## 2 Literature Review
### Wireless Sensor Network Challenges  
(1) _Network scalability:_ the nature of WSN based on a widespread deployment of sensors to cover the largest possible area for monitoring. This makes the whole system very sensitive to failure [20]. To diminish this challenge, the network coverage should be examined to ensure a high Quality of Service (QoS). (2) _Energy management_: the energy (power) is the biggest limitation in any wireless sensors capabilities. Power is one of the main reasons that sensors are subject to failure due to depletion of batteries [21]. Sensors are created to work autonomously for prolonged periods of time in months or years after deployment task. It is not easy to recharge or replace the sensors batteries [22]. Therefore, many aspects that affect the energy management should be examined to minimize the energy consumption of the sensor battery. This can be achieved by examining some issues of the physical layer and protocol layer of the network. (3) _Limited storage and memory:_ the storage in any sensor mostly has the range from 32 KB to 2 GB while the memory (RAM) has the range from 2 KB to 256 KB. This limitation affects the throughput of the sensors [23]. Table 1 lists some available sensor nodes along with their respective storage and memory characteristics [5].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Platform** & \
\begin{tabular}{c} **Microcontroller** \\ **Unit (MCU)** \\ \end{tabular} & **RAM** &
\begin{tabular}{c} **Program and Data** \\ **Memory** \\ \end{tabular} & **Radio Chip** \\ \hline BTnode3 & ATMega128 & 64 KB & 128–180 KB & CC1000/[PERSON] \\ Cricket & ATMega128 & 4 KB & 128–512 KB & CC1000 \\ Imote2 & Intel PXA271 & 256 KB & 32–MB & CC2420 \\ MICA12 & ATMega128 & 4 KB & 128–512 KB & CC1000 \\ MICAZ & ATMega128 & 4 KB & 128–512 KB & CC2420 \\ Shimmer & TI MSP 430 & 10 KB & 48 KB-UP to 2 GB & CC2420/Bluth \\ TelosA & TI MSP 430 & 2 KB & 60–512 KB & CC2420 \\ TelosB & TI MSP 430 & 10 KB & 48 KB–1 MB & CC2420 \\ XYZ & ARM 7 & 32 KB & 256 KB & CC2420 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Available sensor nodes along with their storage and memory characteristics [5].  
4. _Delay of data aggregation_: this challenge is crucial in many WSN applications [24], particularly when dealing with critical data that should be received without any delay. Examples of these data are heart pulses and electrocardiograms of patients [25], disaster detection alarms [26] and power supply requests in smart grids [27].
5. _Interference and fading_: wireless devices mostly operate on license-free bands such as 2.4 GHz ISM band [28]. There are many other devices operate in the same frequency band such as microwave ovens and Wi-Fi routers. This makes the system vulnerable to interference and intrusion by those devices that work on this band [29; 30]. Based on these issues, the network topology planner should keep these aspects in mind.
6. _Security_: the wireless medium is open and accessible to anyone rather than the wired one. This makes transmissions over the wireless medium easily altered, replayed, or intercepted by an adversary. In addition, the intruders may have strong transmitters to block transition that comes from other devices by transmitting many packets through the network to make the network busy. The conflict may be occurred because of packet collaging through the transfer time, which leads to network failure. These issues should be solved using a load balancing technique [31].