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would be typical of the non-AMS(R)S procedure.
8.7.4 AES internorking with nun-compliant GES. There
may be certain AMS(R)S service areas where one or more
GESs are not equipped to support the AMS(R)S air-origination
procedure (e.g. prior to a mandatory lCAO implementation
date). The AMS(R)S procedure as defined for the AES
accommodates this by being inherently compatible with both
equipped and non-equipped GESs (mode switching or crew
intervention are not necessary). Specific attributes of the
AMS(R)S air-origination procedure that allow an equipped
AES to interwork with both types of GESs are as f~llows:~
a) the format of the abbreviated access request SU is
identical to that of the equivalent non-AMS(R)S access
request SU except that the digit No. 2-9 and terminal
ID fields are designated as reserved in the latter SU.
This allows a non-equipped GES to interpret the call
attempt as it would for a non-AMS(R)S call (i.e. the
call information extracted from the SU is identical to
that for a non-AMS(R)S call);
b) equipped and non-equipped GESs will discard any
redundant copies of an access request SU. Therefore,
receipt by the GES of multiple copies of the abbreviated
access request SU will not result in call
processing logic errors (multiple receipt may occur in
the absence of R channel collisions or if timer tA50
has expired before the AES has received a C channel
assignment SU); and
c) the C channel sub-band signalling and continuity test
logic for equipped and non-equipped AESs and GESs
is identical. Of particular note is the -fact that an
equipped AES will repeat the digit No. 2-9 and
terminal ID fields (from the abbreviated access request
SU) on the C channel sub-band in a manner identical
to that of a non-equipped AES. This particular aspect
of the AMS(R)S procedure is key to the ability for an
equipped AES to interwork with any GES.
Note.- The end-to-end access delay performance through
a non-equipped GES will not correspond to that realized when
an equipped GES is in use
8.8 Circuit-mode access
delay performance
Note.- Except where noted, projections of end-to-end
circuit-mode access delay pe@omnce are based on R and P
channels operating at channel rates of 10 500 bits/s.
8.8.1 Air-originated calls. Paragraphs 8.7.2 and 8.7.3
describe the special attributes of the AMS(R)S air-origination
procedure that result in enhanced access delay perFormance for
the safety versus non-safety user. With respect to the concurrent
set-up of both C channel and terrestrial network resources
(8.7.3), the benefits of this improved procedure are most
apparent when the access delay component that is incurred
within the terrestrial network facilities is no greater than that
which is incurred by C channel establishment. As a point of
Attlrchmsnt A to Part 1 Annex 10 - AeronauticaZ Telecommunicohohons
reference it should be noted that the GES will require a minimum
of six seconds (latency) to establish a C channel -
measured from the time at which the C channel assignment
SU is enqueued for P channel service. This implies that to
derive maximum benefit from the enhanced air-origination
procedure, the terrestrial network facilities should impart no
more than an equivalent delay component to the end-to-end
access delay performance - including the time for the answer
indication to flow in the backward direction.
8.8.1.1 GES C channel demodulator acquisition delay.
Annex 10, Volume III, Part I, Chapter 4, 4.3.4.4 allows three
seconds (99 per cent) for the GES C channel demodulator to
recover the received carrier and achieve frame lock (this is
included in the aforementioned six-second minimum for the
GES to establish a C channel after it has received a request).
The AMSS SARPs further recommends that implementors
achieve shorter acquisition times. This delay has a direct effect
on the minimum achievable end-to-end delay in those cases
where the terrestrial network is able to provide an access delay
component of less than six seconds (8.8.1).
8.8.1.2 End-to-end access delay for air-originations. If
one can assume that the delay component which is attributable
to the terrestrial network facilities can be constrained to be no
greater than the component attributable to C channel establishment,
then end-to-end access delay in the absence of any
significant contending P channel traffic can be as short as
eight seconds (95 per cent) for air-originations. IF a GES is
provisioned with C channel demodulators which exhibit
acquisition performance akin to that of the AES's burst mode
of operation, then a five second (95 per cent) end-to-end
　
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