Measure intranet QoS 141
The QoS level is a user-oriented QoS metric which takes on one of these
four settings: excellent, good, fair, and poor,indicating the quality of voice
service. IP Trunk 3.01 (and later) periodically calculates the prevailing QoS
level per site pair,based on its measurement of the following:
•one-way delay
•packet loss
•Codec
When the QoS level is below the fallbackthreshold, any new calls to that
destination are routed over circuit-switched voice facilities.
The computation is derived from ITU-T G.107 TransmissionRating Model.
When the QoS level falls below the fallback threshold levelsfor that
particular destination, that call is not accepted by the originating IP Trunk
3.01 (and later) node; instead the call is re-routed by ESN features over
traditional circuit-switched voice facilities.
Figure 28 "QoS levels with G.729A/AB codec" (page 142),Figure 29
"QoS level with G.711 codec" (page 142), and Figure 30 "QoS level
with G.723 codec" (page 143) show the operating regions in terms of
one-way delayand packet loss for each codec and required QoS level as
determined by IP Trunk 3.01 (and later). Note that among the codecs,
G.711(A-law)/G.711(u-law) delivers the best quality for a given intranet
QoS, followedby G.729AB and then G.723.1 (6.4 kbp/s) and lastly G.723.1
(5.3 kbp/s). These figures determine the delay and error budget for the
underlying intranet in order for it to deliver a required quality of voiceser vice.
Fax is more susceptibleto packet loss than the human ear is; quality starts
to degrade when packet loss exceeds 4%. Nortel recommends that fax
services be supported with IP Trunk 3.01 (and later) operating in either the
Excellent or Good QoS level. Avoid offering fax services between two sites
that can guarantee no better than a Fair or PoorQoS level.
Nortel Communication Server 1000
IP TrunkFundamentals
NN43001-563 02.01 Standard
Release 5.5 21December 2007
Copyright© 2007, Nor tel Networks
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