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efficient to use these connection-mode protocols. Finally, IS0
8208 is the most mature connection-mode protocol available.
7.1.3 SERVICES
7.1.3.1 The packet data interface allows AMSS to
function as the satellite subnetwork of the ATN. The satellite
subnetwork transfers data packets from air to ground and from
ground to air. Packet data transfers are provided in the form
of connection-mode service, using IS0 8208 as a subnetwork
access protocol (see Figure A-14 of this guidance material).
7.1.3.2 The subnetwork user must initiate the set-up of
each connection by sending an IS0 8208 CALL REQUEST
packet. In normal operation, this action results in the establishment
of a switched virtual circuit (SVC) between the calling
subnetwork user and the called subnetwork user. SVCs can be
released by either subnetwork user (when the connection is no
longer needed) or by the subnetwork (when the connection is
no longer supportable).
7.1.3.3 One or more subnetwork users on an aircraft
may be connected to one or more subnetwork users on the
ground, but all ground users will normally interface via a
single ground earth station (GES) at any given time, Handovers
from one GES to another may result in the temporary
intemption of service.
7.1.3.4 The originator of a C&L REQUEST packet
will normally identify both the called add calling subnetwork
user. The originator may also identify quality of service
requirements by invoking the appropriate IS0 8208 facilities.
7.1.3.5 Implementations of a satellite subnetwork will
normally contain elements that are not standardized by ICAO.
One of these elements is a packet switch that interprets DTE
addresses and delivers packets to the appropriate DTE. The
AMSS SAWS permit (but do not require) the connection of
multiple DTEs with an aircraft earth station (AES) or with a
GES, or with both. AMSS SARPs also permit the use of any
DTE address format, subject to the limitations of IS0 8208.
Subnetwork users are responsible for co-ordinating the DTE
address formats (and address compression schemes, if
employed) between the air and the ground.
7.1.3.6 The satellite subnetwork provides connectivity
notification event messages to the attached ATN routers. Since
CN event messages are not transferred across the subnetwork.
the standardization of formats and media for these messages
are considered to be local matters.
7.1.4 ERRORH ANDLING FOR
COMMUNICATION FAILURES
If communication between an AES and a GES is abruptly
terminated (e.g. due to a loss of physical connectivity), it is
possible for the subnetwork layer to remain in its last state
until connections are cleared by the user. The GES subiletwork
layer is particularly susceptible, since it may not immediately
be aware of the loss of communication. Users and/or implementors
may use supplemental means to detect and correct
such conditions.
Annex 10 - Aeronautical Telecommuni~tions Volume III
7.2 Packet data performance
7.2.1.1 The need for ICAO
performance specifications
7.2.1.1.1 AMSS minimum system performance standards
apply over a geographic coverage area to be identified by each
GMSS service provider. In general, the minimum system
performance should be continuously available to each AES
within the identified coverage area. However, AES antennas
are not required to provide full coverage in all directions;
multiple antennas andlor multiple satellites may be needed in
order to achieve the minimum system performance.
7.2.1.1.2 AMSS minimum system performance standards
do not represent the fundamental limits of system performance.
Under favourable conditions. the achievable ANSS system
performance may be significantly better than the minimum
standards reflected in the AMSS SARPs. States are not precluded
from seeking AMSS service arrangements that would
provide system performance that exceeds the minimum
requirements of the AMSS SARPs. However, such anangements
should take into consideration the minimum AES
configurations and the minimum AES performance requirements
of the AMSS SARPs.
7.2.1.2 Physical layer
The basiic performmce standard for the physical layer is the
maximum bit emr rate, which is specified as lw3 for digital
voice and 10.' for packet data and associated signalling. These
system specifications avert the need to specify detailed satellite
link budgets, thus allowing the flexibility to implement a
variety of satellite and GES architectures.
7.2.1 -3 Packet data subnetwork layer
7.2.1.3.1 The specification of subnetwork service
performance will allow flexibility in the design and operation
　
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