## 1 Introduction
Thus, there is a need for specially adapted communication protocols to fulfill the needs of both DSA and WSNs in a CR context. The network layer is fundamental in any network and is significantly affected by the dynamic radio environment created by CR because it addresses the peer-to-peer delivery through other nodes in a multi-hop fashion to the correct recipients in due time. The sending node must address both its dynamic radio environment and that of the next hop node. This phenomenon is otherwise referred to as the \"deafness problem\" and introduces a challenging scenario requiring innovative algorithms that consider the intrinsic nature of the sensor nodes. This scenario necessitates a cross-layer approach for designing spectrum-aware routing protocols. A number of researchers have proposed routing schemes for cognitive radio _ad-hoc_ networks [12]. However, due to the differences in constraints between classical ad-hoc networks and WSNs, these solutions cannot be directly imported to solve the problem of routing in CRSNs. Although CRSNs can also be _ad-hoc_ in nature, they differ from classical _ad-hoc_ networks in the following ways:  
* Sensor networks (SNs) are usually densely deployed, with hundreds of nodes, because the harsh atmosphere to which the nodes are exposed can easily cause node failures. In contrast, _ad-hoc_ networks are not usually densely deployed. * While SNs are highly constrained with respect to memory, energy and computation capabilities, _ad-hoc_ networks usually do not consider these fundamental constraints. * The mode of communication in a SN is usually based on broadcast, whereas _ad-hoc_ networks use point-to-point mode most of the time. * SNs usually have the communication goal of data aggregation, in addition to the plain communication goal of _ad-hoc_ networks. * Addressing schemes in SNs are significantly different from those applied in traditional _ad-hoc_ networks because of the enormous overhead of schemes such as IP addresses and GPS coordinates. * Finally, SNs have periods in which they \"sleep\" to conserve energy, whereas nodes in most _ad-hoc_ networks do not have this property. To the best of our knowledge, specific attention has not been given to routing in the network layer of CRSNs, although recent research has emphasized the transport [10, 11], MAC and physical layers [10, 12, 13]. Hence, there is the need for research to focus on this area. We present a review of WSN routing strategies _vis-a-vis_ CRSN requirements to evaluate the strengths and weaknesses of each strategy. This review is provided to enable protocol designers to use quantitative evidence in selecting the strategies best suited to their application. The paper then discusses the factors affecting routing CRSNs, reviews recent studies in this area and categorizes them appropriately. Open issues in this respect are also identified. The paper further identifies major CRSN routing components and presents a systematic review of relevant studies in each category to reveal the open issues. The main contributions of this paper are as follows:  
* To identify a research gap in the network layer of CRSNs. * To evaluate WSN routing strategies _vis-a-vis_ CRSN requirements. * To propose cross-layer and routing frameworks for routing in CRSNs. * To discuss the main components of routing in CRSNs _vis-a-vis_ recent studies to reveal open areas. The rest of this paper is organized as follows: Section 2 provides a general overview of CRSNs, defining the main building blocks of the field. Recent research trends in this respect are also mentioned. Section 3 examines routing in WSNs, presents issues arising from the introduction of the CR component and discusses the cross-layer design concept. Section 4 presents a performance evaluation of WSN routing strategies with respect to DSA. This paper seeks to be a pioneer in this regard. Section 5 discusses the results of the simulations described in Section 4. Section 6 discusses state-of-the-art routing in CRSNs and describes open research areas.