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M&C and Interfaces
indicated by interrogating the modem and by a green Light Emitting Diode (LED) on the
Interface, and is fed into the receive fault tree per IESS-308. Under certain fault
conditions defined by IESS-308 the receive user data will be replaced by an all ones
pattern. A fault will be signaled when this occurs.
4.2.2.5 Plesiochronous Buffer
User data from the Demux Section is fed into a Plesiochronous buffer. The buffer size is
user selectable in bit increments that correspond to the length of an IESS-308 satellite
superframe. These increments turn out to be in 1, 2, 4, 6, 8, 12, 24, and 32 msec. The
buffer is automatically centered on resumption of service after an outage or may be
commanded to center in the interface configuration Section, from the front panel or
remotely. The start up buffer will overfill upon centering to match the satellite frame to
the terrestrial frame with a maximum slide of 0.5 msec. Manual centering will generally
not be Plesiochronous for obvious reasons.
The fill status is available as a monitor function and is accurate to 1%. Overflow or
underflow incidents will be momentarily indicated by red LED's on the module and are
stored in the 'stored Fault" Section of the Monitor & Control status registers, along with
the date and time of the incident, provided by the modem internal clock. These are stored
in battery backed RAM.
The user will normally select to have the data clocked out of the buffer by the recovered
clock from the Receive Data Input in order to synchronize the Receive data output with
the satellite data. The user may select from 2 other clock sources as a backup, either a
user supplied "External" reference clock, or the internal" clock source. Problems on
either the Recovered Receive data input clock or the external clock (if selected) will
substitute Satellite clock and a fault will be signaled.
4.2.2.6 Engineering Service Channel (ESC)
The engineering service channel (ESC) uses certain of the satellite overhead bits to
implement an asynchronous RS-232 data channel. This channel works by oversampling
input and output RS-232 data so that no clock signal is required. Data rates up to 1/2000
of the satellite rate may be used.
4.2.2.7 Backward Alarm
A backward alarm is included in the overhead. Basically, backward alarms are sent to the
distant side of an satellite link to signal that there is trouble with the receive side which
may be as a result of improper transmission. The M&C computer monitors the receive
side of the link and in the event of trouble sends an alarm over the transmit side to the
distant end. This alarm signal also indirectly includes faults in the downlink chain since
major problems with the antenna, LNA, or downconverter, etc. will cause an interruption
in service which will fault the modem. Reception of a backward alarm is indicated as one
4-4–26
SDM-309B Satellite Modem
MN/U-SDM309B Rev. #
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