## 4 Description of the System
### Incorporation of New Components
#### 4.2.1 On-Board Computer (OBC)
To provide services that require sensors that the vehicle infrastructure does not possess, the OBC has different communication interfaces (serial ports, USB, Ethernet, Bluetooth). For example, one of the cases of service developed for this system consists of a bus stop video surveillance system. For this reason a video-capture device has been connected to the OBC, as described below. Finally, in order to provide a silent power supply that is within the voltage range required by the OBC, an electrical power backup unit (UPS) is connected in series between the vehicle battery and the OBC. The UPS isolates the OBC from micro-outages that occur particularly when starting the vehicle. The key components of the infrastructure providing required data to the proposed system are: the communications system, the positioning system and the vehicle sensors. In the proposed system, the transit company that collaborated in its development, Empresa de Transporte Global Salcai-Utinsa S.A., uses:  
* Communications system: UMTS technology (3G) for short data communications relating to operations and alarms control events in the vehicles. Wi-Fi technology for data communications that require higher bandwidth, e.g., to transfer production data generated in the vehicles, synchronize the databases stored on the vehicles or even for software updates run on the vehicle. These data communications are performed using the existing Wi-Fi infrastructure at certain points on the transport network, in stations and garages, where vehicles are frequent and stop for a set period of time. * Positioning system: GPS system. All vehicles are equipped with a GPS receiver that provides the position, speed and a precise clock signal to synchronize all the clock signals used by the various vehicle devices (driver console and contactless card readers). * Vehicle sensors: sensors to control vehicle temperature, open-door sensors to determine the state of the vehicle doors (open or closed) and cameras to view the vehicle's passengers enter and exit. STH threads follow a standard operating procedure:  
Phase 1: Initiation of service. Checking that the connection to the infrastructure is available and that it can provide the data required by the service. This check is performed by accessing the shared data area and verifying the values contained within these data. The CTH thread is responsible for extracting the values from the protocol frames and storing them in the shared variables. If both checks are successful, the service initiates the data. Phase 2. Announcement of service. In this phase, the service, an STH thread, announces its availability so that the discovery process undertaken by the client applications (ACli) can detect that the service is available. The data required by a service have a variable structure that depends on the service, but all have two common data: the service identifier and a connection address to be used by client applications to connect to the service. A daughter thread, called an Available Services Server (ASS), is responsible for communicating the available service connection data to the ACli applications. Phase 3. In this phase, the service, an STH thread, waits for connections with the ACli applications running on mobile user devices. For each connection established, the STH thread creates a daughter thread which responds to the ACli application by sending the data required by the service using the protocol. This communication is periodical and the duration of the period depends on the service provided, e.g., a period of five seconds has been established for a \"route assistant\" service, while for a \"payment system\" service, this period is 500 ms. The ACli application also follows a standard operating procedure:  
Phase 1. Service discovery. In this phase, the ACli application tries to obtain the connection data from the required service. It does this by trying to establish a connection with the available services announcement system and, more specifically, with the ASS thread.