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The MAC protocol of wireless sensor network can be divided into many different ways from different angles. By channel access, it can be divided into non-scheduling (random/competitive)MAC protocol, scheduling (cooperative)MAC protocol and hybrid MAC protocol1. According to the number of radio frequency used by nodes in the network and the number of channels, it can be divided into single radio frequency single channel MAC protocol, single radio frequency multi-channel MAC protocol and multi-radio frequency multi-channel MAC protocol. According to the time synchronization characteristics of nodes in the network, it can be divided into synchronous MAC protocol and asynchronous MAC protocol (Liangrui Tang(2019)).
The same MAC protocol can also be classified into different classes according to different perspectives. For example, S-MAC, T-MAC, etc.. S-MAC protocol is designed based on IEEE 802.11 to save energy in sensor networks. S-MAC includes energy-saving methods from various energy consumption modes, such as idle interception, collision, crosstalk and control overhead. The T-MAC protocol dynamically adjusts the active time according to the traffic on the basis of keeping the period length unchanged, and sends messages in burst mode to reduce idle listening time. In this section, MAC protocols are divided into three types according to the anti-conflict mechanism adopted the protocol and the organization form of nodes in the network: non-scheduling MAC protocols, scheduling MAC protocols, hybrid MAC protocols and cross-layer MAC protocols (Karuna Babber(2017)).
In the non-scheduled MAC protocol network, the nodes do not exchange information with each other, but compete randomly using the wireless channel resources Carrier Sense Multiple Access(CSMA) (Joseph E. Mbowe(2014)). Because of its simple flexibility and good robustness, CSMA technology has become a common technology in non-scheduling MAC protocol. CSMA technology does not need complex basic technology and information to support: it does not need node time synchronization; No global network topology information is required.
It can adapt to the dynamic change of the number of nodes. However, such problems as high probability of channel conflict, high probability of data transmission failure, excessive data retransmission and long idle listening time will increase the energy consumption of nodes and shorten the network life time. In addition, CSMA technology can only listen to the node signal within the range of one hop, and it is easy to produce the problem of terminal disappearance in multi-hop communication. In order to reduce the energy consumption of nodes, the periodic activation/sleep mechanism is usually adopted, and the RTS/CTS handshaking mechanism is adopted to solve the problem of data collision during communication (Awatef Ben Fradj Guiloufi(2013)).
Scheduling MAC protocol manages the communication process between nodes in the network uniformly according to certain rules, and it widely uses Time Division Multiple Access (TDMA) technology to coordinate the access of nodes to channel resources in the network (Hemanta Kumar Kalita(2010)). The core idea of TDMA is to subdivide the timeline into several different time slots, each node occupies a different time slot, and the nodes wake up in the corresponding time slot and enjoy the exclusive wireless channel without conflict. In order to ensure the uniqueness of the time slot occupied by each node in the network and avoid channel conflict, the MAC protocol based on TDMA technology needs strict global time synchronization (Arun Kumar(2017)).