Exercise 1.11 Solution

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Consider a wireless sensor network that consists of solar- powered sensor nodes operating for 24 hours. A sensor node turns on at 0:00AM, and its initial battery energy is 1450J. The sun rises at 6AM and sets at 6PM. The solar power is 200J per hour during daytime and is zero at night. The power consumption of the sensor node is 300J per hour with 50% duty cycle. a. What is the remaining battery energy after 24 hours? Between 0 AM and 6 AM the battery is discharging for 6 hours, resulting a battery level as: 1450-(30050% hours) = 1450 – 900 = 550 J. Between 6 AM to 6 PM, the battery is charging by solar power, so the resulting battery level is: 550 J + (200- 300*50%)12 = 550 + 600 = 1050 J. Between 6 PM and 0 AM the battery is discharging again for 6 hours, resulting a battery level as: 1050 - (30050%*6 hours) = 1050 – 900 = 150 J. b. When will the sensor node die (energy becomes equal to zero)? The sensor node had 150 J of energy left which is enough for 1 hour of operation. Therefore, the sensor node will die the next day at 1 AM.
Consider a wireless sensor network that consists of MicaZ sensor nodes that are each powered by two AA batteries of 15 kJ. Each sensor node is always in Tx/Rx/LISTEN mode and consumes power 312 J per hour.

a. For how many days each sensor node will be able to work? The total power budget of a sensor node is 2*15 kJ = 30 kJ. The lifetime of a sensor node, T, is a function of the battery capacity, B, and the power consumption, P: T = B/P = 30000/312 = 96 hours = 4 days.

b. Give the answer to a. if the sensor nodes apply 10% radio duty cycling. The power consumption of the sensor node when active is 312 J per hour, while the sensor node consumes only 5 J per hour when sleeping. The total power budget of the sensor node is 215 kJ = 30 kJ. The power consumption of the sensor node when in the active mode is Pactive = 312 J per hour. The power consumption of the sensor node when in the sleep mode is Psleep = 5 J per hour. The duty cycling is D = 0. The lifetime of a sensor node, T, is: T = B/( PactiveD + Psleep*(1-D)) = 30000/(3120 + 50) = 30000/35 = 839 hours = 34 days.

Find a real-life example (using the Internet) where the following technologies are used: a. iBeacon On Korean railways, pregnant women are provided with small beacons that they can wear on their handbags.

1. Consider a wireless sensor network that consists of solar-

powered sensor nodes operating for 24 hours. A sensor node

turns on at 0:00AM, and its initial battery energy is 1450J.

The sun rises at 6AM and sets at 6PM. The solar power is

200J per hour during daytime and is zero at night. The power

consumption of the sensor node is 300J per hour with 50%

duty cycle.

a. What is the remaining battery energy after 24 hours?

Between 0 AM and 6 AM the battery is discharging for

6 hours, resulting a battery level as: 1450-(300*50%*6

hours) = 1450 – 900 = 550 J.

Between 6 AM to 6 PM, the battery is charging by solar

power, so the resulting battery level is: 550 J + (200-

300*50%)*12 = 550 + 600 = 1050 J.

Between 6 PM and 0 AM the battery is discharging again

for 6 hours, resulting a battery level as: 1050 -

(300*50%*6 hours) = 1050 – 900 = 150 J.

b. When will the sensor node die (energy becomes equal

to zero)?

The sensor node had 150 J of energy left which is

enough for 1 hour of operation. Therefore, the sensor

node will die the next day at 1 AM.

2. Consider a wireless sensor network that consists of MicaZ

sensor nodes that are each powered by two AA batteries of

15 kJ. Each sensor node is always in Tx/Rx/LISTEN mode and

consumes power 312.5 J per hour.

Computer Network

Lebanese International University

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