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The SATCOM data channels carry the following traffic types:
Data service traffic
Packet-mode (data) and circuit-mode (voice) signalling traffic
Satellite system management traffic
All data, signalling, and management messages are formatted into uniform signal units (SU).
5.2.3.4 Data Service Traffic
The data channels carry “Data-2” and “Data-3” types of data message traffic. “Data-2” is used for the
FANS datalink messages sent via SATCOM ACARS.
The “Data-2” and “Data-3” traffic use the following channel scheme:
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FANS-1/A Datalink Communications Environment
P-Channel-Channel: All ACARS Uplinks go via the P-Channel
R-Channel-Channel: Downlink Request for T-Channel (See Note)
T-Channel: User Data downlinks sent via T-Channel after R-Channel Request
(See NOTE)
Note: User data/messages less than 33 bytes go via the R-channel. Data messages greater than
33 bytes go via the T-Channel after successfully being assigned access through the R-channel
request.
In the AMSS system, the P-channels are TDM with no possibility of collisions whereas R-channels are
Slotted ALOHA-based which is subject to collisions. However, in the downlink case, all aircraft need to
share the same channels so there will inevitably be message collisions that will increase with the traffic on
the channel. In addition, the downlink traffic goes through a multi-step process when a message is
greater than 33 bytes. The aircraft must first send the GES a request on the R-channel for a time slot on
the T-channel before sending a downlink message. Upon receipt, the GES sends a T-channel time slot
assignment to the aircraft via a P-channel. The aircraft then sends the downlink data message via the
assigned T-channel time slot. The exception to this multi-step process is in the case where the downlink
data message is less than 33 bytes. In this case, the downlink data message is sent via the R-channel in
one step. The average FANS message is greater than 33 bytes.
5.2.3.5 Date and Voice Signaling Traffic
In addition to the actual data messages themselves, the data channels carry the following signalling data:
Acknowledgment Traffic: each data message sent in one direction is acknowledged in the other
direction.
Reservation Traffic: if a downlink data message is greater than 33 octets, it is necessary to
send a T-channel reservation request over the R-channel and the corresponding T-channel
assignment is sent over the P-channel.
Voice Channel Request Traffic: to make an air-to-ground call, an AES sends a voice channel
request over the R-channel and the corresponding voice channel assignment is sent over the Pchannel.
CN86 Traffic: when a voice channel is released, the GES sends a confirmation SU over the Pchannel.
5.2.3.6 System Management Traffic
The data channels carry the following data related to the management of Inmarsat system:
Logon management data: this includes traffic generated by the system logon procedure, which
includes in particular logon and logon/logoff acknowledgment SUs sent over the R-channel,
logon confirm and logon/logoff acknowledgment SUs sent over the P-channel.
System Broadcast data: a GES broadcasts both the partial and the complete (only a Psid sends
the complete system table) of the system table and the spot beam map (this data is sent
continuously over the Psid channel.)
5.2.3.7 Data Channel Data Rates and Loading
There a low speed (600 bps and 1200 bps) and high speed (10500 bps) data rates defined for the P, R,
and T channels.
The AES R-channel and T-channel downlink data rates are determined by both the AES R-channel and
T-channel capability and the GES's R- and T-channel allocation. The T-channel data rate used is
determined by the GES channel assignment uplinked to the AES in response to an R-channel downlink
15
FANS-1/A Datalink Communications Environment
T-channel request for messages greater than 33 octets. The GES will attempt to use the highest data
rate channel available for P-channel uplinks and R and T-channel assignment.
Some AESs are capable of low speed SATCOM only. Other AESs are capable of both high speed and
low speed. However, not all aircraft that are capable of high speed operation have enabled the use of
high speed SATCOM and, instead operate in low speed only. There are two reasons why an airline may
choose not to enable the high speed capability. The first reason is that there were issues associated with
the use of high speed SATCOM when it was first introduced, such, that, while these issues have since
been corrected, a given airline may not yet have confidence in it. The second reason is that use of high
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