VCS VideoJet X.21 Manual page 32

Quick Installation Guide
Connection and therefore transmission of live video images is only
possible if a DCE (data circuit-terminating equipment) is installed at
both ends of the X.21/G.703 leased line. The X.21/G.703 leased
line and the transmission line must be set up and "switched
through". On most terminal adapters where DCEs are used as the
termination of the X.21/G.703 leased line, it is possible to activate
a loop function. The line can be checked with this function. Please
contact your X.21/G.703 network operator or consult the technical
documentation to check the line.
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VideoJet X.21/G.703
Manual