Wavecom W74PC V8.7.0 - Manual (Page 241)

WAVECOM Decoder W74PC, W-PCI/e, W-CODE, W-CLOUD Manual V8.7.0 Transmission Modes

231
for proper bit synchronization. Frame synch is achieved by using an unscrambled unique word at the be-
ginning of the frame.
The coding will correct some errors otherwise ARQ is used.
The forward channels from the LES and NCS are continuous time division multiplex (TDM) channels and
are used for message transmission and signaling.
In the return direction messages are transmitted by the MES on time division multiple access (TDMA)
channels assigned by the NCS. The frame length varies between 2176 symbols and 10368 symbols in
steps of 2048 symbols.
SAT-C TDM
As for the other Inmarsat satellite modes, the Inmarsat C system uses a NCSC (Network Control Station
Channel) for the announcement of new messages for the terminals. At the time of writing these an-
nouncements were transmitted on the following frequencies:
Ocean Region NCSC Frequency Ocean Region NCSC Frequency
AOR-W 1537.70 MHz IOR 1537.10 MHz
AOR-E 1541.45 MHz POR 1541.45 MHz
The NCSC frequency is fixed for each ocean region. This ocean region is user selectable through the ‘Sat-
ellite Settings’ panel.
In the frequency offset bar the Inmarsat C mode has a NCSC-Auto check box.
With this checkbox the user is able to select between two operation modes:
 Fixed Frequency Mode (checkbox not ticked)
 NCSC Auto-Mode (checkbox ticked)
In Fixed Frequency Mode the decoder decodes all messages, both signaling and traffic messages on the
channel tuned to. The traffic messages are transmitted on frequencies announced on the NCSC. This mode
makes it possible to decode for example only the NCSC and display all control information transmitted
there. The user may also leave the decoder on a traffic channel frequency.
In the NCSC Auto-Mode, the decoder is able to recognize announcements for all MES IDs, for a specific
MES ID or a range of MES IDs depending on the watch list settings. The decoder then automatically tunes
to the corresponding traffic channel and decodes relevant data traffic corresponding to the settings. In the
decoder output window only the data content of the messages for the “MES of Interest” is visible. Mes-
sages for any other MES are indicated by the message sequence number and the logical channel assigned
to the MES. However, data files are generated for all messages received, except if a watch list was de-
fined. If a watch list was defined only messages for the MES IDs in the watch list are decoded and stored.
Inmarsat C data content is interpreted according to the codes shown in the table below:
“Announcement” message presentation field WAVECOM decoder output presentation
Pres: 0 ASCII
Pres: 6 Baudot
Pres: [any other] HEX
In NCSC Auto-Mode the decoder interprets the data for the “MES of Interest” according to the value of
the presentation field (“Pres:”) of the Announcement message. For decoded data not for the “MES of In-
terest” the configuration selected in the display dialogue is used.
The NCSC Auto Mode is recommended if the user would like to intercept the traffic for a particular target
MES.
In case the watch list does not contain any MES ID, the decoder will process the first incoming announce-
ment.