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Chapter 7Performance optimisation for the support of TCIS pFrom Figure 7.15b, the maximum sustainable system throughput was ≈ 56% of the ccfor the PRA mechanism. For the mechanism provided by QoS, the maximum systemwas also ≈ 56%. Internet traffic was choked off because in every MCI frame, theheadend schedules first the bandwidth for VoIP traffic and then in the remaining slots IPtraffic. This becomes more evident when the maximum system throughput is achieved(at 56% of the cc, produced by 80 stations).B) Performance analysis for VoIP trafficIn this section the performance is focused on the case in which a fixed-rate connectionis used for the delivery of VoIP streams. Here two different audio codecs are used forthe analysis, G.711 [56] and G.723.1 [57]. So far, most of the VoIP streams examinedhave been based on the codec G.723.1, which encapsulates 4 VoIP frames of 20 bytesevery 120 ms. In order to approach the maximum system performance that theDVB/DAVIC protocol can achieve, we have used four possible configurations for eachcodec. In addition, to increase the number of streams supported, header suppression(which is an advanced functionality in the latest version of this protocol [35]) is alsoconsidered in this analysis.Codec G.711 was considered here to stress the CATV network and also becauseEuroModems (class B) are more likely to use this codec for quality voice calls. G.711 isthe mandatory codec according to the ITU-T H.323 conferencing standard [59], whichuses Pulse Code Modulation (PCM) to produce a data rate of 64 kbps. This audio codeccreates and encapsulates a VoIP frame of 80 bytes every 10 ms. In order to reduce theprotocol overhead involved, a frame size of 30 ms (240 bytes) can also be used,according to the Packet Cable Audio/Video Codecs Specification [80].7-20