## 2 Related Work  
In recent years, quite a lot of articles have been published describing new algorithms, routing protocols and architectures aiming at WSN lifetime maximization through energy awareness. In this section, we provide a brief overview of some related research work. The LEACH protocol [8] is the seminal protocol for both the class of hierarchical clustering protocols and proactive protocols. LEACH protocol defines the concept of round and operates in  
Soft threshold value: The sensed value is transmitted to the cluster head if there is a change compared to the previous sensed value that is greater than the soft threshold value. The nodes continuously sense their environment. When a value greater than the threshold value is sensed, it is stored in an internal variable in the node. In the subsequent sensing activities, it is checked whether the sensed value has changed by an amount greater or equal to the soft threshold value. If so, the data is transmitted to the respective cluster head. Our protocol also uses threshold values, but their use is different than in the TEEN protocol. We have a base threshold which is used to filter out the undesired data and in this way reduce the transmissions to the cluster heads (_i.e._, filtering is done at the sensor nodes). We also have a _critical threshold_ that is used to expedite the delivery of the data form the cluster heads to the sink node. The TEEN protocol's thresholds are not used in the same way as they are used in our protocols. Additionally, we use filtering via a function (like XOR) in the cluster heads to reduce the amount of data sent by cluster heads, not only by the sensor nodes. Therefore, our protocols are different from the TEEN protocol. The authors in [12] proposed a hierarchical reactive routing protocol called SHPER which achieves energy conservation by using both an energy efficient routing strategy and a power aware route selection scheme. This protocol specifies that the election of the cluster heads is not randomized. More precisely, the node elected to be the cluster head within each cluster is the one having the maximum residual energy. Furthermore, the route selection policy proposed takes into consideration both the residual energy of nodes and the energy consumption for all possible paths. In [13], the authors proposed a protocol called EAP. In EAP, a node with a high ratio of residual energy to the average residual energy of all the neighbor nodes in its cluster range will have a large probability to become the cluster head. EAP clusters sensor nodes into groups and builds routing tree among cluster heads for energy saving communication. In addition, EAP introduces the idea of area coverage to reduce the number of working nodes within a cluster in order to prolong network lifetime. In [14], the authors suggested an energy efficient heterogeneous clustering scheme which they call EEHC. According to this scheme, selection of cluster head is determined taking the weighted selection probability in terms of the residual energy in each node into consideration. In [15], the authors proposed an algorithm called EECH similar to EEHC, in which there is a higher probability for a node with more energy to be selected as cluster head than a node with less energy. Furthermore, cluster heads are set to use multi-hop forwarding and routing when they would like to communicate with the base station. [PERSON] _et al._[16] proposed two power efficient data gathering and aggregation protocols based on minimum spanning tree routing scheme, called PEDAP and PEDAP-PA.The basic idea is to minimize the total energy expended in a round of communication while balance the energy consumption among sensors. Simulation results show that PEDAP and PEDAP-PA perform better than LEACH and PEGASIS.