[ih] ARPA initial IMP-IMP line speed

[Craig Milo Rogers]

On 04.02.22, Mike Padlipsky wrote:
> At 05:30 AM 2/22/2004, Noel Chiappa wrote:
> >(I have this
> >vague memory that they were specially conditioned)
> 
> well, i have a clear memory that we called 'em 'leased lines', but since i 
> never did 'do' hardware i had no idea how they were specially conditioned, 
> tho that they were such was clearly implied.  indeed, i strongly suspect 
> that even the dial-up lines to multics, for those lucky enough to have 
> access from home, had to be specially conditioned in some fashion in those 
> times -- and weren't anywhere near 50kb/s.

	My memory is that there were three leased line services that
computer folk would commonly request from the phone company at the
time:

1)	A "conditioned" line was usually a line from the telco's central office
	(CO) to the user's end-point that had been tested, and if necessary,
	adjusted, to provide a relatively flat gain over a certain
	frequency band.  300 bps FSK modems usually worked over ordinary,
	unconditioned phone lines, but a 1200 bps modem was more questionable
	over an unconditioned line.  Conditioned lines would have a
	uniform impedence, eliminating nasty echoes, etc.  Conditioning
	might be a one-time affair, or a continuous "guarantee" by the
	telco.

	They worked well for some people, but were an endless nightmare for
	others.  For example, some lines would lose their conditioning
	whenever it rained (an infrequent occurance in Los Angeles), and
	regain their desired characteristics by themselves when they dried
	(perhaps before the telco serviceman arrived).

	Conditioned lines might be part of the standard dial phone network,
	or they might be provided as part of a private dial network or
	a dedicated circuit between two (or more, "multidrop") end-points.

2)	An "unloaded bare copper" circuit was a leased circuit between
	two user end-points with no filters, no frequency-multiplexed
	shared circuits, etc.  You could use it for simple burglar alarm
	circuits (which used DC signals to indicate a breakin), you
	could attach teletypes (current loop at 75 or 110 bps, perhaps),
	or you could attach computer modems to it (so long as your signal
	didn't exceed certain frequency and energy limits, designed to
	prevent signals on adjacent pairs of wires from coupling to and
	interfering with each other).  Unloaded bare copper circuits
	in the same CO were relatively easy for the telco to provide
	('till the neighborhood pairs ran out), and relatively inexpensive.

	You might use an unloaded bare copper circuit to connect a Host
	to an IMP in a nearby location.  Some organizations (such as, say,
	UCLA) would lay their own bare copper wires to avoid telco hassles
	or provide direct, shorter circuits by bypassing the telco's CO --
	and lose track of them, later.

3)	A "dedicated long-distance circuit", with various caveats about
	the type of modulation to be used on the line and the degree of
	isolation needed between it and other lines.  This was the type
	of line used between most (but not all!) adjacent ARPAnet IMPs,
	and required substantial one-time engineering charges and hefty
	monthly lease fees.

					Craig Milo Rogers