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:
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
Controlled slips
Out of frame signals (severely errored framing defects)
AIS
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.
Severely errored seconds are 1-second intervals with serious problems. Most CSU/DSUs that keep statistics keep records of the number of severely errored seconds. Some of the errors that cause this are:
Out of frame signals (one or more severely errored framing defects)
One or more AIS defects
320 or more CRC errors (ESF only)
If only the Ft framing bits are measured, four or more frame bit errors. When both the Ft and Fs framing bits are measured, eight or more framing bit errors. (SF only)
Severely errored is an appropriate description for a second with 320 or more CRC errors. At 8,000 frames per second and 24 frames per superframe, only 333 superframes are transmitted per second. T1 does not discard severely errored data, but it is usually treated as suspect by data-communications equipment. If the receiver discards all superframes failing the CRC check, 96% of the data is lost!
Ten consecutive severely errored seconds remove the path from use. The path becomes usable after 10 consecutive seconds with no severely errored seconds. While the line is unavailable, all other parameter counts are frozen because they are intended to measure trouble with the line when it is in service.
Table C-5. Performance data
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