Slip error messages indicate that the T1/E1 port is receiving clock information that is out of phase (out of sync). Because the router has elastic store architecture, it can experience controlled slips while it receives clocking from two different time sources. Note: Physical layer issues, such as bad cabling or faulty clocking references, can also cause slips. Eliminate these slips by addressing the physical layer or clock reference problems.
There are five basic timing scenarios that can occur when a digital T1/E1 voice port adapter is connected to a PBX, CO, or both. In all of the examples, the Public Switched Telephone Network (PSTN) (or central office) and the PBX are interchangeable for the purpose of providing or receiving clocking.
Two clock sources can be configured in these three ways:
Dual T1/E1s, both receive clocking from the line
Dual T1s, one receives clocking and one provides clocking
Dual T1s, both clocks from the router
The digital T1/E1 port adapter has an on-board Phase-Lock Loop (PLL) chip that can either provide a clock source to both T1/E1s or receive clocking that can drive the second T1/E1.