From mboxrd@z Thu Jan  1 00:00:00 1970
From: jnc@mercury.lcs.mit.edu (Noel Chiappa)
Date: Tue, 17 Jan 2017 10:32:07 -0500 (EST)
Subject: [TUHS] TUHS Digest, Vol 14, Issue 63
Message-ID: <20170117153207.B39A518C094@mercury.lcs.mit.edu>

    > From: Joerg Schilling

    > Was T1 a "digital" line interface, or was this rather a 24x3.1 kHz
    > channel?

Google is your friend:

  https://en.wikipedia.org/wiki/T-carrier
  https://en.wikipedia.org/wiki/Digital_Signal_1


    > How was the 64 ??? Kbit/s interface to the first IMPs implemented?
    > Wasn't it AT&T that provided the lines for the first IMPs?

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