Avaya IP Telephony Guia de Configuração descarregar pdf (Página 201)

Quality of Service 201

IP Telephony Configuration Guide

Quality of Service parameters

Quality of Service depends on end-to-end network performance and available bandwidth. A

number of parameters determine the VoIP Gateway QoS over the data network. The VoIP

Gateway monitoring function can take about three minutes to respond to marginal changes in the

network condition.

• Packet loss: Packet loss is the percentage of packets that do not arrive at their destination.

Transmission equipment problems and high delay and congestion can cause packet loss. In a

voice conversation, gaps in the conversation represent packet losses. Some packet loss, less

than 5%, can be acceptable without audible degradation in voice quality.

• Packet delay: Packet delay is the period between when a packet leaves and when a packet

arrives at the destination. The total packet delay time includes fixed and variable delay.

Variable delay is the more manageable delay, while fixed delay depends on the network

technology. The distinct network routing of packets are the cause of variable delays. To

minimize packet delay and increase voice quality, the gateway must be as close as possible to

the network backbone (WAN) with a minimum number of hops.

• Delay variation (jitter): The amount of variation in packet delay is otherwise known as delay

variations, or jitter. Jitter affects the ability of the receiving gateway to assemble voice packets

received at irregular intervals into a continuous voice stream.

Fallback to PSTN

If the measured Mean Opinion Score (MOS) for all codecs is below the configured threshold for

any monitored gateway, the Fallback to PSTN activates. This feature reroutes calls to different

trunks such as the Public Switched Telephone Network (PSTN) until the network QoS improves.

When the QoS meets or exceeds the threshold, calls route over the IP network.

Fallback can be caused by any of the following reasons:

• bad network conditions

• remote gateway is out of service

• no network connection

• not enough DSP resources available

The fallback feature can be in the Local Gateway Configuration. With the fallback feature

disabled, calls move across the IP telephony trunks no matter what level of Quality of Service. The

fallback feature is active only at call setup. A call in progress does not fall back if the quality

degrades.

Calls fallback if there is no response from the destination, an incorrectly configured remote

gateway table, or if there are not enough DSP resources available to handle the new call.

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Avaya IP Telephony Guia de Configuração Página 201