Energy-Efficient And Low-Latency Medium Access Control Protocols In Wireless Sensor Networks

With the rapid growth and development of micro-electronics, embedded computation, integrated circuit, wireless communication and sensing technologies, Wireless Sensor Networks(WSNs) that are consisted of large number of sensor nodes are widely used in many areas, such as military, environment monitoring, disaster early warning, intelligent traffic and so on. WSNs have been one of active research areas in the network technologies.Medium access control(MAC) protocol is the underlying basic protocol in the network protocol stacks of WSNs. MAC protocol has a significant effect on the network communication performance, because it decides how the wireless devices share the wireless channel, assigns the limited communication resources and schedules the sensor nodes to access the channel. So that it becomes one of major research directions of WSNs network protocols.Due to energy waste of idle listening, IEEE 802.11 MAC protocol is not suitable for WSNs that are composed of the sensor nodes with finite power. The researchers proposed many MAC protocols based on duty-cycle for WSNs according to the WSNs characteristics. Duty-cycle can reduce significant energy waste of idle listening, but it introduces extra packet delivery latency. In order to satisfy the low-latency requirements of WSNs applications, the proposed MAC protocols adopted some approaches to optimize the latency. However, we still can optimize the latency for various WSNs applications.This thesis focuses on the energy consumption and latency optimizations of MAC protocol in WSNs and pays the most attention on the contention-based MAC protocols.Considering the insufficients in the current contention-based MAC protocols, this thesis proposes four contention-based, energy-efficient and low-latency MAC protocols for WSNs. The major contributions of this thesis are as follows:(1) For forwarding multiple packets on a single node in the event detection of WSNs,a new synchronous SR-MAC protocol based on slot-reserved mechanism is presented.SR-MAC reserves several time slots for the nodes during the Sleep period in one operation cycle, and adopts the cross-layer optimization to schedule the multiple packets of the nodes to be forwarded on the muli-hop path without collision in the reserved time slots. SR-MAC can guarantee the node’s energy efficiency, meanwhile can achieve lower event delivery latency, higher event delivery ratio and higher network throughput.(2) Based on analyzing the reason of the higher data packet delivery latency in asynchronous MAC protocols, a new cross-layer asynchronous REA-MAC protocol is proposed. In REA-MAC, each node decides when it wakes up to send the beacon based on cross-layer routing information. By utilizing this mechanism, the nodes on the packet forwarding path can wake up successively and the packets can be forwarded continuously,such that the packet delivery latency can be reduced. Moreover, in order to reduce the sender’s energy waste of its unnecessary idle listening, the sender adaptively wakes up to listen to the channel, based on the timing relationship between the transmission request time and the wakeup time of its next hop node.(3) A new adaptive beacon time asynchronous OEL-MAC protocol for burst traffics in WSNs is presented. In OEL-MAC, the receiver adaptively adjusts its beacon time based on the sender’s beacon time, so that the sender can forward its received data packets as soon as possible. Therefore, OEL-MAC can reduce the packet delivery latency. On the other hand, when the node wants to transmit the data packets, it can schedule its wakeup time to listen to the channel only before its receiver wakes up, such that it can reduce energy waste of the sender’s idle listening and save its energy.(4) A new asynchronous THO-MAC protocol based on the pseudo-random number is proposed. In THO-MAC protocol, each node determines its periodic wakeup time based on its own pseudo-random number generator. By predicting the receiver’s wakeup time accurately, the sender schedules when it wakes up to listen to the channel on demand to reduce the sender’s energy waste of idle listening significantly, so that it can reduce the node’s energy consumption. Furthermore, in order to achieve the lower packet delivery latency, THO-MAC chooses the best forwarders in two-hop forwarders set to minimize two-hop forwarding latency, in terms of the character of the dense sensor nodes deployment in WSNs.