# MB-OFDM-UWB Based Wireless Multimedia Sensor Networks for Underground Coalmine: A Survey
## 3 Key Supporting Technologies for the WMSNs
### Routing Protocols  
Just like traditional WSNs, the routing protocols, whose function is establishing and maintaining data transmission paths between any two nodes that need to communicate with each other, are also key part of researching WMSNs. However, the design of the routing mechanism for WMSNs is quite different from that for traditional WSNs in the following aspects:  
1. QoS: Most of the routing protocols for traditional WSNs mainly aim at minimizing the energy consumption, but introducing the multimedia contents like videos and images into WMSNs makes QoS a crucial issue. When designing routing protocols, not only the energy consumption, scalability and robustness of the network, but also real-time and reliable transmission of multimedia data need to be taken into consideration. 2. Sensing models: Sensors in WMSNs are universally directional, e.g., video sensors, which make the sensing models or coverage models distinct from those of traditional WSNs. Distinctions in sensing models or coverage models will directly affect the design of network structure and routing protocols. 3. Data aggregation: For the sake of reducing the amount of data and lowing power consumption, a majority of the routing protocols for WSNs have adopted data aggregation mechanism. In WMSNs, sensors nodes have strong directionality and transmits multimedia data which are information-rich and time-sensitive. The redundancy and the relevance of multimedia data need to be considered when aggregating data. At present, based on the design principles of QoS guarantee, energy minimization or network lifetime maximization, stateless routing and so on, routing protocols of WMSNs including SPEED [47], MPMPS (Multipriority Multipath Selection) [48], REAR (Real-time and Energy Aware Routing) [49], and ReInForM (Reliable Information Forwarding using Multi-Paths) [50] have been proposed. Among these, SPEED is a location-based, stateless and real-time routing protocol for sensor networks, whose major object is to provide end-to-end soft real-time guarantee for the data streams with real-time requirements. Nonetheless, the protocol has not taken the energy characteristic of wireless networks or nodes into account. The MPMPS protocol is a WMSNs based routing protocol with multi-priority multi-path selection for video streaming. The core idea of MPMPS is designing multi-paths to increase the bandwidth, and decomposing video streams into audio streams and image streams. Higher priorities are assigned to important data streams which will occupy the paths with higher bandwidth and smaller time delay, and different paths are used depending on the levels of priority. In short, the features of MPMPS are decomposing video streams into sub-streams with different priorities and using multi-paths to efficiently increase the bandwidth. However, energy consumption is also not taken into consideration, and the process of decomposing or composing video streams brings in some time delay and high energy consumption. REAR is an energy-aware real-time routing protocol for WMSNs. The aim of this protocol is solving the real-time routing problem with the limitation of energy and QoS requirements. The superiorities of REAR are introducing metadata to establish multi-path routing for reducing the energy consumption, and constructing a cost function which trades off between energy and delay to evaluate the consumption of the transmission links, making REAR suitable for the QoS routing problem of WMSNs. Nevertheless, in the case of streaming applications, the metadata for streaming data can itself be very huge in mount and result in extra energy consumption [49]. In addition, the metadata may sometimes discard some useful information got from different viewing perspectives which cannot be simply overlooked.