In addition to errors, monitoring performance data allows administrators to spot line degradations before they turn into stressful line-down situations. Most performance data is reported as the number of seconds for which a particular condition occurred. Here is a list of problems you might encounter, which are summarized in Table C-5:
Line errors include BPVs, EXZs, and LOS defects. The number of seconds with at least one of any of those errors is a line errored second.
A controlled slip second is a second during which at least one controlled slip occurred. Controlled slips can be accurately measured only by CSU/DSUs.
As defined for the path (as opposed to the line), an errored second is a second with at least one error off the following smorgasbord:
Path code violations
Out of frame signals (severely errored framing defects)
An errored second for the path may also be abbreviated as ES-P. Early drafts of T1.231 that were used as the basis for the SNMP monitoring of T1 links used a different definition, which is discussed in Appendix D.
An errored second type B is a parameter that measures data corruption. It is a 1-second interval with between 2 and 319 CRC errors, but without severely errored framing problems or AIS. Because framing is functional and AIS is not present, the path appears to be functional.