A critical shortage of spectrum resources and the underutilizafion of the licensed spectrum urge people to discover new approaches to efficiently utilize the spectrum resources. Opportunistic spectrum access (OSA) has been considered to be a good approach to exploit spectral resources and improve licensed spectral utilization. In this paper, based on this idea, we model and analyze one type of network (i.e., main network) overlaid by another type of network. The main network is defined as primary network while the other type of network is called secondary network. A centralized network access protocol is adopted in primary network and secondary network uses a distributed network access protocol. The system performance is investigated through extensive simulations. The obtained results show that channel utilization can be improved by about 50% compared with traditional sharing spectrum model in a single network. We also observe that there is tradeoff between the blocking probability of calls in primary network and the throughput of OSA network. Therefore, the system performance can be dynamically controlled.