The wireless sensor network consists of a large number of inexpensive mini-sensor nodes deployed in the monitoring area. A multi-hop self-organizing network system formed by wireless communication is mainly used to collect, disseminate and process sensor information.
Unlike traditional wireless self-organizing networks, wireless sensor networks have a large number of nodes and dense node distribution; due to environmental impact and energy exhaustion, nodes are more prone to failure; environmental interference and node failures are prone to cause changes in network topology. In addition, the energy, processing capacity, storage capacity and communication capacity of the nodes are limited, so the primary design goal of wireless sensor networks is the efficient use of energy. The Media Access Control (MAC) protocol for wireless sensor networks must focus on energy conservation and use a compromise mechanism to enable users to extend the network life cycle, increase network throughput, and reduce communication latency. select.
At present, for different sensor network applications, researchers have proposed multiple MAC protocols from different aspects. Lack of a unified classification method. According to the use of fixed channel allocation or random access channel, the sensor network MAC protocol is divided into: time division multiplexing (TDMA), random competition and other MAC protocols. The fixed channel allocation TDMA can naturally complete the low duty cycle operation on the node, because they only need to turn on the wireless module in their time slots to complete the transmission and reception, but their scalability is poor, and time synchronization is A large overhead. Due to the low data rate of the wireless sensor network and the low requirement for delay, the current practical energy-saving MAC protocol is basically a competition-based protocol. A lot of experiments and theoretical analysis show that the energy waste of wireless sensor nodes is mainly caused by idle listening, collision, crosstalk and control. Therefore, combining the existing wireless sensor network MAC protocol, introducing the hierarchical topology control idea, establishing an efficient and energy-saving wireless sensor network protocol, and analyzing, simulating and verifying, it has research significance.
1 Analysis of competing MAC protocols
1.1 S-MAC protocol
The S-MAC protocol is a sensor network MAC protocol based on the 802.11 MAC protocol and aimed at the energy saving needs of the sensor network. The S-MAC protocol assumes that under normal circumstances, the sensor network has a small amount of data transmission, and the nodes cooperate to complete common tasks. The network can perform data processing and fusion to reduce data traffic, and the network can tolerate a certain degree of delay. Studies have shown that sensor energy is mainly consumed in the passage between nodes, and idle listening accounts for about 1/3 of the node's communication energy. To achieve the purpose of energy saving, the S-MAC protocol mainly uses periodic listening / sleeping low occupation The air ratio mechanism controls the node to sleep as much as possible to reduce the energy consumption of the node. However, the S-MAC protocol has the following problems: all nodes in the same virtual cluster in the S-MAC protocol must transition from the sleep state to the active state at the same time to start competition for the channel, and a large number of nodes do not have data transmission tasks. The competition and idle listening waste a lot of energy.
1.2 T-MAC protocol
The T-MAC (TImeout MAC) protocol is proposed on the basis of the S-MAC protocol. The cycle length of the S-MAC protocol is limited by the delay requirements and buffer size, while the listening time mainly depends on the message rate. Therefore, in order to ensure the reliable transmission of messages, the node's cycle activity time must adapt to the highest communication load, resulting in the relative increase of the node's idle listening time when the network load is small. To address this deficiency, the literature proposes the T-MAC protocol. The protocol dynamically adjusts the node's active time according to the communication traffic while keeping the periodic listening length unchanged, and sends messages in a burst mode to reduce idle listening time. However, due to the large number of nodes that do not need data transmission, the channel competition and idle listening still cause a lot of energy waste. In addition, the implementation of the T-MAC protocol will cause early sleep problems, resulting in reduced network throughput. To this end, it uses two methods to improve the data throughput drop caused by early sleep: (1) Future request sending mechanism. (2) Full buffer priority mechanism, but the effect is not very satisfactory.
2 MAC protocol design based on topology control structure
All nodes in the S-MAC and T-MAC virtual clusters periodically transition from the sleep state to the working state, participating in channel contention and data transmission, and most nodes do not have data transmission tasks, causing them to be in most of the active time Idle listening state while wasting a lot of energy. Aiming at the inevitable problems of S-MAC and T-MAC, a new MAC protocol GS-MAC (Geo graphical SeNSor MAC) is proposed. After introducing the GAF topology control algorithm in the GS-MAC protocol, the number of active nodes is appropriately reduced, the convergence speed of the algorithm is accelerated, and the idle listening and data collision of a large number of nodes on the channel are reduced, thereby achieving the purpose of energy saving.
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