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end systems (ES) using the ATN and its subnetworks, including
the VDL, for air-ground comml~nicationis provided by the
transport layer entities in these end systems. Transport
connections between airborne and ground end systems shall be
maintained though controlled changes of the precise ATN
intermediate systems (IS) and VDL network elements that
provide this connectivity.
protocol data units transmitted by an ES can pass via any
air-ground ATN compatible subnetwork (such as aeronautical
mobile-satellite service (AMSS) data link, SSR Mode S data
link or VDL) that meets the quality of service {QOS) requirements.
A transport connection between an aircraft ES and a
ground ES shall be maintained as long as there is at least one
air-ground subnetwork connection between the aircraft IS and
a ground IS which has connectivity to the ground ES. In order
to maximize subnetwork connectivity, aircraft are expected to
maintain air-ground subnetwork connections via any subnetwork
(AMSS, Mode S or VDL) with which link layer
connectivity can be established.
3.1.3 The VDL subnetwork provides connectivity in the
form of switched virtual circuits between IS0 8208 data
terminal equipment (DTE) entities of aircraft and ground-based
ATN intermediate systems. Due to the fact that VHF signals
have only line-of-sight propagation, it is necessary for aircraft
in flight to regularly establish link connections with new VDL
ground stations in order to maintain VHF coverage. An established
VDL virtual circuit between an aircraft DTE and a
ground DTE is maintained through a controlled change to a
ground station through which the ground DTE can be
accessed.
3.1.4 VDL virtual circuits may be cleared when the
aircraft or ground IS identifies a policy situation where the
virtual circuit to the ground DTE is no longer necessary but
this shall only happen if another VDL virtual circuit remains
established. A policy situation is a situation where considerations
other than coverage influence the decision to establish a
connection. This could be, for example, a situation where an
aircraft is within the designated operational coverage area of
ground stations operated by different operators and a decision
must be made with which operator to establish a connection.
The case where an aircraft crosses a border between two
States needs special attention. An aircraft has to establish a
virtual circuit to the DTE in the IS of the State entered before
clearing the virtual circuit with the DTE in the IS of the State
left.
3.1.5 The scenarios for subnetwork connection maintenance
are shown in Figure 8-1'. If the ground stations on each
side of a State border do not offer IS0 8208 connectivity to
3.1.2 Transport connections between ATN ES are not
linked to a particular subnetwork and IS0 8473 network 1. All figures are located at the end of this attachment.
ANNEX I0 - VOLUME I11 -MlWI No. 72
Annex 10 - Aeronautical TeZecommunic&'ons Volume IZZ
the DTEs of the IS in both States, aircraft crossing the border
will have to set up a link connection to a ground station in the
State entered before being able to establish a virtual circuit to
the IS of that State. Only after establishment of the new link
connection and virtual circuit, the aircraft will clear the virtual
circuit with the DTE of the IS of the country left over the link
which gave access to that IS. If the VDL aeronautical stations
on both sides of the State border offer connectivity to the IS
in both States, the changeover of the virtual circuits has to
take place over the same link connection.
3.2 VDL quality of service for ATN routing
3.2.1 The use of the VDL system for air-ground
communications will depend on the routing decisions of
aircraft and ground-based AT~iIn termediate systems (IS).
These ISs will decide on the path to be used for air-ground
communications based on quality of service values requested
by transmitting end systems (ESs).
3.2.2 The IS at each end of the air-ground connections
must interpret the requested QOS value and decide which of
the available connections can best be met. It i s important that
the level of QOS which a VDL connection is perceived as
providing is set at a level which corresponds to its true
performance.
3.2.3 In cases where the VDL is the only data link with
which an aircraft has been equipped, all cornrnunications must
be routed via a VDL connection and the value set for QOS to
b e provided. by the connectidn must not block the
communication.
3.2.4 In other cases where aircraft are equipped with
other air-ground data links (such as AMSS and SSR Mode S)
　
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