## 5 IEEE 802.15.4 Technology
### IEEE 802.15.4 Network Topologies and Operational Modes  
To overcome the limited transmission range, multi-hop self-organizing network topologies are required. These can be realized taking into account that IEEE 802.15.4 defines two types of devices: the Full Function Device (FFD) and the Reduced Function Device (RFD). The FFD contains the complete set of MAC services and can operate as either a PAN coordinator or as a simple network device. The RFD contains a reduced set of MAC services and can operate only as a network device.  
Two basic topologies are allowed, but not completely described by the standard since definition of higher layers functionalities are out of the scope of 802.15.4: the star topology, formed around an FFD acting as a PAN coordinator, which is the only node allowed to form links with more than one device, and the peer-to-peer topology, where each device is able to form multiple direct links to other devices so that redundant paths are available. An example of both the IEEE 802.15.4-compliant network topologies is shown in Figure 7.  
Star topology is preferable in case coverage area is small and low latency is required by the application. In this topology, communication is controlled by the PAN coordinator that acts as network master, sending packets, named _beacons_ for synchronization and managing device association. Network devices are allowed to communicate only with the PAN coordinator and any FFD may establish its own network by becoming a PAN coordinator according to a predefined policy. A network device that wishes to join a star network listens for a beacon message, and after receiving it, the network device can send an association request back to the PAN coordinator, which either allows or denies the association. Star networks also support a non beacon-enabled mode. In this case, beacons are used for association purpose only, whereas synchronization is achieved by polling the PAN coordinator for data on a periodic basis. Star networks operate independently from their neighboring networks.  
Peer-to-peer topology is preferable in case a large area should be covered and latency is not a critical issue. This topology allows the formation of more complex networks and permits any FFD to communicate with any other FFD behind its transmission range via multi-hop. Each device in a peer-to-peer structure needs to proactively search for other network devices. Once a device is found,  
Figure 6: Superframe structure.  
the two devices can exchange parameters to recognize the type of services and features each supports. However, the introduction of multihop requires additional device memory for routing tables.  
IEEE 802.15.4 can also support other network topologies, such as cluster, mesh, and tree. These last network topology options are not part of the IEEE 802.15.4 standard, but are described in the ZigBee Alliance specifications [32] (see Section 5).  
All devices belonging to a particular network, regardless of the type of topology, use their unique IEEE 64-bit addresses and a short 16-bit address is allocated by the PAN coordinator to uniquely identify the network.  
Figure 8: ZigBee-compliant tree network topology.  
Figure 7: The two IEEE 802.15.4-compliant network topologies: star and peer-to-peer topology.