This part of ISO 17575 defines how to convey all or parts of the data element structure defined in other parts of ISO 17575 over any communication stack and media suitable for this application. It is applicable only to mobile communication links (although wired links, i.e. back office connections, can use the same methodology).To establish a link to a sequence of service calls initializing the communication channel, addressing the reception of the message and forwarding the payload are required. The definition provided in this part of ISO 17575 includes the required communication medium independent services, represented by an abstract application programming interface (API).The communication interface is implemented as an API in the programming environment of choice for the Front End (FE) system. The specification of the Back End (BE) API is outside the scope of this part of ISO 17575.The definition of this API in concrete terms is outside of the scope of this part of ISO 17575. This part of ISO 17575 specifies an abstract API that defines the semantics of the concrete API as illustrated in Figure 3 and its protocol implementation conformance statement (PICS) proforma (see Annex B). An example of a concrete API is presented in Annex C. Where no distinction is made between the abstract and concrete communications APIs, the term “communications API” or just “API” can be used.This part of ISO 17575 also provides a detailed specification for the structure of associated API statements, an example on how to implement it and its role in a complex toll cluster such as the EETS (see Annex A to Annex E).Media selection policies, certificate handling and encryption mechanisms are outside of the scope of this part of ISO 17575.