Fig. 441. Magneto Intercommunicating SystemView full size illustration.
This system has the advantage of great simplicity and of being about as "fool proof" as possible. It is, however, not quite as convenient to use as the later common-battery systems which require no turning of a generator crank.
Common-Battery Systems.In the more popular common-battery systems two general plans of operation are in vogue, one employing a plug and jacks at each station for switching the "home" instrument into circuit with any line, and the other employing merelypush buttons for doing the same thing. These may be referred to as the plug type and the push-button type, respectively.
Fig. 442. Plug Type of Common-Battery Intercommunicating SystemView full size illustration.
Kellogg Plug Type.The circuits of a plug type of intercommunicating system, as manufactured by the Kellogg Company, are shown in Fig. 442. While only three stations are shown, the method of connecting more will be obvious.
This system requires as many pairs of wires running to all stations as there are stations, and in addition, two common wires for ringing purposes. The talking battery feed is through retardation coils to each line. When all the hooks are down, each call bell is connected between the lower common wire and the tip side of the talking circuit individual to the corresponding station. The ringing buttons at each station are connected between the tip of the plug at that station and the upper common wire. As a result, when a person at one station desires to call another, it is only necessary for him toinsert his plug in the jack of the desired station and press his ringing button; the circuit being traced from one pole of the ringing battery through the upper common ringing wire, ringing key of the station making the call, tip of plug, tip conductor of called station's line, bell of called station, and back to the ringing battery through the lower common ringing wire.
Fig. 443. Push-Button Wall SetView full size illustration.
Kellogg Push-Button Type.Fig. 443 shows a Kellogg wall-type intercommunicating set employing the push-button method of selecting, and Fig. 444 shows the internal arrangement of this set.
Fig. 444. Push-Button Wall SetView full size illustration.
Western Electric System.The method of operation of the push-button key employed in the intercommunicating system of the Western Electric Company is well shown in Fig. 445. When the button is depressed all the way down, as shown in the center cut of Fig. 445, which represents the ringing position of the key, contact is made with the line wires of the station called, and ringing current is placed on the line. When the pressure is released, the button assumes an intermediate position, as shown in the right-hand cut, which represents the talking position of the key and in which the ringing contacts1and2are open, but contact with the line for talking purposes is maintained. The key is automatically held in this intermediate position by locking plate3until this plate is actuated by the operation of another button which releases the key so that it assumes its normal position as shown in the left-hand cut. When a button is depressed to call a station, it first connects the called station's line to the calling station through the two pairs of contacts4and5and then connects the ringing battery to that line by causing the spring1to engage the contact2. The ringing current then passes through the bell at the called station, through the back contacts of the switch hook at that station, over one side of the line, and through the "way-down" contact1of the button at the calling station, thence over the other side of the battery line back to the ringing battery, operating the bell at the called station.
Fig. 445. Push-Button Action, Western Electric SystemView full size illustration.
The circuits of the Western Electric system are similar to those of Fig. 442, but adapted, of course, to the push-button arrangement of switches. Two batteries are employed, one for ringing and theother for talking, talking current being fed to the lines through retardation coils to prevent interference or cross-talk from other stations which might be connected together at the same time.
Monarch System.As the making of connections in an intercommunicating system is entirely in the hands of the user, it is desirable that the operation be simple and that carelessness on the part of the user result in as few evil effects as possible. For instance, the leaving of the receiver off its hook will, in many systems, result in such a drain on the battery as to greatly shorten its life.
The system of the Monarch Company has certain distinctive features in this respect. It is of the push-button type and as in the system just discussed, one pressure of the finger on one button clears the station of previous connections, rings the station called, and establishes a talking connection between the caller's telephone and the line desired. In addition to this, the system is designed to eliminate battery waste by so arranging the circuits that the battery current does not flow through either called or calling instrument until a complete connection is made—the calling button down at one station, the home button down at the called station, and both receivers off the hook. It does not hurt the batteries, therefore, if one neglects to hang up his receiver.
