## 5 Use Cases
### Route Assistant for Travelers with Special Needs
Performance of the Route Assistant System  
For this type of service, in which users use their mobile phones to access the service, the most influential aspect for system performance is communications. Performance analysis has therefore focused on this aspect. This analysis has consisted in studying the following aspects of the communications system: bandwidth consumed to run the service, degree of mobility supported and battery consumption on mobile user terminals. * Bandwidth required by the protocol. The protocol feature that most affects the bandwidth consumed is the length of the frames. The protocol establishes a frame structure whose data field is of variable length, the length depending on the current state of the vehicle. The most demanding scenario arises when the vehicle is on a line service, in which case the frames may have a data field with a length of 256 bytes and a total length of 263 bytes. Taking the routes used by the transit company used to evaluate the system as our reference, the maximum number of frames required to represent the state of a vehicle when it is operating a line service is three frames. Considering the available bandwidth of the Wi-Fi infrastructure (802.11 g), with speeds in open outdoor environments ranging from 54 Mbit/s in a radius of 75 m to 6 Mbit/s in a radius of 400 m, these three frames represent a very small percentage of bandwidth. * Mobility. 802.11 technology was designed to operate in mobile environments in which the mobile stations do not move at speeds above 10 km/h, which is the speed of a person walking. Strictly speaking this is called roaming and not mobility. Because the speed at which a vehicle approaches a stop is variable and depends on various factors, such as traffic or road type, the behavior of the communications system when considering this aspect varies. To obtain precise data on this aspect of performance, we analyzed the behavior of the communications system when the vehicle approaches a stop located at the main station used by the company that collaborated in the development of this system. This station is located in an urban environment with highly variable traffic on the roads depending on the type of day (working or holiday) and the time of day; for example, traffic is heavy on a business day from 07:00 to 22:00. The results show that transmission error rate oscillates between 5% and 10%. * Battery consumption. The efficient use of energy is a key technology challenge for any mobile communications technology. In our case, considering the currently available technologies on mobile devices--UMTS (3G or 4G), IEEE 802.15 (Bluetooth) and IEEE 802.11 g (Wi-Fi)--Wi-Fi technology was chosen because it has a greater range and bandwidth than Bluetooth. The choice of Wi-Fi over UMTS was due to power consumption. In both technologies one of the aspects that most affects power consumption is the registration of mobile terminals at infrastructure access points, due to fluctuations in the strength of the signal owing to the mobility of the mobile terminal. These registration changes trigger an exchange of control packets from the respective protocols and fluctuations in signal strength when sending the packages. In the topology of the proposed system's communications network, the Wi-Fi access points are located in the vehicles. Therefore, from the perspective of signal quality and assuming that the user mostly uses the service when aboard a vehicle, this is the ideal situation: the maximum distance between a mobile terminal and the access point will be the length of the vehicle, which in the case of public transit buses is no longer than 20 m, and the vehicle contains no elements (walls) that attenuate the signal. However, in the case of UMTS access, mobile terminals would use various points of infrastructure access (cell registration) that would cause registration changes and fluctuations in the terminal transmission strength, thus causing the mobile terminal to consume more energy.