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minimum set of protocol interactions necessary to ensure interoperability
of aircraft earth stations (AESs) and ground earth
stations (GESs) operating in the safety service. The protocol
interactions necessary to support air and ground-originated
calls arriving at the AES or G?3 are defined via separate
signalling procedures in each.
8.1.2 Special logic for air-origirr~ti011T~h.e AES and
GES air-origination procedures differ from prevalent aeronautical
mobile-satellite service (AMSS) signalling procedures
for the non-safety service. The aeronautical mobile-satellite
(route) service (AMS[R)S) procedures are specially designed
to achieve enhanced and consistent call set-up times for airoriginated
voice calls at the safety priorities (i.e. distress/
urgency, flight safety, and regularity/meteorological) with
minimal performance impact from other AMSS traffic.
8.1.3 Link layer services. Circuit-mode services use the
R, P, and C channel sub-band link layer protocols defined in
Annex 10, Volume III, Part I, Chapter 4, 4.5 and 4.6 for all
call signalling information exchanged between the AES and
GES. This information is exchanged with the link layer via
circuit-mode link interface data units (CM-LJDUs). Link layer
services perform conversions between CM-IDUs and the
respective AMS(R)S circuit-mode signalling units (SUs). In all
cases, only the direct link service (DLS) is used since circuitmode
services are responsible for recovering from missing
sus.
8.1.4 AIR-ORIGINATIPORNO CEDURES
8.1.4.1 Aircrafl earth station (AES) procedure
8.1.4.1.1 The AES commences an air-origination by
transmitting to the GES a series of fwr, three, or two
contiguous "abbreviated access request" CM-LIDUs. The
series length is determined by whether the call priority is
distress/urgency, flight safety or teg~llarity/meteorological,
respectively. The purpose of the multiple transmissions is to
mitigate the effects of an SU last due to R channel collisions.
The AES then awaits receipt from the GES of a C channel
assignment CM-LIDU for a period of tA50 seconds. This
sequence will be repeated four times before aborting the call
attempt. Calls aborted by the AES in this manner must be
reattempted by the user.
8.1.4.1.2 At the instant at which the call origination
begins, if sufficient AES reSOUKeS for the call are not
available due to blockage attributable to a lower priority call
at the Am, the AES will defer pre-emption of these resources
and proceed as per 8.1.4.1 .I until a C channel assignment is
received from the GES. This will allow the AES to make a
proper pre-emption decision based on the exact EIRP assignment
from the GES. Upon receipt of the C channel assignment,
all required AES C channel resources (i.e. channel unit
and AES EIRP) are pre-empted from a lower priority call (if
necessary) and allocated to the call. This is followed
immediately by further signalling and continuity checks on the
C channel sub-band while the GES is engaged in completing
the call to the ground destination.
Note.- As a secondary benefit, by requiring that the AES
defer pre-emption until a C channel assignment is received,
the f i t u ~de velopment of an alternative procedure incorporating
reuse of an existing C channel is facilitated.
8.1.4.1.3 The "abbreviated access request" CM-LIDU
contains all digits of the fixed-length 10-digit AMS(R)S
ground address. Providing this information concurrent with the
access request allows the GES to begin forward completion of
the call across the ground network while it proceeds simultaneously
with C channel establishment. These address digits
Annex 210 - AemmufrufTre~lle cornmunic~~ons Volume IZZ
are repeated on the C channel sub-band as "call information - service address" C'M-LIDUS which serve as a basis for the
C channel continuity check procedure. However, their presence
in these CM-LIDUs serves no purpose for the AMS(R)S airorigination
procedure other than to allow the AES to interwork
with a GES which supports prevalent non-safety AMSS
circuit-mode procedures rather than the SARPs-specific airorigination
procedure.
8.1.4.2 Cmund earth station (CES) procedure
8.1.4.2.1 Upon receipt of an "abbreviated access
request" CM-LIDU from an AES, the GES immediately allocates
C channel resources and responds with a "C channel
assignment" CM-LIDU. Simultaneously, the GES analyzes the
network-ID contained in the access request and begins forward
call completion across the selected ground network. It is
necessary that the GES perform forward call completion while
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