## 2 Taxonomic and Technical Analysis  
Although the term for communication between machines (M2M) or devices (D2D) is a traditional and clear concept for the entire research community, it does not present always clear definitions when applied to vehicles, infrastructure, or pedestrians. As an example, the terms for car to car (C2C), vehicle to vehicle (V2V), car to everything, or car to all (C2X) are used indistinctly as vehicle to everything (V2X) communication in [16; 17; 18], among others. This is even more ambiguous when the different actors are included in the interaction but the origin of the data source is not properly considered in the communication process. Therefore, it is necessary to deepen their definitions and make a classification of the various existing forms.  
The taxonomy proposed below aims to correct errors, to provide further clarification on frequently misunderstood concepts and to include new acceptations such as for the infrastructure-to-everything (I2X) and pedestrian-to-everything (P2X) communications. To this end, the taxonomy has beenelaborated based on the full spectrum of terms collected around V2X, I2X, and P2X communications. Therefore, we provide functional definitions for the different variants. This classification does not intend to provide specifications or impose technical requirements on the various types of communication, but to harmonize terms with the following purposes:  
1. To provide clarity and stability regarding the role played by the V2X, I2X, and P2X communications;
2. To provide a useful framework that saves time and effort during the development of specifications and technical requirements (e.g., in standards);
3. To respond to issues of scope for the writing of future regulations, laws, and policies.  
As can be appreciated in the following section about the state-of-the-art, the different types of communications can be classified as V2X, I2X, and P2X depending on the origin of the communication. Then, the following taxonomy is proposed (Figure 2).  
* **Vehicle-to-Everything** (V2X): Communication from vehicle to any entity, which includes in-vehicle connectivity (IN-V) with sensors (V2S) or other onboard devices (V2D) such as infotainment systems. This definition often encompasses the terms for car to all/car to everything (C2X), car to car (C2C) or vehicle to other vehicles (V2V) such as motorcycles (V2M). This classification group also includes other more specific types of interactions, such as vehicle to grid (V2G) to communicate with smart grids to receive or return electricity, car to infrastructure (C2I) or vehicle to infrastructure (V2I) to communicate with the road (V2R), such as road side units (RSUs) acting as stand-alone units or relay nodes that provide safety and traffic updates (e.g., traffic lights), vehicle to networks (V2N), heterogeneous vehicular networks (HetVNET) or vehicular sensor networks (VSN). These last also include vehicle to broadband cloud (V2B) or vehicle to cloud (V2C) communications utilized for software upgrades or information updates. V2I also includes vehicle to home (V2H) appliances such as lighting or air conditioners, while the car to pedestrian (C2P), vehicle to pedestrian (V2P), or vehicle to phone (V2P) communications may include smartphones and wearables worn by persons. Note that the term vehicle-to-home must be disambiguated with respect to the term vehicle to humans (V2H).
* **Infrastructure-to-Everything** (I2X): Communication from infrastructure to any entity, which may include other infrastructures (I2I), vehicles (I2V), or pedestrians (I2P). This term must be disambiguated with respect to the term individual to individual (I2I).
* **Pedestrian-to-Everything** (P2X): Communication from pedestrian to any entity including other pedestrians (P2P), infrastructure (P2I), and vehicles (P2V). Note that these terms must be disambiguated with respect to the peer to peer (P2P) and payment to individual (P2I) approaches.  
Figure 2: Diagram of interactions between vehicle, infrastructure, and pedestrian communications.