A Novel Multi-Beacon Superframe Structure with Greedy GTS Allocation for IEEE 802.15.4 Wireless PANs Li-Chun Ko and Zi-Tsan Chou Networks and Multimedia Institute Institute for Information Industry, Taipei 106, Taiwan Email: {lcko, ztchou}@nmi.iii.org.tw Abstract--In IEEE 802.15.4 beacon-enabled wireless personal area networks (WPANs), the PAN coordinator will periodically broadcast beacon frames to let its associated end devices keep synchronized. One of the benefits in a beacon-enabled WPAN is that end devices can thus turn off their transceiver during the inactive periods to preserve their energy. Another benefit is that the PAN coordinator can allocate contention-free GTSs (guaranteed time slots) for admitted devices. In this paper, we first pointed out slot size-induced bandwidth waste problem, which is due to the innate defect of the superframe structure. Then we propose the novel multi-beacon superframe (MBS) structure to overcome this problem. To demonstrate the power of MBS, we also propose a greedy GTS allocation algorithm. Simulation results reveal that a WPAN using MBS can attain relatively higher bandwidth utilization at the expense of only a small increase of the device’s active period. I. INTRODUCTION IEEE 802.15.4 [1] is currently the most popular MAC (media access control) standard for low rate and low cost wireless personal area networks (WPANs). As shown in Fig. 1, 802.15.4 supports star network topology and peer-to-peer network topology as well. A. Superframe Structure 802.15.4 defines two types of devices: the full-function device (FFD) and the reduced-function device (RFD). As compared with an RFD, a FFD is considered to have more powerful processing capacity. Thus in addition to act as an end device, a FFD is able to act as a coordinator or a PAN coordinator. Note that an RFD can communicate (associate) with only one FFD at a time. As shown in Fig. 2, in an IEEE 802.15.4 beacon-enabled network, time is divided into fixed-sized beacon interval, which consists of a superframe duration and followed by an inactive period. The coordinator will periodically broadcast the beacon at the beginning of each beacon interval to let neighboring devices keep synchronized at all times. The superframe further consists of a contention access period Star Topology Peer-to-Peer Topology Full-Function Device Reduced-Function Device Communication Flow PAN Coordinator Fig. 1. FFDs and RFDs in star and peer-to-peer topology networks. (CAP) and a contention-free period (CFP). Any device can employ the slotted CSMA/CA (carrier sense multiple access with collision avoidance) mechanisms to send best-effort data during the CAP. On the other hand, if a device has periodical data traffic to send, it can apply the reservation mechanisms to acquire some GTSs (guaranteed time slots) access right in each CFP. The coordinator controls the superframe structure (shown in Fig. 2) by specifying two parameters in the beacon: BO (MAC beacon order) and SO (MAC superframe order). In other words, BO and SO determine the lengths of beacon interval (BI) and superframe duration (SD) respectively. Specifically, in 802.15.4, the values of SD and BI should strictly obey the following rules. 2 SO SD aBaseSuperframeDuration = × (1) 2 BO BI aBaseSuperframeDuration = × (2) Notice that both of SO and BO must be integers, and their ranges fall between 0 and 14. Besides, SO BO ≤ and the value of aBaseSuperframeDuration is fixed at 960 symbols. As shown in Fig. 2, the length of SD in 802.15.4 can be further divided into 16 identical-sized slots, which may be used for GTS reservations. In particular, only devices that have registered with the coordinator for GTSs usage can transmit data during the CFP. 1525-3511/07/$25.00 ©2007 IEEE This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the WCNC 2007 proceedings. 2330