## 2 Routing Protocols in Large-Scale WSNs
### Energy Balance Algorithms
In GESC, the cluster head is elected depending on the location of the source, the residual energy and the importance of the candidate, thus avoiding the effect of \"hot-spots\". Additionally, the energy consumption is distributed over all the sensors because cluster heads is elected independently at individual nodes and differs at each time and at each node, by this means prolonging the network lifetime. However, all the phases will be performed whenever a node failure occurs, which is prone to produce large overhead. _DCSSC_. In order to utilize the spatial correlation among the sensed data, reference [41] proposed Distributed Clustering Scheme based Spatial Correlation (DCSSC) for grouping sensors based on similarity of data readings. The sensor nodes that have the highest similarity in observations are grouped into the same cluster, and accordingly, they can be scheduled to alternatively report their sensed data for energy saving. Additionally, a dynamic backbone is constructed for efficient data collection. The dissimilarity is defined to describe the degree of difference between the data readings of any two sensor nodes. It is calculated by:  
\[d(s,v)=\omega_{i}\left|s_{1}-v_{i}\right|+...+\omega_{n}\left|s_{n}-v_{n}\right| \tag{6}\]  
where \(s_{i}\) and \(v_{i}\) indicates some types of reading data at the respective node and \(\omega_{i}\) denotes how much it affects relative part. The cluster construction is started with a Cluster Formation (CFRM) message broadcasted from the sink. Upon receiving the message, each initial (INI) node will change state to gateway-ready (GWR) and set two timers, which are \(t_{req}\) for transmitting Cluster head Request (CHREQ) message and \(t_{wait}\) for receiving Cluster head Advertisement (CHADV) message. When the former timer expires, a CHREQ message is sent out. Upon receiving the message every INI node calculates the dissimilarity measure, and will change to cluster head candidate (CHC) if they are proved to be strongly correlated (meaning \(d(s,\,v)\leq\tilde{A}\), the latter part is a predefined value), setting a timer \(t_{adv}\) at the same time. Then the new CHC node declares itself as a CH node by broadcasting CHADV message when the timer expires. After receiving the CHADV message, every temporary state node, INI or CHC node here, calculates the dissimilarity measure with the new CH. If they are strongly correlated, the receiver will become a member (MEM) of the cluster; otherwise, it goes to GWR state. The GWR node upon receiving the message needs to check the predecessor ID (p-ID) of the message with its own. If they are matched, GWR will enter GW state; otherwise, it becomes MEM of the cluster. However, if until \(t_{wait}\) expires the GWR node has not got any CHADV messages, it will change state to CHC. Then the GW and MEM nodes continue to propagate the CFRM message by creating and broadcasting cluster-extend (CEXT) message to discover the rest of the network, and the receiver, if strongly correlated with the originating CH, will become a MEM node of this cluster and the message disseminator becomes the cluster-extend node. The process described above proceeds until the entire sensor nodes are grouped into respective clusters. It should be noted that this scheme constructs a dynamic backbone as an accessory of cluster formation based on the reversed paths of the cluster formation message propagation paths, and no extra overhead is incurred. The backbone consists of CH, GW, and CEXT nodes which are responsible for collecting the sensor reading data and propagating the control messages from the sink to the entire WSN. During the operation of this scheme, the CH could decide whether its cluster should be split or not when it detects the dissimilarity among sensor nodes is enlarged. If it is, the GW of the cluster will initiate a new local cluster construction phase to regroup these sensor nodes into several clusters.