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factor in end-to-end access &lay (after receipt of the
abbreviated access request SU) is the longer of either the
ternstrial network calI completion delay or the C channel
establishment delay - but not the sum total of both delays as
would be typical of the general call origination procedure.
8.7.4 AES interworking with non-compliant GES. There
may be certain AMS(JX)S service areas where one or more
GESs are not equipped to support the abbreviated airorigination
procedure (e.g. prior to a mandatory ICAO
implementation date). The abbreviated 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 abbreviated air-origination procedure that
allow an equipped AES to interwork with both types of GESs
are as follows:
a) the format of the abbreviated access request SU is
identical to that of the equivalent general access request
SU except that the digit No. 2-9 and terminal W 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 general call (i.e. the call information
extracted from the SU is identical to that for a general
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 nonequipped
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 thmugh
a mn-equipped GES will not correspond to that realized whn
an equipped GES is in we.
8.8 Circuit-mode access
delay performance
Note.- Except whem noted, pmjections of end-to-end
circuit-mode access delay pe~~nnancame h e d o n R and P
channels opemting at channel rates of 10 500 birds.
Y11IOo
No. 75
Annex 10 - Aeranuuticul Telecommunic&m Volume IZZ
8.8.1 Air-originated calls. Paragraphs 8.7.2 and 8.7.3
describe the special attributes of the abbreviated air-origination
procedure that result in enhanced. access &lay 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 cham1 establishment. As a
point of 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 c h m l service. This implies that to derive
maximum benefit from the enhanced air-orjgnation p d u r e ,
the terrestrial network facilities should impart no more than an
equivalent delay component to the end-to-end xcess 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. Wlume III. Part I, Chapter 4, 4.3.4.4 allows three
seconds (99 per cent) for the GES C channel demodulatm to
recover the received carrier and achieve frame lock (this is
included in .the aforementioned six-second minimum for the
GW to establish a C channel after it has received a request).
The AMSS SARPs further recommends that implementon
achieve shorter acquisition times. This delay has a direct effect
on the minimum achievable end-t&end delay in those cases
where the ternstrial 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 dehy for air-originations. If
one can assume that the delay component which is attributable
to the terrestrial network facilities can be w11strahe.d to be no
greater than the component attributable to C channel
establishment, then end-to-and access delay in the absence of
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