| The wide use of inexpensive hardware such as CMOS cameras and microphones which can ubiquitously capture multimedia content of the environment has fostered the development of Wireless Multimedia Sensor Networks (WMSNs). WMSNs, the network of wirelessly interconnected devices that can retrieve video and audio streams, still images and scalar sensor data, cannot only enhance existing sensor network applications like environmental monitoring, but also enable several new applications as multimedia surveillance, traffic enforcement, and industrial process control, and therefore become a very important topic with theoretical and practical values.Compared to traditional sensor networks, WMSNs pay more attention to the information-intensive data. The most concerned focus of recent research is placed in tradeoffs between energy conserving and QoS. The Medium Access Control (MAC) layer, which is used to coordinate node access to the shared wireless medium, is the best level to address the energy inefficiency. Because the MAC layer provides fine-grained control of the transceiver, and allows switching the wireless radio on and off. So, the channel utilization from MAC layer provides fundamental support of WMSNs.In this paper, the characteristics and technical challenges of MAC in WMSNs are summarized, the state of the art in algorithms and protocols on MAC layer is surveyed from channel access policies, scheduling, error control, multimedia packet size, and the open research problems are also discussed in detail. In addition, the simulation tools of MAC in WMSNs are analyzed and compared along with their advantages and drawbacks.In view of the characteristics of the node deployment and location-related multimedia data in WMSNs, an Adaptive Round Distributed Cluster Head (ARDCH) scheme has been proposed, which takes both the energy efficiency and communication range into account. According to the communication cost and the remaining energy, ARDCH selects the cluster headers and adjusts their work round dynamically. Furthermore, ARDCH also takes advantages of multi-channel communication between different clusters to avoid inter-cluster interference. Simulation results have shown that ARDCH scheme can satisfy WMSNs and prolong the life cycle of WMSNs compared with other clustering protocols.A back-off scheme for different services is needed because of the different traffic classes that WMSNs have, although Contention-Based Protocols work well when burst multimedia traffic is in a small amount. In this paper, a new MAC layer SDDB back-off algorithm is proposed, which can provide corresponding back-offs for flows with various priorities, thus effectively avoid the second-collision. SDDB is able to meet the demand of real-time transmission without the reducti on of throughput in network. Simulation results have indicated that SDDB algorithm is more suitable than other existing methods for WMSNs.Each type of traffic in WMSNs has its own QoS requirement, real-time traffic services need low-latency while none real-time traffic services pursue high reliability. To solve these problems, the most important tasks are to regulate access to the channel and to allocate bandwidth resources reasonably and efficiently. In this paper, a Different Services Medium Access Control (DSMAC) is presented for WMSNs. A four-channeled protocol can support different services as using TDMA for real-time traffic and Contention-Based access for none real-time traffic, and therefore the transmission of burst multimedia traffic becomes possible through inserting random contention period into frame. Complementarily, a multi-channel inter-cluster transmission is proposed to avoid the hidden terminal problem. Experimental results have shown that DSMAC is fitter for WMSNs than other MAC protocols in real world systems.In summary, to study and design MAC level protocol for WMSNs, this paper proposes ARDCH scheme, different services back-off algorithm, and the channel access policy with both academic and practical values. |
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