Comtech EF Data DMD2401/DMD2401L - Manual (Page 29)

Operation DMD2401/DMD2401L/DMD2401 IBS/IDR Satellite Modem
3-6 TM065 – Rev. 3.3
3.6 Standard IBS Mode
In the first or “Normal” mode, all bit assignments are per the IBS standard. The bits of Overhead
Housekeeping byte 32 are implemented as shown below:
Bit 1 ES to ES Data
Channel This bit is routed directly to the ES to ES Data
Channel. Its data rate is 1/512
th
of the aggregate rate
(0r 1/480
th
of the through terrestrial data rate, and is
normally used to super-sample an asynchronous data
channel.
Bit 2 Frame Alignment Part of the Frame Alignment word.
Bit 3 Backward Alarm Transmit and Receive with main processor to activate
main alarm/LED
Bit 4 Multiframe Message As per IBS
Bits 5 and 6 Spare Not currently utilized
Bits 7 and 8 Encryption Utilization Not currently utilized
The ratio of the through terrestrial data channel rate to the aggregate rate is 15/16.
The standard transmit and receive channels of the ES to ES data channel in standard IBS mode
are raw channels operating at the specific bit rate as controlled by the data channel rate, without
buffering. In addition, no clocks are provided with this channel. Since it would be rare that the
data rate provided was exactly that required for a standard rate device, the only method of
communicating using this channel is to allow it to super-sample the user data.
3.7 Asynchronous Multiplexer Mode
Since many of the frame bits in the standard IBS mode are not used, an “Enhanced” multiplexer
mode has been implemented that can be engaged under software control. Since this mode
changes the use of many of the framed non-data bits, this mode is only usable when the
DMD2401 is at both ends of a link. In this mode, the overhead signaling bytes 16 and 48 can be
used to implement a significantly higher speed ES to ES Data Channel under software control.
When implemented, this rate is 16 times that of the normal IBS standard, or 1/30
th
of the
terrestrial data rate (1/32
nd
of the aggregate rate).
Note: The IBS Async mode MUST be selected for true Asynchronous channel operation to
be available.
3.8 ESC Backward Alarms
When running in IDR Mode and if the modem has the ESC Option, there will be four Backward
Alarms available for use by the earth stations at each end of the link (both ends must have the
ESC option). These alarms are accessed via the ESC ALARMS Port. The four alarms are
controlled by four relays, each having a normally open, normally closed, and a common
connection. The common connections of these relays (referred to as Backward Alarm Inputs) can
be connected to whichever system on the earth station that the user wishes to trigger the
backward alarm. When ground is applied to the common (input) connection of one of these
relays, that relay and associated backward alarm will then be in a “no fault” state. When the
ground is removed, the relay and the associated Tx Backward Alarm will toggle to the faulted
state. When in the faulted state, the receive end of the link will receive that backward alarm that
is initiated at the transmit end of the link.