The N-8400RS substation interface unit uses packet audio technology to permit connection of up to 16 RS-4xx series 4-wire master or door stations. Master stations and door stations use the same dual unshielded twisted pair wiring. Each N-8400RS supports one voice link. The Substation Interface Unit may be used as a stand-alone system (requires at least one master station) or connected to an N-8000 IP intercom system over LAN or WAN.