Furuno FELCOM 12 Service Manual page 28

Shore-to-ship Message Transfer Steps
1. On receipt of the call over the public networks, the LES checks its MES database, to
ensure that the MES is valid and logged-in.
2. The LES sends the result (MES status) to over the public networks.
3. If the MES is logged-in, the LES accepts and stores the incoming message.
4. The LES sends a MES status request over Interstation Signalling Link (ISL) to the
NCS in its ocean region.
5. The NCS checks its database to find the status of the MES, and returns this
information to the LES in a MES status packet over the ISL.
(a) Not in the region
(b) Idle (free)
(c) Busy (not free)
6. If the MES is idle, the NCS transmits a shore-to-ship announcement over its common
channel to the MES.
7. The MES receives the announcement and finds the CES TDM channel, to which it
should tune. And then, the MES tunes from NCS common channel to the CES TDM
channel.
8. The announcement signal contains the bulletin board and signalling channel packets,
which the MES uses to select a slot in one of the signalling channels to transmit an
"Assignment response" packet to the LES.
9. The MES transmits the "Assignment response" packet to the LES. This establishes
the connection between MES an LES.
10. The LES sends a "MES status" packet to the NCS indicating busy status.
11. The LES transmits the message on the LES TDM channel to the MES.
12. After the message is transmitted, the LES sends a "Acknowledgement request"
packet to the MES.
13. In response to the "Acknowledgement request" packet, the MES sends an
"Acknowledgement" packet, which contains a list of any packets that were missed or
received in error. Reception of an "Acknowledgement" packet prompts the LES to
retransmit any messages in error.
14. After all packets have been received error free, the MES sends the
"Acknowledgement" packet.
15. If there are no further calls, the LES begins the call clearing process by sending a
"Clear" packet.
16. After receiving the "Clear" packet the MES synchronizes to the NCS common
channel to go to idle status.
17. The LES tells the NCS that the MES is idle.
18. The NCS updates its data base to show the MES is idle.
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