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114 ITG engineering guidelines
Silence Suppression engineering considerationsSilence 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 averagebandwidth used for the
entire call would be approximately 23 Kbps.
Tocalculate the average bandwidth, perform the following calculation:
Codec bandwidth from Table17 "Silence Suppression disabled TLAN
Ethernet and WAN IP bandwidth usage per IP Trunk3.01 (and later) " (page
113) x (0.6)
When voice services with multi-channel requirements are extensively used
in an IP Trunk3.01 (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 Suppressionis enabled on the IP
trunk card, Nortel recommends using the more conservative bandwidth
calculations of Table17 "Silence Suppression disabled TLAN Ethernet and
WAN IP bandwidth usage per IP Trunk3.01 (and later) " (page 113) with
Silence Suppression disabled to calculate the portion of the bandwidth
requirement caused by simultaneous voice traffic.
Fax engineering considerationsThe fax calculation is based on a 30-byte packetsize 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.
Payloadand bandwidth output for other packet sizes or modem data rates
must be calculated in a similar manner.
Nortel Communication Server 1000
IP TrunkFundamentals
NN43001-563 02.01 Standard
Release 5.5 21 December 2007
Copyright© 2007, Nor tel Networks
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