In order to adopt LPWN technology for use in real life applications, an association of industry companies, the ZigBee Alliance, has specified a full protocol suite that provides efficient high level communication in wireless sensor networks. ZigBee low stack layers (PHY and MAC) are equal to those in IEEE 802.15.4 while high stack layers are designed to support extended networking functionality and to provide a simple interface between network and end user applications. Comparing ZigBee extensions  with IEEE 802.15.4, ZigBee has some features that 802.15.4 does not have. Among other things, the ZigBee extensions have full support of complex network topologies, tree and mesh, while 802.15.4 only supports star networks. In addition to supporting tree and mesh networks, ZigBee offers secure and reliable multi-hop communication, advanced features like binding and fragmentation, public application profile definitions for application-level interoperability.