Nortel-networks NN43001-563 Manual de usuario descargar pdf (Pagina 142)

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101

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), Figure 29

"QoS level with G.711 codec" (page 143), and Figure 30 "QoS level

with G.723 codec" (page 144) 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

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