## 2 Related Work
In the first phase of the proposed framework, known as first-phase RNs, connections were established by RNs among the CHs and sink. Further, in the second phase, extra RNs were deployed randomly and in a dense way for achieving the desired connectivity level. In the second phase, the aim of the Artificial Bees Colony (ABC) algorithm is to find the optimal location for further deployment of RNs. This framework shows effectiveness, but there are few points that need to be highlighted, such as the constraints, namely, the coverage, energy, and energy-harvesting environment have not been considered in the proposed framework. Next, [PERSON] et al. [35] have proposed a framework for providing k-connectivity of the SNs. They also proposed the Genetic Algorithm (GA)-based framework for k-connectivity. However, GA is suffering from the problem of convergence, and extra computational complexity is created to solve the hierarchical problem. Proposed Approach: Fault Tolerance and Reliable Green Communications in Next-Generation Wireless Systems (FRGNWS)  
In EH-WSNs, the routing framework should use some specific strategies, such as node-disjoint routes with shorter route characteristics, for maintaining the QoS requirements in high-traffic scenarios. In these scenarios, multipath routing is the best-suited alternative compared with the single route between each SN and BS. Even with the multipath routing framework, there is a strict requirement of node-disjoint routes so that RN can serve in one particular route and not others. Node-disjoint routes in EH-WSNs possess two unique properties, namely, not having any common SN nor link among all the detected routes. The main advantage of these characteristics lies in the fact that failure of any SN or link in a group of these routes can affect the particular route in that when a failure of any SN or link occurs the other routes are not affected. The second advantage of node-disjoint routes in EH-WSNs is related to maximizing aggregated network resources. These two advantages make node-disjoint routes in EH-WSNs a preferred choice as compared with link-disjoint and partially disjoint routes. Also, it should be noted that it is always a challenging task to explore the large set of node-disjoint routes between SNs and BS since SNs are usually deployed randomly. On the other hand, several common SNs may occur in link-disjoint routes but no shared link is allowed between the routes is the main unique property of these types of routes. Therefore, any SN failure in these types of routes may affect (deactivate) multiple routes, since a failed SN may be shared with multiple routes. Furthermore, partially disjoint routes can be easily constructed since these types of routes have multiple routes in which several links or SNs between the distinct routes may be shared. Therefore, in these types of partially disjoint routes, failure of any link or SN in a group of these routes may have an impact on multiple other routes. For the performance requirements, the density of the network for the particular application is the key factor in taking the best decision for selecting the particular category of disjoint routes. Furthermore, the minimum delay is another requirement for optimal performance of EH-WSNs that can be achieved with the help of the shortest route, since it uses the minimum hops. Further, the routing routes should be as short as possible in EH-WSNs since shorter routing routes result in lower consumption of energy; also, packets delays are always minimum in shorter routing routes, and consequently the lifetime of the network as well as data-delivery ratio will be enhanced. In a multipath routing framework with multiple sources, the density of the RNs should be high towards the BS side compared to the SN side. Therefore, there is a need for careful selection of the distribution strategies for RNs, as the most appropriate distribution strategy for RNs is a controlled random distribution strategy with a non-uniform nature.