When the E1 module of AR28-30 is connected with E1 cables of 120ohm, the interface frequents UP/DOWN.
The alarm that the interface frequents UP/DOWN is prompted often.
Draw the DIP switch BIT1~8 of the module to OFF, and the problem is solved.
1E1/2E1/4E1 modules have internal DIP switches, of which the setting determines the impedance of interface and the grounding.
For the DIP switches of 1E1/2E1/4E1 modules, it is suggested to turn BIT1~8 ON when they are connected with cables of 75 ohm, and OFF when with cables of 120 ohm.
DIP of 1E1/2E1/4E1 modules is set to ON by default before shipment; that is, the impedance of E1 interface is 75 ohm.