Software Defined Radio, and GNU Radio in particular, were conceived for communication systems such as radio and TV, where information is conveyed in a continuous flow, called a stream. Data networks by contrast use small portions of information, called messages, frames or packets according to the context. A further important difference is that in data networks several nodes share the same medium. In order to operate properly, a so-called medium access control must be enforced. GNU Radio was originally stream oriented, but more recently added support for message communications. A block may thus comprise two different types of inputs and outputs: stream ports for continuous flows of data, and message ports for discrete portions of bytes. As a consequence, some projects that strive at implementing data network standards in GNU Radio have emerged, but they are oriented towards specific communication protocols. In this paper we present GWN (GNU Radio Wireless Network), an open and free extension to GNU Radio specifically oriented to data networks, but not tied to any specific protocol. Its aim is to provide a framework for experimentation and development, working either on existing protocols or devising entirely new ones. To this purpose, GWN provides a new generic block (gwnblock) which adds the tools necessary for data network designs, and at the same time decouples all GWN data network blocks from the GNU Radio generic basic-block. This means a new GWN block only needs to inherit from gwnblock and follow GWN design rules to build a data network application, while keeping full compatibility and access to GNU Radio standard blocks. The GWN generic block adds the following facilities to GNU Radio: Message orientation, Events, Handling of time, and Finite State Machines. As a proof of concept, and to illustrate its usage, we briefly present two examples: anARQ(Automatic Repeat reQuest) protocol with its different flavors and a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) protocol