| Recent years, with the rapid development of wireless network and embedded computer technologies, Wireless Sensor Network (WSN) has emerged and already made big progress. Adaptive to be utilized in variety of domains, such as military, industry, agriculture, medical field, environmental field, etc, WSN has shown evident value in practice. In WSN, the protocol in Medium Access Control (MAC) determines how communication channel is manipulated and how limited wireless communicating resources is allocated. In traditional MAC Protocol of WSN, the prime objectives are throughput as large as possible and latency as small as possible. Differed from other wireless networks, all the nodes in WSN are supplied by limited battery power. So, how to improve the energy efficiency and quality of transportation has become a very important research point in WSN research.Even in the WSN the topology of which is unstable and the clock not strictly synchronized, Competition-based MAC is able to show perfect extensibility. But, we need to strike balance between energy efficiency and communication latency. With the endeavor of countless academics and experts, many competition-based MACs, such as S-MAC, T-MAC etc, have been brought into our eyes.The protocol of Sensor-MAC, also known as the S-MAC, is devised specially for WSN to meet its specialties. In the protocol, control frames, such as RTS, CTS, ACK, SYNC, combined with randomly-backing-off mechanism, are used to cut off energy consumption by avoiding collisions. On this basis, periodical sleeping and node synchronizing are also brought in to further-reduce energy wasted in node listening process. Besides, message channel mechanism, also included in S-MAC protocol, can bring down the chances that battery life is wasted by communication collision among nodes in WSN.The prime flaw of randomly-backing-off mechanism in S-MAC is the fixed size of contention window (CW). In the real network, with new nodes being involved in and old nodes dying out, the node amount and network topology may change from time to time. If node amount and the workload grow too much, owing to the fixed contention window size, the possibility of collision may ascend and the energy efficiency may descend. Conversely, if node amount and workload become too small, fixed contention window size may be too large to send out information in time. That is, the communication channel may be wasted and energy efficiency brought down.To meet this problem in S-MAC, in this paper, an improved model is illustrated. In this improved model, counter on time of constant success communication (CS), optimized counter threshold on time of constant success communication (CS opt), counter on time of constant lost communication (CL), optimized counter threshold on time of constant lost communication (CL opt) are brought in. New field CW is also added to the RTS frame. When the counter doesn't reach the threshold, CW gets its initial value CWinit. When the counter overpass the threshold, CWinit either multiplied with 2 or divided by 2,and then CW gets its initial value CWinit. CWinit of one node should be constantly updated according to the old CWinit values of itself and its neighbors'CW.Visual setting deploying tool is also implemented to help making TCL scripting language generating visible and TCL developing convenient. The developing platform is capable of many function features, such as loading, displaying and editing of existing TCL script, auto running TCL script, etc.Afterwards, the improved S-MAC model is carried out in the NS2, a network simulating environment generally approved by academics. At last, the performance of original S-MAC and improved S-MAC will be tested in a number of typical network topologies, and the quantitative analyzing cooperation between them will also be given, that the exact improvement on energy efficiency and communication channel utilizing ratio. |
PDF Full Text Request