PBX Traffic Does Not Launch Because Of Transmission NE Board
Hi,
Today I want to share a case of operating PBX for a customer, the service needed is E1 and the Transmission NE is OSN500 equipped with SP3D board.
Description
A customer has a PBX which needs initially 2 E1s to work, the PBX uses balanced E1s = 120 ohm.
when connecting the first E1 it worked normally but when connecting the 2nd E1 , the whole traffic failed and PBX didn’t work.
Steps of check
1-We started with checking physical connection from the Transmission E1 frame to the PBX E1 frame but connection was correct.
2-Usually we use LED ( light emitting diode ) to check the connectivity of the signal from point to point, when attaching the LED legs to the transmit pair of the E1 ,if there a signal, the LED illuminates.
But in here the LED illuminates even when connecting a single wire of the transmit pair with the grounding wire.
https://www.pcboard.ca/5mm-frosted-lens-led
knowledge
-The E1 could work in 2 modes balanced 120-ohm or unbalanced 75-ohm, when E1 is balanced the connection of the E1 is 2 pair of wires, one pair for transmit and one pair for receive. When E1 is unbalanced the connection is using coaxial cable one for Transmit and other for receive.
A coaxial cable carries a signal which goes across the center copper wire as well as the metal shield. The insulators keep the signals from coming in contact with or cancelling out each other. The insulators also protect the signal from outside magnetic fields. As a result, the signal is carried over long distances with little interference or signal loss.
Coaxial cable
Conclusion & Solution
The LED illuminates with one wire of the Transmit pair with ground only if the impedance of the transmit port is 75ohm.
We checked the board manufacturing information of the SP3D and found that the board is TNH1SP3DB that means 75ohm.
When searching in HedEx, the SP3D has a feature Codes they are A means 120ohm or B means 75ohm.
SP3D Feature Code
-also from NE Explorer > Configuration > PDH interface shows the impedance of the SP3D =75ohm
SP3D IMPEDANCE
-Even when the impedance of the port is 75ohm one E1 can work on 2 pair of wires for short distance but when the 2nd E1 got connected they share the same ground and fails both of them.
by changing the Board by TNH1SP3DA the problem solved and the PBX worked normally.
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