BelllAboratories RECORD 228 Memory Devices R. H. Meinken and L. W. Stammerjohn Two magnetic memory devices, the permanent magnet twistor and the ferrite sheet, VIIlllm,' ;'.1 • Number 6 • June 1965 store the operating intelligence of the system.

236 The Switching Network A. Feiner A very flexible network, it is actually a cross between electromechanical and elec­ tronic, rather than fully electronic, techniques.

Contents 241 Mechanical Design D. H. Wetherell Because its great flexibility stems from the stored progmm, No.1 ESS needs fewer equipment options than any present system. PAGE 246 Power System and Ringing and Tone Plants J. W. Osmun and J. R. Montana 194 Introduction W. A. Mill' N,ti,. Solid state devices help simplify the power system and lead to the introduction of Presentin!J an ;XXII,' '11/. III" No, 110:1''1'11'111/;,' S"';I"/,;II" S,/"",,,, ,/1,,111,,· If'rlmolo!lical precision dial tones and call progress tones. and scientific d;IIWl,';1I //'11;/'11 ;1//"'" /'/1/H"';I''''', 251 A New Approach to System Maintenance R. L. Campbell and W. Thomis 196 The Evolution of Telephone SwikhinJ( 11'. Kf'ilft,·,. Automatic maintenance programs comprise over half the contents of the stored pro­ 'I'f~'VoIIII;on Seemingly a ;11 11'/,,/,11111/" "/1"',.",,, . .VII, 1 1o:SS ;" 111'1111111" HilllfIl'tdy in the gram and make No.1 ESS a "do-it-yourself" machine. tradition of cmnmon ('/11/1,.,,1 H//';lrll;"1/ "/1",,.1/,,,. 257 Some Magnetic Materials D.H. Wenny 204 From Morris to Succa.~unn8 fl. W. Kl'lrhl,·,II/'· A new alloy was created whole cloth for the ferreed switch while two older ones got All the vast chant/"x XI'III/I'III;/lli III,! MII,.,.i" 11'",1 11'11/11 II,,· SIl/·'·I,..~lIuua office were an entirely new magnetic dress for the twistor memory. made to enhance .~tol'l~d 1'1'11111"/1" "0/1/1'01, 262 Semiconductor Devices M. L. Embree and J. Sevick 210 Features and Service .J . .1. YoHtllill,! There are hundreds of thousands of transistors and diodes in the system, but a The stored program. 1I1';nllx nt'tiI "",.v'r,." III Ilt'll S/I"'I'/II "IIHIOI//f'I'X and adds new mere handful of basic types serve all system functions. flexibility to many tnulitimwl xI/xII'/II "·"'''I'I'x. 268 Testing the System R. S. Cooper 214 The Stored Program fJ. /I. Sil'fI,·t, .1,.. 11/1'[ S. Siff,,'" Before No.1 ESS can be tried for the simplest kind of call, it must be tested and Many functional prog'l'am.~, 1Iuildinll 1Il0rh'x 0111,,' Hlllrt'd 11I't1//I'UIn, guide each step evaluated as a machine for executing programs. the system takes in procl'.~.~inf} ('flUX 111Id /II",/0I'IIIi1ll/ IlI/IOlI/llli/' maintenance. 274 Cut-Over at Succasunna 222 The Control Unit A. H. DotJltnaie,. On May 30,1965 in Succasunna, New Jersey, 20 years of research and development The executive part of the systWllt, it interpl''''x ittHi 1'''''' illn.~ lind data from the stores were realized in the Bell System's first commercial electronic central office. and generally keeps things runnin!/.

Devices on this month's cover are all components of the sole subject of this issue: The No.1 Editorial Board G. C. Dacey, Chairman • \Y. M. Bacon • F. H. Blecher • W. A. Cornell Electronic Switching System. From the ferreed A. E. Joel, Jr. • M. V. Mathews • H. G. Och • H. A. Stone switch and the ferrite sheet (large and very small squares at the right) to the logic circuit Editorial Staff B. E. Strasser, Editor • M. W. Nabut, Managing Editor G. L. Bromleigh, Jr., Assistant Editor • J. C. Smith, Jr., Assistant Editor (extreme left) and the ferrod sensor, all are • F. E. Piellusch, Art Editor • T. N. Pope, Circulation Manager Cover unique devices in telephone switching. A part THE BELL LABORATORIES RECORD is published monthly by Bell Telephone Laboratories Incorporated of the twistor memory occupies the center of 463 West Street, ~e~ York, N. Y. 10014, J. B..FISK, P~esident; K. PRINCE, Secretary; and T: J. MONTIGEL: the picture. This device holds the stored program, Treasurer: SubscnptIOn: $2.00 per year; ForeIgn, $2.90 per year. Checks should be made payable to Bell a set of instructions that shapes the system's Laborator~es Record and addressed to the Circulation Manager. Printed in U. S. A. © Bell Telephone responses to the requests of telephone customers. LaboratOrIes, Incorporated, 1965. (Cover painting by Paul Lehr.) Electronic

