SLM-3650 Satellite Modem
Revision 3
Options
MN/SLM3650.IOM
A–13
As a test mode, the transmit data can be replaced with a 2047 pattern. This selection
overrides the AIS. Only user data bits are replaced with the pattern, while the
Engineering Service Channel (ESC) (including framing and alarms) will operate
normally.
The composite MUX data stream is normally fed to the modem for further processing
(scrambling and K=7 Viterbi encoding). The composite data stream may be looped back
at this point as a test function, called interface loopback, when the transmit data rate
matches the receive data rate. This allows the operator to test the entire interface as the
ESC is looped to itself through the DEMUX. The plesiochronous buffer may also be
checked, since user data passes through this circuit.
A.1.3.2.3 Receive
DEMUX
The receive data with overhead is processed in the DEMUX. This circuit checks and
synchronizes to the frame pattern and separates the user data from the Engineering
Service Channel (ESC). If the DEMUX is receiving a correct and synchronized signal, it
will signal the modem that the MUX system is locked (MUXlock) and passing data. This
is indicated by interrogating the modem, a green LED on the interface and sending the
signal into the receive fault tree in accordance with IESS-308. Under certain fault
conditions defined by IESS-308, the receive user data will be replaced by a pattern with
all 1s, and a fault will be signaled.
A.1.3.2.4 Plesiochronous
Buffer
Data from the DEMUX section is fed into a plesiochronous buffer. This buffer size is
user-selectable in bit increments that correspond to the length of an IESS-308 satellite
superframe. The increments range from 1 to 32 ms. Refer to the interface specifications
for a list of valid entries for each of the selected formats. The buffer automatically centers
on resumption of service, either from the front panel or remotely. The startup buffer will
overfill when centering to match the satellite frame to the terrestrial frame with a
maximum slide of 0.5 ms. In general, manual centering will not be plesiochronous.
The fill status is available as a monitor and is accurate to 1%. Overflows or underflows
are stored in the stored fault section of the M&C status registers, along with the date and
time of the incident (which are provided by the modem internal clock). These are stored
in battery-backed RAM.
A normal selection is to have the data clocked out of the buffer by an external clock. This
procedure removes the Doppler from the receive satellite data.
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