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Chapter 6Optimisation of CRA using adaptive CSAs pFor the splitting tree algorithm a more efficient CSA is not necessary, since the systemperformance has already been maximised by fixing the Min. o. of CSs per signallingframe to 1 CSs, regardless of the traffic type and when a medium size network has beenconsidered (under 70 stations).6.2.3.1 Forced-CSA used with the exponential backoff algorithmThis mechanism is based on the dynamics of the splitting tree algorithm. When acollision occurs, the splittting tree algorithm automatically allocates a CS in the nextsignalling frame, which is then split into 3MSs and used only between the stationsinvolved in the collision. The Forced-CSA, on the other hand, allocates a flexiblenumber of CSs in the next signalling frame. We refer to these additional slots as forcedcontention slots (FSs). With this new functionally, stations competing for contentionaccess have more chance to transmit successfully, since more contention slots areallocated when they are needed, reducing considerably packet access delays.The idea of allocating more contention slots, in additional to the unreserved slots thatare also allocated to the contention-based access region, was first reported in [96] and[98]. Here the authors introduced a new contention slots allocator, referred to as‘Forced Mini-Slots CSA’ for the IEEE 802.14 protocol. The main difference betweenour Forced-CSA and the Forced Mini-Slots CSA, is that the latter allocates more CSsaccording to the maximum throughput of the Slotted Aloha System, defined inEquation 6.1. a −1ηmax= a⋅ p ⋅ (1 − p)(6.1)where p is the retransmission probability and a is an estimation of the number ofstations competing for a contention slot.On high traffic loads, the Forced Mini-Slots CSA allocates e(=1/η max ) CSs for each datamessage to be transmitted. Conversely on low traffic loads, it allocates less thane(=1/η max ) CSs. However, our mechanism (Forced-CSA) allocates more CSs when acollision occurs and on high traffic loads, the average number of CSs required perrequested data message approaches very close to e(=1/η max ).6-10