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Chapter 6Optimisation of CRA using adaptive CSAs p6.4 ConclusionsIn this chapter we have shown that the overall system performance of the DVB/DAVICcable communications system can be significantly improved by the use of three novelContention Slot Allocators (Simple-CSA, Variable-CSA and Forced-CSA), whichallocate dynamically the number of contention slots that should be allocated in the nextsignalling frame, based on the current traffic load. Results presented in this chapter havepointed out that the Forced-SCA not only provides the highest system throughput but inmost of the cases also offers the lowest packet access delays for the exponential backoffalgorithm. Simulation results have been shown to be accurate when compared withresults from theoretical analysis.In addition, a performance comparison of the contention resolution algorithms adoptedby the DVB/DAVIC protocol was also presented. The splitting tree algorithm takesadvantage of the exponential backoff algorithm in the sense that feedback and allocationinformation allows a station, (with new incoming arrivals) to compete for contentionbasedslots without the risk of collision with backlogged stations. One more advantageof the splitting tree algorithm is that the use of minislots for reservation requests furtherdecreases the risk of collisions, since one contention-based slot is divided into threeminislots, increasing the probability of successful request transmissions. However,drawbacks of the splitting tree algorithm are higher complexity at the headend,increased processing times of the feedback and allocation information at the station, andsince every contention slot should be acknowledged regardless of whether it is used ornot, higher control information at the downstream channel is assigned.Results presented by both contention resolution algorithms showed that the systemperformance is a trade-off between mean access delay/system throughput and the valuesselected for the initial/truncated backoff values and the Entry Spreading factor.In general, results presented here showed that an increase of over 9% on systemperformance can be obtained by the use of a splitting tree algorithm when backoffvalues (initial/truncated) and the Entry spreading factor have been optimised, fordifferent traffic configurations such as Internet, VoIP and mixed traffic.6-29