[ih] ARPA initial IMP-IMP line speed

[Guy T Almes]

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
>
>