In this paper we present the design of our wireless mesh network testbed (CentMesh), which facilitates experimentation as a service. CentMesh differs from other testbeds in terms of its modular, flexible and extensible design. The CentMesh software suite provides a modular programming library that can be modified and/or extended by the users of the testbed, allowing them to implement their own modules (e.g., routing, scheduling etc.). The basic services such as transport of control messages, broadcast, etc., are provided to experimenters by a set of system modules. Modularity allows the experimenters to implement only the part of network stack that they are interested in experimenting with, while reusing the other readily available CentMesh modules.