E1 interface connection of Quidway R3680&R1760 routers was failed in long distance.
The physical state of E1 interface periodic was up and down.
Change Rx&Tx ;
I changed cable;
Real length of cable is twenty metre but by specification of router this length is normal;
I think polarity can’t be reason of this problem;
At last I find the reason of this problem my self. I changed position of dip switch on the board. By default all switch is on. This is use for 75 ohm. Bat we use 120ohm. So I changed position S1-S5 switch to off. After this every thing was ok.
Problem with wrong connection of Rx and Tx;
Problem in the connection cable;
Problem in the router specification;
Also our specialist sagest me change polarity of Rx&Tx pair.
Always need pay more attention to default setting of equipment. Because some times it can be deferent with our real setting.