Comtech EF Data VMS v3.12.x Vipersat - Manual (Page 482)

Entry Channel Mode Switching MN/22156, rev 12
E-26 VMS User Guide
their SCPC carrier inhibit flag set. The keep alive message is sent once every
two seconds until re-entry is invoked.
Fail-Safe Operation
For Entry Channel Mode switching, it is useful to describe the fail-safe mecha-
nism used for freeing pool bandwidth.
If the VMS loses communications with a switched Remote for more than three
minutes, it will attempt to return the Remote to its home state. If the revert-to-
home state command succeeds (restoring communications), Entry Channel
Mode will cause the Remote to switch to its initial SCPC bit rate.
If the revert-to-home state command fails, the VMS will send a command to
return the Remote and the Hub demodulator to the state where they were prior
to losing communications, but leave the Remote enabled in the STDMA burst
controller. This provides the Remote with 2 paths to rejoin the network:
1. If the outage was the result of a power loss at the site, the Remote will reboot
in its home state (STDMA), then acknowledge the receipt of the first burst
map, causing it to rejoin the network through ECM. The VMS will park the
demodulator previously in use and free the bandwidth slot.
2. If the outage was due to an extended rain fade or other communications
blockage with no loss of power, the Remote will rejoin the network via the
previously assigned SCPC channel. When the VMS receives a PLDM, it will
send a revert-to-home state command and free the bandwidth slot and burst
demodulator. The Remote will then rejoin the network through ECM.
Since it is not possible to know which of the above scenarios caused the
communications outage, the VMS will not free the bandwidth slot except
through operator intervention.
Figure E-15 and figure E-16 diagram the time state differences and the process
of recovery. Note that the times referenced in the diagrams are approximate.