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                                       The Step-by-Step Exchange
 

The goal of this section is to describe the process flow for making a call on a Step-by-Step exchange (SxS). 

Preamble
Throughout this site, many aspects of telephone systems technology are discussed.  Each facet is a part of a big jigsaw puzzle. When the pieces are locked together, an automatic exchange starts to breathe – see image. The “outside plant” cables, poles, and conduits are not covered here but this is a big cost in servicing each subscriber.
 

 





 

 

 

 

 

 

 

Each of these pieces is discussed in the Exchange Anatomy (EA) and associated sections. So, their detailed operation won’t be repeated here. If you are new to exchange workings, and want a basic understanding, it may be best to review the EA sections before continuing here. Interesting historical aspects, notable people and intriguing inventions are also covered under EA. 

What is a Step-by-Step exchange?

Some would define it as one that uses Strowger type (“up and over”) switches. However, more broadly, it is an exchange architecture that uses switches that are directly controlled by the dialed digits, step-by-step. This is also called progressive direct dial control. In contrast, the panel, crossbar and rotary switches are not direct dial controlled. 

The second method of switch control is called common control. The two methods are compared here


So, these switches qualify to be used in a step-by-step exchange:

  • Lorimer switch

  • All Strowger switch types (Western Electric, Automatic Electric Company, others)

  • Rotary uniselector switch 

  • Ericsson XY switch (also called Stromberg-Carlson XY)
     

Because these switches may be used in a Step exchange, they are often called Step-by-Step switches. In fact, some exchanges comingle the XY and Strowger type in the same exchange; they are interoperable for the most part. The SxS method was the most popular worldwide for a period of 90 years. See What's in a Name for more insights. 

All of these switches all discussed on this site. Without doubt, the Strowger type switch was the most popular style used in SxS exchanges. So, this section will focus on a Strowger-based SxS exchange. See Appendix A. 

If you are new to exchange concepts, review the section on Strowger switches first. Understanding how this switch works will enhance your appreciation for how calls are made in an SxS exchange.  

Figs 1-2 provide a glimpse of some SxS system components inside the JKL Museum [JKL].  Fig 3 shows a closeup of a line finder switch. 

telephone exchange puzzle -- salient parts of an exchange
Telephone switches: Stromberg-Carlson XY (left) and Western Electric Strowger-type (right)

Fig 1, Stromberg-Carlson XY (left) and Western Electric Strowger-type (right)

closeup of Strowger style Selector switches and wiring

Fig 2, closeup of Strowger style Selector switches and wiring

 Closeup of 3 line finder terminal banks, wipers and vertical commutator

Fig 3, Closeup of 3 line finder terminal banks, wipers and vertical commutator [Len Hicken]

WYSIWYG Observing

The WYSIWYG acronym, standing for 'What You See Is What You Get,' is simple to understand. It pertains to applications or user interfaces where the on-screen display matches the final output as seen, for example, in Microsoft Word. 

Let’s reimagine this concept to better understand how a Step exchange connects callers inside the exchange.  Physically observing the switches in a working exchange, “what you see,” – the active switches and their movements – provides a clear picture of “what you get” – how the switches make the connections for the talking path.

The SxS selector and connector switches are roughly grouped in the racks by the dialing order; i.e. 4xxx, 46xx, 468x, 4688. So, an observer can walk among the switches and view, one digit at a time, the switches making the talking path connection. This will be developed in a video below.

In the panel and 7A rotary exchanges, WYSIWYG applies also but to a lesser extent because the switches are not grouped by powers of 10 (10, 100, 1000...) . So, observing the physical dialing path across switches is less straightforward.

Visually following the switching chain in a #5 crossbar office is the most difficult because the Marker logic makes control decisions based on the number dialed plus the existing switch traffic and other factors.

Follow the dialing in SxS

Fig 4 illustrates a 4-digit system supporting ~10K lines. There are four different Strowger-type switches:

  1. 200-point Line Finder type. At a minimum (always more), there needs to be 50 of these to funnel one subscriber from a pool of 10,000.

  2. 1st Selector type- provides dial tone and responds to the first dialed digit.

