Home' RTCA Documents for Review : DO-343B Contents © RTCA, 2018
Ground to Air Calls
For Ground to Air calls, the CNPs provide two part dialers 36, also known as Interactive
Voice Response (IVR) systems where the user dials an access number, e.g. an 0800
number, and is then prompted for a password, the call priority, and the ICAO 24-bit
aircraft address of the aircraft. The IVR generates the aircraft phone number of the
form 870 5 p ICAO, where p indicates the call priority and ICAO is the octal
representation of the ICAO 24-bit aircraft address. The CNP delivers the call to the
SBB Voice Meet Me Point using a leased line. Use of the leased line and of a password
for access to the IVR ensures that only bona fide users can make calls to the pilot,
which is a security requirement.
The flood dialer dials the required number for both Classic Aero 37 (94, 95, 96, 97) and
SBB (98) routes, with 98 being added for SBB. The call is eventually bridged to the first
entity that answers the call. The transit switch, on receipt of the 98 number, forwards
this call on to the Soft-Switch for number translation. The soft-switch will extract the
ICAO number and map this to the provisioned MSISDN 38 of the format (870 77 44
ICAO). The soft-switch will also extract the priority digit, p, and prefix this to the Calling
Party Number and also map this to the relevant MLPP value within the call setup
signalling. Once this procedure is complete, the call with the modified numbering is
passed back to the GMSC for onwards routing via the BGAN system. The GMSC uses
the MSISDN and retrieves the terminal location from the HLR based on the mapped
IMSI. Once the location is known, the call is passed to the relevant MSC, using a
roaming number retrieved from the VLR. A call is then alerted via the CS domain (Call
Waiting feature having been enabled at AES activation for the IMSI). If the CS route is
busy, then the call is forwarded towards the VoIP domain and the call is then placed
via the PS domain for presentation to the cockpit.
Upon receipt of the call setup message, the AES determines the appropriate behavior,
depending upon the call priority level, and whether the CS domain is already occupied
in a voice call. If the CS domain is already occupied, and the AES determines that it is
necessary to accept the call, the AES will instruct the CS domain to forward the call via
VoIP operating over the PS domain by returning a busy notification triggering call
forward on busy. In this way the AES can receive multiple calls via the combination of
the CS domain and VoIP over PS domains. If an incoming call is signalled and the AES
is currently utilizing both voice circuits, it will make a pre-emption decision based on
the presented priority level. If there is a lower priority call in progress, the AES will pre-
empt the lower priority call to allow presentation of the higher priority call. If there are
no candidate lower priority calls, then the incoming call will be rejected.
Air to Ground Calls
Processing for A2G calls is simpler than for G2A calls. On receipt of a request to initiate
a call from the cockpit, the AES terminates, if needed, any existing calls to allow a
higher priority call. The AES places the call via the CS domain if the CS domain trunk
is available or via the VoIP over PS domain if the CS trunk is in use. The priority of a
VoIP call is signaled to the ground by prefixing the p digit to the calling party number.
12 The text assumes the CNP provides the 2-part dialling function. Alternatively, some CNPs may elect to
contract Inmarsat to provide such capability on their behalf – this is shown in figure Figure 3-18 as the dashed
box titled Inmarsat 2-part dialler.
13 Classic Aero is not shown in Figure 3-18.
14 The MSISDN is the ‘phone number’ of the AES, and the number that must be presented to the MSC.
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