User guide

Appendix B - Automatic Switching B-3
Load Switching
Load Switching
The next sections describe the principles behind Load Switching and Rate
Adjustment (Step Up/Step Down).
Bandwidth Allocation and Load Switching by the
STDMA Controller
As part of normal STDMA processing, the Hub monitors the traffic levels from
each of the Remotes for which it is allocating bandwidth. This is done using the
STDMA ACK management message (table B-1) that is transmitted at the begin-
ning of each burst from the Remote. The STDMA ACK contains two metrics
that are used by the Hub:
1. The number of bytes received for transmission (Queued Bytes) since the last
cycle.
2. The number of bytes currently waiting to be transmitted (Bytes In Queue).
These metrics are used by the Hub for three purposes:
1. Determine the amount of STDMA bandwidth (slot size) to allocate in the
next cycle.
2. Provide statistics of the amount of activity at each Remote (Average Bytes
Received).
3. Determine if a load switch is needed.
Table B-1 STDMA ACK Message
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)