Cisco 6700 Series Multiservice Access Platforms

Field Notice: QRSS Patterns on DS1 May Cause Invalid Off Hook Events and Cause MCC Switchover

September 13, 2000

Products Affected




Main Control Card, Stratum 4 Clock CPU, Ethernet


Main Control Card, Stratum 3 Clock CPU

Problem Description

Quasi-Random Signal Sequence (QRSS) test patterns inserted on a live digital signal level 1 (DS1) that has a call reference value (CRV) assigned to it may result in the Main Control Card (MCC) switching over on the Cisco 6732, which is configured as a Head Node, and attaching to a Class 5 switch.

Note:?For the purposes of this field notice, QRSS refers generically to all quasi-random test patterns, such as QRSS, 2^15-1, 2^23-1, or 2047. Other test patterns may also cause this anomaly to occur.


QRSS testing is often used to troubleshoot a DS1 circuit. When a quasi-random test pattern is injected onto an active DS1 terminating into the 6732, which is configured as the Head Node, the random bit pattern is misinterpreted as a very quickly changing AB or ABCD signaling stream used in robbed-bit signaling. This condition overloads the MCC GR-303 processing circuits, and the MCC switches over to the standby MCC.

It should be noted that the Cisco 6705 is not affected by this problem because the GR-303 interface functionality is not present in the MCC installed in the 6705.

Problem Symptoms

On the 6732, the MCC switches over. The most apparent symptoms include a possible lack of dial tone, or calls dropping. The user may also notice that phones ring continuously.

Network maintenance staff may observe the MCC cards reset and switchover continually as long as the QRSS-type signal is still applied. These switchovers are due to watchdog timeouts.


Currently, there are two workarounds available:

  1. Apply loopbacks to the DS1 to be tested before injecting a QRSS-type signal.

  2. Use unframed QRSS-type signals. This prevents the 6732 from framing up and finding the AB or ABCD signaling bits.

For More Information

If you require further assistance, or if you have any further questions regarding this field notice, please contact the Cisco Systems Technical Assistance Center (TAC) by one of the following methods: