AT&T DS1/DMi/ISDN-PRI manual Facility Alarms, Excessive Slips, Excessive Misframe and CRC Errors

8-8MAINTENANCE AND ALARMS

Facility Alarms

There are several types of facility alarms that may arise. These are briefly described next.

Excessive Slips

Data received from a DS1 facility is stored (clocked) into buffers on the circuit pack using a clock signal derived from the received signal. Data is read from the same buffers using a clock derived from the master clock. If at any time these two clocks are not phase-locked, data will be stored into the buffers at a rate different from the rate read from the buffers. This results in underflow or overflow of the buffers, called slips.

Slips result in the repetition or deletion of one 8-bit word for every channel. The slip rate is monitored by the circuit pack. The slip rate is used to determine if that DS1 being used as the system clock reference is functioning properly. Slips cause pops in voice and voice-grade data signals and can cause errors in digital data signals.

An example for a type of problem that will cause slips is having both switches (at each end of a DS1 facility) administered as timing masters instead of one being a master and the other using the received DS1’s signal from the master as a timing reference.

Excessive Misframe and CRC Errors

Misframes and cyclic redundancy check (CRC) errors may be produced by marginal or faulty line repeaters, NCTE, noise on the transmission line, or by the circuitry that generates the framing pattern or CRC at the transmit end.

Bit errors, in a DS1’s signal, are detected via misframes when D4 framing is used and via CRC errors when ESF is used.

The microprocessor keeps count of the number of misframe or CRC errors and uses the count to process the minor and major alarms. The misframe or CRC count is used in choosing clock references for the switch. Also, an unterminated transmission line could generate noise that looks like an DS1’s signal. The absence of a framing pattern or continuous CRC errors is used to show that it is not a DS1.

LFA Alarm

The receive DS1’s signal should contain either the D4 or ESF framing pattern. Which framing pattern is determined by administration details. The ANN35 has two green LEDs that function to show local and remote framing status. Normally, both green LEDs will be on when the near end and far end are framing properly.

When the (top) green LED is off, the near end interface cannot frame up on the DS1’s signal. This event is known as the LFA alarm. The LFA alarm is also known as the red alarm, because a red LED lights on the D4-channel bank when this alarm is on.