The ISDN BRI WAN interface cards are shared between the Cisco 1600, 1720, 2600 and 3600 series. Each card supports a single ISDN BRI port is offered with and without the NT1 interface. The S/T WAN interface card module (WIC-1B-S/T) needs external Network Termination 1 (NT1) device where the U WAN interface card module (WIC-1B-U) has an internal NT1 device. These cards are an excellent choice for customers seeking a single primary ISDN BRI connection or for use as a back-up connection.