Fig. 446. Push-Button Wall SetView full size illustration.
Fig. 447. Push-Button Action, Monarch SystemView full size illustration.
Three views of the wall set of this system are shown in Fig. 446, which illustrates how both the door and the containing box are separately hinged for easy access to the apparatus and connecting rack. As in the Western Electric and Kellogg push-button systems, each push-button key has three positions, as shown in Fig. 447. The first button shows all the springs open, the normal position of the key. The second button is in the half-way or talking position with all the springs, except the ringing spring, in contact. The third button shows the springs all in contact, the condition which exists when ringing a station.
The mechanical construction of the key is shown in Fig. 448. Each button has a separate frame upon which the springs are mounted.Any one of the frames with its group of contact springs may be removed without interfering with either the electrical or the mechanical operation of the others. This is a convenient feature, making possible the installation of as few stations as are needed at first, and the subsequent addition of buttons as other stations are added.
Fig. 448. Push-Button KeysView full size illustration.
The restoring feature is a horizontal metal carriage, in construction very much like a ladder—one round pressing against each key frame, due to the tension on the carriage exerted by a single flat spring. The plunger of each button is equipped with a shoulder, which normally is above the round of the ladder. When the button is operated, this shoulder presses against a round of the carriage forcing it over far enough so that the shoulder can slip by. The upper surface of the shoulder is flat, and on passing below the pin, allows the carriage to slip back into its normal position and the pin rests on the top of the shoulder holding the plunger down. This position places the talking springs in contact. The ringing springs are open until the plunger is pressed all the way down, then the ringing contact is made. When the pressure is released, the plunger comes back to the half-way or talking position, leaving the ringing contacts open again.
When another button is pressed, the same operation takes place and, by virtue of the carriage being temporarily displaced, the original key is left free to spring back to its normal position.
Each station is provided with a button for each other station and a "home" button. The salient feature of the system is that before a connection may be established, the button at the calling station corresponding to the station called and also the home button of the station called must be depressed, if it is not already down. The home key at any station, when depressed, transposes the sides of the line with respect to the talking apparatus. The home key also has a spring which changes the normal connection of the line at that station from the negative to the positive side of the talking battery. Unless, therefore, a connection between two stations is made through the calling key at one station and the home key at the other, no current can flow even though both receivers are off their hooks, because in that case no connection will exist with the positive side of the battery. This relation is shown in Fig. 449, which gives a simplified circuit arrangement for two connected stations.
Fig. 449. Monarch Intercommunicating SystemView full size illustration.
Referring to Fig. 449, when the station called depresses the home button the talking circuit is then completed after the hook switch is raised. This is because the talking battery is controlled by the home key. Conductors from both the negative and the positive sides of the battery enter this key. In the normal position of the springs, the negative side of the battery is in contact with the master spring in the home key and through these springs the negative battery is applied to all the calling keys, and from there on to the hook switch. When, however, the home button is operated, the springwhich carries the negative battery to the home key is opened, and the spring which carries the positive battery is closed. This puts the positive battery on at the hook switch instead of the negative battery, as in its normal condition.
In this system it is seen that a separate pair of line wires is used for each station, and in addition to these, two common pairs are run to all stations, one for ringing and one for talking battery connections.
For Private Branch Exchanges.So far the intercommunicating system has been discussed only with respect to its use in small isolated plants. It has a field of usefulness in connection with city exchange work, as it may be made to serve admirably as a private branch exchange. Where this is done, one or more trunk lines leading to an office of the city exchange are run through the intercommunicating system exactly as a local line in that system, being tapped to a jack or push button at every station. A person at any one of the stations may originate a call to the main office by inserting his plug in the trunk jack, or pushing his trunk push button. Also any station, within hearing or sight of the trunk-line signal from the main office, may answer a main-office call in the same way. In order that the convenience of a private branch exchange may be fully realized, however, it is customary to provide an attendant's station at which is placed the drop or bell on which the incoming trunk signal is received. The duty of this attendant during business hours is to answer trunk calls from the main office and finding out what party is desired, call up the proper station on the intercommunicating system. The party at that station may then connect himself with the trunk.
