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Jitter
Issue 6 January 2008 247
Jitter
Jitter is thought of as the statistical average variance of the arrival time between packets
received from the IP network. To compensate for jitter, a de-jitter buffer is implemented in VoIP
endpoints. The purpose of the jitter buffer is to hold incoming packets for a specified period of
time such that voice samples can be played out at a regular rate to the user. In doing so, the
jitter buffer also adds packet delay.
Excessive jitter might cause additional delay if the jitter still fits the size of the jitter buffer.
Excessive Jitter might also result in packet discard creating audible voice-quality problems
when the variation is greater than the jitter buffer size. Dynamic jitter buffers give the best
quality. Static jitter buffers should generally be sized at twice the largest statistical variance
between packet arrivals. However, care needs to be taken in the design of the resizing
algorithm of dynamic buffers in order to avoid adverse effects. Dynamic jitter buffering can
exacerbate problems in an uncontrolled network. The network topology can also affect jitter.
The existence of multiple paths between endpoints with load balancing enabled in routers can
contribute significant amounts of jitter.
The following Avaya products all have dynamic jitter buffers to minimize delay by automatically
adjusting the jitter buffer size:
● Avaya G350 and G700 Media Gateways and G650 Medial Gateways with the TN2302AP
circuit pack
● Avaya IP SoftPhone software
● Avaya 4600 Series IP Telephones
On a VoIP network, the Avaya Converged Network Analyzer (CNA) system is capable of
providing ongoing measurements of jitter (see C
NA Application Performance Rating on
page 254). CNA can also generate alarms when jitter rises to levels that are detrimental for
voice quality. For more information on CNA, see The Converged Network Analyzer
on
page 344.
Packet loss
Packet loss occurs when packets are sent but not received, or are received too late to be
processed by the endpoint jitter buffer. Too much delay or packet mis-order can be perceived as
lost packets. It may appear that the network is losing packets when in fact they have been
discarded intentionally because of late arrival at the endpoint. IP networks are characterized by
unintentional packet loss in the network as well as by discarded packets in the jitter buffers of
the receiving endpoints.