Wireless Ad Hoc and Sensor Networks

Admission Controller Design for High-Speed Networks 161specification is to be admitted, the available buffer space is updated bysubtracting the assigned buffer space for the new source from the availablenetwork resource. Conversely, when an existing source ceases to transmit,the resource used by this source is now available to all other sources, andhence the available network resource is updated.The peripheral schemes for the AC are a congestion controller (Jagannathan2006, Jagannathan and Talluri 2002), a bandwidth estimator aspresented in Section 4.2 through Section 4.4, and a network resourceestimator. The congestion controller generates a congestion indicatoraccording to the measured system statistics, such as the queue length, qk ( ),and its past values, the CLR, and round-trip delays at the egress node.Here, the predictive congestion controller developed in Jagannathan andTalluri (2002) is preferred, but any congestion controller including bufferthreshold can be used. The congestion indicator flag is set when the pastseveral buffer occupancy values are about 90% full, the rate of change ofthe queue length is positive and high, the round-trip delays are large, andCLR occurs consistently. These values are obtained from careful analysis.In other words,If ((q(k)) > 90%) and (q(k − 1) > 90%) and (RTT > 2RTTmin) ) thencongestion_flag = true, else false (4.28)Bandwidth estimator obtains an accurate estimate of the current bandwidththat has to be assigned for the next measurement interval. Initially,the new source has to provide only its intended PBR/PCR, whereas otherschemes request mean bit/cell rate (MBR/MCR), burstiness, delay, andnumber of packets/cells, in addition to PBR/PCR. All admitted newsources are assigned an initial bandwidth value equal to the PBR/PCR.From the next measurement interval onwards, the bandwidth to beassigned for this new source after its admission into the network iscalculated using Equation 4.22.Therefore, the approach presented in this chapter is more adaptive anddoes guarantee the performance in terms of QoS and network utilization.Based on this information, and to make the proposed admission controllersimple and easy to implement, rules are generated as follows:If (congestion flag is true) and (available_capacity > PCR) and(Available_network_resources > 10%), then admit source “i”; else reject.(4.29)The sources are expected to return to wait state, which is typically a verysmall value, and they are allowed to send a request again for admission intothe network after the wait state. At each measurement interval, the congestioncontroller generates the transmission rates for all the sources to meet the QoS.