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termination of the call by the GES. When this happens, the
GES will send six "call progress - channel release" CMLIDUS
during this phase so as to ensure that the AES's call
state is cleared. If the AES camtr were to reappear (possibly
indicative of anomalous propagatian conditions non-conducive
to a usable call) the GES will attempt to again clear the AES
call by sending twelve additional "call pmgress - channel
releasei' CM-JJDUs on the C channel sub-band and one on the
P channel.
8.1.5.1.3 If the GES receives a "call progress - channel
release" CM-LIDU (indicating that the aircraft user has
terminated the call) the GES will forward a "clear back" BI'T'E
to the ground network and terminate the cdl. Similarly, if the
GES receives a "clear forward" FlTE from the ground network
(indicating that the ground user has terminated the call) the
GES will send six "call progress - channel release"
CM-LIDUS over the C channel sub-band and then monitor the
AES to ensure that it has terminated its from-aircraft C
channel carrier.
8.1.5.1.4 Events such as unsuccessful continuity checks
and call clearing will result in theeventual termination of C
channel transmissions fram the AE$. While these various
ternination events are progressing, the GES will monitor the
from-aircraft carrier to ensure that the AES .is performing
similar call termination activities. If the from-aircraft carrier
has not terminated within tG17 seconds after the transmission
of an initial series of "call pmgress - channel release"
CM-LIDUS, the GES. will send twelve additional of these
CM-LIDUs via the C channel sub-band and a thirteenth via the
P channel. If these transmissions are not successful in causing
the M S to terminate its C channel transmission within tGl8
seconds after the last transmission, the GES will terminate all
call activities unilaterally.
8.1.5.2 Aircroft earth station (Am pmedun
8.1 52.1 An incoming call to an AES is announced by the
receipt of "call announcement7' and "C channel assignment"
CM-LDUs. Since these are sent on the direct link service,
circuit-mode services implements an error recovery procedure
whereby the AES can infer that one of these CM-LIDUs is
missing (e.g. due to bit errors on the P channel) and request
that it be repeated by the GES. The AES initiates the request
by transmitting an appropriately encoded "telephony
acknowledge" CM-LIDU via the R channel. When an AES has
received a ''call announcement" CM-LIDU and its associated
"C channel assignment" CM-LIDU it will initially verify that
the called terminal is available and that adequate C channel
resources (i.e. channel unit and AES EEW) are either available
or pre-emptable. If the foregoing requirements are not all met
the AES will send an appropriately encoded "call progress -
call attempt result" CM-LIDU via the R channel and then
terminate all activities for the call. Otherwise, it will activate a
C channel unit on the assigned C channel frequency par and
begin the C channel continuity check procedute by sending a
"call progress - test" CM-LIDU via the C channel sub-band.
8.1.5.2.2 The AES will send an additional "call progress - test" CM-LIDU every tA26 seconds for d 4 1 seconds until
an identical CM-LDU is received fram the GES. This
constitutes a successful continuity check and will immediately
cause the A@S to: (1) enable the voice circuit between the
aircraft voice channel and the C channel unit, (2) forward a
"call origination" FITE to the aircraft destination, and (3) send
a "telephony acknowledge" CM-LDU via the C channel
sub-band to the GES. The AES will then await tA42 seconds
for the aircraft destination to answer the calf. If the call is not
answered within this time period the AES will clear the call
toward the aircraft destination, send six "call -progress -
channel release" C M - W s to the GES, and then terminate
the call. Otherwise, if the aircraft destination answers within
this time limit the AES will send a "call progress - connect"
CM-LIDU to the GES and maintain the C channel during the
talk phase of the call.
8.1.5.2.3 When the "call progress- connect" CM-DU
is sent, the AES will initiate an error recovery pmedure to
ensure its receipt at the GES. (The Lalk phase progresses in the
interim.) Timer tA26 is used to implement a transmission
repeat cycle for this CM-LIDU which must result in receipt at
the AES of a "telephony acknowledge" CM-LJDU within tA30
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