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It Ain't Necessarily So

Even though I was "all PBXed out," the first of my columns Ian Angus printed was about PBXs. It appeared in Telemanagement for April, 1990.

Digital PBXs Have More Features

In the last year, I have read at least half a dozen articles which insist that digital PBXs are better because they have more features. I would love to know where this bizarre idea came from originally, but it seems a lot more important to know where to stuff it. After all, features are a function of the system control, not the technology used to construct the transmission paths through the switching matrix.

The feature revolution is one of those technology-driven events that has taken on a life of its own and now has a cult following. Inexpensive microprocessors and memory chips allow hundreds of features to be added to a PBX for a few cents worth of memory; most of these features are unused, but that doesn't stop people from compiling lengthy lists differentiating between music on hold and music on park, or the naive copying such lists blindly into PBX RFPs.

The important thing to remember is that the processor with its software-based features doesn't really care about the rest of the system. The old Womack, one of the few modern PBXs to use relays from which to construct its switching matrix, had a full array of features stored in its computer brain. Mitel, with the original SX200 and SX-100, made a fortune by not being digital while still offering all the computer controlled features, including system prompts and messaging on the Supersets.

On the other hand, a great many digital systems never did get around to having as many features as the Mitels and the Dimensions; one thinks immediately of the Digital Telephone Systems D-1000 (or D-1200, depending on where you bought it), which eventually overcame many of its limitations, but by no means all. Even the Rolm CBX, when it first came on the market, did not have either ARS or CDR comparable to that of the Womack, and I know of at least one digital Lexar which found itself unable to replace an analog Dimension 2000 for lack of certain very simple features (which it has since added, of course).

So if digital PBXs do NOT have more features than analog PBXs, why have all the analog PBX companies such as AT&T, Mitel, and National Telecom (the present name of the company that made the Womack) gone digital? For exactly the same reason that Rolm, Harris Digital Telephone Systems and Lexar converted from their original versions of digital to the digital format found in their current product line. They all eventually came to the conclusion that telephone switches should use the same technology as telephone transmission systems: T Carrier.

What is the payoff from all this? Principally, the ability to omit large quantities of equipment that is no longer needed to convert a telephone signal from one format to another: codecs, channel banks, individual line and trunk circuits, etc. Further, the signal, once coded into digital, does not change (at least in principle) in its trip from the calling party to the called, and vice versa: no distortion, no level loss, no attenuation as a function of frequency and no delay distortion. Thus better service is possible at lower cost, without regard to possible features.

Once digital modulation is used, it becomes economical to handle signaling on a separate channel, and to locate groups of lines remote from the central switch, taking advantage of the way in which digital techniques permit inexpensive multiplexing for access to remote line groups or to other switches, either PBX or CO.

One might, of course, say that direct digital interfacing with transmission media to other switches is a "feature" that analog PBXs do not offer, but quite a few digital PBXs have not offered it in the past, and may not for some time to come. It should be evident that digital switching, to match digital transmission, is the best way to go in telecommunications today, but let's not kid ourselves about features, which are something else entirely.

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