 introduced by circuit components or natural disturbances, that tend to degrade the performance of a communications channel. nonblocking nonblocking A switch that has a through-traffic path for each attached station. A nonblock- ing switch or switching environment is designed to never experience a busy condition due to traffic volume. nonswitched line In data communications, a permanent connection between computers or devices that does not have to be established by dialing. nonvolatile storage A storage medium whose contents are not lost when the power is removed. NPA (numbering plan area) A geographic subdivision of the territory covered by a national or integrated numbering plan. An NPA is identified by a distinctive area code. NRZ (nonreturn to zero) A digital signaling technique in which the signal is at a constant level for a duration of time. NSIS (Next Steps in Signaling) A network architecture that addresses the introduction of QoS on an end-to-end basis. The design goals of NSIS include applicability across different QoS technologies, such as DiffServ and MPLS, as well as resource availability upon request prior to a reservation request. NSP (network service provider) A very large, global backbone carrier that owns its own infrastructures (e.g., AT&T, Sprint, Verizon). NTSC (National Television Standards Committee) A television broadcasting system that uses 525 picture lines and a 60Hz field frequency. It is used primarily in the United States, Canada, Mexico, and Japan. See also PAL and SECAM. NTU (network termination unit) A device that connects the PSTN with CPE. An NTU marks the final interconnect between the public network and a customer's private equipment. It is owned by the service provider and typically has communication standards, such as voltages and protocols, that allow specific types of equipment to communicate with the PSTN. null attached The operation of an FDDI concentrator without being attached to the back- bone network. This configuration establishes a small, autonomous, single-ring F