## 2 Related Work  
Among the relevant works in this regard is [22], which proposes a cross-layer cognitive routing protocol, which is called the opportunistic service differentiation routing protocol (OSDRP) for dynamic CRNs. The protocol is comprised of the route discovery, route decision, opportunistic routing with transmit power control (ORTPC) and route maintenance modules. The route discovery follows the usual AODV strategy to discover all possible routes from source to sink, after which, the route decision module then chooses the minimum delay route. During the data-transfer phase, the authors sought to switch to a greedy forwarding-based strategy. However, how neighboring nodes become aware of the operating channel of the sending node was never detailed. More importantly, the local minimum case, which is usually a major cause of route failure in such forwarding techniques, was not addressed in the route maintenance module. Rather, the route maintenance was still basically AODV based. Moreover, energy was not considered in the routing metrics. In the case of ROR, although it follows the same method to determine the best route to the sink, it also uses this phase to solve the local minimum issues that can arise during the data forwarding phase by identifying the nodes along the route as local minimum resolution (LMR) nodes, which are finally resorted to in order to forward the data in case the eligible forwarding node happens to be in a local minimum region. In addition, because we are considering CR sensor nodes, energy was a basic routing metric considered. The most-recent work in this area is the spectrum-aware clustering for efficient multimedia routing in cognitive radio sensor networks (SCEEM) protocol for CRSN [23, 24], which was proposed for efficient energy consumption and dynamic spectrum access. SCEEM is based on a hierarchical routing scheme, which organizes neighboring nodes into clusters after the sharing of local spectrum sensing results and residual energy. A cluster head, which usually is the node having the highest energy spectrum rank, is chosen via a self-announcement scheme. The cluster-head is responsible for controlling the spectrum access and data routing. Communication in SCEEM is based on a hybrid time division multiple access (TDMA) and carrier sensing multiple access (CSMA) medium access protocol to relay intra-cluster and inter-cluster packets, respectively. In comparing SCEEM with ROR, both ROR and SCEEM support scalability, because SCEEM is based on hierarchical routing, while ROR is based on geographical routing, and both schemes support scalability [25, 26, 27]. Again, both utilize the concept of clustering. However, while SCEEM utilizes clustering for spectrum opportunity aggregation and energy efficiency, ROR utilizes clustering to ensure the instantaneous link quality guarantee and route stability in order to maintain QoS. ROR maximizes the utilization of idle listening, while SCEEM does not take this into consideration. Even though SCEEM utilizes clustering as a means of energy reduction based on the operational principles of radios, SCEEM will expend more energy. This is because in SCEEM, all nodes have to send and receive control messages for clustering and spectrum opportunity aggregation, after which, additional energy is expended in the route request operation. Additionally, it should also be noted that cluster members, as well, take part in forwarding the route request. In contrast, ROR has merged both route request and clustering operations into one operation to achieve QoS. Thus, in ROR, by virtue of position information, nodes can immediately become a cluster member upon hearing the route selection control message. In terms of medium access complexity, SCEEM utilizes a hybrid MAC, which uses TDMA for intra-cluster communication and CSMA for inter-cluster communication. In the intra-cluster communication phase, all nodes broadcast their spectrum opportunities along with their residual energy to their neighbors via CC.