User guide

Entry Channel Mode (ECM)
G-2 VMS User Guide
After all remotes are processed into ECM, the Burst Controller drops into sanity
mode sending a keep alive map to service remotes which may have 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 a detailed description of the features of VMS applications switching, refer
to Appendix F, "Automatic Switching". As application switching relates to the
ECM mode, it is useful to describe the fail-safe mechanism 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 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 power outage at the site, the remote CDM-570/
570L or SLM-5650A will reboot in its home state (STDMA), 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 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 G-1 and figure G-2 diagram the time state differences and the process of
recovery. Note that the times referenced in the diagrams are approximate.