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Cognitive Radio (CR) has been recognized as a promising solution to solve the current spectrum inefficiency problem. We consider a CR Base Station (CR-BS) has multiple spectrum bands over a wide frequency range and uses channel-dependent data rates to communicate with the Secondary Users (SUs). To maximize the spectrum utilization while supporting the Quality of Service (QoS) of the SUs, various decisions needs to be made during the operational period of the CR system taking into account not only the behavior of Primary Users (PUs) and SUs but also the current utilization of each spectrum band. In this paper, we propose an optimal policy for joint spectrum resource management in multi-band CR networks, which consists of admission control, spectrum band selection, spectrum handover and eviction control. The problem is formulated as a Semi-Markov Decision Process (SMDP) and the optimal decision for each system state is derived to reduce the SU blocking probability while keeping the SU dropping probability upper-bounded. Performance evaluation shows that the proposed policy outperforms other three policies in terms of both the SU blocking probability and the SU dropping probability.