A Score of Tears of Organized Attack

It was inevitable that modern electronic technol­ is an over-all uniqueness of the system to which our partnership with Western Electric has grown ogy would be applied to the switching machines of your attention is directed for a moment. Compare steadily broader and deeper as WE ha produced, the telephone plant as it has to the transmission net­ o. 1 ESS with earlier switching systems. in hardware, Bell Laboratorie ' de igns of apparatus work. There is nothing casual or haphazard, how­ The manual telephone switchboard was designed and equipment, and delivered on tough chedules ever, in the form that electronic switching has on the prior decision that switching would be ac­ to meet dose deadlines. The under tanding, the taken in the Bell System nor in the date of its ap­ complished by a human operator manipulating give-and-take; and the real cooperation in chi un­ pearance as an operating office serving telephone plugs, jacks, and keys. Step-by-step switching was dertaking have been marvelous to behold. customers. This program has been the largest sus­ based on the prior decision that switching would be While the big effort f the operating companies tained Bell System development effort toward a accomplished with step-by-step switche controlled in the u e of electronic witching office to erve single goal ever undertaken by Bell Laboratories. directly by the pulses generated a the customer our cu tomers better i yet to come, many operating Prior to World War II various individuals had dialed. Likewise, panel and cro sbar witches were company people have made individual contribu­ given thought to the possibility of an electronic chosen as the central building blocks for those sys­ tions to the development and de ign effort. Others switching system. Note that this was years before tems. In the case of electronic switching sy terns, are preparing them elve to plan and operate o. 1 the transistor was invented or named. Immediately no such prior decision wa made. Compare one ESS office in their territories. The ignificant con­ after the war, we organized a switching research aspect of No. 101 ESS (RECORD, February 1963) tribution of the operating companies to date has effort to explore ways in which the evolving tech­ and No. 1 ESS. Both sy tems use much common been the coordinated planning to in tall No.1 ESS nology could be usefully applied to the problem of type of equipment. But o. 101 ESS uses time di­ offices throughout the country a fa t a equipment witching telephone calls. In 1953 an electronic vision switches, and No. 1 ESS space division. is made available by the We tern Electric ompany. switching organization was formed in the Develop­ \'<7hen work on electronic switching started, we There i no better example of the neces ity for ment Area, hopefully looking toward a practical understood the principles of common control and and the benefits derived from the a ociation of the electronic switching telephone office. The 12 years its huge advantages. We believed that electronic research and development people, the manufactur­ since 1953 divide into two periods of a half dozen circuits would operate with such high speed that a ing and installation group, and operating com­ years each. The first culminated in the Morris, Illi­ single concrol would serve even a large office, and pany people than this Bell Sy tem achievement of nois, experimental trial office; the second, in the we saw the advantage in this, as distinct from a a common goal of better telephone service in Succasunna, New Jersey, office, giving commercial multimarker arrangement where competition arises timely fa hion. service. Thus, we come to the fruition of 20 years between them. Further, we dearly recognized tb,e This post-war score of year ha brought rapid of Bell System organized effort directed toward an usefulness of large memories with hort access progress in switching technology. ot the least i electronic switching system. times. But the important decision to use memory to a kind of intellectual and organizational maturity The final phase of the effort-the creation of the tore tbe ystem logic evolved as the development which allow the de igner to proceed in orderly detailed design information, fabrication of the first work progressed. fashion from the requirement to be met to the models, tests of the hardware and software-was There are two new and fundamental character­ choice of sy tem and apparatu to meet the tated started in 1959. In the fall of that year, we set the istics of electronic switching: the high speed elec­ objectives. ervice to our en tomers will benefit goal of mid-1965 as the date for the first cutover. tronic central control, and the use of memory to greatly from thi maturity. Bell Laboratories people engrossed in this project, store the ystem logic which in turn determines, view with some pride and much satisfaction the in detail, how the office will perform its functions. successful meeting of this schedule date. And so, electronic switching came into being W. A. Mac air The unique characteristics of No. 1 ESS and its without a single piece of apparatus being prechosen V ice President solutions of technical problems will unfold as you as the preferred solution to any particular problem. Transmissio1z and Switching read the articles in this issue of the RECORD. There In the final phase of No. 1 ESS development, Development

194 Bell Laboratot'ies Reco1'd June 1965 195 The Evolution of Telephone Switching

w. Keister

THE OPENING OF THE No.1 Electronic Switching System office at Succasunna, New Jel'sey in May, 1965 was the culmination of the largest single development project ever undertaken by Bell Laboratories for the Bell System. Because millions of man hours have been spent on this one development, it would appear, at first glance, that nothing less than a revolution in telephone switching has been in the making. In one sense it is a revolution, or at least the first stage of one: Electronic s\vitching systems, in the next few decades, will replace all existing Bell System switching systems. But in a deeper sense, the No, 1 Elec­ '. ',' tronic Switching System (henceforth ESS) is the product of years of accretion of experience along many lines in the evolution of telephone switching systems.

June 1965 INCOMING

TERMINATING ,w,,, 1 NETWORK Among the most significant trends have been a switches. There is no fixed relation between the functional separation of switching actions and the converted digits and the original dialed digits OTHER actions that control them, a clarification of the and the translation can be changed by changing OFFICES roles of logic and memory in telephone switching, cross-connections in the decoder, thus providing and the burgeoning of solid state electronics tech­ flexibility in rearranging trunk connections. nology into an array of extremely high-speed, In the late 1930s, common control took a major O:~i~::~ versatile devices which opened new areas in step forward in the No. 1 crossbar system. The 0"'00"0 J switching techniques. As all these trends matured switching network of this system is not con­ TRUNKS they made the development of No. 1 ESS not strained by the numbering system in any way, merely feasible, but in a sense inevitable. and hence it can be designed purely in terms of 1 Technically speaking, the ancestry of NO.1 ESS traffic requirements. This singular freedom stems OTHER runs back through crossbar systems to panel sys­ from a new device, the "marker". In addition to OFFICES tems. Both of these are strictly electromechanical translating dialed digits, as the panel system de­ ~!'"+J systems, but both are definite stages along the road coder does, the marker locates idle trunks and di­ to common, or centralized, control which reache rects the network to them. If traffic congestion its highest present development in No.1 ESS. The impedes the first attempt to find a connecting basis of this technique is that actual switching path through the network, the marker makes a actions can be separated from the actions that ond attempt. Similarly, if all trunks are busy control them, and call connections through a on th direct route to a desired central office, the CALL MEMORY switching network can be directed for many lines marker choo es an alternate route via another of­ AND CONTROl by one "common" group of control equipment. The fice. Th philo ophy behind these valuable fea­ control equipment routes a call through the net­ tures is that the marker can be made to examine work and is then released to 'act on other calls. the system to if certain components are busy The concept was first tried in the panel system or idle. On th basi of its findings and the in­ ROUTE ROUTE MEMORY TRANSLATOR about 40 years ago. Itwas developed through early formation repr nt d by the dialed digits, it can TRANSLATOR AND CONTROL crossbar systems, and came to full maturity in the then determine th mo t efficient way to make a No. 5 crossbar system. In modern crossbar sys­ desired connection. tems the network has no control function at all; Originating and terminating networks are sep­ PANEL SYSTEM NO. 1 CROSSBAR SYSTEM control is the exclusive function of specialized arate in the '0. 1 crossbar system. Each has its equipment. However, in order to handle traffic de­ own marker to translate between telephone num­ mands effectively, the No.5 crossbar system re­ bers (Le. dialed digits) and control numbers. The quires a number of duplicated groups of control terminating marker is assisted by a number group equipment to serve one office. Electronic speeds translator which can select a particular line from allow No.1 ESS to operate with only one control a group on an equipment frame in the central of­ for an entire office. fice. The line number on the equipment frame need Common control was not born with "automatic" not correspond to the telephone number, a factor switching systems. The first automatic system, that overcomes many constraints in the layout of IN the step-by-step system, is designed for direct the network and permits the network engineering 1 control. Contact arms select terminals on the to take account of the variation in the traffic load COMBINED OTHER SWITCHING } 0,"" switch in direct response to the dialed digits. Be­ between individual customers. NETWORK OFFICES NETWORK OFFiCES OUT cause telephone numbers must correspond to the A further improvement in these principles is J location of particular terminals, direct control embodied in the No.5 crossbar system, first put in allows little flexibility in the layout of the switch­ ing network. The technique is also rather profli­ gate in its use of switching equipment-the These diagrams show how the concept of common amount of equipment tied up in completing one control and the concept of the roles played by call under direct control is inherently capable of logic and memory have evolved through the major completing hundreds of calls in the same time. switching systems from panel to No.1 ESS. A The first elements of common control, units major trend is the removal of any control function SCANNER ADDRESS called registers, decoders, and senders, were in­ from the network. Notice how in the panel system troduced in the panel system. These units are sende1" links are part of the network, as they are never permanently associated with single lines or in the No.1 crossbar system. Also, these two sys­ calls but are used in common by large groups of tems have both terminating and originating net­ lines. Dialed digits are stored in the register and works. In NO.5 crossbar, and in No.1 ESS, the then converted by the decoder from decimal num­ networks have become strictly passive elements bers to a nondecimal number system. The sender in terms of control. Notice how memory and CONTROl uses the converted numbers to control the panel logic in No.1 ESS, are entirely discrete functions. AND ROUTE TRANSLATOR

