Comtech EF Data VMS v3.12.x Vipersat - Manual (Page 468)

Load Switching MN/22156, rev 12
E-12 VMS User Guide
• SCPC Step Down Threshold – Similar to the Step Up Threshold, except
Step Down is used to trigger a switch to a lower data rate when the average
traffic load falls below the set value.
• SCPC Step Delay – This is a built in latency that forces the Remote to
maintain an average load for the specified period (seconds) that exceeds
the switch threshold before a switch to a new data rate is actually initiated.
Similar to the Hub parameter STDMA Switch Delay. However, the Remote
offers two switch delay parameters once the unit has entered SCPC mode:
a Step Up and a Step Down. This provides the operator the option of
specifying, for example, a shorter step up delay and a longer step down
delay to ensure bandwidth requirements are quickly met and sustained
while minimizing repeated switch events due to short-term fluctuations in
the data rate.
• SCPC Step Up Excess – This is an additional amount of bandwidth that is
allocated beyond the calculated traffic rate, and is added to each switch
request.
Note that the value applies to both Step Up and Step Down switches, and
is computed against the average traffic load at the time the switch is
initiated.
For example, if the current average traffic at the time of the switch is
130 kbps, and the Step Up Excess is 10%, then the allocation will be for
130k + 13k = 143 kbps. And because bandwidth is always allocated in
8 kbps blocks, the rate will be rounded up to 144 kbps.
Same as the Hub parameter Percent Allocation.
Determination for Switching
The following process is used to determine if bandwidth utilization warrants a
change, and thus a switch to a new data rate.
The operator defines both a Step Up and Step Down threshold in terms of
percent utilization, a bandwidth margin value, and a latency or averaging
period. Once per second, the modem/router software determines the current
percent utilization by dividing the bits transmitted by the current transmit data
rate.
If the percent utilization exceeds the step up threshold or is less than the step
down threshold for the entire latency period, then a Switch Request is sent to the
VMS. The bandwidth requirement in the request is computed by taking the
average percent utilization over the latency period and multiplying that by the
current data rate to determine the actual data rate used over the measured inter-
val. This number is multiplied by the margin value and rounded up to the near-
est 8 kbps to determine the requested bandwidth.