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NEN-EN 15531-1 may be exchanged between a number of different organizations, or between different systems belonging to the same organization. Key interfaces include the following: - Between public transport vehicle control centres - generally, for fleet and network management. - Between a control centre and an information provision system - generally, to provide operational information for presentation to the public. - Between information provision systems - generally, sharing information to ensure that publicly available information is complete and comprehensive. - Between information provision systems - and data aggregation systems that collect and integrate data from many different sources and different types of data supplier and then distribute it onwards. - Between information provision systems and passenger information devices such as mobile phones, web browsers, etc. Annex B describes the business context for SIRI in more detail. SIRI is intended for wide scale, distributed deployment by a wide variety of installations. In such circumstances it is often not practical to upgrade all the systems at the same time. SIRI therefore includes a formal versioning system that allows for the concurrent operation of different levels at the same time and a disciplined upgrade process. In this general framework, SIRI defines a specific set of concrete functional services. The services separate the communication protocols from the message content (‘functional services’). This allows the same functional content to be exchanged using different transport mechanisms, and different patterns of exchange. Figure 1 below shows this diagrammatically.