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1.1 Interfaces Specified by this Technical Specification 1.1.1 Business Context Real-time information may be exchanged between a number of different organisations, or between different systems belonging to the same organisation. 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. 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.