Home' RTCA Documents for Review : DO-262D Contents Appendix E
© RTCA, 2018
High-Power Amplifier (HPA)
The HPA amplifies the transmitted signal to the appropriate power level. The output
power is carefully managed to achieve near-constant received power density levels at
the satellite. The HPA is linear, to allow transmission of complex modulation
schemes. In the case of a multi-channel AES, increased linearity is required from the
HPA to suppress intermodulation products that may be formed in the HPA.
Modem & Protocol Stacks
The modem performs the modulation and demodulation functions in the AES. For
SwiftBroadband the modem is required to be agile, switching on command between
frequencies, modulation modes and forward error correcting codes. The modulator is
required to transmit bursts of data according to the return burst schedule allocated by
the network. This agility allows the BGAN network to dynamically allocate spectrum
and other resources on demand. It also allows the network to assign resources to
priority services, even when demand is high.
The radio bearer handled by the modem may be used to transfer a multiplexed data
stream, providing multiple services like voice, ACARS and IP data over one transmit
and one receive bearer at a time.
The SwiftBroadband protocol stack implements the upper layers (L2, L3) of the 3G
protocol adapted to satellite communications. The SwiftBroadband priority data
services (including VoIP) use dedicated PDP contexts as described further.
An AES6 or AES7 may have one or more modems for single or multi-channel
operation as required. An AES4 only contains one modem. Priority services only
require one modem. Additional modems typically provide non-priority services.
Segregation & Arbitration
Even though being primarily designated for safety services to the Aircraft Control
Domain (ACD) the AES can be used to provide services also to other domains. The
segregation and arbitration function ensures that each domain and each service is
handled with correct priority within AES. It also ensures that more critical domains
cannot be accessed from lower criticality domains (ACD from AISD or PIESD; AISD
from PIESD). The function can be implemented in hardware, software or combination
The Airborne Datalink Gateway (ADGW) is typically a software function in the AES. In
the air-to-ground direction the ACARS messages and the ATN/OSI Protocol Data
Units (PDUs) sent from the aircraft avionics are encapsulated by the ADGW into the
Inmarsat’s custom point to point protocol data messages. The protocol is AIGI (also
referred as AAP v1) for the Standard ACARS service without VPN and AAP v2 for the
Enhanced Service Set E1 or E2. The data messages are further encapsulated in
UDP/IP packets which are then sent to the IPsec based airborne security gateway
(Enhanced Service set E1 or E2) or directly to the BGAN stack (Standard ACARS
service) for transmission to the ground. The ADGW communicates over a logical
point-to-point link with its peer Ground Datalink Gateway (GDGW). The data packets
exchanged between the gateways contain additional information for error checking
and time and location stamping of messages.
Data packets containing ADGW messages are sent through a dedicated priority
background PDP context. This PDP context is set up when the AES starts up, and
form a virtual connection between the air and ground datalink gateways.
In the ground-to-air direction the reverse operation is performed by the ADGW. Time
stamping of data packets allows measurement of latency between the ADGW and the
Depending on end-to-end ATN/IPS security architecture, which is yet to be defined,
the processing of ATN/IPS packets may either follow the same path as ATN/OSI or
may bypass the security gateway..
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