1.1.2.2 Media Advisory Messages
The uplink routing philosophy mentioned above,requires that the internetworking function has infomation regding aircraft connectivity at a given time.This is obtained through the use of the media advisory message.These messages are transmitted by aircraft upon a change in media and/or service provider.Within the message are flags indicaing the establishment or loss of communication via each media.When commnication is established via a partcular media,the signature line of the media advisory message provides the identity of the service provider.
When received by a service provider,media advisory messages are forwarded to its own internetworking function and the internetworking function of all other service providers with which it has an internetworking agreement.
In general,the first attempt to deliver a message is made using information derived from recently received media advisory messages.This will indicate whether VHF is in use and if so,through which service provider.It will also indicate if the aircraft has logged-on to the satellite facilities of a non-contracted service provider.
In the absecnce of media advisory messages or if the first attempt to deliver message has failed then the internetworking function will forward the message to the DSP of the aircraft's contracted service provider.The information will be contained in configuration tables.
