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terminal area or aerodrome surface communications except in
areas of very low channel loading.
4.5.5 The VDL SARPs call for the provision of a
common signalling channel (CSC) on which access to VDL
service will be guaranteed in all areas where VDL Mode 2
service is available. This is especially important at airports and
on the edge of VDL en-mute coverage zones where aircraft
are likely to establish initial VDL connectivity. Since the
characteristics of Mode 1 and Mode 2 radio frequency transmissions
are not compatible, the CSC cannot be used far
Mode 1 wmmunications. There is no requirement for a CSC
for VDL Mode 1.
4.6 Ground station connection
to intermediate systems
4.6.1 In order to provide access to the ground systems
which are connected to the aeronautical telecommunications
network, a VDL ground station needs to be connected to one
or more ATN IS. The purpose of a VDL ground station is to
interconnect aircrdt with the ground-based ATN via which
communications with terrestrial ATN ES can take place.
4.6.2 The ground-based ATN IS can be co-hosted in the
VDL ground station computer in which case the VDL subnetwork
virtual circuit will end in that computer. This
architecture will have an impact on the exchanges required
when an aircraft establishes a VDL link with a new ground
station. The exact exchange will depend on whether the
ground stations contain separate IS or elements of the same
distributed intermediate system.
4.5.2 As with VHF voice communications, VDL 4.6.3 If the IS is not contained in the VDL ground
communications cannot be limited to propagating only within station, it will be connected to the ground station by one of the
States and frequency co-ordination between States will be following means:
required in the allocation of VDL frequencies. The nature of
the protocol does, however, allow for frequency re-use by a) wide area network (WAN);
several ground stations within the same coverage area and
hence the rules For the assignment of frequencies are not the b) local area network (LAN); and
same as for voice commun~cations. c) dedicated communications line.
4.5.3 The camer sense multiple access (CSMA) media
access control protocol (MAC) layer used in VDL cannot
exclude message collisions if some stations using a frequency
channet cannot receive the transmissions of other stations, a
situation known as a hidden transmitter situation. Hidden
transmitters lead to simultaneous transmissions which can
cause the intended receiver of one or both transmissions to be
unable to decode the received signal.
4.5.4 A frequency will be assigned to providing en-route
coverage and all the en-route stations will be set to operate on
this frequency. In order to minimize the probability of simultaneous
transmissions on the channel by hidden transmitters in
4.6.4 In all cases, in order to be in accordance with the
Manual of the Aeronautical Telecommunication Network
(ATN) (Doc 9578) for providing an open systems interconnection
(OSI) compatible connection-oriented subnetwork
service between the aircraft IS and the ground-based IS, the
VDL ground station computer will be required to extend the
VDL virtual circuit across the terrestrial network or link.
4.6.5 In order to provide simultaneous virtual circuits to
several terrestrial ISs, the VDL ground station computer needs
to contain a VDL subnetwork entity capable of convening
addresses in VDL subnetwork call requests into addresses in
the ground-based network.
-6/13/97
No. 72
Annex 10 - Aermascticul Telecommuni&ns Volume XXI
5. VDL AIRBORNE
OPERATING CONCEPT
5.1 Avionics
5.1.1 VDL avionics. In order to operate in a VDL network,
aircraft need to be equipped with an avionics system providing
the VDL subnetwork user (IS0 8208 DTE) function. The system
providing this function will also provide the subnetwork
user functions for the other air-ground ATN-compatible subnetworks
and the airctaft ATN intermediate system function
and, hence, its development is necessary in order to provide
ATN communications to multiple end-systems or over multiple
air-ground subnetworks.
5.2 VDL avionics certification
5.2.1 The VKF digital radio niay also provide for
double-side band amplitude modulation (DSB-AM) voice
capability for emergency back-up to VHF radios used for
voice communications. It would be necessary in this case to
demonstrate that the VDL functionality of the VDR does not
interfere with the DSB-AM voice functionality.
5.2.2 The VDL function in- the VHF digital radio
provides an air-ground data link service to the VDL sub-.
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