Asynchronous Overhead Operation (Framing/Multiplexer Capability) - Comtech EF Data DMD50 Installation And Operation Manual

DMD50 Universal Satellite Modem
1024
1536
1544
IDR (With 2048
6312
96 Kbps O/H)
8448
1544
2048
8PSK
6312
8448
DVB
1. n = code length, k = information symbols and t = symbol error correcting capability.

3.10 Asynchronous Overhead Operation (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 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 5-3.
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 3-2. Baud Rate Examples for Standard IBS and Enhanced Mode
Baud Rate Example for
Kbps
128
256
384
512
640
768
MN-DMD50– Revision 3
(126, 112, 7)
(126, 112, 7)
(225,
205,10)
(219, 201, 9)
(194, 178, 8)
(194, 178, 8)
(219, 201, 9)
(219, 201, 9)
(219, 201, 9)
(219, 201, 9)
All (204, 188, 8)
2. Design objective.
(Standard IBS), the Baud Rate is approximately 1/2000 of the
Data Rate listed in Table 3-3.
(IBS Async.), the Baud Rate is selectable, but Data Rate is
limited.
Standard IBS
64
128
192
256
320
384
0.125
0.125
0.0976
0.0896
0.0899
0.0899
0.0896
0.0896
0.0896
0.0896
0.0851 12
Baud Rate Example for
Kbps
Enhanced Mode
9.6
19.2
32
64
128
192
Theory of Operation
4 8
4 5
4 9
4 7
4 2
4 <2
8 18
8 13
8 4
8 3
-
300
600
600
1200
2400
4800
3–17