User guide

Load switching
F-4 VMS User Guide
the traffic requirements of the remote, or if the remote has exceeded the maxi-
mum allocation, some data will be held for the next cycle. In this case, the
number of Bytes in Queue will start to grow and will exceed the Queued Bytes.
(In other words, the Bytes in Queue is the sum of the data not yet transmitted
plus the new data received).
If the condition is due to a short burst of data, the backlogged data will eventu-
ally be transmitted and the system will return to a sustainable rate. However, if
the overload condition is due to long term increased activity, then the backlog
condition will continue to grow and eventually trigger an SCPC switch. If the
overload condition lasts long enough, buffer capacity will eventually be
exceeded and some data may have to be discarded.
Note: This is not necessarily bad, as it is often more effective to discard old
data than transmit it after it has become ‘stale.’
The “Bytes in Queue” metric is used to determine the STDMA bandwidth allo-
cated (slot size) for the next cycle; the goal being to keep the data backlog to
zero. The hub uses this metric to compute the slot size for each remote in the
next cycle as follows:
•Fixed Mode - All remotes get the same slot regardless of need; in other
words, the metric is not used.
• Dynamic Cycle Mode - Available bandwidth is allocated to remotes
proportionally based on current need. The bandwidth allocation for
remotes is calculated by dividing the Bytes in Queue for each remote by
the total Bytes in Queue for all remotes to calculate the percentage
bandwidth allocation to be given to 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
NOTE