Energy efficiency is one of the major concerns in wireless sensor networks as the network lifetime depends on it. This article investigates the relationship between the sensor network performance, particularly its lifetime, and the number of active reporting nodes N. The analytical and simulation approaches are both used in this process. decreasing the number of reporting nodes
increases the number of reports which are to be sent to the sink if a desired information reliability about a detected event is to be achieved. If the the number of reporting nodes is reduced, we can reduce the probability of collision occurrence.

INTRODUCTION
As the sensor nodes’ batteries are a limited resource, the energy efficiency can be a major issue in the WSN. scheduling the sensor nodes’ activity so that redundant nodes enter the sleep mode as often as possible is a widely employed method for saving energy. several energy-efficient MAC protocols and energy-efficient routing protocols are available in the literature. But the specific QoS requirements of sensor applications must also be kept in mind while reducing the energy consumption. Thus the average time required to report reliably an event also becomes a key parameter in the wireless sensor network .

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