combination of peer-to-peer groups and star config- urations. There is still just one PAN coordinator in the whole network. One example of such a structure is Two device types, of different complexities, are defined. a cluster-tree network shown in Figure 5.2-23. In this A full function device (FFD) will be able to implement arrangement devices on the network extremities may the full protocol set and can act as a network coordinator. well be out of radio range of each other, but they can still Devices capable of minimal protocol implementation are reduced function devices (RFD). Due to the distinction communicate by relaying messages through the in- dividual clusters. between device types, networks in which most members require only minimum functionality, such as switches and sensors, can be made significantly less costly and have Frame structure, collision avoidance, lower power consumption than if all devices were and reliability constrained to have maximum capability. Flexibility in network configuration is achieved Zigbee frame construction and channel access are similar through two topologies-star and peer-to-peer that are to those of WLAN 802.11 (Wi-Fi) but are less complex. depicted in Figure 5.2-22. A network may have as many The transmitted packet has the basic construction shown as 255 members, one of which is a PAN (personal in Figure 5.2-24. The purpose of the preamble is to area network) coordinator. The function of the PAN permit acquisition of chip and symbol timing. The PHY header, which is signaled by a delimiter byte, notifies the baseband software in the receiver of the length of the STAR TOPOLOGY PEER-TO-PEER TOPOLOGY subsequent data. The PSDU (PHY service data unit) is the message that has been passed down through the higher protocol layers. As shown, it can have a maximum of 127 bytes although monitoring and control applica- tions will typically be much shorter. Included in the PSDU are information on the format of the message frame, a sequence number, address information, the data TWO WAY C