The practice of the Dean Company, for instance, is as follows in regard to trunking between intercommunicating systems and main offices with common-battery equipment. The attendant's station telephone cabinet contains, besides the push-button keys for local and trunk connections, a drop signal and release key, together with relays in each trunk circuit. The latter are used to hold the trunks until the desired party responds.
The main-exchange trunk lines, besides terminating at the attendant's station, are wired through the complete intercommunicating system so that any intercommunicating telephone can be connected direct to the central office by depressing the trunk key, which is providedwith a button of distinctive color. The pressing of the trunk key allows the telephone to take its current from the main-office storage battery and to operate the main-office line and supervisory signals direct, without making it necessary to call on the attendant to set up the connection.
Fig. 450. Junction BoxView full size illustration.
Fig. 451. Typical Arrangement of Intercommunicating SystemView full size illustration.
Incoming calls from the common-battery main office to the intercommunicating system are all handled by the attendant. The main-office operator signals the intercommunicating system by ringing, the same as for a regular subscriber's line. This will operate a drop in the attendant's station cabinet, and through an armature contact, give a signal on a low-pitched buzzer. This alarm buzzer operates only when the main exchange is ringing and, therefore, does not require that the drop shutter be restored immediately. An extra key may be provided for an extension night-alarm bell, for use where the attendant also does work in a room separate from that containing the attendant's station telephone equipment.
The attendant operator answers the main-line signal by pressing the proper trunk button, as designated by the operated drop on the attendant's cabinet. The answering of the trunk connects a locking relay across the circuit so that the attendant may call the desired party on the intercommunicating system without having to hold the trunk manually. The party desired is then notified which trunk to use and the attendant operator hangs up her receiver, no further attention being necessary on her part.
The trunk-holding relay is automatically released when the desired party (with the telephone receiver off the hook) depresses the proper trunk button, thus clearing the trunk line of all bridged apparatus and making the talking circuit the same as in the regular type of private branch-exchange switchboard.
The most convenient way of installing the wires of an intercommunicating system is to run a cable containing the proper number of pairs to provide for the ultimate number of stations to all the stations, tapping off from the conductors in the cable to the jacks or push buttons at each station. These tap connections are best made by means of junction boxes which contain terminals for all the conductors.
Such a junction box, with the through cable and the tap cable in place, is illustrated in Fig. 450. A schematic lay-out of the various parts of a Dean intercommunicating system, provided with an attendant's station and with trunks to a city office, is given in Fig. 451.
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Definitions.Telephone messages between communities are called long-distance messages. They are also called toll messages. Almost all long-distance traffic is handled by message-rate (measured-service) methods of charge. All measured-service messages are toll messages, whether they are completed within a given community or between communities. The term "long-distance," therefore, is more descriptive than the term "toll." The subject of local and long-distance measured service is treated exhaustively in a chapter of its own.
Some telephone-exchange operating companies call their own inter-city business "toll," and use the term "long-distance" for business carried between exchanges for them by another company. The distinction seems to be unwarranted.
Use of Repeating Coil.Most long-distance lines are magneto circuits. If they are switched to grounded circuits, repeating coils need to be inserted. Toll switching equipments contain means of inserting repeating coils in the connecting cords when required. Their use reduces the volume of transmitted speech, but often is essential even in connecting metallic circuit lines, as a quiet local metallic circuit may have a ground upon it which will cause excessive noises when a quiet long-distance line is connected to it.
