The Research Of MAC Protocols And Cross-layer Design In Wireless Sensor Networks

Wireless sensor networks (WSN) are integration of techniques including sensor, embedded computation, networking, wireless communication and distributed computation. The small sensor nodes can collaborate to test, sense, collect and process the information of monitored objects and transfer the processed information to user via multi-hop wireless communication. WSN is regarded as a way to realize"ubiquitous computing"and it has a wide range of applications, such as military, environment monitoring, traffic monitoring, smart building, etc. WSN becomes a new research area and has a bright future in application. However, several technologies related to WSN are still explored, such as networking protocol, energy management, data transmition security and reliability, etc. Therefore, it's meaningful to make scientific researches on WSN.MAC (Media Access Control) protocol is responsible for channel resource allocation. It is the underlying part of network protocols and plays an important role in the network performance. Therefore, it becomes an important research area in WSN. With regard to the specific feature of WSN, the design of MAC protocol aims to reduce energy consumption as much as possible to prolong the network lifetime. Meantime, end to end delay and throughput should also be taken into consideration. On the other hand, compared to wired network, wireless network has limited spectrum resource and high bit error rate. Traditional layered protocol design can't meet the demands of wireless communication, since it hasn't fully utilized network resource to optimize the performance. Recently, the cross-layer optimization of WSN is becoming a hot research topic, the ideas of designing network protocols by sharing information among different layers have been proposed in many works. In this paper, mainly focus on the MAC protocol design and cross-layer design, some discussions and new designs are presented as follows:The main difference between WSN and traditional networks is proposed. The considerations to design MAC protocols are discussed. Sensor nodes use batteries for power supply, so they are power limited. When designing a MAC protocol, several points should be considered, such as network lifetime, energy efficiency, end to end delay, throughput, etc. The existing MAC protocols are classified and both the advantages and disadvantages are analyzed. Finally, some future possible research fields of MAC protocols are proposed.SMAC, the typical MAC protocol in WSN, is analyzed and the disadvantage of SMAC is discussed. Based on SMAC, in order to decrease network delay and improve energy efficiency, a new MAC protocol in WSN named traffic adaptive MAC protocol (TA-MAC) is proposed. SMAC adopts a static duty cycle mechanism, while in TA-MAC, traffic variation is predicted based on the number of packets queued. The traffic adaptive duty cycle scheme in the listen-sleep schedule is adopted. When the network traffic becomes busy, the long listen-sleep cycle is divided into several short cycles, thus the duty cycle is increased and more packets are transmitted during one long listen-sleep cycle. The adaptive listening scheme is also adopted, one packet can be transmitted further to two hops in one basic cycle, and thus the latency is reduced. Simulation results show that TA-MAC can obtain lower latency and reduce energy consumption.Focus on the network load balance, a residual-energy aware MAC protocol (REA-MAC) for wireless sensor networks is proposed. REA-MAC modifies the contention mechanism of SMAC. First, dynamic contention window adjustment mechanism is adopted. The size of contention window is increased when there is a collision and returns to the initial value when the packet is delivered successfully, therefore, the probability of collision is decreased. Second, REA-MAC takes the node's residual energy into consideration to select the deferring time adaptively and let the nodes that have more residual energy to access the channel with priority. The nodes with less residual energy only need to send the data sensed while avoiding the task of packet delivery, thus the network load is balanced and the network lifetime is prolonged. Simulation results show that, compared to SMAC, REA-MAC can prolong network lifetime effectively.Then, the present research of cross-layer design is discussed and future research fields about cross-layer design are proposed. Based on the analysis, a new cross-layer design is proposed. The residual energy information of physical layer is utilized by routing protocol to select a proper route. The nodes with more energy are selected to handle the task of relaying packets, thus the network load is balanced. The information of routing table is utilized by MAC protocol to judge whether I am the next hop of this communication, if not, go to sleep directly. Thus unnecessary idle listening is avoided and the energy consumption is reduced. Simulation results and theoretical analysis show that this cross-layer scheme can reduce energy consumption effectively.At the last part of this dissertation, the whole work of the dissertation is outlined and the further research issues are discussed.