1.1.2 Uplink Message Delivery
1.1.2.1 The Internetworking Function
In general uplink FANS-1/A messages are delivered to a service provider's DSP for delivery to an aircraft.In order to reach the DSP the message may have traversed the networks of a number of service providers.In addition to this,delivery may be attempted through up to two serviceprovider's DSPs.
Each service provider offering FANS-1/A datalink service to a CAA must support an internetworking function.The internetworking function provides the logic that determines the service providers to be used.The internetworking function may be hosted in the service provider's DSP or it may be hosted externally.
ln order to meet the performance requirement for message delivery,the internetworking function will(as much as possible)follow the philosophy that if VHF is being used by an aircraft then delivery will be attempted by one VHF RGS.If the attempt is unsuccssful,then delivery will be a attempted via satellite.If VHF is not in use,then delivery should only be attempted via satellite.Where satellite is used,the choice of provider is usually dictated by the contractual arrangements of the airline concerned.
These rules are only indicative as the choice of possible uplink delivery paths is a product of the uplink routing logic of all service providers who will handle the message.As one service provider cannot dictate the delivery paths that another service provider can use,there will be instances where the philosophy described above may not be strictly adhered to.
