## 2 System Model and Assumptions  
As WSNs are very much application dependent, first of all we like to assert the types of WSNs applications for which the research applies. There are different types of application classifications for WSNs. One of the possible classifications of WSNs applications distinguishes applications according to the type of data that must be gathered in the network. Almost any application, in fact, could be classified into two categories: event detection (ED) and spatial process estimation (SPE) [1]. In ED, sensors are deployed to detect an event, for example a fire in a forest, a quake, _etc_. [2, 3, 4], whereas in SPE the WSNs aim at estimating a given physical phenomenon (e.g., the atmospheric pressure in a wide area, or the ground temperature variations in a small volcanic site), which can be modelled as a bi-dimensional random process. In this case, the main issue is to obtain the estimation of the entire behaviour of the spatial process based on the samples taken by sensors that are typically placed in random positions [5, 6, 7, 8].  
Now, let us describe the basic system model. This is a very familiar system model, and almost any application of event detection (ED) category or spatial process estimation (SPE) category can use this model. Moreover, the same model has been used by many researchers in designing various protocols for WSNs [9, 10, 11, 12, 13]. This system model has been used throughout this paper.  
Wireless sensor networks are very much application and system model dependent. Algorithms/protocols that are designed based on one system model usually do not produce the same results or show the same effectiveness when they are applied on another system model without modification. Thus, it is important to define the system model before presenting any algorithm/protocol/architecture. The assumptions of the basic system model, which would be used throughout the paper, are described below:  
* Assume a large-scale WSN. Large-scale WSNs consist of hundreds to thousands of nodes [14, 15].
* They link the physical world to global communication networks for a broad set of applications. Assume a target field, where a large number of wireless sensor nodes are deployed randomly. The deployed sensors sense the data and send it to the base station periodically. The base station is located outside the target field (see Figure 1).
* Both the base station and the sensors are stationary after deployment.
* All sensor nodes have limited battery power, and recharge of the batteries is impossible. Efficient energy-aware protocols are thus required for energy conservation.
* All the sensors are homogeneous. They have the same initial power and communication and processing capabilities.  
Figure 1: A system model of WSNs.  
* All the sensors have limited sensing range. However, the sensors have the ability to control the transmission range depending on the distance between a sensor node and its next-hop node.
* The radio channel is symmetric such that energy required for transmitting a message from a sensor node A to another sensor node B is the same as the energy required for transmitting a message from the sensor node B to the sensor node A.
* All sensor nodes are sensing the environment at a fixed rate, and thus always have data to send to the end user.