SEMi- TEMPORARY PERMANENT 198 Bell Laboratories Record MEMORY MEMORY LINE NO. TRANSLATOR CAll PROCESSING STORED PROGRAM REGISTER AND TRANSLATIONS

NO.5 CROSSBAR SYSTEM NO.1 ESS service in 1947. A single network controlled by a "0' IF LINE CONDITION CENTRAL single marker serves both originating and termi­ cessfully in many weapons systems during the HAS NOT CHANGED CONTROL second World War. Studies made at Bell Labora­ "1" IF LINE CONDITION nating traffic. The marker handles outgoing and HAS CHANGED incoming calls. tories soon after the war indicated that these A number of parallel evolutionary trends in high-speed electronic techniques could be point­ telephone system design emerged during these 40 ing that new direction. But electronic technology, years. We have been describing the major one, the at that time, rested squarely on vacuum tubes evolution of common control systems themselves. which fell far short of the cost and reliability re­ It was accompanied by an equally significant evo­ quirements of a practical telephone system. The lution in switching devices and apparatus. Origi­ last corner was turned with the invention of the CONDITION OF CONDITION OF nally, the panel system used motor-driven shafts, transistor; an electronic system could now be LINE AT LINE AT built that would be competitive with electro­ PRESENT SCAN LAST SCAN clutches, cams, and innumerable other mechanical devices. But these were difficult to maintain, and mechanical systems. as relays became more reliable they took the place One dilemma remained: A system design usu­ of much of the mechanics. Crossbar systems, ally is current for 10 to 15 years, its service life meanwhile, skirted the mechanical hazards with may be very much longer. New technical ad­ "LAST LOOK" vances are made during a system's "design life," CONDITION the relay-like crossbar switch and with control OF LINE 200 circuits that were almost exclusively relays. and social changes create new demands for fea­ CALL STORE During this time circuit designers began to tures and services that the system is not equipped ~ ADDRESS OF to handle. Often, the best way to effect these fea­ LINES LINE 200 look at their product in a wholly new light. They saw that they were not designing electrical cir­ tures and services is to design a new system to FERROD LINE SCANNER CALL STORE cuits so much as logic circuits and that the intri­ replace the old one, but the long service life im­ cate connective patterns between relay contacts poses a more costly solution. Existing offices must needed to establish a talking path between tele­ be modernized and adapted to changing service PRESENT LAST OUTPUT MEANING requirements even while new installations of a LOOK I LOOK I I phones could be viewed as the stage-by-stage pro­ gression of the simple logical relations AND and more modern system are taking place. It often re­ 0 0 0 STATE UNCHANGED-OFF HOOK OR. For example, consider a lamp plugged into a quires a greater effort to redesign an old system than to design a new one. And it often costs more 0 1 1 STATE CHANGED-CALL START wall socket controlled by a wall switch. The lamp will not light unless both the lamp switch AND to modify the old system for new features than to 1 1 0 STATE UNCHANGED-TALKING the wall switch are turned on. On the other hand, provide these features with a new system. Now, take the action of the dome light of an automobile how do you design a system so flexible that it can 1 0 1 STATE CHANGED-CALL END which lights if one of the front doors OR the other be adapted to features that are not even foreseen is opened. Relays can be wired to open or close at the time? Electronics had the answer-stored contacts in the same fashion and these simple program operation. logical relations can be repeated as often as neces­ The idea of a stored program system sprang sary to form a highly complex system that decides from a close consideration of the roles of logic and complicated logical questions. (See the drawing memory, the twin operators that play such an im­ A highly simplified way to apply logic circuits in line scan­ on this page.) portant part in every telephone system. Memory ning is shown here. The central control directs the line All this was taking place within an all­ is information, what to do. Logic is the decision­ scanner and the call store memory to report on the present encompassing change of the business and social maker, how to do it. Memory knows what tele­ and last states of line 200. The logic function to be per­ environments that telephone systems served. Busi­ phones to connect; logic decides what path to take formed is: if both states are 0, or if both states are 1, then nesses expanded and decentralized; populations between them. In the panel system, the sender the output signal should be a O. (The table shows the out­ clustered around the large urban centers. This provides memory and shares logic functions with put for various combinations of bits.) If the two states do created demands not only for more telephone serv­ the decoder. In crossbar systems, the two are more not agree, however, then the output signal is a 1 to indi­ ice, but for different kinds of services, and tele­ clearly defined; the senders and registers provide cate that central control must take action on the line. If phone system designers began to think of systems memory, the markers make the logical decisions. either the present or the last state, or both states, are 1, that would perform these services and could be In No.1 ESS, the logic procedures for making then the output of the OR gate is a 1. If both states are 1, adapted to new services as the need arose. Circuits telephone connections are written in the form of a the output of the lower AND gate is a 1 which the inverter which began to grow more sophisticated and more stored program which is placed in a changeable changes to a O. This is applied to the second input of the complex to meet the exigencies of the new types memory. And therein lies the system's great flexi­ upper AND gate. Thus, the output of the combined circuit is of services reached a point where the operating bility. Logic was "written" in copper wire in the o if both the present and last states are 1 or if both are O. speed of relays became inadequate. Even crossbar systems, the last word in electromechanical sys­ Wired logic. This intricate pattern of wires in tems, were reaching a practical limit in what they a No.5 crossbar system connects the many relays could be made to do. A new direction was needed. that perform the complex memory and logic func­ Electronic switching techniques first came into tions required of the marker in directing telephone their own in digital computers that operated suc- calls through the maze of a switching system.

