Avaya BCM 2.0 IP Telephony Bedienungsanleitung PDF herunterladen (Seite 27)

Engineering guidelines 27

P0911590 Issue 02 Enterprise Edge 2.0 IP Telephony Configuration Guide

The settings in Table 4 indicate the quality of voice service. IP telephony

periodically calculates the prevailing QoS level per site pair based on the

measurement of the following:

• one-way delay

•packet loss

• codec

When the QoS level of any remote gateway is below the fallback threshold, all new

calls are routed over the standard circuit-switched network.

The computation is taken from the ITU-T G.107 Transmission Rating Model.

Fax is more open to packet loss than the a person’s ear; quality starts to degrade

when packet loss exceeds 10%. Fax services must be supported with the gateway

operating in either the Excellent or Good QoS level. Avoid fax services between site

pairs that can guarantee no better than a Fair or Poor QoS level.

Measuring Intranet QoS

Measure the end-to-end delay and error characteristics of the current state of the

intranet. These measurements help to set accurate QoS needs when using the

corporate intranet to carry voice services.

Measuring end-to-end network delay

The basic tool used in IP networks to get delay measurements is the “Ping”

program. The Ping takes a delay sample by sending an ICMP packet from the host

of the Ping program to a destination server, and waits for the following:

Pinging 10.10.10.15 with 32 bytes of data:

Reply from 10.10.10.15: bytes=32 time=13ms TTL=252

Reply from 10.10.10.15: bytes=32 time=10ms TTL=252

Reply from 10.10.10.15: bytes=32 time=6ms TTL=252

Reply from 10.10.10.15: bytes=32 time=5ms TTL=252

The round trip time (rtt) is indicated by the time field

So that the delay sample results match what the gateway experiences, the Ping host

must be on a correctly working LAN segment on the intranet. The option of

destination host is as important, following the same guidelines for the source host.

Set the size of the Ping probe packets to 60 bytes to approximate the size of probe

packets sent by IP telephony and determine if new calls need to fall back on the

circuit-switched voice facilities.

Notice from the Ping output the difference of rtt. The repeated sampling of rtt

allows you to receive a delay characteristic of the intranet. To get a delay

distribution, include the Ping tool in a script which controls the frequency of the

Ping probes, which timestamps and stores the samples in a raw data file.

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Avaya BCM 2.0 IP Telephony Bedienungsanleitung Seite 27