[ih] Arpanet line speed

[Alex McKenzie]

John,
The Bell 303 modem was specified to run at 50kbps.  Your guess about DS0 sounds plausible for the underlying circuit speed but I don't know if it is correct.
Cheers,Alex


      From: John Day <jeanjour at comcast.net>
 To: Paul Ruizendaal <pnr at planet.nl> 
Cc: internet-history at postel.org
 Sent: Wednesday, January 18, 2017 6:11 AM
 Subject: Re: [ih] Arpanet line speed
   
I believe it was 56Kb. A DS0 out of a T1.

John

> On Jan 18, 2017, at 04:10, Paul Ruizendaal <pnr at planet.nl> wrote:
> 
> 
> I'm wondering about (early) Arpanet line speed. Online sources seem to say that it was 50kb/s on the long haul connections, provided by AT&T. To me, that "50" sounds like a strange number:
> 
> - if the modem from the IMP to the Bell System was analog, the best technology of the time was perhaps 2.4kb/s (and this speed seems to have been considered initially). In this case a speed of 50kb/s is not a multiple of 2.4kb/s, and it would have required 21 parallel lines -- more logical would have been to use a full T1 connection and have 24 parallel lines or 57.6kb/s.
> 
> - if the modem from the IMP to the Bell System was digital, it would most likely have used a single channel of a T1 connection, i.e. a 56kb/s rate. I suppose AT&T would have to run special lines from the IMP to the trunk, as I don't think digital lines were used for the last mile back then (perhaps they were to company/hotel PBX's, I don't know).
> 
> Perhaps the "50" is a rounded number, perhaps some capacity is used for error correction, perhaps it is plain wrong.
> 
> Does anybody have more background information on this?
> 
> Thanks,
> 
> Paul
> 
> 
> 
> 
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