Avaya 555-245-600 manual Jitter, Packet loss

Jitter

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 CNA 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.

Issue 6 January 2008 247