An NT1 is an intermediate device located between the router and the service provider ISDN switch. The NT1 is used to connect four-wire subscriber wiring to the conventional two-wire local loop. In North America, the customer typically provides the NT1, while in the rest of the world the service provider provides the NT1 device.

It may be necessary to provide an external NT1 if the device is not already integrated into the router. Reviewing the labeling on the router interfaces is usually the easiest way to determine if the router has an integrated NT1. A BRI interface with an integrated NT1 is labeled BRI U. A BRI interface without an integrated NT1 is labeled BRI S/T. Because routers can have multiple ISDN interface types, determine which interface is needed when the router is purchased. The type of BRI interface may be determined by looking at the port label. To interconnect the ISDN BRI port to the service-provider device, use a UTP Category 5 straight-through cable.

Caution: It is important to insert the cable running from an ISDN BRI port only to an ISDN jack or an ISDN switch. ISDN BRI uses voltages that can seriously damage non-ISDN devices.

Web Links Integrated Services Digital Network (ISDN) http://www.webwisdom.com/edu_content/ cps640fall00/lectures/ module3/WAN/sld044.htm