[ih] Arpanet line speed

[Paul Ruizendaal]

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