Comtech EF Data VMS v3.12.x Vipersat - Manual (Page 463)

Appendix E - Automatic Switching E-7
MN/22156, rev 12 Load Switching
• Dynamic Slot Mode – The slot size for each Remote is computed based
on the time (at the current data rate) needed to transmit all the “Bytes in
Queue”. If the result is less than the minimum slot size or more than the
maximum slot size, the slot is adjusted accordingly.
• Dynamic Cycle Mode – Available bandwidth is allocated to Remotes
proportionally, based on current need. The Bytes in Queue for each
Remote is divided by the total Bytes in Queue for all Remotes to
determine the percentage allocation of bandwidth for each Remote.
• GIR (Guaranteed Information Rate) Mode – Initially computed the
same as Dynamic Cycle, except there is no maximum limit. After all
Remotes have been assigned slots, the Burst Map is checked to see if the
total cycle length exceeds one second. If not, then all requirements are
satisfied and the Burst Map is complete. However, if the cycle is greater
than one second, then the slots are adjusted proportionally so that all
Remotes receive at least their guaranteed rate plus whatever excess is still
available.
In the current design, when the one second restriction is exceeded,
Remotes without a specified GIR are reduced to the global minimum slot
size and the remaining bandwidth is distributed amongst Remotes that
have been assigned a GIR rate. This approach is based on the assumption
that Remotes that have been assigned a GIR are paying a premium and
should benefit from available excess bandwidth when needed.
Note that the GIR allocations are restricted so that the assigned GIR totals
cannot exceed available bandwidth. If this restriction is somehow violated,
then it will not be possible to properly allocate bandwidth when the
network is overloaded.
• Entry Channel Mode – This is the same as Dynamic Cycle, except that as
soon as the Hub receives an STDMA ACK, it initiates a switch to SCPC
mode based on the policy set for that Remote.
Note that load switching is disabled for ECM Remotes while operating in
STDMA mode.
The important thing to understand about “Bytes in Queue” is that any data that
is not transmitted (i.e., does not fit) in the next slot will be reported again in the
next STDMA ACK. Thus the “Bytes in Queue” is not necessarily an accurate
measure of the actual traffic being passed through the Remote.
The “Queued Bytes” on the other hand, reflects only the data that was received
in the last cycle and thus is never duplicated (not including TCP retransmis-
sions). This is the metric that is used for computing average load and initiating a
load switch as needed.