PRI Primary Rate Interface This ISDN service is used mainly by Internet service providers (ISPs) and businesses. Why? Because it provides a lot of bandwidth: 23 B channels with 64-kbps throughput capability and one 64-kbps D channel for call signaling and setup. It was designed for transmission through a standard North American T1 trunk and has throughput capability to 1.472 mbps. If you see the notation 23B+D, that's another way of saying the same thing.