Research on MAC protocol of wireless multimedia sensor network Zhou Lipeng, Fu Jiangtao, Zheng Guoqiang (College of Electronic Information Engineering, Henan University of Science and Technology, Luoyang, Henan 471003) 0 Preface Wireless multimedia sensor network (WMSN) is a new type of sensor network that introduces multimedia information perception functions such as audio, video, and images on the basis of traditional wireless sensor network (WSN). As shown in Figure 1, WMSN is usually composed of a large number of sensor nodes equipped with CMOS cameras and miniature microphones, which can sense rich audio, video, image and other multimedia information, realize environmental monitoring of button granularity and accurate information, and can be widely used in battlefield The fields of visual monitoring, environmental monitoring, traffic monitoring, smart home and medical and health care have attracted great attention from governments and academia in various countries. Since 2003, the American Computer Association has specially organized international video surveillance and sensor network seminars to exchange related research results. Many famous universities in the United States such as the University of California and Stanford University have begun WMSN research work. Chinese universities and research institutes have also begun to explore this field, but the research results are still in their infancy, which is still far from the actual demand. In WMSN, the Medium Access Control (MAC) protocol is at the bottom of the wireless sensor network protocol stack. Its main function is to allocate limited wireless channel resources between competing sensor nodes, which determines the use of wireless channels and networks Performance is an important technology to ensure the normal operation of the entire network. In view of the characteristics of limited WMSN hardware resources, rich audio and video media information, and complex processing tasks, this article summarizes the characteristics and challenges of the current MAC protocol design, categorizes the existing protocols according to channel access methods, and focuses on the analysis of several typical Protocols, and discussed in detail the support capabilities of these protocols for real-time multimedia applications, and finally discussed the problems that need to be researched and solved in the future MAC protocol design. 1 WMSN media access control protocol features and challenges The traditional wireless sensor network MAC protocol design mainly considers the following three aspects: energy saving; node deployment and network topology must be scalable; network efficiency, mainly including fairness, throughput and bandwidth utilization. Currently, the MAC protocol of the sensor network basically does not support QoS, and cannot provide multimedia service transmission services. The WMSN MAC protocol is a relatively new research topic. In addition to considering the above wireless sensor network MAC protocol considerations during design, the multimedia service determines that the WMSN MAC protocol design faces the following technical challenges: (1) Limited hardware resources. Due to the large number of deployments of miniaturized sensor nodes, the nodes have very limited hardware resources such as energy supply, computing power and storage space, which determines that the MAC protocol design must be based on energy saving. For WMSN, due to the complexity of business transmission and processing tasks, it is necessary to consider how to efficiently use these resources while providing QoS guarantee. (2) QoS guarantee. QoS sensitivity is an important feature of WMSN, which is embodied in audio and video quality, network delay, network energy consumption, and media information processing. Compared with traditional WSN, WMSN, MAC protocol design needs more attention to service quality. (3) Differentiated services. There are audio and video information in WMSN, and text information may exist at the same time. Different applications pay different attention to different parameters of QoS. Therefore, the MAC protocol design should be able to provide differentiated services for the above-mentioned different services, and at the same time realize the effective use of resources within the entire network. (4) Performance trade-offs. MAC protocol design needs to achieve a balance between various performances, and the balance between each performance is often more important than the performance of a single performance. How to balance the multimedia sensor network MAC in QoS, network efficiency, scalability, energy consumption, etc. is also an important issue. (5) The trade-off between complexity and optimized performance. MAC protocol should optimize WMSN performance as much as possible, but often design the protocol too complicated. The sensor node itself has limited energy, storage, and computing power, and cannot perform too many calculations, so the protocol should be designed as simple and efficient as possible. 2 WMSN media access control protocol In WMSN, the MAC protocol is at the bottom of the wireless sensor network protocol stack, and the allocation of wireless channel resources between competing sensor nodes determines the use of wireless channels and network performance. According to the channel access mechanism, these protocols can be divided into three categories: non-competitive occupancy, competitive occupancy, and mixed occupancy schemes, as shown in Figure 2. The support capabilities of various MAC protocols for real-time multimedia applications are discussed in detail below. 2.1 Non-competitive occupation scheme Non-competitive MAC protocols are usually dominated by TDMA, and channel access by FDMA or CDMA can also be used. SMACS proposed by Sohrabi et al. Is a distributed MAC protocol based on TDMA. In the absence of network-wide synchronization, SMACS can discover neighbor nodes, establish transmit / receive links, and merge neighbor node discovery and channel allocation. The communication link consists of a pair of communication nodes randomly selected on a fixed frequency (or frequency hopping sequence). The mechanism of random wake-up when the link is established, and the mechanism of closing the transmission in the idle time slot effectively reduces energy consumption. However, this scheme has two disadvantages: the number of time slots of neighbor nodes is fixed and requires a time synchronization mechanism; the implementation of fixed time slots is not flexible enough to support higher bandwidth. In addition, all TDMA-based solutions require time synchronization between adjacent nodes. Based on the EDF scheduling algorithm, Caccamo et al. Proposed a MAC protocol based on FDMA. The entire network is divided into many clusters, adjacent clusters use different frequencies to communicate in FDM, and TDMA communication is used between nodes in the cluster. Correspondingly, the message is divided into two types of information exchange within the cluster and information exchange between clusters. This scheme guarantees