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7.2.1.3.1 The specification of subnetwork service
performance will allow flexibility in the design and operation of
GESs while assuring a uniform minimum level of performance
for the packet data user. In general, the subnetwork perfonnance
parameters are based on the delinitions given in Section 10 of
IS0 8348, "'Infomation processing systems - Data cornrnunications
- Network service definition" (1987), which are
incorporated into the AMSS SARPs by reference. The mapping
between packers and subnetwork service primitives is defined in
IS0 8878, "Information processing systems - Data communications
- Use of X.25 to provide the OSI connection mode
network service".
7.2.1.3.2 The term "subnetwork service" in the context of
packet data perfonnance refers to the service provided by the
subnetwork to the higher layer user. The term "service
provider" in this same context refers to the protocols within
the satellite subnetwork and to the net effect of their operation.
These terms should not be confused with the services of the
satellite communication provider or GES operator. Packet data
service depends on all elements of the system; AES, satellite,
GES, ancl the physical paths between them.
7.2.1.3.3 The values of the performance parameters in the
AMSS SARPs are intended to permit economical and
spectrum-efficient operation. They are calculated under
nominal worst case conditions, including maximum physical
bit error rate and peak busy hour traffic loading. Improved
speed of service performance (relative to the specified vdues)
may be achieved by increasing the number of physical
channels used to serve a given traffic load, thus decreasing the
traffic load per channel. Using this technique, cost of service
may be expected to increase (and spectrum efficiency may be
expected to decrease) as the speed of service is improved.
Annex 10 - Aeronautical Telecommunk&ns Vdums ZZZ
7.2.1.3.4 AMSS subnetwork performance does not
include the effects of other links in the end-to-end data path.
Subnetwork performance includes the effects of all operations
beginning with the IS0 8208 DCE protocol at the sending end
of the satellite subnetwork and terminating with the IS0 8208
DCE operations at the receiving end of the satellite
subnetwork, including:
a) the satellite subnetwork interworking functions;
requirements. P channels may be operated at utibzation factors
higher than 70 per cent if the total P channel utilization by
traffic at and above the lowest priority associated with safety
and regularity of flight does not exceed 70 per cent.
7.2.2.4 A GES that is intended to serve as a backup to a
another GES should be appropriately sized to provide the
required performance in the event of the failure of the primary
GES.
b) the satellite subnetwork dependent protocols; 7.2.2.5 The term "highest priority service" denotes
priority 14 service, which is reserved for distress, urgency and
c) the satellite data link layers; and certain infrequent networklsystem management messages. The
term "lowest priority service" denotes tbe lowest priority used
d) the satellite physical layers. for safety and regularity of flight, under peak-hour traffic
loading. Performance for priority levels not used for safety or
IS0 8208 operations associated with the DTE are not part of regularity of flight is not specified.
the sateiiite subnetwork, and afe excluded from AMSS packet
data performance specifications, as are data link layer and
physical layer operations between the DCE and the DTE. 7.2.2.6 Ransit &lay a d 95 per cent
doto tmmfier delay
?.2,2 SPEED OF SERVICE
7.2.2.1 . Speed of service is determined by a number of
facturs, including:
a) the system architecture;
b) the channel rate(s) in use;
C) the priority of the traffic for which speed of service is
being measud,
d) the length of the messages for which speed of service is
being measured, the traffic loading on the system; and
e) data processing delays.
Finally, the actual speed of service in the from-aircraft
direction depends on the amount and type of traffic being
transmitted from each aircraft, to the extent that packets may
be delayed on the aircmh while other packets ase being
transmitted to the GES.
7.2.2.2 In accordance with current industry practice,
speed of sewice is specified in terms of packets containing 128
octets of subnetwork user data.
7.2.2.3 There is no specitication for the maximum amount
of traffic to be handled on P or T channels. However, the speed
of service performance standards are based on a maximum
　
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