## 3 Efficient Energy Aware Routing Protocols: R-EERP and S-EERP
### The Use of the Critical Threshold and Base Threshold  
We propose the use of two threshold values to reduce the amount of data sent to the base station, but still react to emergency cases as quickly as possible. Reacting quickly is important for wireless sensor network monitoring applications used for detection of poisonous or detonating gases which may emerge slowly or suddenly in an environment due to various reasons. The two thresholds we propose are: (1) critical threshold; (2) base threshold.  
Critical Threshold: This is a critical value above which we assume that there is an emergency situation that has to be handled as quickly as possible. Hence, when the network senses a value above this threshold (_i.e._, when a sensor node reports a value above this threshold to the cluster head), the data will be sent to the sink node without being kept waiting at the respective cluster head. It is transported towards the base station as soon as possible. What the critical threshold is depends on the application. It may vary from application to application. Therefore, here we do not specify a method about how we can set those thresholds. Critical threshold is important for the rescue teams to detect the emergency situation as soon as possible so that best emergency actions can be taken in the environment without much delay. The precautions that will be taken on time may be very important for human health and life.  
Base Threshold: This is the minimum value desired to be sensed and reported to the sink node. The sensed values above the base threshold will be delivered to the base station, but there is not much hurry in delivering them. Hence the cluster head can wait such data for a while. A value below the base threshold will be ignored at the sensor nodes and will not be delivered to neither the cluster heads nor the sink node.  
The nodes constantly sense their environment. But not all sensed data is delivered across the network. The sensor nodes know the base threshold (they are configured initially to know the base threshold) and filter out all sensed data below the base threshold. Only values above the base threshold are sent to the respective cluster head nodes. Therefore, a cluster head will only receive values above the base threshold. A cluster head may receive such data (values) from one or more sensor nodes in its cluster. Each cluster head collects the incoming values between the base and critical thresholds for certain amount of time. Then each cluster head compresses (or filters out with a filtering function like XOR) the different collected values and sends them to the base station. If there is an incoming data above the critical threshold, however, it is not aggregated or compressed; it is sent to the sink node immediately. Only the values above the critical threshold are treated like that (sent immediately). The use of critical and base threshold is suitable for applications requiring periodic reports and also requiring quick response to emergence cases.  
The TEEN protocol [10] uses also some thresholds. But they are used in a different manner. The main drawback of TEEN is that, if the thresholds are not reached, the nodes will never communicate; the user will not get any data from the network at all and will not come to know even if all the nodes die. The base threshold in our protocol removes this deficiency of [10].