Initiating the Systems Engineering Process for Rural Connected Vehicle Corridors, Volume 3: Model System Requirements Specification (2021)

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37   System Interface Requirements System interfaces describe the protocols, messages, frameworks, and/or APIs used to commu- nicate between internal and external elements within the system. e internal interfaces are those interfaces that are completely contained within the system you are developing. External interfaces are those interfaces between the system under development and other systems, third parties, or other users that you do not control. Requirements documenting these interfaces are important because dierent development teams are likely developing system elements inde- pendently of each other. Having well-formed interface requirements minimizes the risk that communications between system elements will have issues. When documenting interface requirements, a best practice is to use standardized interfaces or APIs. ere are several mature and well-formed ITS and connected vehicle standards that dene the interfaces between dierent system elements. Likewise, most third-party services and other external users now provide well-documented and dened APIs or specications for how to collect and send information. Depending on the size and complexity of the system, a more detailed Interface Control Document (ICD) may need to be developed during the design phase to ensure communications between systems are truly interoperable. 4.1 Internal Interface Requirements Internal interface standards dene the interfaces between internal system elements. For ITS and connected vehicle systems, several standards can be used to dene internal interfaces. e NTCIP family of standards denes the interface between TMCs and most types of ITS infra- structure devices. e SAE J2735 and J2945 family of standards and the IEEE 1609 family of standards dene the interface between connected vehicle devices. Table 18 contains the deni- tions of the internal interface requirements. 4.2 External Interface Requirements External interface requirements dene the interface between the system under development and other systems you do not control (see Table 19). e use of standards is highly encour- aged for these requirements; however, as many external interfaces are to and from third-party services, the use of APIs is likely to be more prevalent. S E C T I O N 4

38 Initiating the Systems Engineering Process for Rural Connected Vehicle Corridors E x ternal Interface Requirements E IR-01 The interface between the Backoffice system and the NW S shall be in accordance with the NW S API W eb Service. E IR-02 The interface between the Backoffice system and other TMCs for sharing Center- to- Center ( C2C) information shall be the TMDD v3.1. E IR-03 The interface between the Connected Vehicle Roadside Equipment and the SCMS shall be in accordance with IEEE 1609.2.1. E IR-04 The interface between the OBU and the SCMS shall be in accordance with IEEE 1609.2.1. Table 19. External interface requirements (EIR). Internal Interface Requirements IIR-01 The interface between the Backoffice system and signal controllers shall be in accordance with NTCIP 1202 v3a. IIR-02 The interface between the Backoffice system and DMSs shall be in accordance with NTCIP 1203 v3. IIR-03 The interface between the Backoffice system and environmental sensor systems shall be in accordance with NTCIP 1204 v3. IIR-04 The interface between the Backoffice system and Connected Vehicle Roadside Equipment shall be in accordance with NTCIP 1218 v1. IIR-05 The interface between OBUs and Connected Vehicle Roadside Equipment shall be in accordance with the IEEE 1609 family of standards, and SAE J2735 and SAE J2945 family of standards. Table 18. Internal interface requirements (IIR).