Comtech EF Data SLM-5650A Vipersat - Manual (Page 102)

Load Switching
B-4 Vipersat SLM-5650A User Guide
switch is initiated. The data rate for the switch is computed by determining the
current bandwidth requirement of the Remote, and adding some percentage of
excess margin.
The main difference between switching from STDMA to SCPC and adjusting
within SCPC is that in STDMA mode, the current available bandwidth is
constantly changing, while in SCPC mode, it is constant between switches.
Furthermore, switches from STDMA to SCPC mode are always caused by the
traffic level exceeding the switch rate threshold. Within SCPC mode, switches
can be caused by traffic exceeding an upper threshold or dropping below a
lower threshold. However, in both cases the new data rate is based on the actual
traffic requirements adjusted up by the margin percentage. Also, based on
policy settings in the VMS, if a Remote requests less than the specified thresh-
old amount of bandwidth, the Remote is put back into STDMA mode. The
exception to this is a Hub controller operating in ECM whose Remotes will
remain in SCPC mode but drop down to the specified entry rate.
Bandwidth Allocation and Load Switching by the Hub STDMA
Burst 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:
• The number of bytes received for transmission (Queued Bytes) since the
last cycle.
• The number of bytes currently waiting to be transmitted (Bytes In Queue).
These metrics are used by the Hub for three purposes:
• Determine the amount of STDMA bandwidth (slot size) to allocate in the
next cycle.
• Provide statistics of the amount of activity at each Remote (Average Bytes
Received).
• Determine if a Load switch is needed.