The Communications Viewpoint provides a framework for identifying the communications protocols necessary to provide interoperability between Physical Objects in the Physical View. These protocols need to meet the requirements on performance and constraints imposed by physical connectivity, environmental and operational challenges and relevant policies (such as an assurance of pseudonymity for mandatory data provision).
Stakeholders that take a user role, application or device developer role, tester, maintainer or standards development role will find many of their concerns addressed by the communications viewpoint. The Communications Viewpoint structure enables the engineer to answer questions such as:
- Do communications protocols exist to provide an interface solution that meets mission needs? What are those protocols?
- If some protocols are missing, at what layers are those protocols missing (what functionality is not provided)?
- What protocols address critical system-wide security issues, such as the provision for protection of data confidentiality, protection of message integrity, defense against in-transit manipulation?
- What organizations are responsible for the maintenance of communications standards and specifications?
The ARC-IT Communications Model is based on previous work in the communications arena, including the Open System Interconnection (OSI) Model, the ITS Station architecture, the NTCIP Framework, and the DSRC/WAVE Implementation Guide. The diagram below shows how the Communications Model for ARC-IT can be related to other communications models. The layers of the ARC-IT Communications Model are described below:
- ITS Application Information Layer: The ITS Application information layer standards specify the structure, meaning, and control exchange of information between two end points.
- Application Layer: The application layer standards define the rules and procedures for exchanging encoded data.
- Presentation Layer: The presentation layer standards define the rules for representing the bits and bytes of information content to be transferred.
- Session Layer: The session layer provides the mechanism for opening, closing and managing a dialogue between application processes. Sessions may be asynchronous as in paired requests and responses (information exchanges), asynchronous as in an unsolicited publication of information, and may require acknowledgement of receipt or not.
- Transport Layer: The transport layer standards define the rules and procedures for exchanging application data between endpoints on a network.
- Network Layer: The network layer standards define the routing, message disassembly/re-assembly and network management functions.
- Data Link Layer: The data link layer standards define the rules and procedures for exchanging data between two adjacent devices over some communications media.
- Physical Layer: The physical layer is a general term used to describe the numerous signaling standards within this layer, typically developed for specific communications media and industry needs.
In addition, the Security Plane identifies standards that specify policies and methods for system-to-system authentication and encryption of data across one or more layers of the communications stack. Security solutions terminate inside the physical device that uses the information flows in question. This means for example, that if a given flow specifies TLS in the security plane, that TLS is terminated inside the device receiving/sending data to satisfy the flow.
ARC-IT currently considers management-related standards within the security plane or, if they provide functions unique to a particular layer, inside that layer. This is in contrast to ISO 24012 (ITS Station). For users that prefer the emphasis on the management plane, all such standards may be considered as part of the security plane. Since the ARC-IT communications model does not delve to the level of interaction points between planes and layers, there are a few differences between the models.
Those standards at the top are referred to as data standards, when considered together make up the data profile. Those standards below that are communications standards, and together make up the communications profile.
The data profile includes all standards at the ITS Application Information Layer, and may include standards at the Application and Presentation layers if those standards define messages, data elements or dialogs. All other standards not related to security or management are part of the communications profile, including standards at the Application and Presentation layers that do not define messages, data elements or dialogs. These divisions between data profile and communications profile are relevant when considering a set of triples; the communications profiles are likely to stay the same while the data profiles vary. This difference allows an implementer to consider the constraints of his communications media separate from the needs of the application, and may simplify some architectural decision making processes.
The communications and data profiles reflect likely implementation choices and have been developed and assigned, where applicable, to triples. Other choices might be made in specific instances, which is why ARC-IT's accompanying SET-IT tool provides the systems architect with the ability to associate any profile with any triple.