In the United States, the customer is required to provide the NT1. In Europe and various other countries, the telephone company provides the NT1 function and presents an S/T interface to the customer. In these configurations, the customer is not required to supply a separate NT1 device or integrated NT1 function in the terminal device. Equipment such as router ISDN modules and interfaces must be ordered accordingly.

To select a Cisco router with the appropriate ISDN interface, do the following:

1. Determine whether the router supports ISDN BRI. Look on the back of the router for a BRI connector or a BRI WAN Interface Card (WIC).
2. Determine the provider of the NT1. An NT1 terminates the local loop to the central office (CO) of the ISDN service provider. In the United States, the NT1 is Customer Premise Equipment (CPE), meaning that it is the responsibility of the customer. In Europe, the service provider typically supplies the NT1.
3. If the NT1 is CPE, make sure the router has a U interface. If the router has an S/T interface, then it will need an external NT1 to connect to the ISDN provider.

If the router has a connector labeled BRI then it is already ISDN-enabled. With a native ISDN interface already built in, the router is a TE1. If the router has a U interface, it also has a built-in NT1.

If the router does not have a connector labeled BRI, and it is a fixed-configuration, or non-modular router, then it must use an existing serial interface. With non-native ISDN interfaces such as serial interfaces, an external TA device must be attached to the serial interface to provide BRI connectivity. If the router is modular it may be possible to upgrade to a native ISDN interface, providing it has an available slot.

Caution: A router with a U interface should never be connected to an NT1 as it will damage the interface.

Web Links Integrated Services Digital Network (ISDN) http://www.cisco.com/univercd/cc/ td/doc/ cisintwk/ito_doc/ isdn.htm