Google is your friend: https://en.wikipedia.org/wiki/T-carrier https://en.wikipedia.org/wiki/Digital_Signal_1 Yes and no. Some details are given in "The interface message processor for the ARPA computer network" (Heart, Kahn, Ornstein, Crowther and Walden), but not much. More detail of the business arrangement is contained in "A History of the ARPANET: The First Decade" (BBN Report No. 4799). Details of the interface, and the IMP side, are given in the BBN proposal, "Interface Message Processor for the ARPA Computer Network" (BBN Proposal No. IMP P69-IST-5): in each direction there is a digital data line, and a clock line. It's synchronous (i.e. a constant stream of SYN characters is sent across the interface when no 'frame' is being sent). The 50KB modems were, IIRC, provided by the Bell system; the diagram in the paper above seems to indicate that they were not considered part of the IMP system. The modems at MIT were contained in a large rack, the same size as the IMP, which stood next to it. I wasn't able to find anything about anything past the IMP/modem interface. Perhaps some AT+T publications of that period might detail how the modem, etc, worked. Noel