200 Bell Laboratories Record June 1965 201 electromechanical systems, and each circuit was Programmed logic, in this description, wears wired to perform one specific operation. To change the same discursive mantle that was attributed to the operation it was often necessary to rework and formal logic. Actual program instructions, how­ rewire circuits extensively. A No.1 ESS program ever, are not written in this way but are cast in is contained on plug-in cards inserted in the mem­ machine language and its alphabet of binary sym­ ory. To change a logical operation, it is only nec­ bols. The operations performed on the binary in­ essary to change a memory card. structions by wired logic in the central control are Formal logic is a discursive process, often mak­ the same as are used in most information process­ ing swift transitions between broad categories of ing machines. They consist of, for example, argument. Programmed logic analyzes a call situ­ shifting information from one register to another ation into discrete steps starting when a calling where it may be compared with the contents of a customer lifts his receiver and proceeding through third, while the difference is stored in another a chain of control actions to establish a talking register, and read out in still another. A number connection. To the customer, the connection seems of special instructions ar highly relevant to call to be made instantaneously; actually, the system processing that would b valu less in conventional deals with each step in turn. With considerable computing. For exampi , the busy and idle states simplification, it works about like this: of trunks can b l' pr s nt d as a binary "1" and Every few milliseconds, the system checks, or a binary "0", l' sp ctiv Iy. If the state of a group scans, each customer's line and records its state­ of trunks is star d in a l' gister, then an instruc­ on-hook, off-hook, dialing, busy, in a talking con­ tion to "find th rightmost zero" locates an idle nection, etc. The state detected at each scan is trunk for a call in progress. compared with the state recorded at the previous Translation and interpretation of the program one. No action is taken if there is no change. When instructions is handled by the wired logic of cen­ the system encounters a change of state-from tral control. This component of No.1 ESS is the on-hook to off-hook, for example-it consults th most far-reaching development of the concept of program to find what action is indicated. In this common control in present switching systems. It case, the off-hook state indicates that the customer is concerned only with the basic information proc­ wishes to place a call and the program directs that essing operations, not with the telephone switch­ dial tone be sent to his line. While th customer ing logic which is all contained in the stored pro­ dials, the program directs that his line is to be gram. Logic and memory are thus completely scanned at regular intervals, and each digit is re­ separated from the switching network and the corded in turn but no further action is taken until trunk equipment which are essentially passive he completes dialing. parts of the system. After the system registers all the dialed digits, Only one central control processes telephone it again consults the program for the next step. calls at any given time, while in a crossbar system For a regular station-to-station call, this step is it is common to furnish up to ten markers. For to make connections through the switching net­ greater reliability, every o. 1 ESS office has a work to the called telephone. That telephone may duplicate central control which is kept up to the be busy or idle and the program is prepared for moment on the progress of all calls in the system, either eventuality. If the phone is busy it directs but this is only for reliability. Electronic speeds, that busy tone be sent to the calling party. If idle, three or four orders of magnitude above the speed the called phone is rung and ringback tone is sent of electromechanical components, allow one cen­ to the calling party. tral control to handle all calls in the system. It When the call is answered, this state is regis­ works on only one call at a time, but at such speed tered and again the line is scanned at regular that it appears to handle all calls simultaneously. intervals but no action is taken until the parties Stored program control had its first trial with hang up, at which time the program directs that the Electronic Central Office in Morris, Illinois in connections be taken down. The whole process is 1960, where it was proved completely feasible. In characterized by a continuous interplay between fact, the stored program concept was the only the programmed logic and the temporary memory thing that emerged from the trial unscathed. Some of the circuit packs that perform the logical function ea kind of electronic slate) that is associated with The components, the system organization, the or­ for the NO.1 ESS central control, shown here slightly larger each customer's line. This memory records the ganization of the program itself, all have been than actual size. Central control is composed of thousands of instantaneous state of the line, and it registers changed and improved for the Succasunna office of these relatively simple logic circuits intricately connected to dialed digits and other transitory information No.1 ESS. In a sense, then, an evolutionary trend perform the necessary steps in processing the information cen­ needed during the progress of a call. When a call in electronic switching has been foreshadowed. In tral control acquires from other parts of the No.1 ESS system. is completed, the slate is wiped clean of informa­ the next article, we will examine some significant tion pertaining to it. details of that trend.

202 Bell Lab01'atories Record June 1965 203 A milestone in telephony, the Morris trial proved beyond .doubt the validity of the stored program concept. But on the level of hardware nothing is left from Morris; Succasunna is a new office grown out of four years of intensive development.

From Morris to Succasunna

R. W. Ketchledge

Morris, Illinois HE ELECTRONIC CENTRAL OFFICE in Morris, design of such a system would, clearly, have to Succasunna, New Jersey T Illinois was a pivotal point in the history take full advantage of the most reliable and ver­ of telephone switching. Turning back to electno­ satile devices and techniques available. This con­ mechanical systems, we can see the Morris office ception inevitably led away from thinking of as the highest point in the evolution of the con­ electronic circuits as a way to perform familiar cept of common control. Turning ahead to a functions faster, and into considering how their switching future that is clearly committed to unique characteristics could be channeled into electronic techniques, we can view it as an arche­ performing new kinds of functions for a new type whose basic outlines may be shadowed in a kind of system. number of future systems, but whose actual For example, from the point of view of a tele­ components already have been transformed into phone customer a switching system that operates the much different components of No. 1 ESS. in tenths of seconds gives perfectly satisfactory Although no components have survived the Mor­ service. Therefore, it would be pointless to con­ ris office in their original form, No.1 ESS em­ trol the switching network in millionths or even bodies the basic idea of the trial system and the thousandths of a second if the object were only changes in components were made because they to make connections at astonishing speed. But served the idea more effectively. That idea­ suppose a cycle of time for the system were sliced stored program control-will become as familiar into infinitesimally small pieces, or slots. In one in the Bell System as the telephone itself. slot the system could hande all network connec­ It would have been a trivial application of tions that had to be made, in the next slot it electronic technology if the Morris office had might diagnose its own internal condition, in the been developed merely as an attempt to "modern­ succeeding one it could direct an intricate new ize" telephone switching by substituting elec­ service. In the twelve seconds it takes the av­ tronic techniques for electromechanical ones. The erage person to dial a seven digit number, the motive behind it was far broader, being nothing system would perform millions upon millions of less than the desire to develop the most flexible separate tasks involving a host of logical opera­ telephone system that could be conceived. The tions. Time, and the manner in which it is used,

