Bell 103 modem standards

From: Fred Cisin <>
Date: Thu Sep 4 17:04:00 2003

On Thu, 4 Sep 2003, Tore S Bekkedal wrote:
> Hell!
> I just picked up a modem for a classmate (I finance the restoration with
> tech support - *sigh*), and on the label on the outside, with the
> supported protocols, and along with the V.24 V.90 yadda yadda there was
> at the end, Bell 103, Bell 212A...! (I may have the numbers slightly
> off, but I'm sure about the 103 part).
> ...Is this a common protocol? In a brochure that came with the DECUS
> newsletter, about Telecommunications, it mentioned these protocols, and
> described them as Low-Speed (defined as less than 2000 baud, I believe).
> This was also mentioned as built-in options for the TTY 33's, and I seem
> to recall them being mentioned several other places.

Bell 103 was the protocol for 300 baud.
I don't think that I've ever seen it successfully used past about 600
baud, and if you look at the frequencies that it used, I don't think that
frequency shift keying COULD be used faster than that without switching to
much higher frequencies. That was why phase shift keying was developed.

Take the frequencies used, and calculate HOW MANY cycles of an audio
tone would occur at those frequencies at various bit rates. Then think
about what method you would use for identifying the frequency of a tone if
you didn't have multiple complete cycles to count.

> Is this me being wrong, has the protocol been constantly upgraded
> speedwise, is the protocol "Just something that stuck around", and
> therefore direly needing support from every new modem? Or is this just a
> case of truly, *truly* excellent backwards compat? I mean, the brochure
> was all in all dated 1967.

There is still some occasional need for "low speed" telecommunications.
The TTYs used by deaf people are still mostly using Baudot (actually
Murray) code with V.18? protocol. The newer fancier ones can now also do
ASCII with Bell 103!
Some "voice modem"s can simulate the V.18 protocol with a bit of software.
When MICROS~1 came out with the "Microsoft Cordless Phone", they claimed
that that software and hardware could do it, but their tech support were
unable to come up with how to invoke that "supported" feature of the

Grumpy Ol' Fred
Received on Thu Sep 04 2003 - 17:04:00 BST

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