Avaya IP Telephony Guide de configuration télécharger pdf (Page 110)

110 Efficient Networking

P0937663 03.1

Adjust the jitter buffer size

The parameters for the voice jitter buffer directly affect the end-to-end delay and audio quality. IP

telephony dynamically adjusts the size of the jitter buffer to adjust for jitter in the network. The

network administrator sets the starting point for the jitter buffer.

Lower the jitter buffer to decrease one-way delay and provide less waiting time for late packets.

Late packets that are lost are replaced with silence. Quality decreases with lost packets. Increase

the size of the jitter buffer to improve quality when jitter is high.

IP telephony fax calls use a fixed jitter buffer that does not change the hold time over the duration

of the call. Fax calls are more prone to packet loss. In conditions of high jitter, increase delay

through the use of a deeper jitter buffer. To allow for this increase, IP telephony provides a

separate jitter buffer setting for fax calls.

Reduce packet errors

Packet errors in intranets correlate to congestion in the network. Packet errors are high because the

packets are dropped if they arrive faster than the link can transmit. Identify which links are the

most used to upgrade. This removes a source of packet errors on a distinct flow. A reduction in hop

count provides for less occurrences for routers and links to drop packets.

Other causes of packet errors not related to delay are as follows:

• reduced link quality

• overloaded CPU

• saturation

• LAN saturation

• limited size of jitter buffer

If the underlying circuit has transmission problems, high line error rates, outages, or other

problems, the link quality is reduced. Other services such as X.25 or frame relay can affect the

link. Check with your service provider for information.

Find out what the router threshold CPU utilization level is, and check if the router conforms to the

threshold. If a router is overloaded, the router is continuously processing intensive tasks.

Processing intensive tasks prevents the router from forwarding packets. To correct this,

reconfigure or upgrade the router.

A router can be overloaded when there are too many high-capacity and high-traffic links

configured on it. Ensure that routers are configured to vendor guidelines.

Saturation refers to a situation where too many packets are on the intranet. Packets can be dropped

on improperly planned or damaged LAN segments.

Packets that arrive at the destination late are not placed in the jitter buffer and are lost packets. See

“Adjust the jitter buffer size” on page 110.

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Avaya IP Telephony Guide de configuration Page 110