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of ground-originated calls to a specific called terminal
being blocked by an engaged condition when an equally
appropriate called terminal is available.
8.2.4 GES TELEPHONY INTERWORKING
8.2.4.1 Relationship of GES signalling systems. Figure
A- 19 of this guidance material depicts the relationship between
the GES circuit-mode procedures, their respective interworking
procedures, and ground-specific telephony signalling implementations.
In particular, the referenced figure defines both the
usage of '~ncoming" and "outgoing" procedures from the
viewpoint of the originating call party and the interworking
interface of the GES circuit-mode procedures.
8.2.4.2 GES interworking telephony event defmition. The
GES circuit-mode procedures interwork with terrestrial
network telephony signalling systems via the forward and
backward interworking telephony events defined in the
Standards. The GES circuit-mode procedures must map
specific interworking telephony events to specific protocol
interactions in the GES circuit-mode logic procedures where
interactions with a ternstrial network signalling system are
required. This mapping must also include parameter mapping
where indicated in the Standards.
8.3 Implementing satellite voice
in the ATS environment
8.3.1 Overview. The AMS(R)S voice service has basic
operational attributes which fundamentally differ with
prevalent very high frequency (VHF) and high frequency (HF)
voice operations. This will require close attention to how
satellite voice is implemented in a data link-oriented ATS
environment. These differences include a statistica1 delay in
speech channel establishment, a perceptible delay in speech
propagation, circuit-switched operation, full-duplex operation,
and the inability for aircraft to monitor communications
between the ground and other aircraft. Additionally, the
AMS(R)S voice service places functional requirements on
terrGstria1 facilities which are external to the GES (e.g.
terrestrial networks and ACF automation equipment) in order
to maximize its utility.
8.3.2 Channel establishment delay. Upon call origination,
each user must provide to its respective AES or GES the
telephone number of the desired destination and then wait for
the system to establish a speech channel. This is in sharp
contrast with conventional radio operations where typically
each user maintains a continuous listening watch on a radio
channel. In addition, the channel establishment delay is statistical
,in nature and is dependent upon the over-all traffic load
on the AMSS system.
8.3.3 Call annunciation. It is anticipated that aircraft
operators will integrate the satellite voice equipment with other
aircraft systems in a manner very similar to current VHF and
HF radio equipment. This may lead to minor inconsistencies
in how voice calls are managed on different aircraft. For
instance, interwiring satellite voice equipment with existing
Altaehment A to Part I Annex 10 - Aeronautical Telecommunications
aircraft audio control panels may lead to inherently half-duplex
operation on certain aircraft even though all of the intervening
speech channels are full-duplex. Incoming air-originated calls
will, in many cases, be annunciated to the aircrew via a
SELCAL chime or other audible indication, and will require
a crew action to answer the call. This means that a ground
user must await a positive answer indication (e.g. crew voice
response) before speech can begin. Otherwise, there would be
no assurance to the ground user that the call is audible to the
crew. This particular aspect is quite important given the
expectation that:
a) satellite voice calls will be very infrequent in a data
link oriented environment; and
b) the inability of an aircrew to monitor satellite voice
communications by other aircraft precludes positive,
routine assurance that the proper aircraft audio panel
selections have been made.
8.3.4 Aircraft call management. Aircraft flight management
computer systems can be useful in managing routine
aspects of voice call management for the aircrew. For instance,
ground number directories and selection menus can be
provided by these systems so that the need for an aircrew to
enter discrete telephone numbers on a controUdisplay device
can be minimized. These systems could also correlate with
directory information the associated data link end-system
address information or the aircraft's position in order to
recommend an appropriate ground address for use in an ATC
call. However, it should be noted that the crew must be able
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