## 7 Routing in CRAHNs  
Unlike the dedicated spectrum communication scenario (WSN), route establishment in CRSNs is more challenging and more complex due to the dynamic nature of spectrum opportunities at each CR node at specific times. The source's spectrum opportunities might be completely different from those available at the sink. Thus, a secured route for data transmission can move across various channels in various spectra as it correlates with the next hop spectrum opportunities. For exchanging spectrum opportunities among neighboring nodes, either the synchronization window or the common control channel (CCC) approach is usually implemented. Based on the generalized routing framework for CR networks presented by [42], in contrast to the classical routing tables of ad-hoc networks that hold only the next hop information, the routing table is usually extended in CRAHNs to contain the full channel information and the characteristics of the next hop. Channel switching decisions are made with respect to the information received from the QoS evaluation block, the learning block and the information obtained from the sensing activity. The learning block systematically arranges the next hop channel history based on previous QoS demands via learning algorithms. Based on this framework, it is expected that the network will gradually become fully aware of its environment to dramatically reduce or eliminate the on-communication delays caused by switching activity. When taking CR into consideration, routing protocols in classical ad-hoc networks can generally be classified as follows: (1) A spectrum with next hop selection-based protocols; in this case, the routing metric taken into consideration is simply the spectrum allocation along the selected route; (2) Spectrum selection with PU awareness/avoidance-based protocols; here, other metrics considered with spectrum allocation include PU location and power adaptation; (3) Spectrum selection with reconfigurable route-based protocols that consider spectrum switching and route recovery as well as spectrum allocation. Moving toward designing effective solutions in this regard, [12,43] extensively review the challenges and directions based on spectrum knowledge categorization. Below, we present some recent studies in this area, including the following. The CRP [44] is a distributed routing protocol built upon the AODV scheme in terms of route discovery. The CRP supports multiple classes of routes based on the application's QoS requirement. Its unique property lies in its spectrum selection mapping metrics and its local PU awareness to delay forwarding packets. Relevant metrics properly depicting these values can be carefully chosen and tuned to fit the protocol requirements. Its use of the CCC favorably reduces computational overhead. The percentage utilization of the CCC has been shown to be approximately 15%-22%, which implies that the control and channel coordination traffic is rather infrequent. The CRP is operated in two phases. First is the spectrum selection phase during which the chosen route is service-classified into two class I routes, whose optimization functions seek to maximize the propagation distance and the longest allowed time for transmission. The class II route, in contrast, gives precedence to complete PU avoidance even at the expense of latency and especially for the PU receivers that are generally not detected. The second stage is the net-hop selection phase, when each node maps its route participation preference to a forwarding delay and the final route is chosen based on the arrival time of the RREQ requests. As is typical with ad-hoc networks, neither computational complexity nor energy was considered in any of its functions; thus, it is completely unsuitable for the sensor network realm. Another solution that also offers service differentiation based on traffic priorities is OSDRP [45], which is implemented for mesh networks.