to a WLAN, the PBX has to be in control of the call-that is, a call to the user's PBX number must be trans- ferred to the user's cellular device. If the user has received a cellular call directly from the cellular network and enters the WLAN coverage area, the PBX has no idea that the call exists and has no way to effect the transfer. Instead, the call is "dragged into" the facility-it simply continues on the cellular network, and cel- lular usage charges apply. Chapter 15 WMANs, WLANs, and WPANs Leading VoWLAN suppliers plan to use Signaling System 7 (SS7) connections between the WLAN and cellular networks, transferring calls between the two wire- less networks in the same manner as calls are transferred between different PSTN carriers. Such SS7 connections can be leased from third-party aggregators rather than from carriers directly, putting even this high-level functionality beyond the car- riers' control. Cooperation is yet to be tested between cellular carriers and WLAN suppliers, not to mention between the voice and data camps within the enterprise. Mesh Networks A quickly maturing alternative to wireless broadband metro and campus area net- works is mesh networking, which is of special interest to those who just can't wait for WiMax. Mesh networking represents an innovative do-it-yourself approach to easily building wireless broadband data networks. It specifically caters to mobile nodes, instant growth, and unpredictable variations in reception and coverage. Mesh networks essentially route voice, data, and instructions between nodes, creating a resilient network in which connections are continuous, reconfiguring around blocked paths by hopping from node to node until a connection is estab- lished. Both wireless and wired networks can benefit from mesh network topologies. Using intelligence embedded in each component, meshing joins the compo- nents into a self-organizing structure, overcoming the limitations of traditional centralized models of wireless networking. Mesh networks turn WAPs into router nodes