Home' RTCA Documents for Review : DO-262D Contents 10
© RTCA, 2018
Safety communications, by ICAO and International Telecommunication Union (ITU)
regulations, are assured of always having the highest priority and are accorded
special measures for protection from interference. A system supporting safety
communications is assumed to have the appropriate priority and preemption control
mechanisms embodied in all elements comprising the system.
Priority, Precedence and Preemption
The primary mechanism for assuring that safety related communications are always
available is the mechanism for priority, precedence and preemption. This MOPS
assumes that the NGSS system design implements the priority structure necessary for
Aeronautical Mobile (R) Service safety services. This includes the mechanisms for
controlling the precedence of both safety and non-safety messages, for the
preemption of system resources as needed to support safety services and to provide
for the added measure of protection to be accorded the safety services. Priority,
precedence and preemption mechanisms are assumed to apply to both data and
voice transmissions, with voice communications taking priority over data transmissions
at the same level of priority.
Assumptions on Higher Layer Packet Data Interfaces
To support the evolving ATN environment, this MOPS emphasizes data
communications. Data link operations are defined in terms of a user application, also
known as a Higher Level Entity (HLE). For the purpose of specifying these interfaces,
this document assumes the HLE performs the following functions:
Is able to initiate, accept and terminate calls via the interface.
b. Is prepared to accept error messages from the AES.
Monitors the AES operation in case of unrecoverable error conditions and
reinitializes the AES as necessary.
Assumptions on Voice Communications Using Digitized Voice
Despite the fact that the evolving ATN environment will utilize digital data
communications, there is still a desire to maintain voice communications for some
operations, especially distress/urgency communications. This document assumes
that the underlying communications infrastructure exists and has not been created
solely for the purpose of AMS(R)S communications. To the extent that the underlying
infrastructure provides voice communications, it is expected that such voice capability
will be provided by digital means, and not by conventional analog modulation
When such "digital voice" technology is implemented, the techniques used to translate
the spoken word into digital form and back will be established by the specific
subnetwork. This document only establishes the high level requirements for such
voice encoding and decoding techniques, which are collectively referred to as "the
Previous RTCA efforts have indicated that the suitability of a particular
vocoder for a particular AMS(R)S function involves a subjective evaluation of voice
communications using that vocoder in a variety of channel environments and ambient
audio noise environments. Furthermore, complete interoperability between two
different vocoder implementations is extremely problematic. Thus, this document
assumes that voice communications, if provided, will utilize the standard vocoder
employed by the satellite subnetwork and sets an extremely high standard for the use
of any alternate vocoder. This is the only manner in which both interoperability and
suitability for a particular application can be assured.
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