Physical layer problem preventing the link layer from coming up.
  Either no incoming signal is being received (LOS-loss of signal) or no framing synchronization is possible (LOF-loss of framing).
   > replace cable.
   > check circuit condition with Telco (or external CSU if one is used).
   > check mismatch of Framing format or Line coding

  A Yellow Alarm is sent by the remote end to indicate that it is not receiving a signal.

   > check Line Build Out
   > replace cable

  No incoming signl is detected.
   > replace cable.
   > check circuit condition with Telco

 When the link layer fails to come up, a common problem is an incorrect clocking setting.
 No physical problems exist and no alarms are present, but no data comes in, either.  
  > check who is providing the clocking.
  > check circuit condition with Telco

In theory, the number of line code errors can indicate whether the line code is mismatched. B8ZS data fed to an AMI device will cause that device to record a large number of bipolar violations. Likewise, AMI data fed to a B8ZS device may occasionally have strings of more than seven zeros and thus lead to a number of excessive zero string events.