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In cluster topology, a group of nodes form a cluster, and the geographical target area is divided into numbers of clusters. Each cluster has one cluster head and one cluster member. In a cluster, two members cannot communicate directly, as shown in Fig. 3(b).

All cluster members send data to the cluster head, and hence the cluster head has the highest priority to allocate the channel for further communication. For any cluster member, inter-cluster and intra-cluster communication takes place via the cluster head.

Node with the highest energy is selected as cluster head as it consumes more energy. Special algorithms and protocols (for example, LEACH) elect the cluster head on the basis of remaining battery life (RBL) of all nodes within the cluster. A cluster that includes a variety of sensor nodes is called a heterogeneous cluster sensor network (Fig. 3b).

A multi-tier architecture is arranged in hierarchy or multiple layers. As shown in Fig. 3(c), one cluster has formed of scalar nodes and the other using video sensors. Both clusters forward the data to the sink through high-end video sensors. The sink is connected to the Internet (IP based network) via a wireless gateway. This way the user can monitor and control targeted areas from a remote location.

Challenges and opportunities
High data rates. WMSNs require high data rates as data is in terms of live video streaming, audio and still images. Achieving high data rates (in terms of Mbps) on available narrow channels is a challenging task. Recently, researchers started to implement ultra-wide band (UWB) technology on sensor nodes for higher data rates.

Signal detection and estimation. Signal detection, estimation, filtering, data gathering and wireless channel separation are still open research challenges for the implementation of WMSNs. Researchers have proposed OFDMs (frequency-division multiplexing schemes) to increase data rates and remove interference between two wireless channels.

Reliability. High information assurance is expected in WMSNs, and for that effective error detection, correction code with accuracy, robustness, resiliency and retransmission policy need to be found out.

Energy efficiency. Nodes have limited battery and, for most applications, charging and replacement of the battery is not possible. Nodes need to serve for many years. To reduce power consumption, researchers have already started designing and implementing different types of MAC protocols [real-time independent channels (RICH) MAC protocols, frame sharing (FRASH) MAC protocol, cluster based on demand multichannel MAC protocol, Q-MAC, RL-MAC and many more]. Energy efficiency can be obtained by implementing new algorithms on the physical layer (modulation techniques, architecture, etc) as well as on the MAC layer. Many image- and video-compression algorithms are also introduced, for instance, distributed video coding (DVC), high-efficiency video coding (HEVC), discrete cosine transform (DCT), discrete wavelet transform (DWT) and curvelet transform. Results show that by using advanced compression techniques, energy consumption can be reduced by up to 50 per cent.

Inherent uncertainty. In most WSNs, mobility of nodes creates problems as it affects topology, routing protocols and synchronisation. Coverage area and coherence bandwidth depend on moving obstacles and the surrounding environment.

Security and privacy. Multimedia industrial monitoring and control kind of applications require high privacy and security. Unauthenticated persons should not be able to access private data.

Quality of service (QoS) and quality of information(QoI). High QoS and QoI are required on each and every communication layer. The resolution should be good enough at remote places. For video streaming, 5fps to 20fps (frames per second) is expected for better quality, and all layers should be designed accordingly.

Applications
WMSNs have numerous killer applications. Multimedia surveillance sensor networks can be used to enhance and complement existing surveillance systems to prevent crime and terrorist attacks. These can be used to locate missing persons, identify criminals or terrorists and record other potentially relevant activities, such as thefts, car accidents and traffic violations. With the help of WMSNs, it will be possible to monitor car traffic in big cities and on highways that offer traffic-routing advice to prevent congestion or identify violations.

Fig. 3: Reference architecture of a wireless multimedia sensor network: (a) single-tier flat homogeneous sensors, distributed processing, centralised storage; (b) single-tier clustered, heterogeneous sensors, centralised processing, centralised storage; and (c) multi-tier, heterogeneous sensors, distributed processing, distributed storage (Courtesy: Akyildiz et al.: Wireless multimedia sensor networks: applications and test beds)
Fig. 3: Reference architecture of a wireless multimedia sensor network: (a) single-tier flat homogeneous sensors, distributed processing, centralised storage; (b) single-tier clustered, heterogeneous sensors, centralised processing, centralised storage; and (c) multi-tier, heterogeneous sensors, distributed processing, distributed storage (Courtesy: Akyildiz et al.: Wireless multimedia sensor networks: applications and test beds)

WMSNs play a vital role in personal health care units. Telemedicine sensor networks can be integrated with 3G and 4G mobile networks to provide ubiquitous health care services. Patients can carry various types of medical sensors to monitor different parameters, such as blood pressure, ECG, breathing activity, pulse oximetry and body temperature. Remote medical centres or personal doctors can monitor the condition of patients to infer emergency situations. This will be more beneficial for villages and rural areas where doctors and health centres are not available all the time.

WMSNs can also be used for environmental and structural monitoring purposes. Video and image sensors may be used to monitor the structural health of bridges and other civil structures. In industrial applications, WMSNs can give visual inspections and automated actions.

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