In ISDN, the 16-kb/s segment of a 144-kb/s, full-duplex subscriber service channel that is subdivided into 2B+D channels, i.e., into two 64-kb/s clear channels and one 16-kb/s channel for the ISDN basic rate. Note 1: The D channel is usually used for out-of-band signaling. The two 64-kb/s clear channels are used for subscriber voice and data services. Note 2: The D-channel specifications are addressed in the CCITT (now, the ITU-T) Recommendation for the Integrated Services Digital Network (ISDN). Note 3: The D-channel may be 64 kb/s for the primary rate ISDN service.