Packet delay variation
Encyclopedia
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). The effect is sometimes referred to as jitter
, although the definition is an imprecise fit.
In computer networking, although not in electronics
, usage of the term jitter
may cause confusion. From RFC 3393 (section 1.1):
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.
-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.
Flow (computer networking)
In packet switching networks, traffic flow, packet flow or network flow is a sequence of packets from a source computer to a destination, which may be another host, a multicast group, or a broadcast domain...
with any lost packets being ignored (RFC 3393). The effect is sometimes referred to as jitter
Jitter
Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
, although the definition is an imprecise fit.
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
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
, usage of the term jitter
Jitter
Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
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
Variance
In probability theory and statistics, the variance is a measure of how far a set of numbers is spread out. It is one of several descriptors of a probability distribution, describing how far the numbers lie from the mean . In particular, the variance is one of the moments of a distribution...
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 ServiceQuality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
-enabled networks to provide a high-quality channel.
The effects of PDV in multimedia
Multimedia
Multimedia is media and content that uses a combination of different content forms. The term can be used as a noun or as an adjective describing a medium as having multiple content forms. The term is used in contrast to media which use only rudimentary computer display such as text-only, or...
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.