Home' RTCA Documents for Review : DO-262D Contents 42
© RTCA, 2018
1. The role of the ATN is to define an environment within which
reliable end-to-end data transfer may take place, spanning the
envisioned airborne, air/ground, and ground-based data
subnetworks, including the satellite subnetwork, while providing
interoperability among those networks. The critical emphasis is on
end-system data transfer interoperability. In the ATN environment,
the Satellite Subnetwork Layer must support the transparent
transfer of Network Protocol Data Units between adjacent
interworking entities (or HLEs as seen by the satellite subnetwork).
This also implies the transparent transfer of global addresses (or
Network Service Access Points (NSAPs) and of quality of service
information (including network priority) as well as of user data.
2. In Data-3 communications, HLEs communicate with the SSND
sublayer by means of a DTE/Data Circuit-terminating (or
Communication) Equipment (DCE) interface, as indicated in Figure
2-4. The DTE contains the DTE portion of SSNAc functions. One
or more DTE/DCE interfaces are provided as logical entities within
the AES. Each DTE/DCE interface is configured to connect to not
more than one DTE. The AES contains and implements the DCE
side of the interface. The satellite subnetwork acts as a relay
between the HLE and its peer(s) on the ground. The need to send
user information to, or receive user information from, the ground is
not determined in any way by the AES but by the HLEs.
3. The HLE (in addition to the incorporation of the SSNAc function)
will also support additional functions. As a minimum, the DTE
incorporates an internetwork function and may also incorporate
transport, session, presentation and application functions. In some
AES implementations, certain HLEs may be contained within the
AES software itself, but this configuration is not illustrated in Figure
Join and Leave Requirements
An AES that provides ATN-compliant packet-mode data service shall provide Join and
Leave event information to the aircraft routing function to indicate that the air/ground
data link is either available or not available, respectively. The Join/Leave information
shall include sufficient information for the routing function to determine the address(es)
of the attached terrestrial DTE(s).
ATN-Compliant Mapping for Priority, Precedence and Preemption
The target and selected values of the priority of data on an ATN-compliant interface
shall be mapped to the equivalent values at the opposite end of the satellite
Data-2 is intended as a legacy interface.
An AES offering packet-mode Data-2 service shall include a Link Service Interface
that provides direct access to the highest level of the Satellite Layer 2 Protocol, so that
bits may be sent in a first-in-first-out manner over the subnetwork. For AES
implementations that provide both ATN-compliant and Data-2 interfaces, it is
permissible for this Link Service Interface to utilize portions of the SSNDP, if desired.
One potential implementation is shown in Figure 2-4. The Data-2
depiction in Figure 2-4 is intended to be informative only.
In Data 2 operations, Link Service Data Units (LSDUs) are injected or collected at the
Link Service Interface in the AES. In order to ensure the forward compatibility of the
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