## 2 Related Work
### Real-Time Routing Protocols  
The real-time transmission is essential for various applications such as multimedia, disaster response, and various studies have been proposed according to the demand. To achieve real-time transmission, the packets should be delivered to their destination within the required time of the applications. In this section, we describe previous work related to real-time routing in WSNs. SPEED [19] has proposed a spatiotemporal approach for the real-time service. SPEED could satisfy the real-time requirement by maintaining a desired delivery speed across the networks. The desired delivery speed depends on not only the end-to-end distance but also the desired time. The intermediate node relays the packet through the node which satisfy the desired speed. As an extension of SPEED, SPEED-Realtime Routing (RR) [20] has been proposed to comprise between real-time and energy cost. To decide the next hop, three metrics, that is, residual energy of two-hop neighbor nodes, routing voidand rate void rate are taken. Furthermore, adaptive control for data rate and cache queue length is employed to distinguish the congestion control. ORRP [21] presented an opportunistic real-time routing scheme. ORRP broadcasts the packet to the neighbor nodes which satisfy the real-time requirement. All neighbors received packet could obtain the priority according to a tolerable time period to be able to satisfy the requirement. As the ORRP could offer the transmission opportunity more neighbors, it could achieve reliable real-time packet transmission. [PERSON] et al. [22] addressed that the remaining time gain could exploit to satisfy the real-time requirement. In the transmission process, the intermediate node accumulates the remaining time to gain through selecting nodes that have a higher speed than the desired speed. The accumulated time would be exploited to pass the area which no neighbors meet the real-time requirement. Unlike the previous work based on single path approach, [PERSON] et al. [23] presented a comprehensive multipath routing approach for QoS in WMSN. Their main strategy for survey was design issues in the aspects of multimedia data. [PERSON] et al. [24] presented a disjointed multipath routing scheme for real-time data transmission in WMSN through hybrid approach with Bluetooth and Zigbee to overcome the bandwidth limitation. Moreover, [PERSON] and [PERSON] [25] proposed Optimized QoS-based Clustering with Multipath Routing Protocol (OQoS-CMRP) to determine cluster heads while considering sink coverage area and energy. In addition, [PERSON] et al. [26] presented the opportunistic multipath routing to improve the performance in WSN which can be applied to multimedia data delivery. However, these approaches do not include real-time delivery of captured multimedia data and would need to be extended. In this regard, [PERSON] and [PERSON] [27] proposed cross layered approach for (_m,k_)-firm stream in WSN. In each layer, transmission power, priority of packet and QoS path is adjusted and found. Moreover, [PERSON] and [PERSON] [28] presented (_m,k_)-firm based routing protocol which is based on a set of fault recovery mechanisms to overcome inherent resource constraint of sensor nodes and instability of wireless communication. A local status indicator to monitor and evaluate their local conditions is introduced on intermediate node. [PERSON] et al. analyzed the challenges presented by real-time communication [29] as part of special issue [30]. Specifically, they introduced a firm real-time model together with hard and soft models. As representative examples, [PERSON] and [PERSON] [31] presented a distributed, measurement-based scheme for the (_m,k_)-firm stream. In addition, they extended this approach to address the scalability problem and lack of architecture for the (_m,k_)-firm stream by using flow aggregation based on a compositional hierarchical model, a velocity-based routing protocol, a hybrid medium access control protocol, and a congestion control scheme [32]. [PERSON] et al.