Comtech EF Data DMDVR20 LBST Rev 1.1 - Manual (Page 29)

Theory of Operation DMDVR20 LBST Satellite Modem
3-8 TM136 – Rev. 1.1
3.4 Loopback Features (Terrestrial)
The modem provides for a number of different loopbacks. The Loopback supported are:
ƒ TX Terrestrial Loopback - Tx Data port is looped back to the Rx Data port after the
interface driver/receiver. (prior to the framing unit)
ƒ TX Baseband Loopback - Tx Data port is looped back to the Rx Data port after the
interface driver/receiver. (after the framing unit)
ƒ RX Terrestrial Loopback - Receive Data from the satellite is looped back for
retransmission to the satellite, providing a far end loopback. (prior to the framing
unit)
ƒ RX Baseband Loopback - Receive Data from the satellite is looped back for
retransmission to the satellite, providing a far end loopback. (after to framing unit)
Usage of the modems loopback capabilities in conjunction with the
Ethernet data interface can produce undesirable network loops. In order
to run any type of data test with an Ethernet interface you must utilize
two modems connected back to back. Simply using one modem and a
loopback will not produce the desired results.