tailieunhanh - Emerging Communications for Wireless Sensor Networks Part 6

Tham khảo tài liệu 'emerging communications for wireless sensor networks part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | A new MAC Approach in Wireless Body Sensor Networks for Health Care 93 synchronized to the network and able to transmit or receive a packet at any time Bourgard et al. 2005 . The beacon mode introduces the so-called superframe structure. The interbeacon period is partially or entirely occupied by the superframe which is divided into 16 slots. Among them there are at most 7 guaranteed time slots GTS . they are dedicated to specific nodes which form the contention free period CFP 2003 . This functionality targets very low latency applications but it is not scalable in BSNs since the number of dedicated slots is not sufficient Zhen et al. 2007 . In the medical field where one illness usually boost-ups other illnesses many body sensors should be able to reach the BAN coordinator via such guaranteed services. Further the current protocol only supports first come first served based GTS allocation and does not take into account the traffic specification delay requirements and the energy resources. Again in medical scenarios many critical events may occur at a time and some of them are more critical and need most urgent response Kumar et al. 2008 . An additional drawback with the current GTS allocation is the bandwidth under utilization. Most of the time a device uses only a small portion of the allocated GTS slots and the major portion remains unused resulting in empty holes within the CFP. In such conditions the use of the contention access period CAP is required where channel accesses in the uplink are coordinated by a slotted carrier sense multiple access mechanism with collision avoidance CSMA CA . Nevertheless in the literature Bourgard et al. 2005 Park et al. 2005 Pollin et al. 2005 it has already been proved that the CSMA CA mechanism has a significant negative impact on the overall energy consumption as the traffic load in the network steadily increases. Thus the appraisal of other existing MAC protocols in terms of delivery ratio end-to-end delay and

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