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where the ATS specialist wishes either to bar incoming calls
or to service arriving 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 ahtomatic
record of the call attempt for the ATS specialist (call
"registration") and advising the aircrew that their failed call
attempt has been registered. This notification to the aircrew
would carry an implied intent on the part of the ATS specialist
to originate a subsequent reciprocal call to the originating
aircraft at the first opportunity.
Note.-- Paragraph 8.3.5 describes the air-originated call
information elements that can be used by the terrestrial facilities
to accomplish call registration. The GES and terrestrial
network facilities are required to convey rhis information to
the gmund user facility.
8.6.2 Signalling of the call mgistration event. The
AMSS SAWS require that a call attempt to a busy destination
be deemed unsuccessful and that a "call unsuccessful - called
party busy" event (BITE 16) then be generated by the enduser's
network facilities. (Intervening telephony signalling
systems, including those of the AMS(R)S subnetwork, convey
this event to the aircrew.) The call registration event is
considered a variation of the BITE 16 event and it is carried
over the AMS(R)S subnetwork by a unique code value in the
cause value parameter of the AMS(R)S "call progress -
channel release" LIDU (Annex 10, Volume IU, Part I,
Appendix 5 to Chapter 4, Figures A5-7 and AS-34 refer). Both
terrestriaI network facilities and network-specific telephony
interworking logic within the GES must support the conveyance
of this backward signal to the GESk air-origination logic
procedure if the recommended call registration facility is
implemented.
8.7 Notes on the AMS(R)S
ai~odgjnationp rocedure
8.7.1 General. The procedures defined in the AMSS
SAWS for use in air-originated AMS{R)S calls differ from
Annex 10 - Aeronauh'cal Telecommu~ications Volume ZZZ
other prevalent AMSS procedures which may be in use for
non-safety calls (e.g. aeronautical public correspondence). The
AMS(R)S procedures provide comparatively shorter and more
consistent access delay performance for calls originated at the
safety priorities.
8.7.2 ACCESS REQUEST PHASE
8.7.2.1 ~bbreviateda ccess request SU. An "abbreviated
access request" SU is used in the AMS(R)S procedures as a
means to deliver all call information (i.e. ground address and
calling terminal-ID) to the GES concurrent with the R channel
access request. This eliminates the need for the GES to await
receipt of this information on the C channel sub-band as is the
case for the prevalent non-AMS(R)S procedure.
8.7.2.2 Series transmission of redundant access requests.
In order to mitigate the effects of R channel collisions on
circuit-mode access delay performance. the AES will send a
short series of identical abbreviated access request SUs upon
both the initial access attempt and each of the four subsequent
attempts (which are initiated by expiry of timer tA50) for a
total of five attempts. The quantity of identical SUs in the
series increases with increasing call priority. The probability
of receiving at least one abbreviated access request SU at the
GES upon each of the five attempts over an R channel loaded
to 15 per cent is depicted in Table A-12 of this guidance
material. (The table brackets the expected performance by
depicting two cases where all conflicting R channel traffic
from other AESs comprises either 1 -SU bursts or 3-SU bursts.)
8.7.3 Simultaneous set-up of circuit segments by the
GES. In addition to the enhanced delivery reliability for the
abbreviated access request SU as described in 8.7.2.2, end-toend
access delay performance is further aided by the GES
beginning forward completion of the call across the terrestrial
network at the same instant at which the GES allocates
C channel resources and begins C channel establishment in the
backward direction. Immediate initiation of forward call
completion is possible because the AM$(R)S abbreviated
access request SU contains all necessary call information as
described in 8.7.2.1. By establishing both the satellite and
terrestrial segments of the end-to-end call concurrently, the
limiting factor in end-to-end access delay (after receipt of the
abbreviated access request SU) is the longer of either the
terrestrial network call completion delay or the C channel
establishment delay - bur not the sum total of both delays as
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