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SDM-309B Satellite Modem
4.2.2.2 Data Interface
The module contains transformer balanced data interfaces supporting CCITT G.703
parameters, (compatible with AT&T DSI service) and de-jitter. Data inputs are named
Drop Send Data Input A and B (DSD-A and DSD-B) and Insert Send Data Input A and B
(ISD-A and ISD-B). Outputs are Drop Receive Data Output A and B (DRD-A and DRD-
B) and Insert Receive Data Output A and B (IRD-A and IRD-B). Frequently the system
will be used with the Drop Receive Data Output signal looped to feed the Insert Send
Data Input. This is to be done at the far end of any redundancy switching as this will
allow Transmit and Receive chains to be switched independently. Zero substitution
codes are AMI, B8ZS, and HDB3 are user selectable during configuration.
A data loop back function on the "Insert" data is available in this Section so that the user
may determine that his T1, or E1 data parameters correctly match those of this interface.
The "Drop" data is always hard wired into loopback.
4.2.2.3 Transmit Multiplexer
The data stream is multiplexed with a 1/15 overhead channel and the resultant
information rates are interfaced to the Mod/Demod/Coder Sections of the modem. A
phase locked loop generates the output clock (with overhead) using the input clock as a
reference. The input clock is normally the recovered clock from the data interface. If no
valid input signal is present the interface will fall back to a 10-5 accuracy reference clock
generated in the modem and transmit a valid IESS-308 framing pattern. This will keep
the link open to the far end. A fault will be signaled if this happens.
The transmit data will be replaced with an all ones pattern (Alarm Indication Signal or
AIS) in the event of certain failures, per IESS-308.
As a test mode the transmit data can be replaced with a 2047 pattern. Selecting this over-
rides AIS. Only user data bits are replaced with the pattern, the Engineering Service
Channel including framing and alarms will operate normally.
The composite multiplexed data stream is normally fed to the modem for further
processing (scrambling and K7 Viterbi encoding). The composite data stream may be
looped back at this point as a test function called "Interface Loopback" provided that the
transmit data rate matches the receive data rate. This allows the customer to test virtually
the entire interface as the ESC is looped to itself through the Demultiplexer (Demux).
User data also passes through the a Plesiochronous buffer in this mode, so this may also
be checked.
4.2.2.4 Receive Demultiplexer
Receive data including overhead is fed into the demultiplexer for processing. This circuit
checks and synchronizes to the frame pattern and separates the user data from the ESC
channel. If the Demux is receiving a correct signal and synchronized, it will signal the
modem that the multiplex system is locked ("Muxlock") and passing data . This is
MN/U-SDM309B Rev. #
M&C and Interfaces
4-4–25
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