1) Perform the loopback test from the DDF to the BTS. If the SWT LED and LIU1 LED of the BTS are off, you can infer that the E1 link between the DDF and the BTS is normal. If the SWT LED and LIU1 LED of the BTS are on, you are required to check the E1 link between the BTS and the DDF to rectify the E1 link faults.
2) Ask the telecom operator to check the transmission link between the ODF and the DDF for faults.
3) Perform the loopback test from the ODF to the BSC. If the LOS LED of the BSC is off, you can infer that the optical transmission link between the ODF and the BSC is normal. If the LOS LED of the BSC is on, you are required to check the link between the ODF and the BSC to rectify the link faults.