## 5 Analysis of Prominent Clustering Routing Protocols in WSNs
### Cluster-Construction Based Clustering Routing Protocols  
#### 5.1.1 Leach  
Low-Energy Adaptive Clustering Hierarchy (LEACH), proposed by [PERSON] _et al_. [14], is one of the pioneering clustering routing approaches for WSNs. The basic idea of LEACH has been an inspiration for many subsequent clustering routing protocols. The main objective of LEACH is to  
Figure 1: Taxonomy of Clustering Methods in WSNs. select sensor nodes as CHs by rotation, so the high energy dissipation in communicating with the BS is spread to all sensor nodes in the network. The operation of LEACH is broken up into lots of _rounds_, where each round is separated into two phases, the set-up phase and the steady-state phase. In the set-up phase the clusters are organized, while in the steady-state phase data is delivered to the BS. During the set-up phase, each node decides whether or not to become a CH for the current round. This decision is based on the suggested percentage of CHs for the network and the number of times the node has been a CH so far. This decision is made by the node choosing a random number between 0 and 1. The node becomes a CH for the current round if the number is less than the following threshold:  
\[T(n)= \begin{cases}\dfrac{P}{1-P\left(r\bmod\dfrac{1}{P}\right)},&\text{if }n \in G\\ 0,&\text{otherwise}\end{cases}  \tag{1}\]  
where \(P\) is the desired percentage of CHs, \(r\) is the current round, and \(G\) is the set of nodes that have not been elected CHs in the last \(1/P\) rounds. When a node is elected CH successfully, it broadcasts an advertisement message to the other nodes. According to the received signal strength of the advertisement, other nodes decide to which cluster it will join for this round and send a membership message to its CH. In order to evenly distribute energy load among sensor nodes, CHs rotation is performed at each round by generating a new advertisement phase based on Equation (1). During the steady-state phase, the sensor nodes sense and transmit data to the CHs. The CHs compress data arriving from nodes that belong to the respective cluster, and send an aggregated or fused packet to the BS directly. Besides, LEACH uses a TDMA/code-division multiple access (CDMA) MAC to reduce inter-cluster and intra-cluster collisions. After a certain time, which is determined a priori, the network goes back into the set-up phase again and enters another round of CH election. Figure 2 showed the basic topology of LEACH. LEACH is a completely distributed approach and requires no global information of network. In the literature, various modifications have been made to the LEACH protocol, which form LEACH family, such as TL-LEACH [19], E-LEACH [56], M-LEACH [57], LEACH-C [58], V-LEACH [59], LEACH-FL [60], W-LEACH [61], T-LEACH [62], _etc_. The advantages of LEACH include the  
Figure 2: The Basic Topology of LEACH. following [63]: (1) Any node that served as a CH in certain round cannot be selected as the CH again, so each node can equally share the load imposed upon CHs to some extent; (2) Utilizing a TDMA schedule prevents CHs from unnecessary collisions; (3) Cluster members can open or close communication interfaces in compliance with their allocated time slots to avoid excessive energy dissipation. However, there exist a few disadvantages in LEACH as follows. (1) It performs the single-hop inter-cluster, directly from CHs to the BS, routing method, which is not applicable to large-region networks. It is not always a realistic assumption for single-hop inter-cluster routing with long communication range. Besides, long-range communications directly from CHs to the BS can breed too much energy consumption; (2) Despite the fact that CHs rotation is performed at each round to achieve load balancing, LEACH cannot ensure real load balancing in the case of sensor nodes with different amounts of initial energy, because CHs are elected in terms of probabilities without energy considerations.