INTERNETWORKING
In simple terms,“intenetworking”means that a downlink message received and processed by a service provider can be delivered to a CAA connected to the network of another service provider.Similarly an uplink message input into the network of one service provider may be processed and delivered to an aircraft using the facilities of another service provider.In the“intenetworked”case,an uplink routing algorithm based on aircraft connectivity derived from Media Advisory messages is required to meet the delivery requirements for FANS-1/A.
Whether handled by a single service provider or a number of service providers operating in an internetworked arrgement,all datalink traffic will be subject to the performance constraints given elsewhere in this document.
1.1 FANS-1/A Datalink Message Delivery
As the message delivery paths for uplink and downlink messages may differ and the routing logic allocated to the internetworking function applies to uplink messages only.Uplink and downlink message delivery will be covered in separate sections below.
1.1.1 Downlink Message Delivery
The downlink message path is detemined by the aircraft.Logic onboard the aircraft will select the appropriate path for message delivery.Typically,an aircraft will use VHF if it is available,otherwise it will use satellite.Messages of greater than 220 characters in length will be broken down into blocks of 220 chracters or part thereof.
All downlink messages received by the air-ground facilities of a given service provider shall also be processed by the DSP of that service provider.Part of the DSP processing includes the re-assembly of multiblock messages.
As there are gateways between each of the service providers’Type-B networks,correctly addressed Type-B messages will always be delivered to a CAA regardless of the network to which the CAA is connected.
