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of GESs while assuring a uniform minimum level of performance
for the packet data user. In general, the subnetwork
performance parameters are based on the definitions given in
Section 10 of IS0 8348, "Information processing systems -
Data communications - Network service definition" (1987),
which are incorporated into the AMSS SARPs by reference.
The mapping between packets 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 performance refers to the swvice 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, and the physical paths between them.
7.2.1.3.3 The values of the performance parameters in
the AMSS SAWS are intended to permit economical and
spectrum-efficient operation. They are calculated under
nominal worst case conditians, including maximum physical
bit error rate and peak busy hour traffic loading. Improved
speed of service performance (relative to the specified values)
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.
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;
b) the satellite subnetwork dependent protocols;
C) the satellite data link layers; and
d) the satellite physical layers.
IS0 8208 operations associated with the DTE are not part of
the satellite subnetwork, and are 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 SPEED OF SERVICE
7.2.2.1 Speed of service is determined by a number of
factors, 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 measured;
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.
Attachment A to Part I
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 aircraft while other packets are being
transmitted to the GES.
7.2.2.2 In accordance with current industry practice,
speed of service is specified in terms of packets containing
128 octets of subnetwork user data.
7.2.2.3 There is no specification 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 utilization of approximately 70 per cent (of theoretical
capacity) for these channel types. Higher utilizations are
permitted, provided that all of the applicable performance
standards are met. For example, a GES that operates multiple
T channels in an appropriate multiserver configuration may be
able to achieve T channel utilization factors of 80 per cent or
more within the constraints imposed by the speed of service
requirements. P channels may be operated at utilization 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 .
7.2.2.5 The term "highest priority service" denotes
priority 14 service, which is reserved for distress, urgency and
certain infrequent networWsystern management messages. The
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