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114 ITG engineering guidelines

Silence Suppression engineering considerations

Silence Suppression/Voice Activity Detection (VAD) results in average

bandwidth savings over time, not in instantaneous bandwidth savings.

For normal conversations, Silence Suppression creates a 40% savings in

average bandwidth used. For example, a single G.729AB voice packet will

still consume 30 Kbps of bandwidth but the average bandwidth used for the

entire call would be approximately 23 Kbps.

To calculate the average bandwidth, perform the following calculation:

Codec bandwidth from Table 17 "Silence Suppression disabled TLAN

Ethernet and WAN IP bandwidth usage per IP Trunk 3.01 (and later) " (page

113) x (0.6)

When voice services with multi-channel requirements are extensively used

in an IP Trunk 3.01 (and later) network, such as Conference, Music-on-hold,

and Message Broadcasting, additional voice trafﬁc peaks to the IP network

are generated due to the simultaneous voice-trafﬁc bursts on multiple

channels on the same links.

In those cases, even when Silence Suppression is enabled on the IP

trunk card, Nortel recommends using the more conservative bandwidth

calculations of Table 17 "Silence Suppression disabled TLAN Ethernet and

WAN IP bandwidth usage per IP Trunk 3.01 (and later) " (page 113) with

Silence Suppression disabled to calculate the portion of the bandwidth

requirement caused by simultaneous voice trafﬁc.

Fax engineering considerations

The fax calculation is based on a 30-byte packet size and a data rate of 64

kbit/s (with no compression) The frame duration (payload) is calculated

by using the equation:

30*8/14400=16.6 ms

where 14,400 bit/s is the modem data rate.

Bandwidth output is calculated by the equation:

108*8*1000/16.6=52.0 kbit/s

Bandwidth output to WAN is:

70*8*1000/16.6=33.7 kbit/s.

Payload and bandwidth output for other packet sizes or modem data rates

must be calculated in a similar manner.

Nortel Communication Server 1000

IP Trunk Fundamentals

NN43001-563 02.01 Standard

Release 5.5 21 December 2007