Research on MAC Protocol for Wireless Multimedia Sensor Networks

0 Introduction

Wireless Multimedia Sensor Network (WMSN) is a new type of sensor network that introduces multimedia information perception functions such as audio, video and images based on traditional wireless sensor networks (WSN). As shown in Figure 1, WMSN usually consists of a large number of sensor nodes equipped with CMOS cameras and micro microphones, which can perceive rich multimedia information such as audio, video, and images, and realize environmental monitoring with granularity and accurate information. It can be widely used in battlefield visualization monitoring, environmental monitoring, traffic monitoring, smart homes, and medical and health care, and has therefore attracted great attention from governments and academia. Since 2003, the American Computer Association has specially organized international video surveillance and sensor network seminars to exchange relevant research results. Many famous American universities such as the University of California and Stanford University have begun research on WMSN. Chinese universities and research institutions have also begun to explore this field, but the research results are still in the initial stage and are far from actual needs.

In WMSN, the media 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, determine the use of wireless channels and network performance, and is an important technology to ensure the normal operation of the entire network.

In view of the characteristics of WMSN such as limited hardware resources, rich audio and video media information, and complex processing tasks, this paper summarizes the characteristics and challenges of current MAC protocol design, classifies existing protocols according to channel access methods, focuses on analyzing several typical protocols, and discusses in detail the support capabilities of these protocols for real-time multimedia applications. Finally, it discusses the issues that need to be studied and solved in the future MAC protocol design.

1 Characteristics and Challenges of WMSN Media Access Control Protocol

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. At present, sensor network MAC protocols basically do not support QoS and cannot provide multimedia service transmission services.

The MAC protocol of WMSN is a relatively new research topic. In addition to taking into account the above-mentioned MAC protocol considerations of wireless sensor networks, multimedia services determine that the MAC protocol design of WMSN faces the following technical challenges:

(1) Limited hardware resources. Due to the large number of miniaturized sensor nodes deployed, the hardware resources of the nodes such as energy supply, computing power and storage space are very limited, which determines that the MAC protocol design must be based on energy saving. For WMSN, due to the complexity of service transmission and processing tasks, it is necessary to consider how to efficiently use these resources while providing QoS guarantees.

(2) QoS guarantee. QoS sensitivity is an important feature of WMSN, which is specifically reflected in audio and video quality, network latency, network energy consumption and media information processing. Compared with traditional WSNs, the MAC protocol design of WMSN needs to pay more attention to service quality.

(3) Differentiated services. WMSN has audio and video information, and may also have text information. Different applications pay different attention to different QoS parameters. Therefore, the MAC protocol design should be able to provide differentiated services for the above different services and realize effective resource utilization across the entire network.

(4) Performance trade-off. MAC protocol design needs to strike a balance between various performances, and the balance between various performances is often more important than the performance of a single performance. How to strike a balance between QoS, network efficiency, scalability, energy consumption, etc. in multimedia sensor network MAC is also an important issue.

(5) The trade-off between complexity and optimized performance. MAC protocol should try to optimize WMSN performance, but the protocol is often designed to be too complex. Sensor nodes themselves have limited energy, storage and computing power and cannot perform too much calculation, so the protocol should be designed to be as simple and efficient as possible.

2 WMSN Media Access Control Protocol

In WMSN, MAC protocol is at the bottom layer of the wireless sensor network protocol stack, allocating wireless channel resources between competing sensor nodes, and determining 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 following discusses in detail the support capabilities of various MAC protocols for real-time multimedia applications.

2.1 Non-competitive occupancy scheme

Non-competitive MAC protocols are usually based on TDMA, and can also use FDMA or CDMA channel access methods. SMACS proposed by Sohrabi et al. is a distributed MAC protocol based on TDMA. In the absence of full network synchronization, SMACS can discover neighbor nodes, establish send/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 randomly waking up when the link is established and shutting down the transmission in the idle time slot effectively reduces energy loss. However, this scheme has two disadvantages: the number of time slots of neighbor nodes is fixed and requires a time synchronization mechanism; the fixed time slot implementation is not flexible enough and it is difficult to support higher bandwidth. In addition, TDMA-based schemes all 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, and adjacent clusters communicate using different frequencies in FDM mode, while nodes within the cluster communicate using TDMA mode. Correspondingly, messages are divided into two types: intra-cluster information exchange and inter-cluster information exchange. This scheme guarantees the transmission, bandwidth and delay limits 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 within 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 the CDMA scheme can provide flexible configuration of bandwidth resources between multiple clusters, better security and higher service throughput, while allowing multi-domain joint communication in time and space. Different CDMA coding sequences are used to replace multiple frequencies on the congruent regular hexagonal cluster structure. When sending information, each node has 1 transmitting module and 6 receiving modules, and 7 receiving modules when listening/receiving. Compared with TDMA and FDMA, CDMA reduces internal channel interference and effectively improves bandwidth utilization, but the disadvantage is that it requires special sensor hardware support and has a high implementation cost.

