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Introduction
To facilitate network management, the CDM-550T incorporates EDMAC, an acronym
for Embedded Distant-end Monitor And Control. In this mode, an additional 5%
overhead is combined with the traffic data, (1.5% in Turbo BPSK modes) which permits
M&C information to be added (transparently to the user), allowing access to the distant-
end modem. This mode does not require any additional cabling at either the local or
distant-end Modems - access to EDMAC is via the standard M&C control port. Full
monitor and control is possible, and importantly, the on/off status of the carrier at the
distant-end carrier can be controlled.
1.1.1
AUPC
An important innovation in the CDM-550T is the addition of Automatic Uplink Power
Control (AUPC). This feature enables the modem to automatically adjust its output
power to maintain the Eb/No of the remote end of the satellite link constant. This
provides protection against rain fading, a particularly severe problem with Ku-band links.
To accomplish this, the framed (EDMAC) mode of operation must be used, and the
distant end modem constantly sends back information about the demodulator Eb/No
using reserved bytes in the overhead structure. Using the Eb/No, the local modem then
adjusts its output power, and hence, a closed-loop feedback system is created over the
satellite link.
A benefit of this feature is that whenever framed operation is selected, the remote
demodulator's Eb/No can be viewed from the front panel display of the local modem.
Note that both EDMAC and AUPC can be used simultaneously.
1.1.2
Software
The internal software is both powerful and flexible, permitting storage and retrieval of up
to 10 different modem configurations. The modem uses 'flash memory' technology
internally, and new firmware can be uploaded to the unit from an external PC. This
simplifies software upgrading, and updates can now be sent via the Internet, E-mail, or on
disk. The upgrade can be performed without opening the unit, by simply connecting the
modem to the serial port of a computer.
1.1.3
Verification
The unit includes many test modes and loopbacks for rapid verification of the correct
functioning of the unit. Of particular note is the IF loopback, which permits the user to
perform a quick diagnostic test without having to disturb external cabling. During the
loopback, all of the receive configuration parameters are temporarily changed to match
those of the transmit side. When normal operation is again selected, all of the previous
values are restored.
1–2
CDM-550T Satellite Modem
Rev. 1.3
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