Comtech EF Data SDM-300A - Manual (Page 276)

SDM-300A Satellite Modem Revision 6
Open Network Operations MN/SDM300A.IOM
10–20
Problems occurring on any of the selected clocks will cause the modem to substitute the
satellite clock and a fault will be signaled.
10.4.7 Engineering Service Channel (ESC)
The ESC uses certain bits of the satellite overhead to implement an EIA-232 data
channel. The two types of available data channels are asynchronous and synchronous.
The asynchronous channel works by over-sampling input and output EIA-232 data, so
that a clock signal is not required. Data rates up to 1/2000 of the satellite rate may be
used. Synchronous data channels are also allowed at a rate of 1/512 of the data rate of the
modem. The synchronous channel requires use of the TX clock provided by the modem
for operation.
10.4.8 Backward Alarm
A backward alarm signal is included in the D&I overhead. The signal is sent to the distant
side of a satellite link to indicate trouble with the receive side, which may be a result of
improper transmission. The M&C computer monitors the receive side of the link. In the
event of trouble, the M&C sends an alarm over the transmit side to the distant end. This
alarm signal indirectly includes faults in the downlink chain, since major problems with
the antenna, Low Noise Amplifier (LNA), or down converter, etc., will cause an
interruption in service and fault the modem. Reception of a backward alarm is indicated
as one of the events that causes a secondary alarm. The modem may be interrogated from
the front panel or by using the EIA-485 or EIA-232 interface to identify the cause of the
alarm.
10.4.9 D&I Framing Formats
The D&I framing formats used in the modem can be divided into T1 and E1 categories, as
follows:
T1_ESF T1 Extended Super Frame
T1 T1 D4 Framing
T1_ESF_S T1 Extended Super Frame Special
T1_S T1 D4 Framing Special
E1_CCS E1 Common Channel Signaling
E1_CAS E1 Channel Associated Signaling
E1_31_TS E1 31 Time Slots
Each format is further explained in Figures 10-5 and 10-6.