The ALERT-C protocol is designed to provide mostly event-orientated road end-user information messages. Many "hooks" have been left for future development and indeed a few status-orientated road end-user information messages were included.
The presentation section of the ALERT-C protocol specifies messages that may be presented to the user in accordance with the general requirements set out above. It defines the message structure and content, and its presentation to the end-user.
RDS-TMC messages are language-independent, and can be presented in the language of the user's choice. The ALERT-C protocol utilises a standardised Event List (EN ISO 14819-2) of event messages with their code values, which also includes general traffic problems and weather situations.
ALERT-C defines two categories of information within messages: basic and optional items. In principle, basic information is present in all messages. Optional information can be added to messages where necessary.
Standard RDS-TMC user messages provide the following five basic items of explicit, broadcast information: Event description, Location, Direction and Extent, Duration and Diversion advice.
Optional information can be added to any message using one or more additional RDS data groups. This optional addition can give greater detail or can deal with unusual situations. Any number of additional fields can in principle be added to each basic message, subject only to a maximum message length of five RDS data groups.
The message management component deals with the message management functions of RDS-TMC. The ALERT-C protocol distinguishes between user messages and system messages. User messages are those potentially made known to the end-user, as defined in the presentation section. System messages are of use only to the RDS-TMC terminal, for message management purposes.
The transmission component conveys the messages over-air. The ALERT-C protocol, which RDS-TMC uses, retains the fundamental approach of earlier work, which aims to code most messages entirely within a single RDS group.
RDS-TMC information comprises both ´system information´ and ´user messages´. System information relates to the particular TMC service, and details the parameters that the terminal needs to be able to find identify and decode the TMC information. System information is transmitted in type 3A groups and in type 8A groups.
User messages contain the details of the traffic events; these may use one or more type 8A groups. Most messages may be transmitted using a single type 8A group, however messages with more detail (e.g. diversion advice) may use up to a total of five, type 8A groups.
The ALERT-C Event List contains all event descriptions. It is described in EN ISO 14819-2.