Basics of telephone exchanges | Tribute to Relays

Basics of Telephone Exchanges

This section provides a brief technical review of the four major electromechanical telephone exchange types in operation from ~1890 until early 1980’s in North America (NA).

Since the beginning, Bell System engineers, and countless others, have looked for novel ways to automate the connecting of any two phones with the minimum of equipment in between, no operator needed.

Putting aside the actual steps involved, and there are many, at a minimum all systems need to provide "dial tone", register the dialed digits, ring the called phone if idle, and provide a talking path using a “switch” technology of some kind. The manual switchboard is not covered here.

Greatly simplified automatic exchange switching concept

The figure illustrates a simplified representation of an automatic dial-based exchange where caller #288 is calling #315. The Switch part is key to making the talking connection. The switch shown in the figure is generic and not meant to convey how the switch actually works. The Call Control part is the "brains of the outfit" and orchestrates most of the calling process. Not all exchange systems used a separate Call Control means. Nonetheless, this model is instructive for how calls can be made.

Two roads diverged...

At this point, you can select the course of the narrative. If the technical aspects of telephone exchange operations intrigue you, please navigate to the the Exchange Anatomy section. Here the exchange is explained from the top down, in layers. The coverage breaks down the workings using a diagram for navigating around several exchange landscapes. Fifteen individual switch inventions are reviewed. There are many pictures and some explainer videos.

On the other hand, if you're more interested in a broader more visual perspective then continue your journey directly below. The visual approach offers minimal technical explanation. The goal here is to get a feeling for what electromechanical switching technology looked like, not how it operated in detail.

Peeking under the hood

Beginning about 1891, and over many years, telephone companies developed four major, and many minor, automatic exchange types. The "Big Four" types are Step-by-Step (SxS), Rotary, Panel, and Crossbar (Xbar). For the purposes of this section, the internals and pros/cons of these exchange types are not explained.

The exchange names are reflective, for the most part, of the switch technology used to make the talking connections. For North America, SxS was mostly used in smaller cities while Panel was designed for large metro areas. Rotary was deployed mainly outside North America (NA). Crossbar systems were initially used in Sweden in 1926. In 1938 Western Electric designed a crossbar system to replace Panel. Crossbar-style switching became popular worldwide.

In 1970, AT&T had 100 million subscribers in North America running across ~14,500 Central Offices (exchanges) supporting up to 10K lines each. A mix of SxS, Panel, and Xbar exchanges were connected to form the largest domestic phone network in the world. These systems were the workhorses of the Bell System for ~70 years. Of course, there was a thriving international telephone market and AT&T played a major role. Check out this exchange timeline for North America. The Western Electric company and Bell Labs contributed mightily to AT&T's success.

The figures immediately below show closeups of the switch part for four main methods. The purpose of a telephone switch is to make connections (~talking path) and the closeups show this aspect for each switch.

basic telephone switching concept with call control

four main telephone switches - panel, crossbar, step, rotary

The Step-by-Step (SxS) Exchange -- Strowger type

This exchange is the granddaddy of them all beginning life in 1891. The first version looks nothing like the picture below. It had humble beginnings and had huge growth spurts in the early 1900's. It is called a Step exchange because of the way the call progresses -- step by step. It proved to be ideal for smaller cities but it struggled to scale to metro areas. There were exceptions but other future exchanges became more successful in metro. To see more on SxS and the Strowger switch, visit the Exchange Anatomy page.

The Panel Exchange

The Panel system was the most mechanical of the three exchange types. True, the SxS system was mechanical in nature too, but not on the same scale as a panel office. The first 7-digit Panel office was installed in NYC in 1922.

Panel was the world's first metro exchange designed to route calls to remote exchanges in large volume. In 1930 there were 128 panel exchanges in metro cities (NYC, San Francisco, Chicago, Boston, Seattle,...) supporting ~1.2 million subscribers. Panel was well suited to handle intra-office and inter-office calls better than any prior system.

The Crossbar Exchange

Source: atlantatelephonehistory.org

The #1 Crossbar system was designed to replace the Panel system starting in 1938. When the electromechanical age of telephone switching ended in the 1970's, the #5 Crossbar exchange was dominating. The crossbar switching device was very popular worldwide.

The exchange was named after its namesake crossbar switch seen in the picture above. With the equivalent of 10 horizontal rods and 10 vertical rods connections could be made at each of the 100 intersections. Several generations of crossbar office were built -- #1 Crossbar, #4 Toll Crossbar, #5 Crossbar and others, each with a feature-set the others did not possess.

Western Electric 7A Rotary Automatic Machine Switching Exchange

Rotary System b.1914

rotary 7A telephone switch -- Ferrymead Post & Telegraph Historical Society display at Ferrymead Park, in Christchurch, New Zealand

The "Rotary 7A" was invented by Western Electric. It was manufactured and used primarily in Europe from the 1910s. The first exchanges were installed in England at Darlington in 1914. Zooming into the picture, you can see the individual rotary switches.

Like some other exchanges, it had motor driven switches so was a hearty mix of electrical and mechanical technologies. The picture above is from a working system at the Ferrymead Post & Telegraph Historical Society display at Ferrymead Park, in Christchurch, New Zealand (thanks to Brian Cameron).

The pictorial coverage above just scratches the surface of electromechanical exchanges. Try also the six demos associated with this site. For more on the exchanges pictured above check out Exchange Anatomy.

Before ending, a short note follows on the migration to automatic systems.

The transition from manual to automatic

By 1905, the manual switchboard, supporting approximately 2.2 million domestic subscribers, had around 27 years of development history. Many were not keen on moving away the "number please" girls and embracing the automatic exchange. Some reasoned, "Why would anybody want an "automatic switchboard" (exchange) when asking an operator to connect a call was so very easy." There is a long history of opposition to automation including the case when some weavers (often called Luddites) in the 19th century destroyed textile machinery to save jobs.

Kempster Miller (1870-1933) was an American telephone engineer and prolific author. He wrote several well illustrated books on early telephony. He considered this question in his paper, "The Automatic vs. The Manual Telephone Exchange", delivered at the International Electrical Congress, St. Louis, in September 1904.

You may enjoy, as I did, reading the speech and following his logic for and against the automatic exchange. From our modern perspective it seems that automation should have been a slam dunk. It wasn't in 1904. The speech's text starts in the second paragraph, page 727, in Kempster Miller's book American Telephone Practice. The book is hosted on the HathiTrust website.

The migration to fully automated took many years and depended on the community. Automatic and manual lived together in relative harmony (automatic plus manual for many long distance calls) until about 1960 when almost every domestic call was dial-based.

Some manual boards bucked the trend in small cities, like Kerman, California. Their board was removed in 1991. The last known electromechanical Step-by-Step exchange in North America was replaced with a digital (DMS-10) switch in Nantes, QC Canada in 2002. So, from its inception in 1891 until at least 2002, the Strowger-type switch was king in the small exchange (telephoneworld.org/).