As shown in Figure 1, the state transition diagram of GAF has three states, namely, discovery,
active, and sleeping. When a sensor enters the sleeping state, it turns off its radio for energy
savings. In the discovery state, a sensor exchanges discovery messages to learn about other
sensors in the same grid. Even in the active state, a sensor periodically broadcasts its discovery
message to inform equivalent sensors about its state. The time spent in each of these states can be
tuned by the application depending on several factors, such as its needs and sensor mobility. GAF
aims to maximize the network lifetime by reaching a state where each grid has only one active
sensor based on sensor ranking rules. The ranking of sensors is based on their residual energy
levels. Thus, a sensor with a higher rank will be able to handle routing within their corresponding
grids. For example, a sensor in the active state has a higher rank than a sensor in the discovery
state. A sensor with longer expected lifetime has a higher rank.