PSM-500/500L/500LT SCPC Satellite Modem
Description
PSM-500/500L/500LT - Rev. 0.91
Page 1-15
1.2.7.1 Modem Control Channel (MCC)
The MCC is available in both the Enhanced and Custom Multiplexer modes. In the Enhanced mode
the MCC provides for an AUPC channel. In the Custom mode the MCC provides for the AUPC plus
the Remote Modem Control (RMC) Channel and the Auxiliary control bits (RFC).
1.2.7.1.1 AUPC Control Channel (AUPC)
When the modem is placed into either the “Enhanced” or “Custom” modes the AUPC control
channel becomes available. The AUPC operation itself is under control of the modem while the
AUPC facility in the MCC provides the channel for the information. This channel provides a minimum
300 baud control channel in each direction to allow the modems at two ends of a link to interactively
maintain the receive Eb/No by controlling the power output at the transmit site.
Refer also to the AUPC operation description of the main manual in Section 3.8 titled “
Automatic
Uplink Power Control (AUPC)” Operation.
1.2.7.1.2 Remote Modem Control Channel (RMC)
When the modem is placed in the “Custom” mode the Remote Modem Control Channel becomes
available. This channel allows the control of a far end modem from the near end site. This control is
not however allowed from the near end front panel, but only via the remote control interface port.
The command protocol for remote unit control is explained in Appendix B
, “Remote Control
Protocol”.
Note that the Automatic Configuration Recovery or ACR is partially designed as a safety feature to
be used with the remote programming of modems. It can help prevent “losing” the modem at an
unattended site. Refer to the ACR section of the main manual in 3.14 “
Automatic Configuration
Recovery”.
1.2.7.1.3 Auxiliary Bit Control Channels (RFC)
When the Multiplexer is placed into the “Custom” mode the auxiliary bit control RFC channels
becomes available. These consist of two single line or “one-bit” control channels that can be used to
send control information independently in both directions over the link. The input signals on these
channels can be either a contact closure or a logic type signals while the output is a form C relay
contract set whose state depends on the state of the input signal. The low input logic level is 0 to 0.4
VDC, while the logic high level can be from 2.4 to approximately 20 VDC. The input is current limited
to accept this wide voltage range without damage. Higher voltages may damage the inputs however
and caution should be exercised. Pin connections for these one bit channels are shown in the RS
Appendix.
1.2.8 Standard and Optional Modem FEC Cards
All of the M500 class modems have two card slots on the main PCB for two FEC function boards.
One board is normally installed containing the standard Forward Error Correction set including
Viterbi (with Trellis Code Modulation mode when in 8PSK mode only) and Reed-Solomon
concatenated codecs. The same standard card has several variations which includes either a 4k or
16k block size TPC or both on the same board if ordered that way initially. The second slot can be
used for optional FECs as desired, for example the
FlexLDPC
FEC featuring exceptional
performance at very low Eb/No.
These two FEC slots are wired in parallel and the control processor on the main board searches for
a requested FEC on the first card containing that capability. The same FEC functions can exist on
both cards, but only the first card will be used in that case.
FEC technology is in many ways a matter of tradeoffs. In most cases the tradeoff is between
bandwidth and performance, which also relates to power and performance. FECs are normally
specified by “Rate”, which is the ratio of data information bits to transmitted bits, and coding gain,
which is the Eb/No reduction able to achieve a specified BER as compared to an unencoded signal.
The extra bits required for a given rate are redundancy processing bits needed to perform forward