204 Bell Laboratories Record I

II ::::::::::::::::::::::::::~::::::::::: .. ~~ ~ ~~ ~~ ~~ ~ ~ ~ ~ ~ ~ ~ ~~ ~ ~ ~ ~ ~ ~ ~ ~ ~~ ~~~~ ~ ~ ~:~ ~~~ ~ ~ ~ is thus one of the most intricate problems in the I: ferrite sheet-the choice was clearer. The ferrite design of an electronic switching system. I: ::::::::::::::::::::::::::::::::::::::::::: I . sheet memory can store a longer word than the It would not be difficult to design each com­ :; ;::::::~:;:::::;::::::;;:::~::E::::T;:: barrier grid store, it is a more economical de­ ponent in the office to operate at the highest I .. vice, and again, it is a fully solid-state device. ,I :::::::::::::::::::::::::::::::::::::::: '0' speed that can be achieved electronically, but it J ::::::::::::::::::::::::::.:::::::::::::••• : I··········· .. ···.. ······ ..··········.. ······ The switching network of the Morris office might be prohibitively expensive and somewhat represented the first study of fully electronic I ~~ ~ ~ ~ ~ ~~::~ ~~ ~ ~~~~~ ~~ ~ :~~~:~::: ~: ~;; ~~ ~~;:; ~ pointless. A more exacting task is to define the I . switching under conditions closely resembling range of speed needed in each of the various ': ::::::::::::::::::::::::::::;:::::::::::::;:: commercial operation. From this point of view, components, and then to design the office so that >:::::::::::::::::::::;:::::::::::;::::::: ::: I I···· .. ···· .. ·· ······················· . . the Morris network was a technical succes , but they all work synchronously. The memory de­ ,I :::::::::::::::::::::::::::::::::::::::::::: it had certain drawbacks. One of the most seri­ vices, for example, must operate in micro econds I ::::::.:::::::::::::::::::::::::::::::::::" ous was the inability of gas tubes to carry either because of the great number of instructions that I . high amplitude 20-cycle ringing signals or direct must be read from the program and converted " ::::::::::::::::::::::::::::::::::::::::::':: I . current from the telephone lines. In keeping I······.. ··· .. ················..·············· to control information that is sent to the switch­ with the inclination toward solid-state devices, a ing network. The network itself, however, does PNPN diode that had many of the characteris­ not have to match the operating speed of the tics of gas tubes was considered for commercial memories. If it operates in milliseconds, or even application. However, a study of the operation in fractions of seconds, there is only a remote :::!! of a remote line concentrator which employed possibility that it will be overpowered by traffic the P PN diode showed that it had the same demands. The judicious use of buffers to hold in­ difficulty with ringing signals and direct cur­ formation until the network is ready to act upon rent. DC switching through the concentrator was The photographic plate used in the serniperrnanent rnernory in it helps to synchronize the very high speed This twistor card is inserted between folded stacks of twistor proposed and this led to the consideration of memories and the slower network. the Morris office is exarnined by R. W. Ketchledge (right) rnernory planes, the serniperrnanent rnern01'Y of the Succasunna switching devices with metallic contacts. These One of the most important lessons learned and . A. Lovell. Each of the squares on the plate can hold ESS. Each of the 2,816 vicalloy spots contains one bit of in­ can handle wideband signals including dc sig­ during the Morris trial was that the components 99,000 bits of inforrnation recorded as dots or absence of dots forrnation, a zero if the spot is rnagnetized, a one if it is not. nals, they have only negligible transmission of an electronic switching system should be de­ in pa1'ticular locations. Four plates rnade up the serniperrna­ There are 128 cards in a twistoT rnodule and 16 rnodules in a losses, and they can handle higher power than signed to perform a particular function; speed n nt rnernory containing oveT 2 rnillion bits of inforrnation. twistoT store. An ESS central office contains two to six stores. either gas tubes or diodes. All these properties of operation is only a characteristic of that func­ are even more valuable in switching network tion, it is not an end in itself. To illustrate this crosspoints than they are in a line concentrator. more explicitly, we will discuss, in turn, thr permanent magnet twi tor, the temporary mem­ development of a solid-state memory was shown Thus, the invention of the ferreed crosspoint, the examples of the changes made between Morris ory is the ferrite sheet. to be feasible, the decision was, in a sense, al­ basic element in the No.1 ESS switching net­ and Succasunna in which different decision w l' The decision to develop the solid-state devices ready made. work, grew out of these remote line concentrator made in terms of speed. In the first xample, was not made lightly. The flying spot store and In other areas of comparison, however, the studies. the memory devices, speed remained ssentially the barrier grid tube operated quite succe sfully distinction between the flying spot store and the The ferreed consists essentially of two mag­ the same but other considerations influ nced the in the Morris office, and although the ferrite twistor was not at all clearly drawn. In the mat­ netic reeds sealed in a glass envelope mounted decision to develop a new type of d vice for sheet has clear advantages over the barrier grid ter of capacity and physical size, for example, between plates of a two- tate magnetic alloy. Succasunna. The econd example is the switching tube in size and in accessibility of information, each has advantages and disadvantages. The The alloy can be very rapidly switched from one network; the network at Succasunna is inher­ the flying spot store is quite competitive with capacity of the store used at Morris was under state to the other with a relatively short pulse ently slower than the network at Morris. Finally, the permanent magnet twistor in these charac­ five million bits, but Morris was a small office of current, and it has the property of high the method of switching between duplicate con­ teristics. In all, the really clear area of choice serving only 435 lines. The smallest size planned remanence-it will remain magnetically satur­ trols is an operation that was made much faster was in the greater reliability of the solid-state for a o. 1 ESS office is about 3,000 lines and it ated until another pulse witches it back to the for Succasunna because the Morris trial showed devices. has the capacity to grow to 65,000 lines. Vast first state. The switching can be done in milli­ that faster operation in this case was necessary When the design of the Morris central office amounts of memory are needed to control an seconds, as it actually is, or even in microseconds, for completely reliable service. began, solid- tate technology could not yet offer office at its ultimate size. so it is much faster than an ordinary electro­ A heavy burden is placed on the system's the range of reliable devices needed in a tele­ Two proposals were made. One was for a fly­ mechanical switch. It is, however, slower than memory devices. Their reliability cannot be over­ phone office. Therefore, sections of the Morris ing spot store with a capacity of 25 million bits, switching performed by gas tubes or diodes, one stresed, they must have large storage capacity system, such as the switching network and the the other for a permanent magnet twistor of 6 reason being that the inertia of the reeds must in the smallest feasible volume; and information memory devices, were constructed with electron million bits. The permanent magnet twistor was be overcome each time the device is switched. stored in them must be efficiently arranged and tubes even though the limitations of this tech­ chosen because it eliminated the problems in­ However, as we have noted, the switching net­ readily accessible. In the Morris office, the mem­ nology were well known. Electron tubes were herent in the high voltages and hot cathodes of work is one place in the system where some speed ories were electron beam devices; the semiper­ judged to be adequate for a trial that would last the electron tube device, it required les devel­ could be traded for other important characteris­ manent (or program store) memory was the little more than one year, but in terms of the opment (at the time it was proposed) than the tics, so the ferreed crosspoint switch was devel­ flying spot store, and the temporary (or call long life predicated for commercial telephone flying spot store, and it offered the superior oped for No.1 ESS. store) memory was the barrier grid store. In the systems designers were already thinking ahead reliability of a fully solid-state memory. Control techniques of a ferreed switch are quite Succasunna office these have been replaced by to the development of highly reliable solid-state In the competition between the temporary different from those of diodes and gas tube and solid-state devices; the program memory is the devices. Thus, when after the Morris trial, the memory devices-the barrier grid store and the the new techniques necessitated changes in the

