Nera Inmarsat-c, Service Manual

Page: 134 / 228

AP1.2 Message & Signal Transfer
AP1-13
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 “MES Status Request Announcement” packet to NCS via
“Interstation Signaling Link (ISL). This packet requests the calling to the MES.
5. The NCS checks its database to find the status of the MES, and returns this
information to the LES.
MES status
- Not in the region
- Idle (free)
- Busy (not free)
6. If the MES is idle, the NCS transmits “Announcement” packet over its common
channel to the MES to inform the message. LES is also informed the message.
7. The MES receives the announcement and finds the message, which LES sends it
and the LES TDM channel to be used. And then, the MES tunes to the LES TDM
channel to be used.
8. The MES finds the frequency of the signaling channel and the slot number from BB
(Bulletin Board) and SCD (Signaling Channel Descriptor) to send “Assignment
response”.
9. The MES transmits the “Assignment response” packet to the LES. This establishes
the connection between MES and 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 “Acknowledgement request” packet
to the MES.
13. The MES sends “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.