What is the difference between a PBX, CO, and DCO? I was under the impression a Central Office (CO) is just a large PBX. And, a DCO is one type of CO? Is this fundamentally correct? (I don’t need the nitty gritty details).
A PBX is a Private Branch Exchange. It serves the same purpose as a CO but usually the CO is owned by the phone company and a PBX is privately owned (that’s why it’s called a PBX).
DCO is a digital central office, so as you suspected it’s a type of CO.
A PBX is a privately-owned (or leased) switch that serves an individual business or building. It is connected to the PSTN via trunks.
A CO is a building owned by a Telco that houses a switch and other ancillary equipment needed to serve its subscribers.
I never heard the term DCO before, but upon googling, it appears to be the name Stromberg-Carlson and later Siemens gave to its digital central office switching equipment, much like Western Electric’s ESS.
It used to be that a PBX would be a compact low-capacity switch and the type of switch housed in a CO, like a step-by-step (analog) or ESS (digital), would be a high-capacity behemoth. But with the latest technology, large CO switches can be employed as PBXs when needed and there have been cases where telcos have employed traditional PBXs to be their CO switches.
It’s worth noting that some PBX systems can be as large as a CO - the PBX at my office supports roughly 10,000 telephones.
Some acronym translation - the PSTN is the Public Switched Telephone Network. Basically, it’s the whole (landline-based) system that your phone plugs into. If you have VOIP phones, there’s a whole different scheme there.
Western Electric’s ESS is the Electronic Switching System. When this came out about 30 years ago, it was seriously hot stuff as it replaced the din of thousands of mechanical switches with the soft whirr of cooling fans.
PBXs typically allow in-system calling with a shortcut number, and COs don’t - but they used to.
In the Bell System days PBXs were built in Denver and 5ESS was built in Oklahoma City.
#4 ESS was introduced in 1976.
#5ESS was introduced in 1982. My group and I were heavily involved with testing the ICs and boards that went into this monster. It was build in Oklahoma City but designed at Bell Labs Indian Hill in Naperville IL. Very cool system, and manufacturing paid a good chunk of my salary for a long time. Our base computer in our lab was a simplex version of the 3B-20 supermini which was the heart of #5ESS in a duplex version.
First, let me emphasize that a CO is a building (or part of a building) in which a switch is located. It is not a switch.
But most Class 5 switches come with a Centrex feature package that basically provides a virtual PBX-like service that can include features like shortcut dialing.
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#4 ESS was introduced in 1976.
#5ESS was introduced in 1982. My group and I were heavily involved with testing the ICs and boards that went into this monster. It was build in Oklahoma City but designed at Bell Labs Indian Hill in Naperville IL. Very cool system, and manufacturing paid a good chunk of my salary for a long time. Our base computer in our lab was a simplex version of the 3B-20 supermini which was the heart of #5ESS in a duplex version.
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Ah, thanks for the clarification on dates and adding the missing “5.” My father was one of the many middle managers looking for a home post-divestiture, and he wound up working 3B support out of Napervile. Think he focused on the 3B20D machines, and at some point was involved with a couple 5ESS hot slides.
PABX would be the normal term here - PBX implies a manual switchboard.
The term PABX never caught on here in the United States. Automatic switches are referred to as PBXs here ranging from the early Western Electric 101 ESS PBX (pdf) to contemporary Cisco IP-based systems.