Wireless Ad Hoc and Sensor Networks

Distributed Fair Scheduling in Wireless Ad Hoc and Sensor Networks 341be met. The net result is an energy-efficient fair MAC protocol for WSNs.The performance of this MAC protocol is demonstrated via simulation,which, in turn, demonstrates that the energy-aware protocol results inenergy savings during channel uncertainties for different node densitiesand traffic patterns, while being fair. Comparison of this protocol withthat of the 802.11 is also included in the simulation.7.6.1 Sleep ModeThe CHs in a WSN remain powered all the time. They can accept anincoming packet from the sensors or data forwarded by other nodes (CHs,BS, etc). Thus, CHs consume energy. At the same time, sensor nodes saveenergy by turning off their computational and RF circuits. To maximizelifetime of the network, nodes volunteer as CHs.7.6.1.1 Sensor Data TransmissionWhen there is any data to be reported to the BS or not, the sensing circuitof a sensor node wakes up the processing and RF circuitry. The sensornode transmits the data to its CH and switches itself to the sleep modeafter transmission. At the same time, the wake-up timer is set to thepredefined interval. Thus, when the sensor does not wake up during thistime instant (e.g., there is no event detected), the timer will expire, and itwill wake up the sensor. Next, the sensor will communicate to its CH tocheck if there is any incoming packet waiting for delivery. Afterwards,the sensor again switches into sleep mode by setting the timer.7.6.1.2 Delivery of Data to a Sensor NodeThe communication between a sensor and its CH is always initiated bythe sensor during its wake state. The CH never starts the transmissiontoward a sensor. Instead, the CH buffers packets to its sensors and waitsfor the sensors to request packets from the CH. When the sensor sends aMAC frame, the CH checks the buffer for any packet to be delivered tothis sensor node. If the packets are found, the CH notifies the sensor andpiggybacks the packets to the ACK frame so that the information isreceived by the sensor. In consequence, sensor nodes do not have to listenfor any incoming traffic and can be in sleep mode.In summary, the sensors wake themselves up and communicate withthe CH in two ways:• Event occurrence: A sensing circuit will wake up the sensor toprocess and send data towards the CH.• Periodic wake up: It is to enable incoming communication duringlong idle intervals.