Basic Rate Interface is an Integrated Services Digital Network configuration that is used in subscriber lines similar to those that have long been used for voice-grade telephone service. Its configuration includes 2 bearer channels at 64kbits each known as B channels and 1 data channel at 16 k bits at D channel. The B channels are used for voice or user data, and the D channel is used for any combination of data, control or signaling, and X.25 packet networking. The BRI ISDN service is commonly installed for residential or small business service (ISDN PABX) in many countries. In contrast to the BRI, the Primary Rate Interface (PRI) configuration provides more B channels and operates at a higher bit rate.