Switching through Local Board.In the simplest form of long-distance switching, the lines terminate in switchboards with local lines and may be connected with each other and with the local lines through the regular cord circuits, if the equipment be of the magneto type. The waystations on such a line are equipped with magneto generators. These waystations may signal each other by bell ringing; the central office may call any waystation by ringing the proper signal and may supervise in a way all traffic on such lines by noting the calls for other stations than the supervising exchange.
Operators' Orders.By Call Circuits.Where the long-distance traffic between two communities is large, economy requires that the sending of signals by ringing over the line, waiting for an answer, and then reciting the details of the call, be improved upon. If the traffic is large and the distance between communities small, call circuits are established in the same way as between the switchboards in several manual central offices of an exchange. The long-distance operator handling the originating call passes the necessary details to the distant operator by telephone over the call circuit. Such circuits also are known as order circuits. They are accessible to originating operators at keys and are connected directly and permanently to the telephone sets of receiving operators. One call circuit can handle the orders for a large number of actual conversation circuits. The operator at the receiving end designates the conversation circuit which shall be used, the originating operator following that instruction.
By Telegraph.Where traffic and distance are large, conversation lines cost more than in the case last assumed. It then is of greater importance to use all the possible talking circuits for actual conversations in order that the revenue may be as high as possible. A phantom circuit good enough for call circuit purposes would be good enough for actual commercial messages, therefore, it is customary to furnish such originating and receiving operators with Morse telegraph sets. The lines are obtained by applying composite apparatus to the conversation circuits. Two Morse circuits can be had from each long-distance line without impairing any quality of that line except the ability to ring over it. As one Morse circuit can carry information enough between two operators to enable them to keep many telephone circuits busy, they do not need to ring upon the composited lines, so that nothing is lost while revenue is gained.
Two-Number Calls.In cases where the traffic between communities is large, where the rate is small, and where the conversations are short and more on the general order of local calls, it is usual to handle the switches exactly as local calls are trunked between central offices of the same exchange. That is, the subscriber's operator who answers the call trunks it, by the assistance of a call circuit and an incoming trunk operator. The subscriber's operator records only the numbers of the calling and called subscribers.No long-distance operators at all assist in these connections. They are known as "two-number calls." The calling subscriber remains at his telephone until the conversation is finished.
Particular-Party-Calls.In cases where the traffic is smaller, and where the rate is large, it is customary to handle the calls through long-distance operators. The ticket records the particular party wished, and the calls are named "particular party" calls. In such connections the calling patron is allowed to hang up his receiver, after his call is recorded, and is called again when his correspondent is found and is ready to talk. This makesall calls for conversationsoutgoing ones. Only recording operators receive callsfrompatrons. Line operators make callstopatrons.
Trunking.Long-distance lines entering a city usually terminate in one office only, no matter how many offices the local exchange may have. It is possible to terminate these long-distance lines on a position of the multiple switchboard for local lines. For a variety of reasons this is not practiced except in special cases. The usual method is to terminate them in a special long-distance board and to provide trunk lines from this board to the one or more local switchboards of the exchange. In common-battery systems these toll trunks are so arranged that the called local subscriber receives transmitter current from the office nearest to him, yet is able to show the long-distance operator the position of his switch hook and is able to be called by the long-distance operator without the intervention of the switching operator in the local office, even though two repeating coils may be in the trunk circuit.
Through Ringing.There is a distinct traffic advantage in having the ringing of the subscriber under the control of the long-distance operator. The latter may call for the subscriber by stating her wish over the call circuit associated with the long-distance trunk. The connection having been made by the switching operator, the long-distance operator may withhold ringing the subscriber's bell until all is in readiness for the conversation.
