The 8020 terminal is installed and debugged well in branch office, and two-way 6b join the conference. There is no line bit error at beginning but only one-way 4b can be allowed to join conference and having pixelation several times later.
Connect terminal with ISDN phone, but we cannot call that ISDN private line. Sometime, the communication has large noise, so we think it is the line problem.
1. Asking telecommunication office for help to test line, the switch has no problem.
2. Connect 3 ISDN private lines with terminal 8020 by NT1. Exchanging NT1 but the problem exists still. So it is not the problem of NT1.
3. Testing the jumper condition segmented, which belonging to 3 distribution frames using by 3 private lines. So eliminate the causes of interference and imperfect contact.
4. Connect terminal with 3 ISDN private lines through the DSL board of ONU. Test it by ISDN phone before connecting ONU. We find that the connection is good and no noise. So the problem occurs in ONU.
5. Restore DSL board and the fault disappeared. So it is DSL board problem. Changing a new DSL board, then the problem solved.
Line problem occurs frequently in the debugging of private video-conferencing system. We should locate it by section testing and the steps as shown below.