Overview & Background on Electronic & Digital Switching Systems


An overview and historical background on the different types of electronic (analog) and digital telephone switching systems that are used on the North American telephone network.

Development of Electronic Telephone Switching

After the invention of the transistor, Western Electric & Bell Labs worked hard to develop an “electronic” computerized switching system. Bell spent 10 years and $500 million to develop the Number 1 Electronic Switching System (#1ESS), which was first installed in Succasunna, NJ in 1965. The #1ESS uses 160,000 diodes, 55,000 transistors and 266,000 resistors, capacitors and other components. These were mounted on unique plug-in circuit boards. The #1ESS, though not a “digital” switch, since calls were still handled in an analog audio format using miniature reed relays in a crossbar-like matrix format. But the major advantage of the #1ESS was a new concept called Stored Program Control (SPC), which enabled the switch to have an electronic memory. This enabled new features such as call waiting, call forwarding and speed dialing. Other advantages were the ability to change the “programming” the switch when needed to change parameters (call routings) and add additional features when they were developed. Existing crossbar and step-by-step switches worked in real time and executed commands as they happened. The #1ESS could store all the dialed digits and act upon them after the dialing was complete.

Though the switch was highly successful, there were some unforeseen problems. When a #1ESS became overloaded, the whole switch would fail all at once instead of breaking down bit by bit. Eventually this was corrected.

In 1976, WE upgraded the processor of the #1ESS to give it more capacity and to handle calls faster. The new upgraded switch was then known as the #1AESS.

The Race to Digital – Northern Telecom vs. Western Electric

[Telephone World wishes to thank a retired telephone employee who contributed to this article.]

The move to full digital telephone switching systems was a complicated one on many fronts. Western Electric/Bell Labs in the 1960s and 1970s was the leader in telephone switching technology. Most other manufacturers of telephone switching equipment either licensed the rights to build their own equipment from WE, or they attempted to copy and mimic the technology as much as possible.

But by the late 1960s and early 1970s, there were companies who were attempting to break out of the “WE mold” and forge out on their own. Northern Electric (later Northern Telecom) and engineers with Bell Northern Research (BNR) of Canada had a raging debate on whether to make an electronic #5XB (not unlike the #1ESS which they briefly manufactured under WE license before switching to their on version of stored program control, the SP-1/SP-1E switch which used mini-crossbars as the switching matrix) or go fully digital. The digital team was successful in getting their project direction approved, and the rest is history.

However, the DMS-10 and DMS-100 development groups were entirely different teams with vastly different objectives! In fact, when a new team began to work with the SL-1 developers with the objective to develop a small Class 5 switch, they had to fight the political pressure of the DMS-100 team to even be permitted to continue. Consequently, the majority of the DMS-10 work moved to Research Triangle Park, NC while (until the late 80’s) the majority of the DMS-100 family work was in Ottawa! Sadly, the DMS-10 has had to struggle its entire life under pressure to eliminate the product. The prejudice against the product caused many delays and restrictions in the development of the DMS-10. There was even an attempt to eliminate the DMS-10 by producing a “modular” DMS-100 for small office applications, but the higher cost of manufacture and operating system (software) overhead could not be reduced enough to make that a viable pricing alternative!

Where was Western Electric in all this? WE/Bell Labs tried to keep the lid on their Class 5 (local end office) digital switch, as they wanted to maximize the life of their electronic (but not digital) #1AESS! This effort was a modern example of a policy that goes back to the earliest days of the Bell organization and was one of the major reasons why the Bell Companies lagged so far behind the independents in adopting automatic switching.

