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Appendix B - Automatic Switching B-3
Load Switching
3. Determine if a load switch is needed.
Table 2.1
STDMA ACK Message
Table B-1
If there is adequate upstream bandwidth available, the values of these two
metrics will be the same. However, if there is not enough bandwidth to satisfy
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; i.e. the
metric is not used.
• Dynamic Cycle Mode - Available bandwidth is allocated to remotes
proportionally based on current need. (The Bytes in Queue for each
Data
Type
Size in
Bytes
Description
Unit of
Measure
Notes:
IP 4 IP address of Remote N/A Used by remote to identify
itself
Unsigned 4 Queued Bytes Bytes Total number of bytes queued
since last cycle (includes
possible buffer overflow)
Unsigned 4 Bytes in Queue Bytes Number of bytes currently
queued
Unsigned 1 Group Number N/A
Unsigned 1 Dropped Buffers Packets Number of packets dropped
(due to limited bandwidth)
NOTE