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Exchange Floor Plans



In 1926 AT&T engineer Elsworth Goldsmith delivered a series of lectures for training purposes. One of his masterpieces is “The Panel Type Dial Telephone System”, part of his curriculum. Some of his figures and descriptions are worthy of repeating here for explaining how exchanges are crafted from the ground up. His introductory coverage is uncomplicated and sufficiently broad to cover exchange concepts at a high level.


A floor plan, for our use, is the basic layout of an exchange without all the details included. It's a blueprint of the switching flow design. Several examples are provided below with the goal of illuminating how 2 subscribers are connected using a combination of switches.


In the beginning…


Fig 1 shows an elementary 1-digit system with a dedicated switch per calling subscriber. Not cost effective, but it works. A subscriber may dial one digit (1-5) and be connected to 1 of 5 phones.  







Fig 1

One-digit system dialing phone #3


For instance, the switch, often referred to as a selector switch, advances mechanically by 3 steps in response to the dialed number 3.  The switch’s connecting finger lands on level 3 to create the speaking path to phone #3. This basic system is what began to replace the “Number please” girls.

Next for consideration is the 2-digit system shown in Fig 2. The caller is dialing phone #33. The first switch responds to the first digit and the second switch responds to the second digit. Not shown is the relay logic support to make this happen seamlessly.

one digit telephone switch example from 1926
icon for more info
two digit automatic telephone exchange example from 1926

Fig 2

Two-digit system dialing phone #33


With 5 positions per switch this figure supports 25 callers. As with the first example, this is woefully inefficient since the caller still has a dedicated first switch as in Fig 1. Imagine how the figure would expand with 25 supported callers or even 99.


The Line Finder is inserted

In figures 1 and 2, each subscriber requires a dedicated first selector switch. A major advance was to insert a Line Finder (LF), or a Line Switch (LS), between the subscribers and the first selector switches. The Keith Line Switch is a similar line concentration switching idea. This concept reduced the total number of first selector switches needed in an exchange. 

In Fig 3 the LF is inside the blue dashes. This is a 3-digit system. The LF does not respond to any digits. Its purpose is to connect an active caller to an available first digit switch in the red dashed box. 

Adding the LF concept was a big step towards a more efficient, scalable architecture because it eliminated the dedicated and costly "first switch". Compare Figs 1, 2 and 3 in this regard. Statistics show that 10-15% of the subscribers are using the system at any one time. The LF is not dedicated per subscriber but each one is connected to many subscribers such that when one goes off-hook, a LF can direct the caller to a free "first switch" to accept the first digit. Including the LF required a dial tone too so subscribers knew when to start dialing.  

three digit dial exchange example from 1926 telephone

Fig 3

Three-digit system dialing phone #333

Exchange example showing 3 stages of switching

The caller (dialing #333), after going off-hook, is immediately attached to the first digit switch. The central switch fabric handles digits 1 and 2 and the final switch (green dashed box) processes digit 3.


This example includes connectivity to other offices, across town for example. The caller is attached to Office A and the called subscriber is attached to Office B. There are trunk wires (speaking paths) connecting the offices.


Interconnecting many Offices with trunk lines enables large networks, eventually spanning the globe. Note the parallels to Fig 2 in the Anatomy section. See Endnote 1.  

The Prototype Exchange


Using Figure 3 as a prototype, it's conceivable to expand the system to accommodate more callers by adding Line Finders, enhancing central switching, and increasing the number of final Connectors. The larger the exchange the more each stage expands to support 4, 5, or more dialed digits.

However, scaling a small exchange by multiplying its parts to make it larger can work but this approach is usually costly and inefficient. Larger systems require more sophisticated architectures, switching devices and control methods to save cost and space. 

Key elements of the prototype exchange are further explained in the Anatomy section.  You can link directly below or go to the Anatomy section master diagram to link.

•    Switching Fabrics -- Larger systems require more sophisticated switching designs to save cost and space. See how to scale switches creatively to meet demand.  

•    Common Control -- The control logic for the examples above is not described. See what methods are used to control many thousands of relays to connect calls. 

•    Switching Devices -- What is the best electromechanical switch type to use for implementing Figs 1-3? There is no one answer. This section explores the choices. 

•    Exchange types -- Many elements come together to create an exchange type. Some named automatic exchanges are Step-by-Step, Panel, Rotary and Crossbar. These four systems share characteristics of the prototype exchange developed here. 




Endnote 1


Upon examining Figure 3, one can identify a drawing error in the wiring of the final connector stage.  The bottom phone on the left is dialing #444 but gets connected to #334 by virtue of the error. Somehow the document proofreader missed this one.

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