User guide

Load Switching
B-6 Vipersat CDD-56X Series User Guide
• Load Switching - This is a type of Automatic Switching that is based on
the amount of traffic at a Remote. If this mode is not set, then no Remote
will be switched based on load.
• STDMA Slot Capacity - This is a threshold value. When the amount of
outbound traffic at a Remote exceeds this percentage of the current
STDMA slot capacity, a load switch is initiated. It is important to
understand that in most STDMA modes, the amount of bandwidth
allocated to a Remote varies with need and thus from cycle to cycle. Thus
the amount of traffic that constitutes X% will also vary from cycle to
cycle.
• STDMA Switch Delay- This is a built-in latency that forces a Remote to
maintain an average load over some number of seconds after reaching a
switch condition before the switch is actually initiated. This prevents
switches due to momentary traffic bursts.
• Percent Allocation - This is an excess amount of bandwidth that is
allocated beyond the current traffic rate when the switch to SCPC is made.
For example, if the current average traffic at the time of the switch is 60K,
and the Percent Allocation is 10%, then the allocation will be for 60K +
6K = 66K.
Note that the Hub always allocates bandwidth in 16K blocks, so 66K
rounded up will actually be 80K in this example.
Load Switching Process
Each time the Hub receives an STDMA ACK, it computes the average load for
that Remote. This average is then compared to the bandwidth currently allo-
cated to the Remote.
For example, if a Remote gets a 50 ms slot in an upstream that is running at
512000 bps, then it can transmit 0.050 * 512000 = 25600 bits = 3200 bytes. If
the Queued Bytes was 3000, then for that cycle, the Remote was at 3000/3200 =
93.75% of capacity. If the current cycle time is exactly 1 second, then the effec-
tive data rate of the Remote is also 25600 bits per second. However, if the cycle
time is only 500 milliseconds, then the effective data rate is actually 25600 / .5 =
51200 bits per second.
The effective data rate is important for calculating switch data rates. If the aver-
age bandwidth used exceeds the threshold percentage of available bandwidth,
then a flag is set indicating a switch is pending. At this point, the statistics are
reset and the traffic load is then computed for the time period specified by the
switch delay. At the end of this delay, if the threshold is still exceeded, a switch
is initiated. The data rate specified for the switch is determined by taking the