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Many incompatible wireless protocols proliferate in unlicensed bands, creating complex coexistence and connectivity problems. If the trend continues, such problems will continue to exist in future unlicensed bands. We take a different approach to spectrum sharing. Instead of proposing a distinct medium-access control (MAC) protocol for each type of application, we propose a family of parameterized MAC protocols called WiFlex, which can be tailored to different application needs, ranging from wireless sensors to media centers. However, these protocols allow communication and spectrum-sharing coordination among different types of devices. We envision this family to be based on a multichannel physical layer. The contributions of this paper include a novel asynchronous split phase (ASP) protocol with dynamic priority support, even in multiple-collision-domain environments. When links are bidirectional, WiFlex resolves the multichannel hidden-node problem, achieving collision-free data exchange in a multihop environment. ASP provides a mechanism for prioritization and fairness. The performance of the protocol is evaluated through analysis and simulation. The coexistence of heterogeneous devices and the support for different applications with high performance are demonstrated.