Admission control is one of the key elements for ensuring the quality of service (QoS) in modern mobile wireless networks. Since such networks are resource constrained, supporting multimedia traffic guaranteeing its QoS levels is excessively challenging for call admission control (CAC) design. CAC is the most important radio resource management (RRM) function in wireless networks as its efficiency has a direct impact on network performance and QoS provision to end users. The goal of this chapter is to provide a thorough study of the basic concepts considering CAC design and a comprehensive analysis of the fundamental CAC schemes employed in wireless networks. The basic performance criterion considering CAC schemes is the probability of denying the access to the network for an arriving call, which is extensively studied in this chapter. Moreover, additional performance criteria are presented and discussed, which may help to provide an overall efficiency estimation of the available CAC schemes.
Quality of Service (QoS) provision in wireless networks is closely related to the exploitation of network available resources and the maximization of the number of users. The increasing demand for multimedia services combined with the limited resources of wireless networks urge for efficient admission control schemes that achieve a competent resource management along with appropriate QoS levels for end users. Call Admission Control (CAC) algorithms are employed to ensure that the admission of a new call into a resource limited network does not violate the Service Level Agreements (SLAs) concerning ongoing calls. Any service session supported by the network can be considered as call. An efficient admission control policy should accomplish the following:
establish a robust priority assigning mechanism for handoff calls and different Service Class (SC) calls,
exhibit a low Call Blocking Probability (CBP),
manage a fair resource allocation,
achieve a high network throughput and
CAC schemes for wireless networks have been widely studied under different network architectures and network administrator policies. To apply a specific call admission policy, the various call types are classified into SCs with precise characteristics and QoS demands. Each SC call is treated differently depending on the criteria set according to the operating principles adopted for the admission procedure. The majority of CAC schemes base their admission criteria on an efficient resource management, accounted for either in terms of channels or of bandwidth units. The methods proposed usually set thresholds related to the desirable QoS for high priority SCs and handoff calls. The last trend in such schemes proposes the use of adjustable admission probabilities for low priority SC calls combined with thresholds for the dynamic handling of the input traffic.
Other CAC schemes examine Signal to Noise Ratio (SNR) levels to determine an admission criterion satisfying the QoS demands of end users. Such schemes have to deal with propagation and mobility issues. Under high traffic network conditions, an efficiency enhancing module may be incorporated into the CAC schemes employed renegotiating the resource allocation of ongoing calls. Through QoS re-negotiation and resource re-allocation, available resources can be retrieved to serve a high priority SC call request. The admission decision may also be controlled according to dynamic changes in call pricing, providing at the same time a stream flow management.
Finally, a critical issue in admission control is the performance evaluation of the proposed schemes to assess the provided QoS. The evaluation is performed under appropriate criteria. The metric studied most in the literature is CBP. Assigning priorities to certain call types causes a serious side effect, as certain SCs may monopolize the network resources leading to an unfair treatment of low priority SC calls. Thus, in shared resources networks, fairness among users of the same SC subjected to different channel conditions with different mobility characteristics and among users of different SCs should be investigated employing appropriate metrics. A call admission request represents potential revenue for the network operator. Thus, CAC schemes should be further evaluated employing performance metrics based on optimal pricing to maximize revenues. The above criteria constitute a thorough performance evaluation of admission control policies providing QoS in wireless multiservice networks.