# Survey of Smart Parking Systems
## 3 Types of SPS Reported in the Literature
### Crowdsourcing in Intelligent Transport Systems  
CITS, as so coined by [PERSON] and [PERSON] [37], is a recent type of SPS which establishes crowdsourcing as an alternative to developing ITS applications. The purpose of this type of system is to use the community of motorists with a smart mobile phone (crowd) to collaborate and feed data to the SPS in real time and almost real time. The motivation is to provide this data to the system without the need to deploy sophisticated sensors on the roads or complex and expensive communication devices inside the vehicles. This is the main difference from the rest of the SPS classified.  
With this type of SPS, the data are collected mostly by a large number of users [38]. This term is used when users of various types of systems are the main engines feeding the databases, rather than using sensors to identify and manage various statistics, events, and parking availability. In CITS, these tasks, together with others, are performed by the crowd, that is to say, by the active users of the system, i.e., any driver or passenger of a vehicle can send the system information about the status of the parking slots, in different ways such as a voice recording [37], GPS [39], or both [40]. The rest of the users, according to their interests, receive this localized information. In addition, it is possible for the system to collect information from users passively, without direct user action. By using geo-positioning data obtained directly from users' smartphones or Google Maps, these data can be processed using distance and speed measurements from an origin and, thus, a vehicle can be determined as parked [37].  
CITS implements client-server communication, where the active users use geo-positioning data about the state of parking slots sent to driver and client devices. These users are called \"smart parkers\" [41] and are those who can send information to the system by answering questions about the availability of parking slots. The other users will receive this information to employ in their decision-making process. It is also possible to use smartphone sensors, such as an accelerometer and/or gyroscope, to detect when a driver parks their vehicle in a certain position. To this purpose, all drivers must have the application installed and activated when driving to a parking place. The information captured is transmitted dynamically [42]. The success of such systems depends largely on the density of users using their mobile devices with the application.  
CITS architecture is composed of three parts: interface, information processing, and localized device messaging. Interface refers to the application that is used by drivers in which the status of car parks and spaces sent by the crowd is displayed. Information processing refers to the system-processing server where tasks such as information retrieval and storage, logging and authentication, and localized messaging to users' mobile devices are executed. In addition, additional services can also be developed, e.g., archiving and forecasting of traffic, verification of information, statistics, etc. Localized device messaging refers to the strategy established for sending notifications to system users. The push mechanism is one such example and is energy-efficient with less-overloaded mobile phone communications.