Comtech EF Data VMS v3.12.x Vipersat - Manual (Page 465)

Appendix E - Automatic Switching E-9
MN/22156, rev 12 Load Switching
• Load Switching – This is a type of Automatic Switching that is based on
the amount of traffic at a Remote. If this feature is not enabled, then no
Remote in this STDMA group 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.
Note for Dynamic Cycle mode:
Since Dynamic Cycle mode tends to provide no more bandwidth than is
needed, Remotes will typically appear to be near 100% capacity whenever
they are passing real traffic. Thus, in this mode, if the threshold is set too
low, switches will occur unnecessarily.
• 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
60 kbps, and the Percent Allocation is 10%, then the allocation will be for
60k + 6k = 66 kbps.
Note that, because the Hub always allocates bandwidth in 8 kbps blocks,
the 66 kbps will be rounded up to 72 kbps in this example.
Hub 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.