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Particular switching network architectures are not mandated
for the terrestrial safety network. For instance, individual
agreements between AMSS service providers and either
administrations or aircraft operators may provide for the
integration into the GES equipments of some or all of the highlevel
routing functions. This would then require that the GES
perform the routing tasks described in 8.4.4.1 and route calls
to individual facilities via dedicated GES-to-facility trunk
P P S .
Annex 10 - Aeronautical Telecommunic~ns Volume III
8.4.6 Pre-cmption. The terrestrial safety network, andlor
its individual Landem subnetworks are required to pre-empt
and reallocate any resource assigned to an existing call when
that call is blocking the completion of a higher-priority call
attempt. The incidence of preempted calls can be minimized
by reserving one or more channels within all voice trunk group
for high priority calls (e.g. distressfurgency).
8.5 Implementing the gmp
cdl/broadcast functions
8.5.1 Overview. Under certain conditions a ground user
may desire to establish a ground-originated conference call (a
group call) with several aircraft at once. Similarly, there may
be occasions where there is a need to establish a one-way
broadcast to a group of aircraft (a group broadcast). Although
the group call and group broadcast functions are not explicitly
provided for in the AMS(R)S system protocols, equivalent
functions can be readily catered for by implementing several
basic call origination functions in the facility automation
system.
8.5.2 Gmup call. The group call function can be effected
by requiring that the ground user (or an associated automation
service) place independent calls to each aircraft designated by
the user to be in the group call or "conference". Separate,
parallel calls through the terrestrial network and AMSS
satellite service would then be established for each aircraft in
the confem~ice. The use of individual voice calls for each
aircraft in the group can be easily implemented and it also
facilitates the centralized management of aircraft entry-to and
exit-from the conference by he user. Exapt for the facility
automation functions and the low-level voice switching
equipment serving the ground user, no other intervening
tandem network (terrestrial or AMSS) need do any specialized
call processing is order to establish a group call.
8.5.2.1 The tenesthid network equipment immediately
adjacent to the ground user should provide an audio
conference function on behalf of the user whereby all aircraft
in the conference can hear speech audio generated by other
aircraft as well as the m u d user. This will provide a passive
attention to flight deck duties. This latter human factors issue
has the potential to cause considerable confusion as various
aircrew answer the call and enter the group conference,
especially if the aircrew are not aware that an incoming call is
a group call. The risk of this confusion can be mitigated if the
aircrew can be advised that an incoming call is actually a
group call so that they can remain quiet on the circuit until the
ground user begins speaking. One way to accomplish this is
for the facility automation fundion to transmit a repetitive
audio alert tone sequence or recorded verbal advisory until at1
aircraft have answered the call and are. ready to participate in
the conference.
8.5.3 Gmup brwadc~lst. The procedures for group
broadcast can be identical to those of p u p call except that the
conference function provided by the facility should not convey
or relay any air-to-ground audio that might inadvertently arrive
from aircraft in the broadcast p u p . As in the case of grwp
call, human factors considerations may require that there be
some way to indicate the arrival of a one-way group broadcast
to the aircrew.
8.6 Implementing the call
@stration fbction
8.6.1 Geneml. Under normal circumstances, an ATS
specialist who has available an AMS(R)S voice
communications service should be able to receive and maintain
concurrent air-originated c d s from a reasonable number of
aircraft. (Paragraph 8.3 provides further guidance on how this
may be implemented.) However, there may be operational
situations where the ATS specialist wishes either to bar
incoming calls or to service aniving calls one at a time - in
other words return a "busy" indication to calling aircraft. In
these instances, good operational practice should include both
making an automatic record of the call attempt for the ATS
speciakt (call "registration") and advising the aircrew that
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