# An Intelligent Parking Management System for Urban Areas
## 3 Description of the System
### System Architecture
#### 3.1.1 Sensor Nodes
To count vehicles entering and leaving the car park, two photoelectric sensors were used to avoid false positives. If a person or an object other than a vehicle interrupts the sensor IR beam, it could be incorrectly counted, but using two sensors and debugged controlling software, the count will not increase unless the pair of sensors actually detect a vehicle. This pair of sensors is fitted on speed bumps located on the entry and exit roads to the parking zones, as shown in Figure 5.  
Figure 4: SHARP-GP2Y0D02 YK0F sensor alongside a standard AA battery to give an idea of its size. Figure 3: Block diagram of the sensor board. To develop the software for the sensor nodes we used the Contiki [29] operating system and the specific library for the CC2530 system-on-chip. Contiki is an open-source operating system geared towards the Internet of Things. It enables communication with microcontrollers via the Internet and can run on a wide range of wireless low-power devices, providing mechanisms to estimate the total energy consumption of the system and to identify the units that consume the most. In addition, Contiki is designed for small systems with only a few kilobytes of memory available; it is very efficient at memory management and provides different memory allocation mechanisms. With regard to communications, Contiki is fully compatible with the IPv6 and IPv4 standards, providing a full IP network stack, with standard protocols such as User Datagram Protocol (UDP), Transmission Control Protocol (TCP), and Hypertext Transfer Protocol (HTTP). Furthermore, it also supports low-power wireless communications on the 6 LoWPAN, Routing Protocol for Low-Power and Lossy Networks (RPL) and CoAP standards. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sensing Mode** & \multicolumn{2}{c}{**Advantages**} & \multicolumn{2}{c}{**Disadvantages**} \\ \hline  & & & - & The range depends on the characteristics of the object (colour, reflectivity, _etc._). \\ \multirow{3}{*}{DIFFUSE} & \multirow{3}{*}{-} & Ideal for short-range applications. & - & A highly reflective \\  & & Does not need a reflector. & & background may cause false \\  & & Easy installation/alignment. & & triggering of the sensor. \\  & & & - & Relatively short \\  & & & sensing range. \\ \hline \multirow{3}{*}{RETROREFLECTIVE} & \multirow{3}{*}{-} & Moderate sensing range. & - & Shorter sensing range than \\  & & Easy to align. & & through-beam. \\ \multirow{3}{*}{Reflector} & \multirow{3}{*}{-} & Requires assembly and & - & May detect reflections from \\  & & wiring of only one & & shiny objects. \\ \multirow{3}{*}{Object} & \multirow{3}{*}{-} & emitter/receiver unit. & - & Requires reflector. \\ \hline \multirow{3}{*}{THROUGH-Beam} & \multirow{3}{*}{-} & High margin for & - & Requires proper alignment. \\  & & contaminated environments. & - & Not recommended for \\ \multirow{3}{*}{Emitter} & \multirow{3}{*}{-} & Longer sensing range than & & detection of \\  & & other technologies. & & transparent objects. \\ \multirow{3}{*}{Object} & \multirow{3}{*}{-} & Most reliable sensing mode & - & Requires space to mount and \\  & & for highly reflective objects. & & wire the emitter and \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Standard sensing modes of the sensor. Figure 5: Installation of the sensors in two speed bumps (**left**) photoelectric sensor located at the centre of the speed bump; (**Right**) Pair of speed bumps with the photoelectric sensors.