Comtech EF Data CDM-570/570L Vipersat - Manual (Page 131)

Appendix B - Automatic Switching B-13
Load Switching
3. The STDMA Controller compares the Remote’s pre-selected buffer limits
with its buffer status and, if the buffer status exceeds the preselected limits,
the STDMA Controller increases the time-slot allocated to that channel. If
this brings the buffer status within established limits, no further changes are
made.
4. If the buffer status continues to exceed the preselected limits, the STDMA
Controller sends an ASR to the VMS.
5. The VMS processes the switch request by checking for available resources:
first determining if there is a free demodulator, and then determining the
channel space (bandwidth) requirements to accommodate the data flow
requested by the STDMA Controller.
6. If the VMS finds available resources, it processes the switch request and
sends tuning commands that switch the Remote out of STDMA and into
SCPC mode.
The modem/router continuously monitors traffic flow volume. Whenever a
preset upper or lower limit is exceeded, the modem/router sends a request to the
VMS to change bandwidth by the amount needed to meet the new requirement.
By this process, the bandwidth is continuously optimized in real time, precisely
accommodating circuit traffic volume.
The ideal condition is for utilization of the channel to reach approximately 90%,
thus optimizing the use of available bandwidth. The ability to actually accom-
plish this is limited by the currently available carrier bandwidth and, ultimately,
the power output and antenna size available at the transmitting Remote site.
If the requested bandwidth is not available, the STDMA Controller will
continue to receive buffer status reports from the Remote indicating that buffer
flow is continuing, and the STDMA Controller will, in turn, continue to request
additional bandwidth from the VMS. When bandwidth does become available,
the VMS will perform the switch the next time that the STDMA Controller
makes the request.
If the video data stream ends before the switch in bandwidth is completed, the
channel is closed, the bandwidth which had been allocated is made available
again to the pool, and no further action is taken.
Reduced Data Flow in Switched Mode (SCPC)
In the event the data flow is reduced—for example, a streaming file transfer
terminates—the SCPC switched demodulator detects the reduced flow and noti-
fies the VMS. The VMS will then send a switch command to reduce the size of
the carrier bandwidth to the newly calculated requirement.