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OM20 Universal Outdoor Modem
4.11 OM20 Automatic Uplink Power Control (AUPC Operation)
Refer to Appendix F
4.12 Asynchronous Overhead Operation J3 (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 ratio of
overhead & data to data rates.
For Regular Async:
For Enhanced Async:
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 are 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 4-2.
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.
Kbps
Baud Rate Example for
128
256
384
512
640
768
896
1024
1152
1280
1408
MN-OM20– Revision 5
(Standard IBS), the Baud Rate is approximately 1/2000 of the
Data Rate listed in Table 4-2.
(IBS Async.), the Baud Rate is selectable, but Data Rate is
limited.
Table 4-2.
Standard IBS
64
128
192
256
320
384
448
512
576
640
704
Kbps
Baud Rate Example for
Enhanced Mode
9.6
19.2
32
64
128
192
256
320
384
448
512
OM20 Theory of Operation
300
600
600
1200
2400
4800
4800
9600
9600
9600
9600
4–18
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