Comtech EF Data SLM-5650A Vipersat - Manual (Page 118)

Entry Channel Mode (ECM) Switching
B-20 Vipersat SLM-5650A User Guide
Entry Channel Mode (ECM) Switching
STDMA Entry Channel Mode provides a method for Remotes requiring SCPC
access channels to enter/re-enter the network initially or after a power or other
site outage. The switch time will be variable based on the burst rate (bps) of the
STDMA group, the number of Remotes with slots in the group, and where in the
burst cycle the Remote is when it acknowledges receipt of the Burst Map.
Initial SCPC rates are settable for each Remote in the STDMA group(s). Upon
detection of a Burst Map acknowledgement from a Remote, the STDMA Burst
Controller will send a switch request to the VMS with the operator-specified
initial SCPC rate. Upon determining that there is an available demodulator and
pool bandwidth, the VMS will send a multi-command to remove the Remote
from the STDMA group, tune it and the switched demodulator to the specified
initial bit rate and selected pool frequency. The Remote will stay at this initial
rate unless an application (such as VTC) or consistent load cause it to request
additional bandwidth from the VMS.
Entry Channel Mode is not driven by the presence or absence of customer traf-
fic. Once in ECM, the switched initial data rate becomes the new temporary
Home State. This temporary Home State sets the low limit data load threshold,
where the Remote will stop sending load switch request commands. Remotes no
longer require Burst Maps to maintain SCPC transmission.
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 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 power outage at the site, the Remote will
reboot in its Home State (STDMA), then acknowledge the receipt of the first