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and SSND protocols. Furthermore, for the satellite
subnetwork. it is more efficient to use tbese connection-mode
protocols. Finally, IS0 8208 is the most mature cmectionmode
protocol available.
7.1.3.1 T ~ E packet data interface allows AMSS to
function as the satellite subnetwork of the ATN. The satellite
subnetwork transfen 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, tbis 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 mare subnetwork users on the ground,
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A-hment A & Part I Annex I0 - Aerorurarticul Tekcommunic4dions
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 interruption of
service.
7.1.3.4 The originator of a CALL REQUEST packet will
normally identify both the called and 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 SARPs 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 matkrs.
7.1.4 ERROR HANDLING 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 subnetwork
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.
7.2 Packet data performance
7.2.1.1 The ~ eford IC AO
perfomnce specifccations
7.2.1.1.1 AMSS minimum system performance standards
apply over a geographic coverage area to be identified by each
AMSS 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 and/or 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
AMSS 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
arrangements 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 basic performance standard for the physical layer is the
maximum bit error rate, which is specified as 1 om3 for digital
voice and 1 w5 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
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