Comtech EF Data SLM-5650A Vipersat - Manual (Page 108)

Load Switching
B-10 Vipersat SLM-5650A User Guide
Same as the Hub parameter STDMA Switch Delay.
• 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.
Load Switch Example
An automatic load switching example, illustrated in the schematic diagram in
figure B-3, illustrates how a network can respond to changes in traffic volume
or load conditions. The network’s capability and method of response to load
changes is determined by the setting and capability of each of the components in
the system, such as the transmitter power output, the antenna capabilities for
each of the sites in the network, and the policies set in VMS.
The elements for determining policies and their interactions are covered in this
section.