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When configuring basic timing for a digital T1/E1 voice port adapter connected to a PBX, CO, or both with two clock sources, the user receives slip errors

Core Issue

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.

For more information, refer to Configuring T1/E1 High Capacity Digital Voice Port Adapters.

Problem Type

Gateway or module

Common Software and Product Issues



Failure Type

Error message

Cannot be configured correctly

End Where Problem is Heard or Device Type Involved

PSTN/PBX or voice gateway connected device

IOS Show Commands

Slips secs

TDM Signaling Type



Clocking and Time


Voice Servers and Gateways

Voice port

Voice Gateway Hardware Components



Voice and Voice/WAN Interface cards