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Low spectrum utilization and lack of spectrum for new wireless services led to think of unlicensed devices that may operate on spectrum holes as and when required with a constraint of preserving the rights of licensed users. Cognitive radios have been proposed as one of such promising solutions for increasing spectrum utilization and in turn helping to resolve spectrum scarcity problem. As the spectrum availability will differ in both time and space for such scenarios, dynamic spectrum allocation is considered as key enabling technique for the success of cognitive radios. This paper assumes a centralised architecture where the central controller maintains the record of all vacant spectrums and decides the spectrum to be allocated to particular users based on spectrum overlay strategy. We model the functionalities of media access based on distributed semi Markovian network of queues where the first queue is equivalent of sorting overlapped detections received from all SU in the network, updating its database, contention and conflict management. We call this queue as spectrum analysis queue. The requests once sorted will be considered for channel allocation which is modeled as channel allocation queue. This paper presents throughput analysis of unlicensed users for opportunistic spectrum access based bandwidth allocation where the paper first presents the formulation of access latency which is then used for evaluating the throughput of unlicensed users.