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Description
1.2.7 Standard Framing and IBS Multiplexer
The 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, ESC, service channel, "asynch" channel or, in IESS terminology, an ES to ES
data channel. A simplified block diagram of the data multiplexer is shown in Appendix RS.
The basic frame structure used by the multiplexer is that specified in the IESS-309 standard, Page
60, Figure 10, resulting in a 16/15 aggregate to through data ratio. This means that when the
multiplexer is enabled the modem aggregate operating data rate is computed as the terrestrial
connection (through) data rate multiplied by 16/15. The user sets only the desired through data rate
while the modem computes the aggregate rate required. The multiplexer is also capable of
expanded operating modes which include custom setting of the ratio of data to framing bytes.
The Multiplexer provides the following modes of operation. They are described more fully in The IBS
Multiplexer/Reed-Solomon Appendix RS.
Mode
Standard/
Compatibility
Disabled
N/A
Standard
IESS-309
Enhanced
Modified IESS-309
Custom
Modified IESS-309
The ESC Data Channel can be set under software-control to either RS-232 or RS-485 mode. The
pin assignments for both modes are shown in Section 2.3 and Appendix RS. These pin assignments
appear on the rear panel "AUX" (Auxiliary) connector J4 only when the Multiplexer function is
enabled. The RS-485 Transmit Data Drivers can set to "RS-485" or "RS-485 ON" when in
"Enhanced" mode. The "ON" setting forces the driver continuously on while 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. In the standard IBS mode only the "RS-485 ON" mode is
available.
In Enhanced or Custom mode a 2 wire receive operating mode can be selected for the receive data
into the ESC channel. In this mode the receive input is muted while the transmit data output is
active. In 4 wire mode the receive is always enabled. In the standard IBS mode only the 4 wire mode
is available. Note that the transmit and receive pairs are physically separate wires and must be
connected together if true RS-485 2 wire connectivity is desired.
The processor on the main board performs software/hardware assignment of bits to specific
purposes in the Custom mode and buffers the ESC Data Channel to standard asynchronous data
rates.
The user does not have to compute data framing variables to use the Custom Multiplexer Mode.
When placed in this mode the entry parameters are the ESC and MCC channel rates selected from
standard asynchronous data rates (300 to 38,400 bps). The modem then computes the proper
relationship between the framing and terrestrial data rates to achieve the proper operation. The
modem also displays the terrestrial data to aggregate ratio.
Page 1-14
PSM-500/500L/500LT SCPC Satellite Modem
Overhead
Ratio
1/1
16/15
Fixed synchronous ESC, No AUPC, No MCC
16/15
M4 compatible ESC and AUPC (limited
MCC).
Variable
Full ESC and MCC including AUPC, Remote
Modem Control, 2 one=bit control channels.
Also variable data load per frame.
Notes
PSM-500/500L/500LT - Rev. 0.91
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