DMD20/20LBST Universal Satellite Modem Theory of Operation
MN-DMD20-20LBST Revision 14 3–17
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: (Standard IBS), the Baud Rate is approximately 1/2000 of the
Data Rate listed in Table 3-2.
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 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 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.
Standard IBS
Enhanced Mode
To Next Page
