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any significant contencling P channel traffic can be as short as
eight seconds (95 per cent) far air-originations. If a GES is
provisioned with C clumml 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 access
delay figure is possible provided that the terrestrial network
facilities can provide an equivalent improwment in
performance.
8.8.2 Gmund-originated culls. The AES and GES logic
pn>cedu~s for gmund-originated AMS(R)S calls are
essentially identical to those of the prevalent nm-safety AMSS
procedures. The expected access delay component that is
attributable to the AMS(R)S subnetwork in the absence of any
significant contending P channel traffic is projected to be six
seconds (95 per cent). (Inclusive of three seconds for GES C
channel demodulator acquisition overhead). A terresuial
network facility delay component of fwr to six seconds will
result in an expected end-twnd access &lay of 10 to 12
seconds (95 per cent). Improvements in eitlrer terrestrial
network or GES demodulator overhead will yield an equivalent
improvement in the end-to-end delay performance.
8,8.3 Projections of AMS(R)S subnetwork cat 1 set-up
delay. The call processing delays in Annex 10, Iblume UI,
Part I, Chapter 4, 4.8.4.1 specify only the components of
AMS(R)S call set-up delay attributable to the performance of
AES and GES equipment. These performance parameters,
while not entirely deterministic in nature, are not likely to
exhibit a significant statistical variance. This is because the
parameters are specified as maximum internal processing
delays exclusive of any transmission delays or queuing delays
for link layer service such as that which might be experienced
by a telephony signalling CM-LIDU awaiting P channel
transmission. However, the over-all AMS(R)S subnetwork
delay to establish an air or ground-originated call, which will
be subject to the statistical petformame of the link layer, is
likely to be of interest to system planners. Based on simulation
studies using a traffic model identical Po that used to determine
the packet-mode performance requirements in Annex 10,
Volume III, Chapter 4, 4.7. AMS(R)S subnetwork call set-up
delay performance f6r"abbreviated air and ground-originated
calls is projected to be as depicted in Table A-13 of this
guidance material.
Note.- Each pe$otmmrce purumeter is applicable co all
AMS(R)S priorities except for tbse panmeters expressed QS a
mge 4 W ~forS hig hest b lowest p~ority.
8.8.4 PSTN end-to-end call set-up delay. Ead-to-end call
set-up &lay m y be excessive when the AMS(R)S subnetwork
is interconnected with the PSTN unless a specific perfmnance
is specified for the PSTN.
8.9 Subjective vdce
quslity walnatlon
8.9.1.1 Cbcoders employing the algorithm described in
Appendix 7, among others, were evaluated by BT Research
hboratories (BTRL) and the U.K. CAA. The ~ u l ouf t hese
evaluations were used in the selection of this aigorithm from
the three f i d s t contenders.
8.9.1.2 Intelllgibilisy. The BTRL evaluation used the
mean opinion score (MOS) assessment methodology. In the
BTRL evalustim, an MOS of about 3.1 was obtained under
conditions of optimized input and listen levels, no channel
noise and no ambient aircraft noise. When a channel bit error
rate (BER) of 0.001 or greater was introduced and aimaft
ambient noise was W, the MOS ranking decreased.
8.9.1.3 The U.K. CAA evaIuations used a specially
constructed test environment in which the test subject, a
controller, was placed in a simulated work situation. A
''pseudo-pilot" read typical ATC messages to the controller
and responded to the controller's queries and instructions.
Varying levels of channel bit e mra tes were introduced. The
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No. 75
A#achment A to Parl I Annex 10 - Aerorcau~uI Telecorronrunkntions
qualitative conclusion was that the vocoder was acceptable for
A X purposes in low-density airspace, such as oceanic.
8.9,2 DVSI 4.8 KBITS/S AMBE CODE
8.9.2.1 The acceptability of the DVSI codec as a suitable
alternative was assessed based on comparative tests against the
BTRL 9.6 kbitsls LPC codec. These comparative tests were
&ed out by Comsat Laboratories bmd on test requiremenb
agreed at RTCA Special Committee 165 and had the objective
of determining whether this codec had an equivalent or better
performance than the BTRL codec. Diagnostic rhyme test
@RT) and MOS were used in this assessment.
8.9.2.2 The results of the comparative tests led to the
　
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