206 Bell Labo?'atories Record June 1965 207 overall system and development of new control de­ vices. The control circuits for the first ferreed were composed of semiconductors, but as the switch was developed it became apparent that the operating speed of semiconductors, in this case, is gratuitous. Therefore, the diodes were replaced with relays which reduced the cost of the control network. A new control device grew out of the problem of line scanning. The ferreed gives the switching network the ability to extend the metallic wire path from a customer's telephone to any part of the central office. To start this action, the system must sense the presence or absence of current in the customer's line which indicates if the tele­ phone is off-hook or on-hook. A fast scanning de­ vice is needed to examine all the lines in the office at frequent intervals so that a customer does not wait for service. Furthermore, for transmission reasons the line must remain balanced and undis­ turbed by the scanning action. The ferrod-actually a saturable core trans­ The basic 8-bY-8 cTosspoint aTmy of ferreed former-grew out of these requirements. It con­ switches that make up the switching network at sists of a rectangular ferrite stick surrounded Succasunna. To connect two customer's lines for by two solenoid windings connected in a balanced a telephone call eight of the individual feTreeds arrangement on each side of a customer's line. in the array close in a specified pattern and Two single turn loops pass through two small they remain closed without any holding power. holes in the center of the stick. If the customer's The gas tubes used in the switching netwoTk at Morris. The system contained more telephone receiver is on-hook there is no current than 23,000 of these tubes which operated successfully throughout the year's trial. flow in the line, and a pulse applied to one wind­ ing produces a corresponding pulse in the other. any calls in progress, the No.1 ESS bus system However, if the receiver is taken off the hook, cur­ was devised. rent flows in the line and saturates the ferrite This system is simply a set of high-speed data stick. Thus, when the pulse is applied, the satura­ channels (actually, for reliability, each office has tion blocks the output pulse and the telephone is a duplicate set) that permanently interconnect known to be off-hook. all equipment units in an office. Electronic gates As the first article in this issue has pointed out, control the flow of data, selecting the particular No. 1 ESS has gone about as far as it is pos­ channel over which any equipment unit can send sible to go with common control: Only one group or receive. The gate settings can be changed in The feTrod, a magnetic curTent sensing of control circuits in the system is used to give microseconds in order to establish a new pattern device, is the basic element of ESS line telephone service at any time. However, to guard of information flow between the units. The buses scanneTS. Each telephone line is con­ against loss of service in the event of a fail­ are easily extended to new equipment that may be nected to a ferrod senSOT. FeTrods aTe ure, each office has twin control systems. Both added as the office grows. aTranged in 61,. TOWS, 16 to a TOW. EveTy keep up to the minute on the progress of all calls There is no question that electronic switching one tenth of a second, centml control in­ through the office because each actually processes has come of age. Indeed, the myriad changes be­ StTUCtS the scanner to check all the feT­ every call, although only one actually controls the tween Morris and Succasunna almost justify that 1'ods in a single TOW simultaneously. switching network. If the active control should this article be subtitled The Evolution of Elec­ The device is intTOduced at Succasunna, develop trouble, the standby is immediately tronic Switching. In Morris, electronic techniques switched in to take its place. The switching must were used as widely as possible to explore all their be done fast enough to keep from losing any calls. potential. The experience showed where the tech­ Relays performed the switching in the Morris niques were superior to electromechanical tech­ office. They required tens of milliseconds to op­ niques, where they were inadequate, and where erate and at this speed they occasionally caused a they just were not necessary. In the following ar­ Ovemll view of part of the switching network at Morris. dial pulse to be lost if a customer was dialing ticles we will turn from the ancestry and history The dots of light at Tight are tubes that weTe being during the changeover. To raise the switching of electronic systems to a direct consideration of used in talking connections when this pictuTe was taken. speed to the range required to avoid the loss of No.1 ESS itself.