High-Voltage Toll Trunks.In some systems, the long-distance trunks are further specialized by being enabled to furnish transmitter current to subscribers at a higher voltage than is used in local conversations. With a given construction of transmitters there is a critical maximum current which can be carried by thegranular carbon of the instrument without excessive heating, consequent noises, and permanent damage. The shortest lines and the longest lines of an exchange district being served by a source of current common to all, the standard potential of this source must be such as to give the longest lines current enough without giving the shortest lines too much. The very longest local lines, however, do not receive current enough from the standard potential to give maximum efficiency when talking over long distances, though they get enough for local conversations. By providing a battery with a voltage twice that used for local conversations and connecting it into the current supply element of the toll trunk through non-inductive resistances, not too much current may be given to the shortest lines and considerably more than normal current to the longest lines.
Ticket Passing.When only one operator is necessary in a town, her duty being to switch both local and long-distance lines, she may write her own tickets and execute them entire. In larger communities with larger long-distance traffic, the duties need to be specialized. The subscribers' wants as to long-distance connections are given by themselves to recording long-distance operators, who write them on tickets and pass these to operators who get the parties together. The problem of ticket-passing becomes important and many mechanical carriers have been tried, culminating in the system which utilizes vacuum tubes. This is in some ways similar to vacuum or compressed-air tube systems for carrying cash in retail stores. The ticket is carried, however, without any enclosing case and the tubes are flat instead of round,i. e., they are rectangular in section. By suitable means a vacuum is maintained in a large common tube having a tap to a box-like valve at each line operator's position. A ticket tube connects this valve with a distributing table at or near which the tickets are written. The tickets are of uniform size and are so made as to enable a flap to be bent up easily along one edge. The distributing operator has merely to insert the ticket, bent edge foremost, in the open end of the tube, whereupon the air pressure behind it will drive it through to its destination, near by or far away. The tickets travel thirty feet a second. The tube may be bent into almost any required form. The ticket, on arriving at a line operator's position, slides between two springs, breaking a shunt around a relay and allowing the latter to light the lamp.
Waystations.Waystations on long-distance lines may be equipped in several ways. Most of them have magneto sets and can ring each other. Some are equipped with common-battery sets and get all current for signaling and transmission from a terminal central office. In the latter case, there is the advantage that the ringers are in series with condensers, assisting greatly in tests for fault locations. Such tests are hindered by the presence of ringer bridges across the line, as in magneto practice. Condensers can be inserted in series with ringers of magneto sets if the testing advantage is valued highly enough. A disadvantage of the use of common-battery sets in waystations on long-distance lines is the lessened transmission volume of the stations farthest from the current source.
Center Checking.An operating advantage of common-battery sets on long-distance lines is that all calls are forced to be answered by the terminal station. Waystations can not call each other, as they have no calling means. With magneto sets, waystation agents sometimes call each other direct and neglect to record the call and to remit its price. When they can not call each other direct, the revenues of the company increase.
A traffic method which requires all calls from waystations to be made to a central switching office is called a center-checking system. It is so called because all checking for stations so switched is done at the central point instead of each waystation keeping its own records of calls sent and received. In such practice it is usual to bill each station once a month for the messages it sent. Where center checking is not practiced, the agent makes a report and sends a remittance. Center checking comes about naturally for waystations having no ringing equipment.
Center checking originated long before the invention of common-battery systems. It requires merely that no waystation shall have a generator which can ring a bell. The method most widely used is to equip the waystations with magneto generators which produce direct currents only; such a generator cannot operate a polarized ringer. It is not usual to produce the direct current by actually rectifying the alternating current, but merely by omitting half the impulses, sending to the line only alternate half-cycles of the current generated. Any drop or relay adapted to respond to regular ringing current will respond to this modified form of generator.
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The term "traffic," with reference to telephone service, has come to mean the gross transaction of communication between telephone users. This traffic may be expressed in whatever terms are found convenient for the particular phase considered.
Unit of Traffic.With reference to payment for local telephone service, the conversation is the unit of traffic. In the daily operations of telephone systems there are fewer conversations than there are connections and fewer connections than there are calls, because lines are found busy and all calls to subscribers are not answered.
For these reasons, in traffic inquiries which have to do with the amount of business which subscribers attempt to transact, the total traffic in a given time usually is considered as so many calls originated by the subscribers in the community. From this condition arises the term "originating calls."