When New York Telephone began a historic contract negotiation with Northern Telecom for supplying the DMS-10 in 1981-82, the WE/Bell Labs team rushed their “small switch” out the door (the #5ESS originally had only one Switch Module) and as the NY Tel attentions turned to the DMS-100, the labs added a space matrix to allow multiple Switch Modules to operate as a single switch. This multi-module version of the #5ESS was their answer to the Nortel DMS-100 threat (and a damn good switching concept to boot!) The first Multimodule #5ESS telephone-switching center was installed in Sugar Grove, Illinois in late 1983. I suspect that had NY Tel and the other Bell companies limited their outside WE interest to the DMS-10, the #5ESS would have also been a small market switch for years to come!

The First Digital Local Office Switching System?

[Telephone World wishes to thank a retired telephone employee who contributed to this article.]

Some telephone historians have claimed that Vidar developed the first digital telephone switching system for Class 5 (local end) offices. The first Vidar ITS-5 did not go into service until early 1978. However, Vidar was the first independent supplier to market with a digital toll switch, but not the first switch of either persuasion!

Vidar installed their first ITS-4 in the North Carolina Telephone Co Marshville, NC exchange in late 1976 replacing a Stromberg-Carlson SCAMA ticketing system earning the title of the first independent digital toll switch to public service. However, the Western Electric #4ESS was the *first* digital toll switch and went into service in Chicago in early 1976.

But the title of the first Class 5 Local End Office Digital Switch is a “split decision” depending on your interpretation of the actions of the parties. Here is the story.

Northern Telecom started the public “digital awareness/fever” ball rolling with their “Digital World” announcement in Feb. 1976 at Walt Disney World near Orlando, Fl. That announcement included a family of products from the access line to the long distance network including; SL-1 PBX, DMS-256 subscriber line carrier (later changed to DMS-1) and the DMS-100 “family of large switches”

A few days after the Digital World announcement, I received an invitation from Northern Telecom to a briefing on another proposed switch (identified in the letter as the SX-10), later announced as the DMS-10, the small exchange digital switch member of the Digital World Family of products.

Our briefing came as a result of a bid associated with a major exchange upgrade program in N. Florida where the Florida Public Utilities Commission had ordered my telephone company (the North Florida Telephone Company), to replace all of our TPL exchanges in the next two years! In response, we put out invitations for replacement solution quotes to Automatic Electric, Northern Telecom, North Electric and Stromberg Carlson for response in January.

Automatic Electric responded with the CXP-5 a “baby” crossbar using the Leich Crosspoint Switch.

Northern Telecom proposed replacing the small TPL offices by remoteing these offices via larger nearby exchanges using DMS-256 subscriber carrier. We later learned this had been a “place-holder” bid because they were not allowed to bid the new digital switch before the “Digital World” announcement in February!

North Electric proposed replacing some of our larger NX-2A offices with NX-1 and moving the NX-2A equipment to the smaller TPL exchanges.

Stromberg-Carlson made two proposals, the electronic ESC-2 Crossreed and alternatively, XY Step by Step!

After the Digital World announcement, we (a group of North Florida & Mid-Continent Telephone Company engineers) were flown to Ottawa where Northern Telecom’s Bell Northern (BNR) engineers gave us a preview of the DMS-10. This switch was to have core architecture and OS commonality with the SL-1 and because of that, it would be able to be ready for market before the DMS-100! This meeting was followed by a bid from Northern Telecom to replace our first five TPL conversions with the new DMS-10!

After lengthy review, the decision to purchase five DMS-10 offices was recommended by my office (I was at the time Chief Engineer of the North Florida Telephone Co), and supported by our Regional Engineering office of the Mid-Continent Telephone Co. (At this time, North Florida Telephone Company was a Mid-Continent property and is now Alltel and soon to become Valor.) Mid-Continent Corporate Engineering agreed and I signed the purchase order for the first production delivery of the DMS-10 to be installed in our Fort White, Florida exchange!

Almost immediately, we were deluged with calls from our Stromberg-Carlson sales team! They revealed that they also had a “digital project” and were not allowed to present that as a solution in January and urged us to rescind the decision and consider their product instead. After review of the technical differences, (as much as either could tell us at that time) we decided to “stay the course” and stick with Northern Telecom and the DMS-10.

