Network Equipment-Building System (NEBS) describes the environment of a typical United States Regional Bell Operating Company (RBOC) central office. NEBS standard GR-1089-CORE (e.g., section 10.7) specifies requirements for voice band conducted noise emissions returned by telecommunications equipment back to direct current (DC) power lines. Particularly, the electrical noise fed back from the telecommunications equipment into its input power source is required to be limited to prevent interference to other equipment sharing the same power plant and distribution system. The level of the voice band noise generated by the telecommunications equipment shall not exceed a particular threshold specified in GR-1089-CORE, section 10.7.1.
Any network device (e.g., router, switch, etc.) may be a source of low frequency voice band noise that is required to be filtered out. At the low frequency range of this noise, inductors and capacitors needed for conventional passive L-C filters become unsuitably large and heavy.