For the reason that the purpose of the switching equipment in a central office is to make connections, the abilities of operators and of equipments frequently are measured in terms of connections per hour or per other unit of time.
For the reason that in charging for service all unavailing calls are omitted, the conversation is the unit of traffic.
Traffic Variations.Telephone-exchange traffic is subject to such general variations as are noted in the way a compass needle points north, the migrations of birds, the blowing of the trade winds, and other natural phenomena. There are variations in traffic which occur each day, others which change with the seasons, and still others which are related to holidays and other special commercial and social events. For instance, the day before Thanksgiving Day, in many regions, is the busiest telephone traffic day in the year.
The daily variations in telephone traffic are closely related to commercial activities and certain general features of this dailyvariation are common to all telephone systems everywhere. Fig. 452 is a typical graphic record of the traffic of a telephone exchange and represents what happens in almost every town or city. The total calls in this figure are not given as absolute units but would vary to adapt the figure to a particular case. The figure shows principally that the traffic in the night is light; that it rises to its maximum height somewhere between 10 o'clocka.m.and noon; that though it is never as high again during that day, the afternoon peak is over 80 per cent as great; and that two minor peaks appear about the dinner hour and after evening entertainments.
Fig. 452. Load CurveView full size illustration.
Busy-Hour Ratio.If the story told by Fig. 452 were to be turned into a table of calls per hour, the busiest hour of the day would be found to correspond to the highest portion of the figure, and in that busiest hour of the day, if a number of selected days were to be compared, would be found a very constant traffic. The number of calls made, or the number of connections completed, in that particular hour, day by day, would be found to be much the same. The ratio of the number of units in that hour to the number of units in thatentire day would be found to be practically the same ratio day by day. This ratio of busy hour to total day would be found to be much more nearly constant than the gross number of calls per hour or per day.
In a large, busy city, about one-eighth of the total daily calls are in some one hour; in a smaller, less active city, probably one-tenth are so congested. This is reasonable when one remembers that in the larger city the active business of the day begins later and ends earlier.
Importance of Traffic Study.A knowledge of the amount of traffic in an exchange, and its distribution as to time and as to the divisions of the exchange, is important for a number of reasons. Traffic knowledge is essential in order that the equipment may be designed and placed in the proper way and the total load distributed properly on that apparatus and its operators.
For example, in an office equipped with a manual multiple switchboard, the length of the switchboard is governed entirely by the number of operators who must work before it. It is mechanically possible to make a switchboard for ten thousand lines only 15 feet long, seating seven operators. The entire multiple of ten thousand lines could appear three times in such a switchboard. The seven operators could not handle the traffic we know would be originated by ten thousand lines, with any present system of charging for service. Even a rough knowledge of the probable traffic would enable us to approximate the number of operators needed and to equip each position, not only with access to the ten thousand lines to be called, but also with just enough keyboard equipment, serving as tools, and just enough answering jacks, serving as means of bringing the traffic to her. It is foreknowledge of traffic which enables a switchboard to fit the task it is to perform.
Rates of Calling.The rates of calling of different kinds of lines vary. The lines of business stations originate more calls than do the lines of residences. Some kinds of business originate more calls than others. Some kinds of business have a higher rate of calling in one season than in others. Flat-rate lines originate more calls than do message-rate lines. When a line changes from a flat rate to a message rate, the number of originating calls per day decreases. An operator's position, handling message-rate lines only, can serve more lines than if all of them were at flat rates. The number of message-rateor coin-prepayment lines which an operator's position can care for depends not only on the traffic but on the method of charging for service, whether by tickets or meters and upon the kind of meters; or it depends on the method of collecting the coins. In some regions, the rate of calling, on the introduction of a complete measured-service plan, has been reduced to one-fourth of what it was on the flat-rate plan.
