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124 Quality of Service
P0937663 03.1
The QoS level is aligned for user QoS metrics to provide an acceptable Mean Opinion Score
(MOS) level. The administrator can adjust the fallback thresholds to provide acceptable service to
the users.
The settings in Table 20 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 if fallback is enabled.
The computation is taken from the ITU-T G.107 Transmission Rating Model.
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. Ping takes a
delay sample by sending a series of packets to a specified IP address and then return to the
originating IP address. Ping then displays statistics for the packets. High packet times can indicate
network congestion. If the packets time out, then the remote device is unreachable.
The round trip time (rtt) is indicated by the time field
So that the delay sample results match what the gateway experiences, both the Ping host and target
must be on a functioning LAN segment on the intranet.
Set the size of the Ping probe packets to 60 bytes to approximate the size of probe packets sent by
IP telephony. This determines if new calls need to fall back on the circuit-switched voice facilities.
Table 20 Quality of voice service
MOS Range Qualitative Scale
4.86 to 5.00 Excellent
3.00 to 4.85 Good
2.00 to 2.99 Fair
1.00 to 1.99 Poor
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