[ih] Six modems

[Steve Crocker]

Jorge,

Your comment about 6 modems got my attention.  I had not paid a lot of
attention to BBN report 1763.  It was written at the very beginning of
the Arpanet contract work in January 1969.

By the time BBN released BBN 1822 in April or May 1969, the IMP was capable
of handling up to four hosts.  I believe this was a change from the
original plan to have just one host on an IMP.

Alex or others can comment more authoritatively, but it seems clear -- or
at least plausible -- that seven places existed within the chassis to put
interfaces.  When the focus was on just one host, that left room for a
maximum of six modems.  As the design progressed, it seems they changed
their design to a max of four hosts and a max of four modems, with a limit
of seven for both.

I also recall that as the interfaces became more varied -- host interface,
distant host (DH) interface, very distant host (VDH) interface -- there
were other constraints.  IIRC, the number 19 figured into the equation,
Each interface consumed some number of whatever the quantity was.  I think
the VDH interface consumed something like four units, but a regular host
interface consumed only two.  But I haven't checked the documentation and
am not certain of the specifics.

Steve


On Mon, Nov 15, 2021 at 10:00 PM Jorge Amodio <jmamodio at gmail.com> wrote:

>
> Weren't the BBN modems capable of handling up to 6 "phone lines" at
> 50,000bps. So basically analog dedicated phone lines.
>
> I'm sure the early lines were not DDS 56K
>
> Regards
> Jorge
>
>
> On Mon, Nov 15, 2021 at 8:35 PM Steve Crocker via Internet-history <
> internet-history at elists.isoc.org> wrote:
>
>> Arpanet lines were 50 kbs, not 56 kbs.
>>
>> Sent from my iPhone
>>
>> > On Nov 15, 2021, at 9:11 PM, Timothy J. Salo via Internet-history <
>> internet-history at elists.isoc.org> wrote:
>> >
>> > I think we all agree that the IMP was a pretty limited machine.  From
>> > the backup slides of a presentation of mine:
>> >
>> > Early ARPANET router, Interface Message Processor (IMP), (1969):
>> > o 16-bit words, 12-16 K-word memory
>> > o 100-μsec clock (10 KHz)
>> > o Early ARPANET links: 56 kbps
>> > o 0.18 clock cycles per bit
>> >
>> > I have argued that this, 12-16 K words of memory, is why we had the
>> > end-to-end argument (which morphed into a principal and then into a
>> > canon).
>> >
>> > (The rest of the presentation pretty much ignores the end-to-end
>> > argument.)
>> >
>> > Also from this presentation:
>> >
>> > Early NSFNET router: DEC LSI-11/73 (1983) with Fuzzball router
>> > o 512 KB memory
>> > o (15.2 MHz)
>> > o 271 clock cycles/bit
>> >
>> > -tjs
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>