Comtech EF Data SDM-300A - Manual (Page 263)

SDM-300A Satellite Modem Revision 6
Open Network Operations MN/SDM300A.IOM
10–7
10.3 Intermediate Data Rate (IDR)
IDR operation adds 96 kbps overhead to the terrestrial data. The terrestrial data rates
supported are:
• 1.544 Mbps (T1) + 96k = 1640 Mbps on the data rate menu
• 2.048 Mbps (E1) + 96k = 2144 Mbps on the data rate menu
The terrestrial data is passed through the baseband loopback relay and is translated from
the G.703 format to TTL with a recovered clock.
Overhead data can include two ADPCM audio channels or one 64 kbps data channel,
four backward alarms, and an EIA-422 format 8 kbps synchronous data channel
multiplexed with the dejittered terrestrial data. If the 64 kbps option is selected, the data
path is through the normal ADPCM audio path. The audio or 64 kbps option is selected
from the Utility Interface menu (Chapter 6), using the IDR TX ESC TYPE and IDR RX
ESC TYPE commands.
The interface for the audio is a 600
Ω transformer-balanced input. The ADPCM audio
channels are encoded according to CCITT G.721.
The backward alarms are level-translated to TTL by threshold comparators set to 2.5V. A
1K pull-up resistor to +5V is connected to each alarm input.
The multiplexed data stream is output to the modulator through the interface loopback
device. The receive data from the demodulator is input through the same interface
loopback device. The receive data is demultiplexed to produce:
• Receive terrestrial data
• 2 ADPCM audio channels or one 64 kbps data channel
• 4 backward alarms
• 8 kbps synchronous data channel
• 1 octet path
The 8 kbps synchronous data channel is formatted for EIA-422. The terrestrial data
output from the DEMUX is input to a plesiochronous buffer. The front panel interface
provides four clock selections clocking the data out of the buffer:
• Internal Clock (SCT)
• RX Recovered Clock (RXCLK)
• External Reference Clock (EXT)
• TX Clock (TXCLK)
If either RXCLK or EXT is selected and then fails, the interface automatically switches
to SCT recovered clock as the source.