## 5 Data Communication Phase
### Inter-Cluster Transmission
#### 5.2.1 Multi-Hop by CHs  
A centralized routing protocol called Base-station Controlled Dynamics Clustering Protocol (BCDCP) is proposed by [130] in which the BS carries out all the energy-intensive tasks including CH selection, balanced cluster formation and constructing the routing path. BCDCP is considered as a contribution to the LEACH-C protocol with multi-hop transmission from CH to the BS. BCDCP introduces CH-to-CH routing to transfer data to the base station using minimum spanning tree approach (MST) [131]. The paper addresses two issues in multi-hop data transmission in cluster-based algorithms: the heavy burden of data relaying for the CHs closer to the BS and radio interference caused by the neighbouring clusters. To alleviate the first problem, one CH is randomly selected to forward the data to the BS and thus the burden of routing is evenly distributed among all CHs. The paper also addresses the issue of radio interference caused by neighbouring clusters either in data transmission to CH or in relaying data towards the BS. To counteract the issue, BCDCP utilizes CDMA codes that each CH using its unique spreading code distinguishes the data transmission from the nodes within the boundaries of neighbouring clusters. Thus the CHs without hindering the operation of other clusters, route back the data to the BS. The random selection of a CH to transmit other clusters' data to the BS is modified in [132] that in each round, the CH, which has the maximum remaining energy and minimum distance to the BS, is selected as super CH to relay data. The weighted spanning tree is introduced in [18] as a backbone to relay data. The algorithm improves the formation of the spanning tree from a single-factor distance-based decision to a multi-criteria one. Selected CHs form a backbone to relay data towards the BS. Therefore, in selection of CHs not only the residual energy and degree of node are considered, but a node's distance to the constructed spanning tree is another determinant factor. The weight of a CH in the spanning tree is evaluated proportional to its remaining energy and the number of its neighbouring nodes, while it is inversely proportional to the distance of the node to the spanning tree route. The algorithm optimizes the inter-cluster data transmission path and extends the network lifetime, especially after demise of half of the nodes within the network. The authors of [133] also propose a weight-based system for selection of relaying path through the CHs. The algorithm calculates the weight of each CH according to the remaining energy of node and its distance to the sink node to construct a routing tree. Although the algorithm may select the optimum next hop from a local perspective, data may be transmitted back and forth in the way of routing to the sink node, which imposes an extra energy expenditure on the network. An algorithm using multi-layer clustering is presented in [134] that reduces the average distance of each CH from its upper level CH. While cluster formation at the lowest level takes place through a distributed scheme, cluster discovery at the upper levels is done by help of the BS. In each round, the nodes respond to the BS broadcast message at their default low power level with their own ID's and thus, the reply message of the closest CHs only is received by the BS. For the next level, the BS broadcasts another message, which includes the CHID's of the first layer clusters, and nodes reply to this message at their default low power level. Nevertheless, this time the first layer CHs relay received responses to the BS. Following the process until no new CH is discovered, the BS generates a table of the CHID's, level of the CH and ID of the forwarding CHs. In this way, the BS selects the CHs for the lower layer CHs from its immediate upper layer CHs and the algorithm constructs a clustering hierarchy that any node in the given layer reaches the BS in equal number of hops and hence minimizes the energy consumption of the network.