Nortel Networks NN43001-563 User Manual
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142
ITG engineering guidelines
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 fallback threshold, any new calls to that
destination are routed over circuit-switched voice facilities.
The computation is derived from ITU-T G.107 Transmission Rating Model.
When the QoS level falls below the fallback threshold levels for 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 143)
,
"QoS level with G.711 codec" (page 143)
, and
show the operating regions in terms of
one-way delay and 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, followed by 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 voice service.
Fax is more susceptible to 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 Poor QoS level.
Nortel Communication Server 1000
IP Trunk Fundamentals
NN43001-563
01.01
Standard
Release 5.0
30 May 2007
Copyright © 2007, Nortel Networks
.