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Operation
DMD2401/DMD2401L/DMD2401 IBS/IDR Satellite Modem
Function
Description
AUPC ENABLE/DISABLE
Enables/Disables the AUPC to function locally
AUPC Eb/No
Desired E
/N
of remote modem
b
0
AUPC MIN LVL
Sets minimum output power to be used
AUPC MAX LVL
Sets maximum output power to be used
AUPC DEF LVL
Sets default output power to be used
The basic AUPC operation is described as follows: Assume that the two modems, one at each
end of the link, are set to AUPC operation. Only one direction is discussed, but the same
functions could be occurring in both directions simultaneously. Modem "A" is transmitting to
modem "B" under normal conditions and modem "B" has a receive Eb/No of 7.5 dB. Modem "A"
has been set to an AUPC Eb/No on the front panel of 7.5 dB, and is currently outputting –15 dBm.
Next, it begins raining at location "B", and the Eb/No drops to –7.0 then –6.8 dB. Modem "B" is
constantly sending update messages to "A" and reports the current Eb/No. When "A" sees the
drop in Eb/No, it slowly begins to raise the output power, and raises it again when it sees further
drops. As the rain increases in intensity, and the Eb/No decreases again, "A" continues to
increase its power level to compensate, and when the rain diminishes and quits, it lowers its
power level to compensate. The operation is therefore a feedback control loop with the added
complication of a significant time delay.
There are safeguards built into the AUPC system. First, the Modulator has two additional
parameters, which allow control of the maximum and minimum power output levels. Second, a
default power level is specified which takes precedence over the output power level during signal
loss or loss of AUPC channel communication. The default power level should normally be set to a
high enough level to reestablish communication regardless of rain fade. The other controls are
built into the operating control software to limit response times and detect adverse operating
conditions.
3.5 DMD2401 Asynchronous Overhead Operation
3.5.1 Asynchronous Framing/Multiplexer Capability
The Asynchronous Framing/Multiplexer is capable of multiplexing a relatively low-speed overhead
channel onto the terrestrial data stream resulting in a slightly higher combined or aggregate data
rate through the modem. The overhead channel is recovered at the far end. This added channel
is termed variously "An Overhead Channel", "Service Channel", "Async Channel" or in IESS
terminology an "ES to ES Data Channel." The basic frame structure used by the multiplexer is
specified in the IESS-309 standard, Page 60, Figure 10, resulting in a 16/15 aggregate to through
data ratio.
For Regular Async. (Standard IBS), the Baud Rate is approximately 1/2000 of the Data Rate
listed in the table below. For Enhanced Async. (IBS Async.), the Baud Rate is selectable, but
limited by the Data Rate. The maximum Baud Rate is 19,200 bps for IBS Async.
Two software-controlled modes are designed into the card to best utilize the available bits;
"Standard IBS" and "IBS (Async)". The characteristics of the Channel Interface is also determined
by the standard or Async mode.
The Async Channel can be set under software-control to either RS-232 or RS-485 mode. The pin
assignments for both modes are shown in Table 1. The "RS-485" setting controls the output into
tri-state when the modem is not transmitting data, allowing multiple modem outputs to be
connected together.
3-4
TM065 – Rev. 3.3
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