DOWNLOAD MY Ph.D Thesis - UNAM

Chapter 5Upstream channel capacity and characterisation preleased in October 2001), a new advanced mechanisms is used for header suppression.By using this feature, all constant components of the different headers can besuppressed, thus the effective data rate of isochronous streams (e.g. multiplexed voice,audio and/or video) is remarkably increased.5.3.5 System parameters and assumptionsIn all simulations, one upstream channel with a capacity of 3.088 Mbps and onedownstream channel with 42 Mbps were considered. Another important simulationparameter is the choice of the network size. According to [100] the number of activesubscribers per upstream channel on European cable networks is approximately 50 usersin peak hours of traffic.In our analysis we use cluster sizes of up to 70 stations per upstream channel in order toanalyse the performance of the DVB/DAVIC protocol during high periods ofcongestion. Some scenarios will approach an increased number of stations (up to 340stations), with a reduced traffic load to analyse the maximum number of nodes that canbe supported. The complete set of simulation parameters used in order to produce theresults presented in this chapter are summarised in Table 5.4.Table 5.4 – Simulation parameters.Simulation ParameterValueUpstream data rate (QPSK)3.088 MbpsDownstream rate (64-QAM)42 MbpsDownstream signallingIn BandMaximum number of active NIUs (EuroModems) 340*Minimum and maximum backoff values for the exponential backoff algorithm 3 and 5*Entry spreading factor of the splitting tree algorithm 6*Minimum contention-based slots per signalling frame3 slots*Buffer capacity per NIU3000 ATM cells*Transmission time (cycle) of the signalling frame3 ms*Simulation time for each run60sMaximum reservation request message length32 ATM cells (slots)*Maximum contention access message length0 ATM cells (slots)*Distance from nearest/farthest NIU to the Headend10-16 KmSpacing between closest and farthest NIURandomly distributedHeadend and NIU processing delay2 microseconds eachPropagation delay (coax and fibre)5 microseconds/Km* These parameters will be ranged as indicated by the traffic scenarios5-8