Abstract: A telephony network with a digital central office and one or more remotely located digital satellite units. Each digital satellite unit connects to a number of remote subscriber lines and monitors these lines to establish communications through interconnecting spans and a digital satellite interface at the digital central office. The digital satellite interface includes a programmed controller that communicates with the remote digital satellite units connected to it, with call processing equipment in the digital central office and with maintenance processing equipment in the digital central office. Supervisory information and voice data information, in digital form, pass through span equipment that interconnects port group highways in the digital central office and the interconnecting spans. The span equipment strips messages from and inserts messages into signalling time slots on the interconnecting span.