[GreenKeys] TWX-TELEX history

[Jim Haynes]

On Tue, 29 Dec 2015, Jeffrey D Angus wrote:

> Looks like the big difference between switch telephone and switched
> teleprinter equipment was the end-to-end continuous copper
> connection. Excluding the use of tape equipment in the central office.

There are three basic kinds of TTY switching, which may be used in
combinations.  Circuit switching is similar to telephone switching,
in that circuit segments are connected together until there is a
real-time two-way connection between the originator and the answerer.
The circuit segments may be copper wire or carrier channels.

Message switching, or store-and-forward, passes a message from one
switching center to the next until it reaches the destination.  In
transit the complete message is stored at each switching point. 
Originally messages were stored as hard copy, later in punched paper
tape, and still later in computer-technology storage such as magnetic
disk or drum or tape.

Selective calling, or way operation, or multistation lines, has a number
of stations connected to one line with arrangements so that only the
addressees of messages get copies.  Originally done by operators ignoring
messages not addressed to them, then by mechanical means of suppressing
printing at stations not designated to receive a message.  The Model 28
stunt box was a major facilitator of this kind of operation.  The
multistation line may be half or full duplex, and may or may not include
a control station that polls the stations for traffic waiting to be sent.
>
> Towards the last periods of operation of the Telexs and TWX networks,
> didn't they switch over to modems, so they could send the data over
> traditional voice circuits?
>
I don't know about the last days of Telex, whether it used DC current loop
signaling to the customer's premises or whether modems were sometimes 
used.  Of course carrier channels could be used for trunks between 
switching offices.

TWX was at first manually switched, and could use DC current loops or 
modems between the switching offices and customers' premises.  What kept
TWX manually switched for so long was the fact that users expected bills
like U.S. telephone bills, showing the date and time of a call and the
destination and duration.  In Europe, where Telex originated, the billing
system did not provide all this detail.  Rather a customer register was
pulsed at intervals determined by the distance of the connection and
the customer was charged so much per pulse.  Having to do U.S. style
billing adds a lot of complexity to the switching process.

By the late 1950s the U.S. telephone network was approaching a state where
customer-dialed long distance calls and automatic message accounting were
widely available.  The Bell System developed a message system for Delta
Airlines that simulated a store-and-forward system but actually involved
dialing connections over the switched network.  (I have written a lot 
about this previously so won't repeat it all here.)  While a real-time
end-to-end connection was made in this system, it did not allow two-way
conversations to take place.  It used modems and autodialers which,
while smaller than refrigerators, were quite a bit bigger than breadboxes.
The stations had to be connected to #5 crossbar switching offices, which
in some cases might require long foreign-exchange circuits.

Meanwhile the ability to do direct distance dialing continued to spread,
and further development produced a modem that would fit inside a Teletype
cabinet.  At this point TWX service could be converted to dial operation.
The Delta Airlines system was taken out of service because the FCC would
not permit a tariff for what was to have been Wide Area Data Service.
It was replaced by a store-and-forward message system using computer
technology for switching.