## 5 Analysis of Prominent Clustering Routing Protocols in WSNs
### Cluster-Construction Based Clustering Routing Protocols
#### 5.1.6 Ucs  
Unequal Clustering Size (UCS) model [20] was proposed by [PERSON] and [PERSON] for network organization in order to balance energy consumption of CHs, thus increasing the network lifetime. UCS is the first unequal clustering model for WSN organization. It is assumed that the positions of the CHs are determined _a priori_, with all CHs arranged symmetrically in concentric circles around the BS which is located in the center of the network, thus it's easy to control the actual sizes of different clusters.  
In UCS, the sensing field is assumed to be circular and is divided into two concentric circles, called layers. In order to simplify the theoretical analysis, the authors approximate the sensing field as pie shaped field with a multiple-layer network, shown in Figure 6. It is assumed that all clusters in one layer have the same size and shape, but the sizes and shapes of clusters in the two layers are different. The position of a CH within the cluster boundaries determines the overall energy consumption of nodes that belong to the cluster. To keep the total energy dissipation within the cluster as small as possible, every CH should be positioned at the center of the cluster. CHs are deterministically deployed in the network and are assumed to be super nodes which are much more expensive than MNs.  
Figure 5: The Two-level Hierarchy in TL-LEACH.  
By varying the radius of the first layer around the BS, while assuming a constant number of clusters in every layer, the area covered by clusters in each layer can be changed, and accordingly the number of nodes contained in a particular cluster can be changed. Data transmission is done through multiple hops, where every CH chooses to forward its data to the closest CH in the direction of the BS.  
The advantages of UCS are discussed as follows: (1) By changing the number of nodes in every cluster with respect to the expected communication load, UCS can maintain more uniform energy consumption among the CHs. Therefore, the total energy dissipated for every CH is similar and UCS can prolong network lifetime compared with the model of Equal Clustering Size (ECS); (2) Using the two-layered network model and two-hop inter-cluster communication method, UCS results in a shorter average transmission distance compared with LEACH, thus effectively reduces the total energy consumption.  
However, there exist a few limitations in UCS as follows: (1) UCS is constrained by the assumption that the network is heterogeneous, and CHs are performed by super nodes all the time and are deployed at pre-determined locations. That is to say, it lacks universality [68]; (2) CHs are required to locate in the center of the cluster, thus a key factor, residual energy of nodes, is not considered in UCS; (3) Similar to TL-LEACH, despite that the average transmission distance is decreased in comparison with LEACH, the two-hop inter-cluster routing of UCS is still not applicable to large-range networks, because it uses only two hops for data transmission from sources to the BS, and long-distance communications need much energy consumption.