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Direct Inward Dialing

By Bruce Crawford

From the October 1998 Switchers' Quarterly

The independents provided it for years. See why Bell took so long to catch up.


One of the nice things about (all) Step-by-Step equipment is its intrinsic flexibility. While the pro-rnanual Bell System was still bumbling along trying to design a suitable "dial" switching system, the independents had large (for the tirne) multi-office systems, up and working, quite satisfactorily; and when a new office was needed, it was only necessary to "open" an unused selector "level", add some "pulse repeaters" (to be covered in a future article) and the existing office could then dial the new office. Likewise, it was easy to add a shelf (or two or three) of "incoming" selectors, to receive calls from the new office! (The trunks between the offices could be one-way or two-way, depending on traffic, and the different types of "pulse repeaters" that were provided.)

Spare levels could be opened "anywhere" in the switch, depending upon need; for instance, as more telephone numbers were required, a new "connector shelf" was provided, and the preceding spare selector level was "graded" into the new connector shelf. NOTHING the Bell had, including #5 crossbar, was so flexible.

What about a "remote" connector group? Why not? Of what use would a remote connector group be? Well, as you know, we often call business establishments...and dial the normal seven (or sornetimes ten, these days) number of the person, or department, we wish to speak to...thus, we don't have to go through the switchboard "attendant". (Well, there used to be an attendant. Today, it's "press one" if the call is regarding a new order. "Press three" if your account is in arrears, and you wish to make payment arrangements, ETC. And if all else fails, "Press 0" and an attendant will be with you, after a 12 minute wait to the tune of recorded music, or better yet, with a captive audience, how about a promo?)

Well, back in 1905, AECo was rnarketing a better idea. How about a PABX in an apartment building? To the caller, there was no difference. That person simply dialed the regular five or six (in those days) digit number, that they got from the phone directory, or "information" (also known as inquiry).

The third to last digit dialed was on the last selector in the switch train; and it found an outgoing pulse repeater, to the connector in the apartment complex; the connector received the last two digits dialed, seized the called line, and rang the correct telephone. (All this before Ma Bell had a successful switch "up and running".)

In the apartment building, probably in the basement, was located a simple exchange, known, in those days, as a "PAX" (Private Automatic Exchange). If the tenant wanted to call his neighbor, he lifted his receiver, operated the plunger line switch associated with his line, and the lineswitch seized an idle connector-selector. (These switches are a somewhat more complicated than your usual 8-relay local connector. Usually, levels one to eight act as a normal connector. Level "9", when dialed, converts the switch to a selector, and level "0" [in this instance] to a "ring down" trunk.) The selector-connector returns dial-tone (provided in later years) and the tenant simply dials the last two digits of the listed number of his neighbor down the hall! The connector functions in the normal manner.

If our tenant wants to call a party outside his own apartment complex, he simply dials "9". He receives a second dial tone(if provided) (from the exchange) and then dials the full, listed number. The dialing of the digit "9" converted the connector to a selector, which then seized an outgoing pulse repeater, much like calling from one exchange to another! If the tenent wishes to call the superintendent, he dialed the digit "0"; the connector then rotates automatically (as a selector) but instead of repeating dialing impulses, it simply rings out, as when a two digit-number had been dialed. (The "super" would also have a regular two-digit number bridged to this special "0" number.)

These "internal" systems had a lot going for them. They were very well received by the owner of the apartment building, and the tenants. (Having to dial an additional digit for "outside" calls meant little; the two digit inside service meant a lot... .on top of that, usually the apartment numbers were the same as the last two digits of the telephone number.)

The main saving to the telephone company, however, was cost. In individual line practice, each tenant would require a single cable pair back to the serving central office. Using the (long defunct) 10% trunking factor, an apartment complex of 80 suites would require 8 two-way trunks; and two or three pair for battery charging (at the PAX); ringing; and possibly alarm trunks. Although the figures in the preceding sentence are hypothetical, the tremendous savings in cable pairs is obvious. Outside plant was, and is, still the most expensive portion of the entire plant, based on a percentage applied to each subscriber. (Outside plant [cable] is declining, somewhat. Today, copper is being replace by fibre optics, with coaxial drops from repeater points. This new outside plant construction allows for extremely high speed data, and is making the world-wide web capable of even greater things [not necessarily including Bill Clinton's nefarious life style].)

So, what happened to this early form of Direct-Inward-Dialing? Well, a couple of important things. First, in the late teens, Bell started buying up large ("big-city") independents, served by AECo step-by-step dial. And the first thing they did was pull out the dial, and convert everything back to manual! (Incidentally, these purchases and conversions didn't make a dent in Pro-manual Bell's bottom line!). Naturally, out went the PAX systems and another fortune was spent replacing the outside plant, (the capacity of which had to be greatly increased to handle the apartment complexes previously served by the PAX systems).

Eventually Bell came to realize that, in order to handle the big city problem, "automatic" was the way to go. They developed their own "Machine system of switching, Panel method (Bell Jargon)" and for the smaller exchanges, they adopted Automatic Electric's Strowger System, and, being loathe to use the word "Strowger" they called the AECo equipment "Machine system of switching, step-by-step method."

Eventually, as Bell bought more and more independent big city systems, they had run into opposition....city administrations would not allow a Bell take-over unless the AECo step switching systems remained intact. (This is probably what led to Bell's adopting dial.) However, what Bell would not do, was to standardize the AECo D-I-D (PAX) apartment house systems. Why? For one very simple reason: Bell's "Machine system of switching, Panel Method" could not be adapted for D-I-D (in fact, the panel system of the early 20's was so crude that automatic reverting call was impossible; one dialed the operator for such services!). So, much to the consternation of happy, satisfied users, these PAX systems were pulled out as soon as Bell took over (even though, by this time, the serving AECo step-by-step equipment was retained).

Here is another interesting point: During 1924, Automatic Electric developed a new PAX, the type 25 (and this will be covered in the January 1999 SQ). The type 25 was developed specifically for DID usage.....and yet nowhere have I noted any AECo advertising to this intended usage. I do not have an iota of proof of this, but I strongly suspect that part of Bell's contract arrangements with AECo included the stipulation that AECo could manufacture D-I-D equipments, but would NOT promote (or sell) same, unless specifically requested by the independent telephone company.

What is even more important is that Bell did not supply D-I-D until they introduced CENTREX® (c. 1959).....and actually, it was not called D-I-D until (c.) 1967. Even at this point in time, D-I-D was supplied to very large customers only, due to the extremely high cost of modifying the #5 cross bar to outpulse the last two or three digits (this was akin to opening up another, nearby central office, to which the existing #5 had "EAS"). Because of this, Bell continued to refuse to supply small customers in areas served by SXS (where the cost was usually relatively inexpensive) until near the end of the eighth decade!

Today, of course, many large apartment complexes and college campuses, actually operate systems similar to the original D-I-D systems of the first decade, using digital switches (and interestingly enough, many of these are supplied and maintained by the local phone company, but leased or owned by the complex/campus). (Full exchange service, complete with its own NXX, is supplied to Cellular companies, and now, to companies supplying "competing" local dial-tone — what comes around, goes around!) Of course, many business have forms of D-I-D; and most telephone answering services now use apparatus, electronic or electro-mechanical, which convert the last two or three digits received from the telephone company to activate a display on a computer terminal, or "mark" the appropriate jack on a cord board.

Note: The forgoing article is intended as a technical explanation of D-I-D. Stan Swihart has indicated that, as time permits, he will provide the SQ with (what I expect will be)a very interesting article on the "rise and fall" of early D-I-D systems. BC