The circuit between the MSC (Mobile Switching Center) and BTS (Base Transceiver Station) passes through the central office, or through multiple central offices. Within each office, it may also pass through multiple pieces of transmission equipment. An interface unit is located at the MSC and BTS. The interface unit may be a simple device having only a remote loop back capability, or it may provide sophisticated performance monitoring capabilities. Its capabilities may or may not be accessible to the wireless service provider technician.
