## 5 Data Communication Phase
### Inter-Cluster Transmission
#### 5.2.2 Multi-Hop by RNs  
Considering the redundancy of the sensor nodes in WSNs, MELEACH [19] constructs a backbone of sensor nodes which collect data from CHs and transmit the aggregated data to the BS. The paper uses the idea presented in [136] called Energy-aware Virtual Backbone Tree (EVBT) as a scalable algorithm with low overhead to form the backbone. The EVBT algorithm introduces a parameter known as _fitness indicator_ to estimate the nodes' competence to join the backbone. The fitness indicator couples the parameters: energy level (e), radio characteristic distance (d\({}_{\text{char}}\)) and direction of the link (\(\beta\in(-\pi,\,\pi)\)) into a single parameter presented in Equation (27):  
\[f_{i}=c_{1}f_{d}+c_{2}f_{e}+c_{3}f\beta \tag{27}\]  
where \(d_{u}\) is upstream link length,\(f_{d}\), \(f_{e}\) and \(f_{\beta}\) are fitness parameters defined in Equation (28) and C\({}_{1}\), C\({}_{2}\) and C\({}_{3}\) are the weights of parameters, respectively:  
\[\begin{split} f_{d}=& 1-\min(1,abs(d_{u}-d_{char})/d_{char})\\ & f_{e}=E_{r}\\ & f_{\beta}=& 1-abs(\beta)/\pi\end{split} \tag{28}\]  
Thus, with the help of EVBT, the MELEACH algorithm constructs the backbone tree route using no-CH nodes, and each CH selects the closest RN as its upstream node to relay its aggregated data to the BS in the data transmission phase.  
An adaptive power-aware multi-hop routing algorithm is presented in [137], which defines link costs as a function of node energy and number of hops to find the optimum path to route the CHs' data towards the sink. The cost metric function of the algorithm provides weights for the links based on node residual power, hop count to sink and link quality. Before starting the data communication stage, the sink node lunches a _discovery_ message towards the entire network and each node generates a local routing table including its parent node ID and its path cost towards the sink and broadcasts its own _discovery_ message. Then, using the graph search [PERSON]'s algorithm [138], the nodes update the _path cost_ value chosen previously, if it is smaller than the value saved in the local table of the node. The iteration of the paths discovery is dependent on the level of the topology and energy changes of the network, which maintains the costs of the links up-to-date. The simulation results by the paper show improvements in terms of network lifetime and hop-count distance, even though the parameters such as channel delay and interference are not considered. In Balanced Energy Consumption and Cluster-based Routing Protocol (BECCRP) [139], the same factors of number of hops to the sink and energy level are considered to select the gateway nodes and construct the routing path to relay packets. The only difference of this scheme is the precedence in selection of gateways over CHs selection. With the aim of balancing the network load and extending the network lifetime, the nodes firstly compete for gateway position and constructing the multi-hop path to the sink and then the rest of the nodes compete to be as CH.  
In [140], the duties of CHs are reduced to path finding and data transmission responsibility is done by gateway nodes. In the cluster formation stage, the network is divided into clusters while the adjacent clusters are overlapped; and the gateway nodes are selected among the nodes located in the overlapped regions. Hence, when a node has data to transfer to the BS, it informs the CH to find the optimum gateway node and CH returns the requested information to the node. However, the paper does not discuss the parameters and conditions of the selection of the gateways and finding the next hop.