It is mainly concerned with receiving data from other sensor nodes. It can be part of the wireless sensor network or an external device such as a laptop or PDA. Typically there is only one base station, but depending on the application multiple base stations are possible in a network. Sensory information is communicated to the base station via hop-by-hop wireless transmissions. Source-sink interaction models include the following: Event detection: If a certain event occurs, sensor nodes report the measurement of that event to interested sinks. This is called event detection in WSN. Periodic measurement: It is necessary to periodically report events to interested receivers to obtain the latest data on the event. Tracking: This feature of the wireless sensor network reports the location of an observed intruder in the network. As can be understood, most of the data from the sensor network will be directed to the sink and this transmitted data will pass through many intermediate nodes. before reaching the well that is interested in the data. Communicating data directly from the node to the receiver will be very expensive in energy, and also very expensive, which is why multi-hop communication is preferred in a wireless sensor network. Fig1.3: Basic example of a path from the node to the receiver