(2) Network in half duplex mode, but the network needs to be in
full-duplex status.
(3) Network itself result in error.
(4) E1 interface cable problems or poor contact.
(5) E1 itself has line error.
(6) Both sides of E1 equipment in asynchronous state, but both ends
of the device work in loopback clock recovery mode.
(7) Different sources clock of both ends of the device, but both ends
of the device set to be at system clock operating mode.
(8) One end of the device choose adaptive mode and the other end
choose differential mode.
(9) Both ends of the device set differential mode while their public
reference clock from different sources.
(10) make sure that both ends of the device have good ground.
