[ih] ARPA initial IMP-IMP line speed

[David L. Mills]

Guy,

You are correct; the eigth bit was for scrambling and the supervisory
bits were every 193 bits. This was the original DS-1 specification;
later the Extended Superframe spec got even more ingenious. The real
lesson of the analog era was that whatever you did had to fid in the
multiplexor hierarchy at the group, super and jumbo bandwidths. That's
why the silly breakout at 50 kbps and 230 kbps.

The really interesting issues were how the transition between analog and
digital domains were managed and, for example, how TV transitioned from
4.5 MHz analog to 2xT3 (92 Mbps) with basically old technology. That was
clever. One of the digital breakout rates is 6.3 Mbps, originally I am
told, to support Picturephone. Remember that?

Dave

Guy T Almes wrote:
> 
> Dave,
>   As I recall, you got only 56 Kb/s from a 64-Kb/s DS0 circuit by wasting
> every 8th bit (to avoid long strings of zeroes that messed up the
> encompassing T1 circuit).
>   The supervisory bit was one bit in every 193-bit T1 frame (24 8-bit
> chunks plus that pesky supervisory bit).
>   But this was the era of digital service, at least.
>   I'm really enjoying learning more about the pre-digital service era.  I
> recall the monster IMP equipment in the CMU CS machine room and always
> imagined that squeezing 50 Kb/s out of an analog circuit was pretty tense.
>   Regards,
>         -- Guy
> 
> --On Wednesday, February 25, 2004 06:52:41 +0000 "David L. Mills"
> <mills at udel.edu> wrote:
> 
> > Kent,
> >
> > Depends who you talk to. AT&T had several special-order circuits,
> > including 48, 50 and 56-kbps circuits, each individually engineered. The
> > 48 and 50 kbps circuits were defintely ananlog, but the 56-kbps circuits
> > were digital derived from 64-kbps voice circuits with a supervisory bit
> > every 192 bits. It might not make much difference from the viewpoint of
> > today and especially because of the ubiquitous nature of AT&T
> > penetration of the 1970s.
> >
> > Dave
> >
> > kent at icann.org wrote:
> >>
> >> On Tue, Feb 24, 2004 at 09:39:40PM +0000, David L. Mills wrote:
> >> > Chris,
> >> >
> >> > The IMP interface is described in BBN Report 1822, which I have. There
> >> > were companion documents for the Digital Equipment IMP-11A interface
> >> > for the PDP-11 Unibus which I don't have. My first ARPAnet connection
> >> > at COMSAT Labs was a 4800-bps analog link to BBN; later I connected
> >> > with 56-kbps DDS and IMP 29 at Mitre. Toward the end of ARPAlife I
> >> > connected from UDel via 56-kbps DDS to IMP 112 somewhere in WashDC.
> >> > Scary to think now I have 100-Mbps desktop-desktop just about anywhere
> >> > in Internet2. How the heck did we get along with only a 50-kbps
> >> > network?
> >>
> >> I wasn't closely connected at the time, and my memory is dim, but I
> >> always thought that the early long line arpanet connections were all 56
> >> kbs.  Where does 50 kbs come from?
> >>
> >> Kent
> >>
> >> --
> >> Kent Crispin
> >> kent at icann.org    p: +1 310 823 9358  f: +1 310 823 8649
> >> kent at songbird.com SIP: 81202 at fwd.pulver.com
> >
> >