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Operation DMD15/DMD15L IBS/IDR Satellite Modem
3-24 TM051 – Rev. 5.8
3.20 DMD15 Asynchronous Overhead Operation
3.20.1 Asynchronous Framing/Multiplexer Capability
The Asynchronous Framing/Multiplexer is capable of multiplexing a relatively low-speed overhead
channel onto the terrestrial data stream resulting in a slightly higher combined or aggregate data
rate through the modem. The overhead channel is recovered at the far end. This added channel
is termed variously “An Overhead Channel”, ”Service Channel”, “Async Channel” or in IESS
terminology an “ES to ES Data Channel.” The basic frame structure used by the multiplexer is
that specified in the IESS-309 Standard, resulting in a 16/15 Aggregate to Through-Data Ratio.
For Regular Async: (Standard IBS), the Baud Rate is approximately 1/2000
of the Data Rate listed in Table 3-3.
For Enhanced Async: (IBS Async.), the Baud Rate is selectable, but Data Rate
is limited.
The maximum Baud Rate is 19,200 bps for IBS Async. Two software-controlled modes are
designed into the card to best utilize the available bits; “Standard IBS” and “IBS (Async)”. The
characteristics of the Channel Interface is also determined by the standard or Async mode.
The Async Channel can be set under software-control to either RS-232 or RS-485 mode. The pin
assignments for both modes are shown in Table 3-3. The “RS-485” Setting controls the output
into tri-state when the modem is not transmitting data, allowing multiple modem outputs to be
connected together.
Table 3-3
Kbps Baud Rate Example for
Standard IBS
Kbps Baud Rate Example for
Enhanced Mode
128 64 9.6 300
256 128 19.2 600
384 192 32 600
512 256 64 1200
640 320 128 2400
768 384 192 4800
896 448 256 4800
1024 512 320 9600
1152 576 384 9600
1280 640 448 9600
1408 704 512 9600
1536 768 576 9600
1664 832 640 19200
1792 896 704 19200
1920 960 768 19200
1920 960 768 19200
2048 1024 832 19200