the transmission, bandwidth and delay limitations of real-time services, but it is difficult to implement multiple frequencies on the existing wireless sensor hardware platform, and the periodic information scheduling of nodes in the cluster accelerates energy consumption. Liu et al. Proposed a CDMA-based MAC protocol to support real-time services in wireless sensor networks. They believe that adopting the CDMA scheme can provide flexible configuration of multi-cluster bandwidth resources, better security, and greater service throughput, and at the same time allow joint communication in space and time domains. On the congruent regular hexagonal cluster structure, different frequencies are replaced by different CDMA coding sequences. When sending information, each node has 1 transmitting module and 6 receiving modules, and has 7 receiving modules when listening / receiving. Compared with the TDMA and FDMA methods, the CDMA method reduces the internal inter-channel interference and effectively improves the bandwidth utilization, but the disadvantage is that it requires special sensor hardware support, which is expensive to implement. Essentially, the non-competitive occupancy scheme effectively reduces packet information collision, increases network throughput, reduces latency, and guarantees real-time service transmission, and is particularly competitive in supporting streaming media applications. However, the disadvantage is that the structure of this scheme is relatively complicated and requires centralized control. In actual deployment, it is difficult to adjust the frame length and time slot. It cannot effectively respond to node failure and changes in network topology, and requires multi-channel communication. The hardware requirements are high. 2.2 Competitive occupation scheme For wireless Ad Hoc networks, literature [9-12] proposes several MAC protocols based on contention and carrier sensing. Due to the similarity of wireless media, these algorithms can also be applied to wireless sensor networks. IEEE 802.11e has stipulated that the MAC layer differentiates the service levels, which is the main basis for the design of the competition occupation scheme. In these schemes, according to the packet priority, differentiated services can be achieved by changing the corresponding IFS duration and CW size. For example, Veres conducted research on distributed algorithms and implemented differentiated services through improved IEEE 802.11 DCF. First, the algorithm determines the contention window ranges CWmin and CWmax according to the grouping priority, and then determines the backoff time according to its value. In this way, the CWmin and CWmax values ​​of high-priority packets can be set lower than that of low-priority packets, shortening the backoff time. Lu et al. Considered the distance and time constraints comprehensively and proposed a RAP packet scheduling strategy. Using RAP's MAC protocol to improve IEEE802.11. Similar to IEEE 802.11e, it uses priority-based inter-frame values ​​and back-off window values. The simulation results show that this strategy is suitable for communication scheduling of wireless sensor networks monitored by nodes in real time. Other schemes based on IEEE 802.11 also follow this principle. Generally speaking, the competitive occupancy scheme is easy to use and has good scalability, and is suitable for processing multiple service flows, which is different from the non-competitive occupancy scheme that needs to accurately estimate the business volume. But the disadvantage is that it can not provide real-time guarantee to the business like the non-competitive occupation scheme. Therefore, this type of protocol is more suitable for networks that do not require high predictability. If they are to be successfully used in multimedia wireless sensor networks, these solutions need to provide probability guarantees for access services. 2.3 Hybrid scheme The MAC hybrid scheme effectively combines the advantages of non-competitive occupancy and competitive occupancy schemes. The scheme divides the transmission cycle into two sub-cycles: the reservation (contention) cycle and the transmission (non-contention) cycle. During the reservation period, sensor neighbors compete for transmission opportunities and transmission cycles based on traffic. Once the transmission time slot is acquired, the transmitter and receiver will communicate. The static Ad Hoc / WSN proposed by Adamout and others is a model of such a hybrid scheme. Adamout divides the entire network into several grids. Nodes in the same grid can communicate with each other. At the same time, the time is divided into fixed frames of the reservation cycle and the transmission cycle. During the reservation cycle, the grid nodes exchange data to send / receive data The time slot is reserved. Once the reservation is successful, the node will send / receive data during the non-contention period. If the reservation and data transmission are successfully completed within the allowable delay range, the delay requirements for real-time services are guaranteed. The advantages of the hybrid scheme are good scalability, low control overhead and collision overhead, and can effectively save network resources. But the disadvantage is that in order to make a successful reservation, neighboring nodes need to be synchronized. Therefore, compared with the competitive occupancy scheme, the hybrid scheme requires a large amount of communication overhead between nodes. 3 Conclusion As a new research direction of sensor network, WMSN shows broad application prospects in many fields such as military and civil. Under the premise of ensuring the transmission of multimedia services, how to design an efficient and energy-saving MAC protocol is one of the key technologies to ensure the normal operation of the entire network. This article focuses on the analysis of several typical MAC protocols and discusses their support for real-time multimedia applications. It can be seen from the analysis that the hybrid scheme is more suitable for supporting WMSN real-time communication. Because the hybrid solution not only provides real-time business guarantee, but also improves energy efficiency and bandwidth utilization, and has good scalability. However, the above MAC protocol still has many open issues that need to be resolved. For example, the protocol does not take into account issues such as data redundancy and energy-loss delay tradeoffs. At the same time, other issues such as end-to-end packet delay, channel quality, power control, and node heterogeneity in wireless networks are mostly ignored in the design. The high-efficiency and energy-saving MAC protocol should strike a balance between distinguishing the complexity of service assurance and efficient application of resources. These are all issues that need to be considered in the design of the MAC protocol for wireless multimedia sensors in the future, and I hope to play a certain role in promoting future research in China.
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