in-band signaling ISDN normally uses a separate wire called the D channel to set up calls and handle their signals. In some locations, this third wire isn't available, and bandwidth must be "borrowed" from the other two data-carrying B channels. For Basic Rate Interface (BRI) service, this means that each B channels is limited to 56 kbps throughput (instead of 64 kbps with out-of-band signaling).