## 2 Related Works and Contribution
### Time-Driven Cluster-Head Rotation Methods  
A distributed algorithm for organizing sensor nodes into clusters was introduced in the hierarchical routing protocol called LEACH (Low-Energy Adaptive Clustering Hierarchy) [15]. According to this protocol, the CH role is randomly rotated among the sensor nodes to not drain the battery of a single sensor node. New CH node is selected after a certain time (round), which is determined a priori. The CH node creates TDMA (Time Division Multiple Access) schedule. Thus, the sensor nodes send their data in time slots allocated by the CH node. The CH node collects data and sends them to the base station. Operation of the LEACH protocol is divided into rounds and each round consists of two phases: set-up phase and steady phase. In set-up phase the new CH is selected and sends an advertisement, which includes information about its role. In the steady phase data transmission begins and sensor nodes send their data to the CH node according to allocated TDMA slots. LEACH allows the CH nodes to perform local aggregation of data in each cluster to reduce the amount of data transmitted to the base station. In centralized version of the LEACH protocol (LEACH-C) [16; 17] each node sends information about its current location and residual energy level to the sink, which uses a centralized clustering algorithm to select CH nodes. The sink computes average node energy, and the sensor nodes with energy below this average cannot be selected as CH for the current round. This method aims at evenly distributing the cluster-head nodes throughout the network. However, the centralized approach requires additional data transmissions form sensor nodes and involves a complex clustering procedure, which must be performed by the sink. LEACH-D [18] is a modification of the original LEACH protocol, which assumes that after initialization of the network, the sensor node with the shortest distance to center of the cluster becomes CH. After each round, the closest neighbor of current CH node is selected as a new CH, provided that its residual energy is more than a threshold. Otherwise, the same condition is verified for the next closest neighbor. In case when there is no neighboring sensor node with residual energy higher than a threshold, the sensor node with minimum distance to the current CH is selected as the new CH. Fixed clusters of sensor nodes are considered in LEACH-F [19]. In this approach the clusters are formed once, using the centralized clustering algorithm developed for LEACH-C. The sensor nodes within a cluster are numbered and the CH position rotates among these nodes according to the round-robin method. It means that the first sensor node becomes CH for the first round; the second node is CH in the second round, and so forth. This protocol eliminates the communication overhead of the cluster formation procedure at the beginning of each round. However, it is not suitable for dynamic networks. In HEED (Hybrid Energy-Efficient Distributed Clustering) protocol [20] the new CH nodes are also selected after a predetermined time, corresponding to duration of one round, which is similar to the methods discussed above. However, in case of HEED the CHs are selected by taking into account the amount of residual energy. For each sensor node the probability of becoming CH is proportional to its battery level. The distributed CH selection procedure requires only local information about neighboring sensor nodes, but involves considerable energy expenditure to periodically rebuild clusters. Another time-driven method is ANTCLUST [21]. In this method the CH nodes are selected at the beginning of each round, as in previous approaches. To improve the network lifetime ANTCLUST selects the new CHs according to battery level and distances to neighboring sensor nodes. A specific feature of this method is the application of the clustering algorithm based on a model of the chemical recognition system of ants.