ViaSat ArcLight, User Manual, Page: 61 / 368

ArcLight Hub User’s Guide
General System Description
Rev. 001 1081006 1-25
The protocol includes provisions to reduce and maintain the VMT
frequency error – as observed at the Hub demodulators – to less than
0.2 ppm.
1.5.2.3 Congestion Control Messaging
The RtNMS notifies all VMTs under congestion control when
adjustments need to be made to the congestion threshold (CT). The
congestion control loop sends a CT update only when measured traffic
conditions require adjustment. The CT data is transmitted in a non-
periodic asynchronous manner. The CT update message has the ability
to target the appropriate population of VMTs under control. Users
operating on an access type that is part of a particular congestion
control group must respond and adjust to CT updates for that group
only.
1.5.2.4 Power Control Messaging
IP traffic from individual VMTs is sporadic in nature. At times the
traffic represents a deluge of large IP packets and at other times only a
trickle of small packets separated by several seconds or minutes of
silence. The power control scheme needs to handle the widely variable
conditions of incoming traffic. Additionally, the rate and volume of
feedback control messages transmitted over the forward link needs to
be considered with respect to the current forward link overhead.
By itself the loop is insufficient to control all of the fluctuations in VMT
amplitude in all situations. For example, when no packets arrive to
drive the loop then no adjustments will be made. If the VMT has
traveled through several G/T contours during this silent period then
the VMT’s transmit power level could be significantly skewed when
transmissions resume. Therefore the loop is supplemented with a state
machine that will periodically poll the VMT to transmit dummy packets
at some minimum rate to keep the amplitude level “fresh.”
The state-machine ensures the VMT transmits a minimum number of
packets in the specified time period. As long as traffic packets arrive
within T
timeout1
then the VMT will not be polled to send dummy packets.
If a silent period longer than T
timeout1
occurs, then the state-machine
indicates the VMT should be polled to send N
refresh
dummy packets. If
no packets arrive within T
timeout2
seconds from polling, then the state-
machine indicates that the VMT should adjust its amplitude by +Delta
dB and be polled again to transmit dummy packets. The process
repeats until packets arrive or the maximum number of attempts (jmax)
is reached. If the process has repeated jmax times with no success, the
state machine indicates that the VMT should be forced to logout/login
again. Typically, the VMT has either powered down, or has moved into
a disadvantaged area.
To accommodate this power-control scheme, the VMT needs to take
action in response to messages from the RtNMS via the Forward Link
that may or may not already exist.
One power control loop and state-machine combination is running for
each logged-in VMT. The RtNMS knows how to route estimates to the
right control loop combination based on the VMT’s VCI within the