PAC: Perceptive Admission Controlfor Mobile Wireless NetworksIan D. ChakeresDept. of Electrical & Computer EngineeringUniversity of California, Santa [email protected] M. Belding-RoyerDept. of Computer ScienceUniversity of California, Santa [email protected] approaches to guarantee quality of service(QoS) work well only with predictable channel and net-work access. In wireless mobile networks, where conditionsdynamically change as nodes move about the network, astateless approach is required. As wireless networks be-come more widely used, there is a growing need to sup-port advanced services, such as multimedia streaming andvoice over IP. Since shared wireless resources are easilyover-utilized, the load in the network must be controlledso that an acceptable QoS for real-time applications canbe maintained. If minimum real-time requirements are notmet, these data packets waste bandwidth and hinder othertraffic, compounding the problem. To address this issue, wepropose the Perceptive Admission Control (PAC) protocol.PAC monitors the wireless channel and dynamically adaptsadmission control decisions to enable high network utiliza-tion while preventing congestion. Through discussion andsimulations, we show that PAC achieves this goal and en-sures low loss and delay for all admitted flows.1. IntroductionWireless devices are becoming prevalent because of theirability to provide mobile communication. Since many com-mon applications, including voice and multimedia, requirelow packet loss and delay, quality of service (QoS) is be-coming an important requirement for these networks. Incontrast to traditional wired networks, mobile networks op-erate under harsh conditions that include mobility, a sharedwireless channel and limited bandwidth. Traditional at-tempts to provide guaranteed QoS are unable to cope withthe constantly changing network conditions. Meeting hardreal-time QoS constraints in wireless mobile networks isunrealistic because of node mobility and shared medium ac-cess. Instead, solutions that provide a stateless service andoffer better than best-effort packet delivery for high prior-ity packets are more successful, such as DiffServ and IEEE802.11e. Unfortunately, these solutions still fail to providethe low loss and delay that real-time applications require ifthe network becomes congested.High quality of service without fully coordinated chan-nel and network access is achievable. The wireless chan-nel must be kept from reaching the congestion point, sinceloss and delay increase rapidly once this point is reached.Maintaining the utilization below the congestion point isdifficult because the channel is shared between nodes thatmay not be able to communicate directly; therefore, nodesneed to passively determine the network utilization. Oncethe amount of available bandwidth is determined, nodes canthen adapt their data traffic to keep the channel from becom-ing congested.We propose the Perceptive Admission Control (PAC)protocol to control the amount of traffic in the network andprovide high quality service to all admitted traffic. PAC en-sures the network congestion point is not reached throughthe requirement of call admission for all new flows. To makean admission decision, PAC considers not only the limitedarea within a sender’s transmission range, but the entire areathat a new flow may impact. We show that the time that thewireless channel is sensed as busy is a good estimator ofavailable bandwidth. Using this measure, PAC performs ad-mission control for new flows to avoid congestion. We beginour discussion by focusing on single hop admission control.We then describe how to easily extend PAC for multihoppaths.The rest of this paper is organized as follows. Section 2provides background on wireless transmissions, includingmethods for determining the available bandwidth and previ-ous approaches for providing high packet delivery and lowdelay in wireless networks. In Section 3 we describe PAC,our approach for admission control. In Section 4 we demon-strate the performance of PAC in simulation and describehow it avoids the shortcomings of previous approaches. Fi-nally, Section 5 concludes the paper.2. BackgroundTo perform admission control in wireless networks it isimportant to understand how a wireless transmission im-pacts other nodes. In Section 2.1 we describe the importantdistances for packet transmission and reception. Since ad-Proceedings of the First International Conference on Quality of Service in Heterogeneous Wired/Wireless Networks (QSHINE’04) 0-7695-2233-5/04 $20.00 © 2004 IEEENeighbor (N)CarrierSensingNeighbors0.50"ReceptionRange(CSR)(RxR)CarrierSensingRange(CSN)Figure 1. Approximation of reception range (RxR) and carriersensing range (CSR). Nodes within reception range are calledneighbors (N), while carrier sensing neighbors (CSN) are allnodes within carrier sensing range.mission control decisions depend on accurate estimation ofthe available bandwidth, we examine several methods forcalculating the available bandwidth in Section 2.2. In Sec-tion 2.3 we categorize related work and discuss why mostproposed solutions are insufficient. In Section 2.4 we de-scribe the solution most closely related to our proposed ap-proach.2.1. Impacted AreaFor admission control purposes, there are multiple no-table ranges for wireless communication. Each distance isimportant for the measuring channel utilization and predict-ing the available bandwidth. At a short range, we assumethat nodes are capable of direct communication. We refer tothe maximum separation between a sender and receiver forsuccessful packet reception as RxR, as shown in Figure 1.Nodes within RxR of a particular sender are considered itsneighbors (N).Nodes that are within carrier sensing range of a sendercan sense packet transmissions. The nodes inside a sender’scarrier sensing range are called carrier sensing neigh-bors (CSN). These nodes detect a transmission but may notbe able to decode the packet. The maximum distance thata node can detect an ongoing packet transmission (carriersignal) is called the carrier sensing range (CSR). This rangeis typically much larger than the reception range. In wirelessMAC protocols based on CSMA, such as IEEE 802.11, allCSN of the sender are unable to initiate a packet transmis-sion while the sender is transmitting because they sense thechannel is busy. In CSMA networks, a large CSR preventsmultiple transmissions