There are many potential ways for passenger transport operations centres to interact. The approach taken by SIRI is for an open-ended set of standard data structures, carried over a communications channel constructed using one of a small number of specific options.Part 2 of this Technical Specification specifies the communications channel. This section specifies a number of functional modules, based on the ‘use cases’ identified in Annex B to Part 1:- Production Timetable (PT): this service enables the provision of information on the planned progress of vehicles operating a specific service, identified by the vehicle time of arrival and departure at specific stops on a planned route for a particular Operational Day.- Estimated Timetable (ET): this service enables the provision of information on the actual progress of Vehicle Journeys operating specific service Lines, detailing expected arrival and departure times at specific stops on a planned route. There will be recorded data for stops which have been passed, and predicted data for stops not yet passed. In addition the Estimated Timetable service allows Vehicle Journeys to be cancelled, added or changed.-Stop Timetable (ST): this service provides a stop-centric view of timetabled vehicle arrivals and departures at a designated stop. It can be used to reduce the amount of information that needs to be transmitted in real-time to stops and displays, as reference data for a Stop Monitoring Service; and provides a data feed of the static timetables.- Stop Monitoring (SM): this service provides a stop-centric view of vehicle arrivals and departures at a designated stop. It can be used by displays and other presentation services to provide departure board and other presentations of timetable and real-time journey information both at stops and at a distance.- Vehicle Monitoring (VM): this service enables the provision of information on the current location and status of a set of vehicles.