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speed data channels requires additional power over that required for low speed channels. The result of
this is that fewer SATCOM voice channels are available when operating high speed data channel units at
the edge of coverage in high latitudes.
The INMARSAT sponsored SATCOM Improvement Team is recommending to those airlines that are
using low speed SATCOM channels to change to the high speed channels. Low or High speed channel
use is selectable by individual airlines in the aircrafts Operational Requirements Table (ORT). This is
complicated by the fact that some Satellite Data Units (SDU) on the aircraft will always use the high
speed channel regardless of the setting in the aircraft ORT.
Data channel loading is the percentage of effective channel capacity used to transmit user data and
signalling traffic. Figure 9 illustrates the T-channel loading effects on delay.
Figure 9. Delay Versus T-Channel Load
T-Channel load (%)
Channel load
increases as traffic
increases
Congestion appears
if the instantaneous
load on a channel
exceeds its capacity
Congestion causes
delay:
– Data must be
queued up for
Traffic (Kbit) Delay (Sec) transmission
100 %
80 %
Traffic vs T-Ch load
T-Ch load vs Delay
0
No Congestion
Aera
T-channel capacity limit
Congestion
Aera
16
FANS-1/A Datalink Communications Environment
5.2.3.8 Capacity Planning
The GES capacity is not infinite so users need to plan the addition of new applications to identify if
required performance can be achieved. There is ongoing work on how to best achieve accurate capacity
planning.
Global capacity planning by all stakeholders is necessary to draw up a plan for maintaining the availability
of the classic aeronautical service at an acceptable level of performance through 2018.
The capacity planning approach to date has generally been a top down approach of setting targets for the
percentage of data link messages to be delivered within given target times. This approach could only
work so long as there was no capacity issue on the AMSS Inmarsat network. The top down targets may
remain but the current problems show that the ISPACG and other FANS implementation groups around
the world also need to start using a bottom up approach to predict the performance that will be provided
for additional traffic.
The solution would be to feed any planned additional traffic into a model of how the Inmarsat/ICAO
Aeronautical Mobile Satellite Service (AMSS) protocol works with its P, R, and T channels for the actual
traffic on the existing channels assigned by Inmarsat and the data rates supported by the aircraft using
the service. The planned additional traffic from AOC, APC and ATS traffic from all airlines and all ATS
provider regional groups needs to be fed into a central model because they all share the same satellites
and GES. This can be compared with the actual capacity.
Accurate ATS traffic forecasts are needed to have more accurate traffic forecasting to input into capacity
plan models. An Excel based traffic forecast form was trialled but it soon became apparent that it was
impractical to get such details from ATSP’s. Currently under investigation is a recommendation that each
datalink service provider determine historical FANS growth and that Boeing in their role as CRA, identify
any given ANSP FANS plans and airline equipage plans that Boeing views will result in a more than
typical FANS traffic growth. The Boeing CRA will quantify such non-typical growth per ANSP and airline.
More than typical growth is defined as growth that is greater than what would be predicted based on using
historical data only. Datalink service providers can then take this information to refine their traffic forecast
information.
17
FANS-1/A Datalink Communications Environment
5.3 ATS Internetworking, Routing and Media Selection, and ATS/AOC
Prioritization
CSP’s supporting FANS services support ATS internetworking. As previously described above, the
internetworking enables 1) an airline to have their choice of CSP network/media to use when communicating
with a given ATSP FANS end system, regardless of what CSP network it is connected to, and 2) enables a
FANS ATSP to send uplinks to a single address without needing to know what CSP network a given aircraft
is currently using. Downlinks received by the CSP that is not connected to the ATSP are sent to the ATSP
via internetworking with the CSP connected to the ATSP. Uplinks from a FANS ATSP are sent to the
internetworking function of the CSP to which the ATSP is connected. The internetworking function then
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