Comtech EF Data Vipersat CDM-570 User Manual page 102

L o a d S w i t c h i n g
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.
Before discussing how load switching is determined, it is necessary to explain
the user parameters that control the switch. The menu shown in figure B-1
shows the entries in the automatic switching menu at the hub that are used to
control load switching.
B-4
remote is divided by the total Bytes in Queue for all remotes to give the
percentage allocation for each remote.)
• 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.
• 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 1 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 1 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.
V i p e r s a t C D M - 5 7 0 / 5 7 0 L U s e r G u i d e