BTW, Vidar, like Stromberg-Carlson did not reveal their digital program (ITS-4) until after the Northern Telecom “Digital World” announcement!

Subsequent to our decision not to switch to the Stromberg-Carlson offer, SC signed an agreement for the delivery of “their” first switch to Coastal Telephone’s (Hinesville, GA) Richmond Hill, GA exchange.

Now here is where you get to pick the “real” first!

Stromberg-Carlson put their switch into service in July 1977 in Richmond Hill, GA. Northern Telecom put their switch into service in Oct 1977 in Fort White, FL.

Clear winner huh?

No not exactly, you see Richmond Hill’s Stromberg-Carlson Digital Central Office (DCO) was a pre-production (lab) switch and not the product later delivered to the market under the DCO banner. (NOTE – The former CEO of Stromberg-Carlson told Telephone World in January 2013 that this was a production switch, not a lab prototype.) That switch was also suppose to be a Rural Electrification Administration (REA) field trial, but that pre-production switch at Richmond Hill was never “accepted” by the phone company and was removed and eventually a DMS-10 was installed in Richmond Hill in the early 1980’s! I heard that Coastal never actually paid Stromberg-Carlson for the switch so even though it was in public service; it was never the property of the phone company!

Fort White’s DMS-10 on the other hand, was actually the “first production digital Class-5 exchange” to go into public service on October 21, 1977. The switch was followed over the next year with four identical (other than line size) switches, which were installed in our other target Florida location. Those exchanges were Orange Springs, Jennings, Citra and Wellborn. Northern Telecom also shipped switches to Continental Telephone and many independents during that period.

Fort White’s DMS-10 is still in service and in 1997, to celebrate its 20th anniversary; the switch was upgraded to the 400 generic and ISDN BRI capabilities, which was another Fort White DMS-10 first! (The 400 generic also included DMS-100 6X type line cards) Even with the addition of the (6X) line equipment in 1997, the switch still has line equipment in service from the original installation. With respect to the ISDN “first”; BRI had been around in since 1987-88 in larger switches (DMS-100, 5ESS and GTD-5), but this was the first install in a DMS-10!

So, pick a winner. Stromberg-Carlson shipped a “pre-production” switch, which was never owned by the Telco to be first to the field, but Northern Telecom shipped and installed the first “production” switch and it was accepted, bought and paid for by the customer and best of all, is still operating today after over 28 years of service!

[To add to the controversy, another telco engineer says otherwise]

The information on your web page concerning TRW Vidar has a few factual errors. The information probably came from someone repeating information they may have heard. I don’t have the once removed filter. I was there.

The country’s first digital Class 4 toll switch went into service in Bishop, CA in February of 1976. It went into revenue production with the Continental Telephone Company. It also was the first digital Centralized Automated Message Accounting (CAMA) system in the US, supporting both operator assisted and direct dial AMA billing data collection. Ten additional systems including the County’s first Class 5 in Ora Loma, CA and first Class 4/5 in Pella, IA were in place and in revenue production at Commonwealth Telephone Company, Continental Telephone and GTE, before the first DMS-10 went into service. TRW also introduced the First 2 Gig Digital Radio System in the United State and the first Digital Loop Carrier directly connected to a Class 5 digital switch. I also think they introduced the first D2 and D3 T1 channel banks in the independent carrier market. Not sure what happened, but they flamed out in competition, as Nortel, GTE, Stromberg-Carlson and Siemens finally got their digital products on line in the 80s.

Other Early Digital Switches of Note

North Electric DSS-1 (later called the ITT-1210 digital switch which played a major role in the early years of digital, especially in the United Telephone System (now part of CenturyLink).

Also, Nippon Electric sold NEAX-60 Digital switches to a few Bell and Contel properties before retreating from the U.S. Market.