## 5 Analysis of Prominent Clustering Routing Protocols in WSNs
### Cluster-Construction Based Clustering Routing Protocols
#### 5.1.4 Panel  
Position-based Aggregator Node Election protocol (PANEL) [17,18], presented by [PERSON] and [PERSON], is a position-based clustering routing protocol for WSNs. With respect to other CH election protocols, PANEL supports asynchronous sensor network applications where the sensor node readings are fetched by the BSs. The main goal of PANEL is to elect aggregators, _i.e._, CHs, for reliable and persistent data storage applications. PANEL assumes that the nodes are deployed in a bounded area, which is partitioned into geographical clusters. The clustering is determined before the deployment of the network, and each node is pre-loaded with the geographical information of the cluster to which it belongs. PANEL introduces a notion of reference point. At the beginning of each epoch, a reference point \(R_{j}\) is computed in each cluster \(j\) by the nodes in a distributed manner in terms of the epoch number, as follows:  
\[\vec{R}_{j}=\vec{Q}_{j}+\vec{Q} \tag{4}\]  
where \(\vec{Q}_{j}\) is the position of the lower-left corner of cluster \(j\). Furthermore, the current epoch number \(e\) is known by every node and the computation consists in calling a pseudo-random function \(H\)(e) that maps \(e\) to a relative position \(\vec{Q}\) inside the cluster, _i.e._,:  
\[H(e)=\vec{Q} \tag{5}\]  
where \(\vec{Q}\in(-\delta d,d+\delta d)\times(-\delta d,d+\delta d)\), \(d\) is the size of the cluster, and \(\delta<1\) is a parameter which expresses the magnitude of this re-sizing operation in percent of the original cluster size \(d\). Once the reference point is computed, the node that is the closest to the reference point will be elected the CH for the given epoch. The reference points of the clusters will be re-computed and the CH election procedure will be re-executed in next epochs. This CH election procedure ensures load balancing in PANEL because each node of the cluster can become CH with almost the same probability. The illustration of the geographical clustering in PANEL is shown in Figure 4.  
Figure 4: Illustration of the Geographical Clustering in PANEL. The CH election procedure needs intra-cluster communications. PANEL takes advantage of these communications to establish routing tables for intra-cluster routing. Especially, at the end of the CH election procedure, the nodes also are conscious of the next hop towards the CH elected for the current epoch. Moreover, a position-based routing protocol is introduced in PANEL. The intra-cluster routing is used to route a message to the aggregator of a given cluster if that messages is already inside the cluster. The intra-cluster routing of PANEL takes advantage of the fact that the nodes within the cluster communicate during the aggregator election procedure. The following are the main merits of PANEL: (1) This protocol is an energy-efficient protocol that ensures load balancing because each node is elected aggregator, _i.e._, CH, nearly equally frequently. Besides, data aggregation is performed and communication load is reduced, accordingly PANEL can prolong the network lifetime; (2) The outstanding feature of PANEL that makes it different from other data-aggregation based clustering protocols is that besides synchronous scenes, it also supports asynchronous applications. The main limitations of PANEL are discussed as follows: (1) The assumption that the clusters are determined before deployment and thus cannot be applied to WSN dynamics; (2) Geographical position information of the nodes is used to determine which node should be the aggregators. This is a restriction in WSNs, because the geographical position is not always available without special condition, such as GPS-like hardware and software; (3) A crucial assumption of PANEL, described by the authors of PANEL, is that the nodes within a cluster form a connected sub-network. If this assumption is not satisfied, and the sub-network within a cluster is partitioned, then some nodes will not hear the announcement of the node closest to the reference point, and they will elect another node as aggregator.