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groups.
8.4.6 Pre-emprion. The terrestrial safety network, and/or
its individual tandem 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 uunk
group for high priority calls (e.g. distresslurgency).
85 Implementing the group
calYbmadcast 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
AWhment A to Part I Annex LO - Aeronautical Telecommunications
functions can be readily catered for by implementing several
basic call origination functions in the facility automation
system.
8.5.2 Group 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 conference. 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 entryto
and exit-from the conference by the user. Except 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 in order to establish a group call.
8.5.2.1 The terrestrial 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 ground user. This will provide a passive means
to serialize access to the ground user that is identical to that of
VRF radio except for the satellite delay effects. Additionally,
the facility automation function should also manage the "list"
of aircraft in the group so that the ground user can be
constantly aware of the presence - or absence - of each
aircraft in the group call.
8.5.2.2 During group call origination, the individual
aircraft speech channels in a group call cannot be expected
to begin operation simultaneously. This is because; (1) the
call announcement signalling arriving over the P channel
will not reach all aircraft in the gmup call simultaneously,
(2) C channel establishment delay after receipt of this
signalling will vary, and (3) all aircrew in the group may not
be able to answer the incoming call simultaneously due to
primary 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 function to transmit a repetitive
audio alert tone sequence or recorded verbal advisory until all
aircraft have answered the call and are ready to participate in
the conference.
8.5.3 Croup broadcast. The procedures for group broadcast
can be identical to those of group 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 group. As in the case of group
call, human factors considerations may require that there be
some way to indicate the anival of a one-way group broadcast
to the aircrew.
8.6 Implementing the call
registration function
8.6.1 General. 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 calls from a reasonable number of aircraft.
(Paragraph 8.3 provides further guidance on how this may be
implemented.) However, there may be operational situations
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