User guide

Load switching
F-2 VMS User Guide
Bandwidth Allocation and Load Switching
Load Switching is the mechanism by which the Vipersat network switches a
remote terminal from STDMA to SCPC mode or SCPC-to-SCPC dynamic
based on traffic levels at the remote. There are two components of load switch-
ing in a Vipersat system: VMS (Vipersat Network Management), MODEM
(CDM-570/570L, SLM-5650A). The VMS component receives switch
requests from the MODEM based on policy settings and available resources,
either grants or denies the request. Within the MODEM component, load
switching is managed at either the Hub or the Remote, based on the current
mode of operation. When a remote is in STDMA mode, load switching for that
remote is managed by the Hub STDMA controller. After a Remote has been
switched to SCPC mode it manages its own switching (or Step Up / Step Down)
requests.
The basic concept for all load switching is that a running average of current
utilization is maintained, and when that utilization exceeds a pre-set threshold, a
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 band-
width 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 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 policies set in the VMS, if a remote requests less than some threshold
amount of bandwidth, the remote in put back into STDMA mode.
Note: If the Hub STDMA mode is GIR (Guaranteed Information Rate) or Entry
Channel, normal load switching is automatically disabled. In GIR mode,
the remote is switched to SCPC as soon as the GIR threshold is
reached, if there is a switch rate defined. In Entry Channel mode, the
remote is switched to SCPC as soon as the hub receives the first trans-
mission from the remote.
Load switching
The next sections describe the principles behind Load Switching and Rate
Adjustment (Step Up / Step Down).
NOTE