## 5 Analysis of Prominent Clustering Routing Protocols in WSNs
### Cluster-Construction Based Clustering Routing Protocols
#### 5.1.9 Ace  
Algorithm for Cluster Establishment (ACE) [24], presented by [PERSON] and [PERSON], employs an emergent algorithm, which is any computation that achieves formally or stochastically predictable global effects, by communicating directly with only a bounded number of immediate neighbors and without the use of central control or global visibility. One of the main distinguishing characteristics of emergent protocols over other localized protocols is the existence of feedback during protocol operation.  
The main idea of ACE is to allow a node to assess its potential as a CH before becoming one and retire if it is not the best CH at the moment. The algorithm works in iterations that do not have to be synchronized at the individual nodes. ACE has two logical parts: the spawning of new clusters and the migration of existing clusters. When a node decides to become a CH, it spawns of new cluster by broadcasting an invitation message to recruit its neighbors. Upon getting the invitation, a neighboring node joins the new cluster and becomes a follower of the new cluster. At any moment, a node can be a follower of multiple clusters while the protocol is running. However, the node can be a loyal follower, _i.e._, a member which belongs to only one cluster. Migration is a process in which the best candidate for being CH is elected again. Each CH will periodically check the ability of its followers to determine which is the best candidate for the new leader of the cluster. The CH will retire if one of these followers has more followers than it does. The node, considered as the best candidate for CH, would have the largest number of followers while minimizing the amount of overlap with existing clusters. Once the best candidate is determined by the current CH, it will assign the best candidate as the new CH and abdicate its position as the old CH. Thus, the position of the cluster will appear to migrate in the direction of the new CH. Accordingly, some of the former followers of the old CH do not belong to the new cluster, while some new nodes near the new CH become new followers of the new cluster.  
The characteristics and advantages of ACE are summarized as follows: (1) ACE is an emergent algorithm that uses feedback to induce the formation of a highly efficient cover of uniform clusters over the network; (2) Minimizing the number of CHs would not only generate efficient cover of the whole network but also minimizes the cluster overlaps. This also improves the efficiency of the algorithms that executes at the level of the CHs; (3) ACE is very robust because it can easily repair structure damage caused by node failure and can also integrate new nodes in the network.  
Nevertheless, there are several limitations in ACE as follows: (1) The important factor, energy, is not considered during the process of CH election. That is to say, those nodes with low residual energy may be elected CHs and work until death; (2) It is hard to decide the number of iterations for cluster formation while satisfying the communication cost requirements and energy consumptions; (3) It is obvious that migratory mechanism needs a large amount of information exchange among sensor nodes, thus this protocol adds additional overheads.