In the integrated service digital network (ISDN), a high-speed channel comprising multiple aggregated low-speed channels to accommodate bandwidth-intensive applications such as file transfer, videoconferencing, and high-quality audio. An H channel is formed of multiple bearer (B) channels bonded together in a primary rate access (PRA) or primary rate interface (PRI) frame in support of applications with bandwidth requirements that exceed the B channel rate of 64 kbps. The channels, once bonded, remain so end-to-end, from transmitter to receiver, through the ISDN network. The feature is known variously as multirate ISDN, Nx64, channel aggregation, and bonding. ISDN standards define H channels at the following levels.