The design of complete and seamless networking of vehicles and infrastructure represents significant technological and challenges as many different protocols and data formats are required and many stakeholders are involved 29. In a Vehicle to X communications, Vehicles share information with each other. To do so V2X communication technology utilize roadside stations to enhance communication range and to connect infrastructure based entities to the vehicles 30. In general, V2X communication system architecture could be divided into two domain: Vehicle domain and Infrastructure domain. Vehicle domain contains hardware and software for the vehicular subsystems.On the other hand, Infrastructure domain represents services relevant to infrastructure.Figure 3 illustrated system architecture of V2X communication 29.
Figure 3: System Architecture of V2X Communication 29
Vehicles use IEEE 802.11p based Dedicated short-range communication technology to enable themselves to communicate with each other. Therefore, Vehicles are equipped with so-called Vehicle ITS station (VIS) which consists out of a Communication Control Unit (CCU) and Application Unit (AU). CCU is responsible for handling all the communication from the physical layer to the Network layer which includes the implementation of respective sender/receiver modules for each communication Channel.Application unit provides basic services required by other system components or applications 29.
The Computation unit is attached to an interface e.g display with a touchscreen which enables interaction between driver and V2X system. Driver gathers warning messages through this interface. This unit is also connected to Controller Area Network (CAN) bus to get access to the vehicles internal data and local sensors.In this way, various services are available in V2X communication systems e.g vehicle state and dynamic data, local sensors data, GPS data, vehicle speed, exterior light state etc 30.
In V2X communication DSRC is used at frequency 5.9 GHz that why the coverage area of individual VIS is not so big.Therefore, multi-hop communication technology is used to enlarge communication range of VIS. The V2X message is forwarded i.e received and retransmitted by multihop station until reaching the destination. Roadside ITS stations (RIS) are also involved in V2X communication. RIS also enlarge communication area especially in the low frequented area by retransmitting received message 30.
Besides IEEE 802.11p based Vehicle ad-hoc communication, V2X also support cellular communication.Therefore, Vehicles also equipped with cellular communication hardware that provide IP based services to the backend services and allow to communicate over long distance 29.
1.1.1 Forwarding Types
V2X supports several message forwarding technologies 29. In the following basic forwarding types are described.
o Unicast: A Unicast transmission refers one to one transmission that passes from a single source to a single receiver.Though Unicast is the point-to-point transmission, in a succession of one-to-one communication, the source node can send information to any number of other nodes on the network 31. Basically, this type of transmission is used between Vehicles and RSUs 29.
o Topologically-Scoped broadcast: Topologically-Scoped broadcast refers a data transmission technology where data transport from one node to all nodes within the coverage area of Vehicular Ad-hoc Network.Topologically-Scoped broadcast is mapped into broadcast service at data link layer 7 and its service is restricted in the number of wireless hop.