# Prototype to Increase Crosswalk Safety by Integrating Computer Vision with ITS-G5 Technologies
## 2 State of Art
### ETSI ITS-G5 Technology  
The ITS-G5 technology is arranged in protocol layers, such as Access, Networking and Transport, Facilities, Applications, Management and Security [10]. The Access layer defines the physical and data link components. The physical layer (PHY) implements the IEEE 802.11p standard and the data link layer includes two sublayers: Logical Link Control (LLC) and Medium Access Control (MAC). LLC is capable of distinguishing protocols at the network layer and MAC is responsible for the transmission schedule, to minimize interference between ITS-G5 stations. Decentralized Congestion Control (DCC) is a management entity which controls the network load, regulating the transmission opportunities based on its load. Although the DCC appears in the layers of the ITS model, it is only an algorithm for controlling the network flow and not a protocol [10]. The network and transport layers use two main protocols, the Geo-Networking (GeoNW) and the Basic Transport Protocol (BTP). GeoNW is a routing protocol which provides packet delivering features on ad-hoc networks without a coordination infrastructure and using geographic coordinates for the addressing and routing functionalities. BTP adapts the concept of ports and works over the GeoNW providing an end-to-end connectionless and non-guaranteed delivery transport, similar to the User Datagram Protocol (UDP). BTP multiplexes messages from the Facilities layer and sends them to GeoNW [10]. ITS-G5 Facilities layer defines the messages for the communication between vehicles, infrastructures and people, namely the Cooperative Awareness Messages (CAM), Decentralized Environmental Notification Messages (DENM), In-Vehicle Information Messages (IVIM), Signal Phase and Timing Extended Message (SPATEM) and Map Extended Messages (MAPEM) [11]. CAM messages contain information about the presence, position, and status of vehicles [11] that are sent periodically and received by other cars and fixed stations that are at a single hop of distance. Upon receiving a CAM message, the ITS-G5 stations get to know all cars in its vicinity, their position, speed, basic attributes and other data. The CAM message manager, inside the Facilities layer, generates information collected from the clock, vehicle monitoring system and other sensors. Then, the CAM message is sent to the lower layers and relayed to the physical network. At the receiving side, the CAM messages are validated by the Local Dynamic Map (LDM) module, which analyzes the message and updates the LDM, a georeferenced database that contains information about the neighbourhood vehicles. The CAM periodic transmission rate is between a minimum limit (T min = 100 ms) and a maximum limit (T max = 1 s). A CAM message is also trigged when a vehicle exceeds its predefined dynamic thresholds for heading, movement and acceleration. When the wireless channel load is high, the minimum period can be increased. If required by any safety situation the maximum period can be decreased [12]. DENM messages contain information related to hazard warnings or anomalous road conditions [13] and are disseminated to ITS-G5 Stations in the neighbourhood. The DENM message manager is responsible for the creation, managing and processing of these messages. The creation is triggered by an ITS application. On the other hand, when receiving a DENM message, the Facilities layer validates it and sends the information to the application layer and when it is considered relevant it is presented to the driver. IVIM is an infrastructure-to-vehicle (I2V) message type with information about the traffic infrastructure. The purpose of these messages is to provide drivers access to traffic sign information in any environmental conditions [11]. SPATEM is another I2V message type used for communications at road intersections and contains dynamic information about the intersection status in a given instant, such as traffic lights, predictions of future states and speed advice [11].