What do the E1/T1 L1 Error counters represent?

Slip

The number of positive and negative slips. Slips are covered in more detail in later questions.

Typical causes:

The operator's PDH network is not maintaining correct internal sync. Understanding the operator's sync topology is a pre-requisite to troubleshooting this problem.

GTH is (mis-)configured to use its internal frequency reference when it should be configured as a sync slave to one of the E1/T1 inputs.

GTH is unable to find a sync source, for instance because the framing configuration is incorrect. One example is configuring GTH for multiframe when the link is actually doubleframe.

Frame Error

The incoming signal did not conform to the expected framing.

Typical causes:

Incorrect framing configuration, e.g. GTH configured for multiframe, whereas the link is actually doubleframe.

Incoming signal is not an E1 signal, e.g. attempting to monitor a DSL line can give this symptom.

Incoming signal contains too much noise to decode correctly.

Code violation

The incoming signal did not conform to the expected signal coding rules.

Typical causes:

Incorrect coding configuration, e.g. GTH configured for HDB3 coding (used on all E1 links) but the incoming signal is actually T1 (which uses B8ZS coding). In most cases, this will cause hundreds or thousands of code violations per second.

Incoming signal is not an E1 signal, e.g. attempting to monitor a DSL line can cause this symptom.

Excessively weak incoming signal. Use a handheld signal tester or an oscilloscope to verify that the signal strength is as expected.

Incorrect monitoring configuration, e.g. GTH is configured for -20dB monitoring, but the incoming signal is not attenuated.

Line noise/interruption. A small number of code violations is normal. In some installations, this can be as low as five or ten code violations per link per month. Error rates greater than a few tens of violations per day are usually worth investigating.

CRC error

The Layer 1 data integrity check (a CRC check) failed. The counter shows how many times the check failed.

This counter is only used when the GTH is configured for multiframe framing.

Typical causes:

Line noise. A small number of CRC errors is normal. In some installations, this can be as low as five or ten per link per month.

Incorrect framing configuration, example when a link is configured for multiframe but used as doubleframe.