For network administrators who require Basic Rate Interface (BRI) access to ISDN communications, Cisco offers BRI port adapters for the Cisco 7200 Series routers. The BRI port adapters come in two versions: one with four ports, the other with eight. The four-port BRI port adapter supports U-interfaces by including an on-board NT-1 device for network termination. The eight-port version supports S/T interfaces, and used in environments where NT-1 support is not required; This relaxed requirement enables the doubling of port density.Both port adapters offer basic "two-B-plus-D" connectivity, supporting 2 B-channels, each operating at 64 or 56 Kbps for a maximum data rate of 128 Kbps. A single D-channel is supported for signaling. BRI port adapters are particularly useful for aggregating traffic from relatively low-speed ISDN networks into high-end routers, as well as providing back-up functionality for high-speed links in the case of network failure. Cisco's BRI port adapters are designed for use with the Cisco 7200 Series router only, and are not supported on the Cisco 7500 Series routers.