[ih] ARPA initial IMP-IMP line speed

[David L. Mills]

Dave & Co.,

Little bit of history here. In 1969 AT&T (Western Electric) was the only
source for analog and digital lines and most high-speed modems used a
48-kHz analog group circuit normally used to multiplex 12 4-kHz voice
channels. There were several modems used by AT&T at 40, 48 and 56 kbps.
In theory, AT&T could provide a supergroup at 6.3 MHz or jumbogroup at
44 MHz with digitronics to match and I bet they did on special order.

Before 1968 the higest speed you could get on an ordinary telephone line
was 300 bps full-duplex (Bell 103 FSK), 1200 bps half-duplex (Bell 202
FSK) and 2000 bps half-duplex (Bell 201 DQPSK). The last could go at a
blinding 2400 bps full-duplex on a 3002 leased circuit. You couldn't get
anything higher on an ordinary phone or leased line until the middle
eighties. Starting in 1986, I had a full-duplext 3002 at 9600 bps
between my home and campus, so that may calibrate the curve.

Until 1968 you couldn't put anything on an ordinary phone line except
AT&T devices. But, the FCC told AT&T to provide a coupling device called
a Data Access Arrangement (DAA) so the customer could use non-AT&T
modems. I had the first one installed in Michigan. They wanted to get it
right, so they sent a fleet of trucks to install it. But, there were
lots of special order things AT&T could provide, like analog program
radio and TV lines and on these you didn't have to use AT&T modems.
That's how I tested early spread-spectrum digital telephone gadgets as a
grad student.

Dave

"Toru Nohzawa ," wrote:
> 
> Dave,
> 
> Thank your for the information.
> 
> Do you know what modem they did use ?
> 50Kbps or 56Kbps whichever at that time, seems 10 or more times
> faster than Chronologically told modem technology.
> Who could provide it with what modulation method ?
> 
> Toru,
> 
> -----Original Message-----
> From: Dave Crocker [mailto:dhc at dcrocker.net]
> Sent: Monday, February 23, 2004 9:53 AM
> To: Toru Nohzawa ,
> Cc: internet-history at postel.org
> Subject: Re: [ih] ARPA initial IMP-IMP line speed
> 
> Toru,
> 
> For simplification, all initial Arpanet links were 56Kbps.
> 
> This avoided having to deal with differential performace stemming from
> different line speeds.
> 
> d/
> 
> TN> Hello, all,
> TN> I have a following question about the subject.
> TN> If you know the fact, please let me know it
> 
> TN> Question:
> TN> There are lots of documents mentioning the initial line speed of ARPAnet
> in
> TN> 1969 was 50Kbps.
> TN> As far as I know, the line speed at that time over the voice band
> TN> transmission was
> TN> less than 9,600bps. Most probably 4,800bps or 2,400bps.
> TN> In addition, no modems for 50Kbps become available.
> TN> Some document says 50Kbps over analog wideband. But this causes other
> TN> question of what modem was used and who provided that.
> 
> TN> How many line speed is used with what modem at initial ARPA IMP-IMP?
> 
> TN> Regards,
> 
> TN> Toru Nohzawa
> TN> nohzawa at w-tri.com
> 
> d/
> --
>  Dave Crocker <dcrocker-at-brandenburg-dot-com>
>  Brandenburg InternetWorking <www.brandenburg.com>
>  Sunnyvale, CA  USA <tel:+1.408.246.8253>