Comtech EF Data SLM-5650A Vipersat - Manual (Page 106)

Load Switching
B-8 Vipersat SLM-5650A User Guide
• 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.
If the average 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 spec-
ified 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 deter-
mined by taking the current load, as indicated by the bytes queued during the
delay period, multiplying it by the percent allocation and rounding up to the
next 8 kbps.
A key point is that in most of the STDMA modes, the bandwidth allocated to
each Remote is constantly being adjusted to the needs of the network. As long
as the network is running below capacity, most Remotes will get the bandwidth
they need and a switch will not be required. Only when a Remote requires more
bandwidth than is available in STDMA will a switch occur.
In Dynamic Cycle mode, each Remote will always appear to be running at near
100% capacity, even when there is actually excess bandwidth available. This is
because in this mode, the Remotes are almost never given more bandwidth than
they need. As a result, the algorithm for this mode uses a maximum allowed slot
size rather than the actual allocated slot size to calculate the effective data rate.
This results in a more accurate estimate of the available STDMA bandwidth.