which does not vary from packet to packet can be ignored, hence if the packet sizes are the same and packets always take the same time to be processed 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.
Instantaneous packet delay variation is the difference between successive packets--here RFC 3393 does specify the selection criteria--and this is usually what is loosely termed "jitter", although jitter is also sometimes the term used for the variance of the packet delay. As an example, say packets are transmitted every 20 ms. 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.
Limiting PDV or its effects
For interactive real-time applications, e.g., VoIP, PDV can be a serious issue and hence VoIP transmissions need Quality of Service-enabled networks to provide a high-quality channel.
The effects of PDV in multimedia streams can be removed by a properly sized play-out buffer at the receiver, which may only cause a detectable delay before the start of media playback.
See also
References
- ^ Demichelis, C.; Chimento, P. (November, 2002). IP Packet Delay Variation Metric for IP Performance Metrics (IPPM). The Internet Society. http://tools.ietf.org/html/rfc3393.
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