Comtech EF Data SDM-650B - Manual (Page 264)

Asynchronous Overhead Channel Unit SDM-650B Satellite Modem
F–6 Rev. 5
F.1.2 Multiplexer
The data processor (AS/1289) forms the bottom of the 2-board module. This board
contains the data multiplexer and demultiplexer.
The synchronous data stream is multiplexed with a 1/15 overhead channel, and the
resultant information is interfaced to the modulator/coder section of the modem.
The input clock is normally the recovered clock from the terrestrial interface board
(SCTE), but the modem reference clock can be used by selecting the internal clock.
The 1/15 overhead channel is composed of:
• Framing signals
• Flags
• Spare bit positions
• ASYNC data stream
The maximum ASYNC data rate is limited to 1.875
% of the synchronous data rate.
F.1.3 Demultiplexer
Receive data and clock from the demodulator/decoder are input to the DEMUX. The
DEMUX synchronizes to the frame pattern, and separates the synchronous and ASYNC
data. Synchronization is indicated by the demux lock LED on the front edge of the board.
The synchronous data is dejittered, and interfaced either to the optional plesiochronous
buffer or to the terrestrial interface board.