In essence, the non-competitive occupation scheme effectively reduces packet information collision, increases network throughput, reduces latency and guarantees real-time service transmission, especially in supporting streaming media applications. It has strong competitiveness. However, the disadvantage is that this scheme has a complex structure and requires centralized control. It is difficult to adjust the frame length and time slot in actual deployment, and it cannot effectively cope with node failure and changes in network topology. It also requires multi-channel communication and has high requirements for sensor node hardware.

2.2 Competition occupation scheme

For wireless Ad Hoc networks, literature [9-12] proposed several MAC protocols based on competition occupation and carrier sensing. Due to the similarity of wireless media, these algorithms can also be applied to wireless sensor networks. IEEE 802.11e stipulates the distinction of service levels at the MAC layer, which is the main basis for the design of competition occupation schemes. In these schemes, according to the packet priority, the distinction of services can be achieved by changing the corresponding IFS duration and CW size. For example, Veres studied the distributed algorithm and achieved service distinction through the improved IEEE 802.11 DCF. First, the algorithm determines the contention window range CWmin and CWmax according to the packet 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 those of low-priority packets, shortening the backoff time.

Lu et al. comprehensively considered the distance and time constraints and proposed the RAP packet scheduling strategy. The IEEE802.11 was improved by using the RAP MAC protocol. Similar to IEEE 802.11e, it uses priority-based inter-frame values ​​and backoff window values. Simulation results show that this strategy is suitable for communication scheduling of wireless sensor networks with real-time node monitoring. Other schemes based on IEEE 802.11 also follow this principle.

Generally speaking, the competitive occupation scheme is easy to use and has good scalability. It is suitable for processing multiple business flows, which is different from the non-competitive occupation scheme that requires accurate estimation of business volume. However, the disadvantage is that it cannot provide real-time guarantee for business like the non-competitive occupation scheme. Therefore, this type of protocol is more suitable for networks with low predictability requirements. If it is to be successfully used in multimedia wireless sensor networks, these schemes need to provide probabilistic guarantee for access services.

2.3 Hybrid Scheme

The MAC hybrid scheme effectively combines the advantages of non-competitive occupation and competitive occupation schemes. This scheme divides the transmission period into two sub-periods: reservation (competition) period and sending (non-competition) period. In the reservation period, sensor neighbors compete for sending opportunities and sending periods according to business volume. Once the transmission time slot is obtained, the transmitter and receiver will communicate. The static Ad Hoc/WSN proposed by Adamout et al. is a typical example of this hybrid solution. Adamout divides the entire network into several grids, and nodes in the same grid can communicate with each other. At the same time, the time is divided into fixed frames of reservation period and transmission period. During the reservation period, the grid nodes reserve the time slot for sending/receiving data by exchanging three pieces of information. Once the reservation is successful, the node will send/receive data in the non-competition period. If the reservation and data transmission are successfully completed within the allowed delay range, the delay requirement of the real-time service is guaranteed.

The advantages of the hybrid solution are good scalability, low control overhead and conflict overhead, and can effectively save network resources. However, the disadvantage is that in order to successfully make a reservation, neighboring nodes need to be synchronized. Therefore, compared with the competitive occupation scheme, the hybrid scheme requires a lot of communication overhead between nodes.

3 Conclusion

As a new research direction of sensor networks, WMSN shows broad application prospects in many fields such as military and civilian. 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 paper focuses on analyzing several typical MAC protocols and discusses their support capabilities for real-time multimedia applications. Through analysis, it can be seen that hybrid solutions are more suitable for supporting WMSN real-time communications. Because the hybrid solution not only provides real-time service guarantee, but also improves energy efficiency and bandwidth utilization, and has good scalability.

However, there are still many open problems that need to be solved in the above MAC protocols. For example, the protocol does not take into account issues such as data redundancy and energy consumption delay balance tradeoffs. At the same time, other issues such as end-to-end packet delay, channel quality, power control and node heterogeneity of wireless networks are mostly ignored in the design. Efficient and energy-saving MAC protocols should strike a balance between distinguishing the complexity of service guarantee and efficient application of resources. These are all issues that need to be considered in the design of MAC protocols for wireless multimedia sensors in the future, and I hope they can play a certain role in promoting future research work in China.