The functionality in each Functional Area is decomposed into several Functions that form a hierarchical set of High-level and Low-level Functions.
Low-level Functions contain simple (un-complex) functionality which is easy to describe, and thus contain the lowest level of functionality in each Functional Area. The Functional Requirements provide details of what the Functions actually do with the data that they receive in order to produce the data that they send out.
High-level Functions are groups of Low-level Functions. They are used to represent these Low-level Functions at higher levels in the hierarchy in order to make it easier to understand the overall functionality within a Functional Area.
Each Function has a number and a name. The numbers are used to identify the position of each Function in the hierarchy within each Functional Area, whilst the names are a simplified expression of what the Function does.
An example High Level Function:
6.6 Provide Traveller Information
This High-level Function shall provide facilities that enable information to be provided to Travellers, either at the roadside, or by other means. Information about traffic conditions shall be provided at the roadside, but other forms of information, e.g. about Points of Interest, shall be available to Travellers through other mechanisms, e.g. a nomadic device.
An example Low Level Function:
6.6.3 Output Travel Information
This Function shall be capable of providing the following facilities:
(1) The ability to take responsibility for the output of information about road conditions, PT services, conditions of other transport modes, Points of Interest (POI) and Personal Services (PS) to Travellers.
(2) The information output to the Traveller shall be provided by the Travel Information Operator through the Produce Traveller Information Function.
(3) The ability to output the information in a form that is easy to understand and be suitable for those with disabilities.
(4) The ability to continuously display the particular information that is being output until replaced by other information or the output is cancelled by the Operator.