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How to configure the TDM Switching feature on an IOS voice gateway router with an ISDN PRI for video calls


Core Issue

On the Cisco 2800 and 3800 Series Integrated Services Routers (ISRs), Time-division Multiplexing (TDM) switching feature can be selected with digital interface cards. Install the cards in either the High-speed WAN Interface Card (HWIC), the Extension Voice Module (EVM), or the Network Module (NM) slots across the platform. On the Cisco 2600 and 3700 Series routers, the digital interfaces that use the TDM Switching feature must be on the same NM; on these routers, the non-voice traffic cannot be switched across a router backplane to a different NM.

Note: Cisco IOS  Software does not necessarily support all features that some ISDN service providers provide. The information in this case applies to basic call switching only, which includes ISDN speech or data calls between voice ports. Do not assume that there is support for any other supplementary ISDN feature.

If a two-port E1 Voice/WAN Interface Card (VWIC) is used, these default the clocking on both ports to line. As a result, the port connected to Telco runs clean. However, the port connected to the Multipoint Control Unit (MCU) sees regular slips, which prevent data applications from working due to errors in the packets.


In order to resolve this issue, set the port connected to the video conferencing unit as the clock source internal. This ensures that there is a consistent clock source from Telco used on the router and the MCU.

Refer to ISDN Voice, Video and Data Call Switching with Router TDM Switching Features for more information.