[TUHS] TUHS Digest, Vol 14, Issue 63

Paul Ruizendaal pnr at planet.nl
Thu Jan 19 00:29:26 AEST 2017

I asked over at the internet history list

Short of it is that it used Bell 303C modems which operated at 50kb/s operating
over an analog "broadband" channel predating the T1. It used the space of 12 voice
channels and some fairly fancy modulation techniques. Connection to the trunk
exchange was over a leased line.

On 17 Jan 2017, at 16:32 , Noel Chiappa wrote:

>> 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

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