208 Bell Laborato?'ies Record June 1965 209 Although No.1 ESS brings many new eight numbers he calls frequently, or he can have features and services to telephone customers, a list of 30 numbers that require 4-digit codes. it must also compete with existing systems. TOUCH-TONE® telephones will have an extra Stored program control puts No.1 ESS (eleventh) button to replace the first 2 digits of ahead of today's systems in many ways by each code which are the fixed prefix "11". expanding established services and giving Dial Conference lets the customer control his them a new versatility. own conference hookup without the services of an operator. He dials a code that results in the con­ nection of his line to a 4-port conference trunk, then dials two or three telephone numbers which are connected to the trunk in turn. Add-on, a modified form of conferencing, lets a customer turn a 2-way conversation into a 3-way conference. A momentary "flash" of the switch­ hook brings dial tone and the customer dials Features and Services the code digit "2" and the number he wishes to The master control center at the Succasunna No.1 ESS office. "add-on". His first call is automatically held and Controls and lamps on the panels at the far 1'ight are used for all three lines are connected in a conference trunk. various maintenance and administrative functions. The tele­ J. J. Yostpille Automatic Transfer and Preset Automatic typewriter supplements the panel controls. The tape reels at Transfer, variations on one theme, permit a the left are used for automatic message accounting recordings. customer who is visiting friends for the evening, ROM THE PRINCIPLES of telephony discovered ity it derives primarily from the high-speed cen­ or making a business call, to reroute incoming in a Boston laboratory 90 years ago, to the in­ tral control which is directed by the program. F calls from his telephone to his host's or his col­ vestigations leading to the development of No.1 Maintenance and administration are more easily porary memory together with the number he league's telephone if it is in the same local area. ESS, the progress of the communications indus­ performed than on any other system. Guided by wishes calls transferred to. The system refers to For Automatic Transfer, the customer dials a try has been linked tightly to research. We have maintenance programs, the system continually the temporary memory when a call comes in to the code and the number he wishes calls tran ferred seen how various streams of research came to­ checks its own internal condition and, over a tele­ customer's number, and instead of completing a to. For Preset Automatic Transfer, he has a list gether in o. 1 ESS to produce, at their conflu­ typewriter, reports any discovered faults and call there it reroutes it to the transfer number. of eight numbers, each represented by an abbre­ ence, a switching system that embodies a major their locations. Most maintenance jobs are thereby If a customer with Add-on flashes his switch­ viated code. To transfer calls to any number on conceptual advance in telephony-stored program reduced to a matter of replacing faulty circuit hook for a period less than 1.5 seconds while he is the list, he merely dials the appropriate code. operation. What are the practical results? packages. Administrative personnel can commu­ in a talking connection the system interprets it as To the Bell System customer, No.1 ESS may nicate with the machine over the teletypewriter These are only a few samples of the many serv­ a request for this service. Central control, under mean a whole new range of optional services de­ and instruct it to change, cancel, or add to infor­ ices No.1 ESS can provide. Their execution is direction of the program, then connects the cus­ pending on the results of customer trials. With mation in its memory that is pertinent to growth based on a continuous exchange of information be­ tomer who flashed to a dial pulse receiver and these services he can reach a 7- or 10-digit number or additions to an office. tween the switching network and central control, holds the second party. The customer then dials by dialing only 2 or 3 digits, he can arrange a tele­ In all these features, the program can be viewed and between central control and the temporary and the party he wishes to add and the system estab­ phone conference simply by dialing two or three as the moderator in a dialogue between the cus­ semipermanent memories. For example, abbrevi­ lishes a 3-way connection in the network. other conferees, he can have calls routed from his tomer and the system, and between maintenance ated codes are not contractions of the actual tele­ These special services can be arranged in elec­ own telephone to any other nearby telephone by or administrative people and the system. Each phone numbers, but consist of the 2-digit prefix tromechanical offices by adding special equipment dialing a short code. To the Telephone Operating special service, for example, is delineated in a se­ "11", and any third digit. When the customer dials which, in some cases, may be electronic. Pos­ Company, these and other services we will discuss quence of actions in the program. A customer the code, it is stored in the temporary memory. sibly an even better measure of the versatility of j n this article are commercial assets, but they are starts the sequence by dialing a 2- or 3-digit code (See the drawing on page 213.) Central control is stored program operation lies in the flexibility it only the public face of No.1 ESS. Behind that which is referred to the program by central con­ then directed by the program to examine and in­ provides for some of the most commonplace fea­ there are the hard practical requirements that a trol. The code is recognized as the signal for a terpret the code. The program, recognizes tures of a telephone switching system. Take the new telephone system must be economically com­ special routine and central control, directed by the the prefix followed by the third digit as an Ab­ example of a hunting group. This is a familiar petitive with existing ones, and it should be easy program, sets it in motion. Some routines are breviated Dialing code. Alternatively, it recog­ service found in most business and PBX systems to maintain and to administer. No.1 ESS has sev­ started by a momentary "flash" of the switchhook nizes the multifrequency TOUCH-TONE signal in which an incoming call that encounters a busy eral things in its favor along these lines and each instead of a code. Five optional services are being of the eleventh button as equivalent to the prefix. line is routed to another line, then to another if reflects the versatility of stored program opera­ tried on an experimental basis by 200 selected cus­ A list stored in the memory translates between the the second is busy, and so on until a connection is tion for a telephone system. tomers in the Succasunna office of No.1 ESS. They code and the wanted telephone number. Central made or all lines in the group are found busy. One thing in the system's economic favor is that are called Abbreviated Dialing, Dial Conference, control takes the full number from the semiper­ In electromechanical systems, this is a wired new services can be added by changing program Add-on, Automatic Transfer, and Preset Auto­ manent (program store) memory, transfers it to equipment function and, therefore, it has certain cards, a far more economical process than the ex­ matic Transfer. the temporary (call store) memory as if the limitations. A hunting group is usually restricted tensive rewiring and equipment modifications that Abbreviated Dialing replaces a 7- or 10-digit customer had dialed it, and directs the system to to certain blocks of lines and the hunting sequence accompany most changes in electromechanical number usually with a 2 or 3-digit code. A cus­ "dial" or outpulse the complete number. is arranged consecutively. In other words, if the offices. Another asset is that No. 1 ESS can serve tomer with an ordinary dial telephone can have a In the case of transfer service, a record of the first telephone number in the group, or block, is more customers than any other system, a capabil- list of eight abbreviated 3-digit codes to represent customer's transfer reque t is stored in the tem- number 1111, the second is number 1112, the third