  3. 2nd Selector type – responds to the second dialed digit.

  4. Connector type – accepts 3rd and 4th dialed digits, tests for idle/busy, rings phone, on answer ceases ringing and enables talking.  At a minimum (always more), there needs to be 100 of these to connect to  any one subscriber from a pool of 10,000.  

Step-by-Step dialing chain

Fig 4, Step-by-Step dialing chain

Importantly, this diagram only describes intraoffice calling. Naturally, for a real system callers will often dial subscribers outside of their home office. In this case, access to remote offices over trunk lines is needed. The basic ideas remain the same only there is more switching equipment to accommodate both incoming and outgoing talk paths.

 

Let’s follow a subscriber dialing #4688 (Fig 4):
 

  • The line finder searches for and finds the new off-hook subscriber. Each LF is permanently paired with a 1st selector switch.

  • The 1st selector provides dial tone and registers the first dialed digit, a 4. The switch responds by ascending to level 4 then immediately hunts, along its horizontal contracts, for an idle 2nd selector.

  • The 2nd selector responds to the second digit, a 6, and ascends to level 6 then immediately hunts, along its horizontal contracts, for an idle Connector.

  • The chosen Connector accepts the dialed digits 8 + 8. If not busy, ringing power is provided. If someone answers, the Connector finalizes the talking path.

  • All the switches in the chain are held during the call. On the call release, all the switches return to their home position.

 

Observing SxS dialing in action

Here is a video with switching actions and animations showing the making of a 4-digit call at the JKL Museum of Telephony [JKL]. The purpose is to demonstrate how the talking path is established, during dialing, through a series of Strowger-type switches. 


References

AEC, “This is Automatic Electric”, pamphlet, 1955.

Fagan, M.D., A History of Engineering and Science in the Bell System the Early Years (1875-1925), 1975.

​JKL: JKL Museum of Telephony, www.jklmuseum.com  (opens a new browser tab)
. Many thanks to the Museum staff and especially Remco Enthoven. 

​Korn, F.A., Level Hunting Connectors, Bell Laboratories Record, March 1929

​Miller, Kempster, & McMeen, Samuel: Telephony; a comprehensive and detailed exposition of the theory and practice of the telephone art, 1912.

​Pferd, W., Bell System Technical Journal, February 1979, page 427.

​Sam Hallas, informative site: http://www.samhallas.co.uk/telecomms.htm

Appendix A

Long Live Step-by-Step

During the long life of step-by-step, many apparatus and manufacturing improvements were devised and incorporated in production. However, except for the addition of some common control equipment in certain special situations, the basic architecture for the small and medium-sized cities has not significantly changed. [Fagan]

The main impact of the common-control system was on large cities; see Director. For many years step-by-step remained a system functionally and economically attractive only for small and medium-sized exchanges.

Table I [Pferd] shows the relative importance of the different Bell System exchange types. From 1950 to 1977 SxS was the dominant exchange type in North America. It should be noted that SxS exchanges were relatively smaller in size, used in cities where panel or crossbar would have been excessive.

 

 

Note: Some buildings are multi-entity facilities containing more than one central office code. Therefore, the number of Bell System central offices exceeds the actual number of central office buildings. A central office code is normally the first 3 digits of a 7 digit phone number. By example, for 552-1677, the 552 is the office code. ESS is "Electronic Switching System" and, over time, it replaced electro-mechanical systems. 

In 1955 about 65% of all automatic exchanges worldwide were SxS based using switches from Western Electric, Automatic Electric Company, and other manufactures [AEC].

Number of central office codes 1950 to 1977

Here are a few pictures illustrating Step systems training and maintenance operations. 

Central Training School, Strowger-type exchange, July 1959.

P 7100 Central Training School, Strowger-type exchange, July 1959.
Source: Sam Hallas collection:British Post Office photos.

Elstree, UK, Strowger telephone exchange,1935, adjusting Line finder switches

Elstree, UK, Step telephone exchange,1935, adjusting Line finder switches. IET Archives.

Strowger switch maintenance, Liverpool, UK, 1947

Strowger switch maintenance, Liverpool, UK, 1947

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