| The nodes in Wireless Sensor Networks (WSNs) have constrained energy and limited hardware sources. Especially, in monitoring applications, such as community monitoring, WSNs often experiences global or local congestion due to these constraints on resources and the communication with the many-to-one and multi-hop pattern. This congestion usually leads to the increase of network transmission delay and the loss of data, even network collapse. In WSNs monitoring applications, effective congestion control is one of the key techniques to prolong the network system lifetime and improve the network Quality of Service (QoS) performances. Medium Access Control (MAC) protocol is in the bottom layer of the network protocol stack. It controls and assigns radio channel for sensor nodes. In WSNs, MAC protocol affects the response speed of network congestion directly. Improving the jam-resistant performance of MAC protocol is one of the challenging tasks in the field of research.This dissertation analyzes the cause of congestion in multi-hop gathering-topology network. This dissertation proposes and improves the jam-resistant methods of MAC protocol. The main work and innovations are as follows:(1) Improved transmission mechanism controlled by father node, designed SRI-MAC protocol:When the data is many-to-one gathered in the network, the competition of the son nodes can lead to the decrease of the transmission performance, and the sleep mechanism causes the effective bandwidth to become small. The network congestion caused by both of them. In SRI-MAC, the father node invites several son nodes, and can receive the multiple data packets orderly in an active cycle. It relieves the congestion of data transmission in gathering-topology network. The simulation results show that, when the traffic load becomes heavier, the packet delivery ratio of SRI-MAC is higher than S-MAC and R-MAC.(2) Proposed an adaptive contention window strategy, designed CA-MAC protocol:When the data is multi-hop gathering transmission in the network, the data load is not balanced and the data load density becomes large hop-by-hop, it is easy to cause congestion, resulting in the parent node buffer queue overflow and packet loss. In CA-MAC the node detects congestion state according to its buffer queue occupancy rate and congestion trend, uses adjust contention windows strategy and burst transmission mechanism, relieves the congestion. The simulation results show that, the packet delivery ratio of CA-MAC is higher than S-MAC and RL-MAC with packet loading time interval narrowing.(3) Improved neighbor nodes selection strategy, designed MACA-MAC:In the multi-hop gathering-topology network, a parent node is unable to receive the data because of congestion, resulting in the problem of the data backlog of its son nodes, which makes congestion worse. In MACA-MAC, a node can choose the neighbor node with minimum transmission pressure to forward data packets when congestion occurs on its parent. It achieves the goal of alleviates congestion. The simulation results show that, the packet delivery ratio of MACA-MAC is higher than PW-MAC with packet loading time interval narrowing.Base on the research above, in this dissertation, we design and implemented test nodes using CC2530 chip. Then we implement jam-resistant mechanisms in MAC protocols on hardware platform, and verified in a real environment scene. Experimental results show that, the packet delivery ratio of SRI-MAC is higher than S-MAC. Adding adaptive contention window strategy on SRI-MAC, the packet delivery ratio is higher than SRI-MAC. The packet delivery ratio of MACA-MAC is higher than PW-MAC. The experiment results show that the proposed MAC mechanisms can improve the performance of packet delivery ratio for monitoring applications.
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