210 Bell Laborato7'ies Record June 1965 211 in an area-and still others may be unrestricted. indications of normal or subnormal ervice. If The feature is also used to route and charge calls ervice falls below an acceptable standard, rou­ !f!i:~ on various kinds of customer service. Again, tine maintenance and similar low priority jobs are •"' , ...., SWITCHING SWITCHING TRUNK NETWORK NETWORK CIRCUIT wired equipment puts certain bounds on this fea­ dropped. Then, if further adjustment is neces­ ture for electromechanical offices. In No.5 cross­ sary, processing of new calls is deferred until bar offices, for instance, there can be only about call in progre s are completely processed_ Thus, 113 802-991-4312 100 classes of service. o. 1 ESS increases it ten­ line-load control is initiated by degree. E sential FERROO fold; a total of 1024 classes of service can be line (indicated by clas of service) receive pri­ LINE SCANNER stored in the memory. The large number of classes ority, but the system denies service to nonessen­ will be particularly useful in offices that elect large tial lines only at moment of extreme overload. numbers of features and services. Each special Thus, when line load control mu t be put into service can be given a class of service and then force, No.1 ESS still serve all essential calls and allowed on some lines and denied to others. as many others as it can. ervice i not flatly de­ Features and services can be added to No.1 ESS ni d to any line, and when the overload subside, PROGRAM offices almost without limit. They seldom curtail the ystem automatically return to normal. STORE the system's normal call-processing ability which All these features and services are delineated in 111 =219-423-3769 is affected mainly by traffic conditions. In any the program. Still, the e things and their ramifi­ 112= -366-2712 113=802-991-4312 given traffic situation, however, No. 1 ESS can cations require only about half the program con­ 114=818-4244 serve more customers than any other switching tent. The other half is concerned with mainte­ A implijied diagmm of the No.1 E S system showing the bilat­ III = II 11'1 II III eml exchange between the central control and the other elements. III = III lilt .11 system. The upper limits on No.1 ESS are 65,000 nance, for it is a simple but undeniable fact that 802-991-4312 lines and 100,000 tel phone numbers-party lines the mo t important task of a telephone switching Th t letypewriter is the instrument of communication between CALL STORE PROGRAM STORE account for the di crepancy. In an area with aver­ ystem is to keep running. A computer may break the system and the administrative and maintenance personn l. age calling rates-a suburban community like Suc­ down in the middle of a problem and the problem Any telephone call through No.1 ESS involves a constant ex­ ca unna, for instance-an office can serve the max­ may be rerun because all the vital data remain in change of information between central cont?"ol and the two mem­ imum number of lines. But in an area with a high the computer. But a switching sy tem breakdown fice. A 0.1 ES grow in th Bell y tem, it will O?"y blocks. Abbreviated dialing is illustmted here. The abbrevi­ calling rate, like Washington, D.C., No.1 ESS may is di a trous because reque t for ervice are ir­ be possible to trace interoffic calls automatically. reach its maximum call-carrying capacity with ated number "113" dialed by the custome?" is stored in the call revocably lost. Furthermore, it is a fair rule that ommunication over the teletypewriter between store memory. Central contTOl then ?"efers the number to the pro­ only about 30,000 lines. In this case, the calling the more complex a machine is, the more difficult admini trative personnel and th system eases gram memory which checks it against a list of eight possible rate determines the size of the office. it is to maintain, and o. 1 ESS i an e:\.'tremely many administrative tasks. Many of these task numbers. The full number represented by "113" (it may be 7 A rare event that does act to degrade service is complex electronic machine. Yet we have said that are recurrent one in all telephone offices. For or 10 digits) is transfe?"red to the call store memory where it an extreme overload on the system. The cause is No.1 ESS is easier to maintain than any existing example, translation data - which include such replaces the abbreviated number. Finally, the complete number usually external to the switching system-floods telephone system. A major rea on for thi is that thing a the location of a trunk group that may is sent through the switching network to outgoing trunks to the or hurricanes may damage the outside plant, for the ystem participates in it own maintenance. be u ed for an outgoing call, the number of digits distant office just as if the customer had dialed it in its entirety. example, severely reducing the number of trunks To accomplish thi, o. 1 ESS continually cru­ to be outpul ed, the equipment location of the available to an office and causing overloads. tinizes its own performance. If a fault develop, called line, the type of ringing sent to a party At such times, electromechanical offices may be the affected unit is switched out of service and a line, etc.-is frequently changed. In electro­ number 1113, etc. A call to number 1111 can hunt forced to invoke line-load control, completely cut­ duplicate takes over. At the same time, a diag­ mechanical switching system, this data is "re­ through the whole group if necessary, but a call ting off service for all but essential lines such as nostic program is called in to locate the circuit corded" in wired cross-connections and it is to number 1113 can only hunt to higher numbers, fire departments, police, and hospitals. Nonessen­ package causing the malfunction. The location of changed only by rewiring. In o. 1 ESS, transla­ it can not hunt back to the lower ones. tiallines can only receive calls. Manually operated the package is typed out on the teletypewriter so tion records are stored in the memory, and they In No.1 ESS, the lines in a group and the order switches govern the action. When they are that the majority of system trouble can be cor­ are chang d by typing the new information into of hunting are stored in the memory, the lines are thrown, all nonessential lines associated with them rected quickly and ea ily. the sy tem. Thi information an be stored and not associated through wiring. A call can be made are denied originating service and the action is This "dialogue" between man and machine is held in the temporary memory until the semiper­ to hunt through a group of arbitrary lines in any revoked only when the switches are reset. the key to all maintenance problem . It is often manent memory twistor card are changed. desired sequence. Alternatively, if the call is not Stored program control, on the other hand, can necessary, for example, to trace a call in order to At first thought, stored program control could completed in the first group it can be shifted to deal with degrees of overload, continually reor­ find a reported trouble on a connection, but the be con idered as a step that make the operation another and made to hunt through that. Custom­ ganizing and readjusting the system so that serv­ ferreed switch in the No.1 ES network make it of a witching ystem completely automatic_ How­ ers can operate control keys in their offices which ice continues at the most efficient level despite impo ible to visually follow a connection between ever, as we have een, it actually provides a means will cause the system to hunt in these patterns, or severe changes in internal or external circum­ line and trunk. However, the temporary memory of closer communication between the system and not hunt at all when that is desired. stances. The program recognizes a certain hier­ records the locations of all lines and trunks as 0­ the people who maintain and admini tel' it. In the Another example of how No.1 ESS imparts new archy in the jobs the system must do and acts ciated with a call in progress, and a pecial call final analysis, unique as o. 1 ESS is in the his­ versatility to an established feature is classes of always to maintain it by periodically measuring tracing program will render all these locations to tory of telephony, its design look to e sentially ervice. This tested and indispensable feature per­ the efficiency of service against a certain stand­ a maintenance man. He merely types in an in- the ame goal a the de ign of any telephone sys­ mits different treatment on PBX lines. Some can ard. The standard includes such things as permis­ truction to trace the call to a specified line. The tem. That goal, the ability to proce telephone be limited to local calls, others to toll calls within sible delay in returning dial tone, the number of system will identify the calling line or trunk. call rapidly, efficiently, and economically, has a limited area-even to a specified exchange with- calls blocked in the switching network, and other This feature is now limited to call within an of- been attained through stored program operation.

212 Bell Laboratories Record June 1965 213