The Information Act Brian Hayes Annan, Octopus, 1990 The Numbering Crisis in World Zone 1 i carcity is no stranger in this land of I ten-digit numbers are possible telephone or a ladder without rungs—I couldn't .plenty. From time to time it seems I numbers. Indeed, more than 90 percent fathom the use of it. Then my grand • we are running out of fuel, out of wa of them are unacceptable for one reason mother demonstrated. She picked up the ter, out of housing, out of wilderness, out or another. A telephone number is not receiver and said, "Jenny, get me Mrs. of ozone, out of places to put the rubbish, just an arbitrary sequence of digits, like Wilson, please. Thank you, dear." out of all the stuff we need to make more the serial number on a ticket stub; it has My grandmother's telephone was al rubbish. But who could have guessed, as a surprising amount of structure in it. As a ready quite an anachronism when I first the millennium trundles on to its close, matter of fact, the set of all valid North saw it in the 1950s. Automatic switching that we would be running out of num American telephone numbers constitutes gear—allowing the customer to make a bers? That was one resource everyone a formal language, analogous to a com connection without the help of an opera thought was infinite. puter programming language. When you tor—had been placed in service as early as The numbers in short supply are tele dial a telephone, you are programming 1892. The invention of the first telephone phone numbers. In some parts of the the largest machine on earth, the global switch comes with a story worth telling. United States they are already quite telephone network. According to legend, Almon B. Strowger scarce, and they will have to be carefully A look at the grammatical structure of was a Kansas City undertaker who found conserved over the next few years. At first telephone numbers reveals a lot about he was losing business to a rival. Potential the idea of such a shortage seems prepos how the telephone system works and how customers would telephone Strowger but terous. A standard North American tele it evolved. And modifying that grammar "mistakenly" be connected to his com phone number has ten digits: three for turns out to be the key to solving the petitor. Strowger noted that the competi the area code, three for the central-office numbering crisis. The solution is dis tor's wife was the switchboard operator for code and four for the local line number. A cussed in a document released earlier this the local telephone system. His revenge ten-digit format allows for ten billion dis year by Bellcore, one of the surviving cor was to invent a device that would eventu tinguishable telephone numbers, from porate fragments of the dismembered ally displace operators almost everywhere. 0()()-000-0()00 through 999-999-9999. Bell System. The document has an im Strowger's invention was a ten-posi Even if every person in North America posing title: "North American Number tion rotary selector switch with a pivoting had a telephone at home and at work, as ing Plan Administrator's Proposal on the central arm that could rotate to connect well as separate numbers for a car phone, Future of Numbering in World Zone 1." with any of ten electrical contacts. The a fax machine, a modem and a beeper, pivoting arm was moved by an arrange there would still be more than enough ment of electromagnets, springs and numbers to go around. When diallessI was atelephone boy, my wasgrandmother's an object of ratchets. Each time the electromagnet re The flaw in this analysis is that not all mystery. It was like a clock without hands ceived a pulse of current, it advanced the 12 THE SCIENCES • November!December 1992 arm by one position. In the first network the street? The question is a miniature to do with the operation of the switching to try Strowger's idea, the customer oper version of a problem that has plagued network; instead it was a matter of map ated the switch by pressing a button. If telephone switching for at least forty ping letters to numbers. Central-office you wanted to dial a 7, you pressed a but years. In the first place, KL 5 is simply codes were introduced with names rather ton seven times, thereby sending seven 555; the alphabetic encoding of numbers than numbers because the telephone pulses of current to the electromagnet exists only on the dial of the telephone. company thought IH'tterfield 8 would be driving the selector arm. The push but Thus the first four digits of KL 5-2345 are more memorable than 288. On the tele tons were soon replaced by a rotary dial, the same as the local number 5552. When phone dial, 0 and 1 are not assigned any which automated the counting of pulses. you have dialed those four digits, what alphabetic equivalents, and so they could A single switch of that kind could in should the switch do? Should it connect not appear as the second letter of a cen terconnect ten subscribers. If you were you to your sister, or should it wait to see tral-office name. That subtle constraint, one of those subscribers, when you if you dial more digits? imposed to help avoid confusion between picked up your receiver, your line would The Strowger switch allows little flexi 0 and 0 and between I and 1, has had re be connected to the central selector arm. bility in resolving such ambiguities. It is markably far-reaching consequences for Dialing a one-digit number would then called a step-by-step switch, because once the telephone system. Named exchanges ring one of the other nine telephones. it has made a selection, it cannot go back to are gone, but their influence on the for Adding a second stage of switching could revise the choice. A Strowger switch must mat of telephone numbers remains. expand the service to a hundred sub determine on the basis of the first digit di h'or a long time 0 and 1 were avoided scribers. Now the original switch, instead aled whether to set up a local call or to se even as the third digit of central-office of being connected directly to ten sub lect a trunk line for a call to another ex codes. There was no compelling reason scriber lines, would be linked to a bank of change. If the switch were to establish a for the practice, although again it helped ten more identical switches. Each sub tentative routing to your sister as you di avoid mistaking 0 for O or 1 for I. scriber would be identified by a two-digit aled 5552, there would be no way to undo In any case, for some decades most telephone number. When you dialed the that connection if you continued dialing. North American telephone numbers fol first digit, say a 3, the first selector switch Telephone switching gear has changed lowed a pattern that can be expressed as would connect your line to the selector a great deal since Strowger's time. Mod .V.V.V-.V.V.V.V, where X represents any of arm of the switch leading to lines 30 ern switches are fully electronic rather the eight digits from 2 through 9 and A' is through 39. Dialing a second digit would than electromechanical, and they are ca any decimal digit at all, from 0 through 9. move the selector arm of the second-stage pable of holding a series of digits in a The maximum capacity of this number switch to the appropriate contact. buffer before determining what to do with ing system is equal to 8x8x8x10x1 Ox them. Nevertheless, the architecture of 1 Ox 1 0, or 5,120,000. In practice, it is an telephone numbers is still strongly influ Upper limit that can be approached but not It ingis easy network to see could how abe Strowger expanded switch to enced by decisions made to accommodate reached. A few lines in each central office handle 1,000 lines (with three banks of the peculiarities of early step-by-step are needed for testing and similar purpos switches) or 10,000 lines (with four switches. Moreover, in some rural tele es, and a few exchange codes, such as 555, hanks). In principle, such growth could phone office there may still be a Strowger have traditionally been reserved. More be continued indefinitely, but the quanti switch clanking and clunking away. over, telephone companies try never to fill ty of switching gear would become im a central office completely, since that practicably large. The telephone compa would leave no flexibility when customers ny adopted a different plan. It set up Seven-digitanalysis, dialingto give wouldeach telephoneseem, on direct first move or request a change in service. switching offices that could each accom access to ten million others. Actually, the modate as many as 10,000 subscribers, number of lines available is only about then provided trunk lines to connect the half that. The reason is that some num By to1950 much seven-digit of the I'.S. dialing (though had not spreadto my various offices. At first, calls between cen bers count for more than others. grandmother's house). A telephone con tral offices were completed by operators, "Dial 0 for Operator" has been stan nected to the network had the theoretical but soon that task too was put in the dard telephone practice almost from the potential of reaching five million other hands of the customer.