114 ITG engineering guidelinesSilence Suppression engineering considerationsSilence Suppression/Voice Activity Detection (VAD) results in averagebandwidth savings over time, not in instantaneous bandwidth savings.For normal conversations, Silence Suppression creates a 40% savings inaverage bandwidth used. For example, a single G.729AB voice packet willstill consume 30 Kbps of bandwidth but the average bandwidth used for theentire call would be approximately 23 Kbps.To calculate the average bandwidth, perform the following calculation:Codec bandwidth from Table 17 "Silence Suppression disabled TLANEthernet and WAN IP bandwidth usage per IP Trunk 3.01 (and later) " (page113) x (0.6)When voice services with multi-channel requirements are extensively usedin an IP Trunk 3.01 (and later) network, such as Conference, Music-on-hold,and Message Broadcasting, additional voice traffic peaks to the IP networkare generated due to the simultaneous voice-traffic bursts on multiplechannels on the same links.In those cases, even when Silence Suppression is enabled on the IPtrunk card, Nortel recommends using the more conservative bandwidthcalculations of Table 17 "Silence Suppression disabled TLAN Ethernet andWAN IP bandwidth usage per IP Trunk 3.01 (and later) " (page 113) withSilence Suppression disabled to calculate the portion of the bandwidthrequirement caused by simultaneous voice traffic.Fax engineering considerationsThe fax calculation is based on a 30-byte packet size and a data rate of 64kbit/s (with no compression) The frame duration (payload) is calculatedby using the equation:30*8/14400=16.6 mswhere 14,400 bit/s is the modem data rate.Bandwidth output is calculated by the equation:108*8*1000/16.6=52.0 kbit/sBandwidth output to WAN is:70*8*1000/16.6=33.7 kbit/s.Payload and bandwidth output for other packet sizes or modem data ratesmust be calculated in a similar manner.Nortel Communication Server 1000IP Trunk FundamentalsNN43001-563 02.01 StandardRelease 5.5 21 December 2007Copyright © 2007, Nortel Networks.