In manual switchboards of early types, wherein the position of the subscriber's answering jack was fixed by his telephone number, the inequality of traffic became a serious problem. Most of the subscribers who first installed telephones when the exchange was small, retained their telephones and numbers; as their use of the telephone grew with their business, it was customary to find the positions answering the lower numbers much more busy than the positions answering the higher numbers, the latter belonging to later and usually less active business places.
Functions of Intermediate Distributing Frame.The intermediate distributing board was invented to meet these conditions of unequal traffic upon lines and of variations in traffic with changes of seasons and of charges. The intermediate distributing board enables a line to retain its number and its position in the multiple, but to keep its answering jack and lamp signal in any desired position. If a flat-rate subscriber changes to a message rate, his line may be moved to a message-rate position and be answered, in company with others like it, by an operator serving many more lines than she could serve if all of them were flat rate.
Methods of Traffic Study.The best way to learn traffic facts for the purposes of designing and operating equipment is to conduct systematic series of observations in all exchanges; to record them in company with all related facts; and to compare them from time to time, recording the results of the comparisons. Then when it is required to solve a new problem, the traffic data will enable the probable future conditions to be known with as great exactness as is possible in studies with relation to transportation or any other human activity.
TABLE XIII
Calling Rates
Kind of ServiceCalls per Day with Different Methods of ChargeFlat RateMessage RateResidence84Business12 to 208 to 14Private Exchange Trunk4025Hotel Exchange Trunk5030Apartment House Trunk3018
There are three general ways of observing traffic. A record of originating calls is known as a "peg count," because the counting formerly was done by moving a peg from place to place in a series ofholes. The simplest exact way is to provide each operator with a small mechanical counter, the key of which she can depress once for each call to be counted. A second way is to determine a ratio which exists, for the particular time and place, between the number of calls in a given period and the average number of cord circuits in use. Knowing this ratio, the cord circuits can be counted, the ratio applied, and the probable total known. The third method, which is applicable to offices having service meters on all lines, is to associate one master meter per position or group of lines with all the meters of that position or group, so that each time any service meter of that position is operated, the master meter will count one unit. This method applies to either manual or automatic equipments.
Representative Traffic Data.For purposes of comparison, the following are representative facts as to certain traffic conditions.
Calling Rates.The number of calls originated per day by different kinds of lines with different methods of charge are shown in Table XIII.
Operators' Loads.The abilities of subscribers' operators to switch these calls depend on the type of equipment used, on the kind of management exercised, and on the individual skill of operators. With manual multiple equipment of the common-battery type, and good management, the numbers of originating calls per busy hour given in Table XIV can be handled by an average operator. The number of calls per operator per busy hour depends upon the amount of trunking to other offices which that operator is required to do. In a small city, for example, where all the lines are handled by one switchboard, there is no local switching problem except to completethe connection in the multiple before each position. In a large city, where wire economy and mechanical considerations compel the lines to be handled by a number of offices with manual equipment, some portion of the total originating load of each office must be trunked to others. Table XIV shows that an increase of 90 per cent in the amount of out-trunking has decreased the operator's ability to less than 70 per cent of the possible maximum.
TABLE XIV
Effect of Out-Trunking on Operator's Capacity
Per Cent Originating Calls Trunked To Other OfficesCapacity of Subscribers' Operator's Position in Calls Per Busy Hour02401023030200501857517090165
Trunking Factor.In providing the system of trunks interconnecting the offices, whether the equipment be manual or automatic, it is essential to know not only how much traffic originates in each office, but how much of it will be trunked to each other office and how many trunks will be required. An interesting phase of telephone traffic studies is that it is possible to determine in advance the amount of traffic which can be completed directly in the multiple of that office and how much must be trunked elsewhere. Theoretical considerations would indicate that if the local multiple contains one-eighth of the total lines of the city, one-eighth of the calls originating in that office could be completed locally and seven-eighths would be trunked out. In almost all cases, however, it is found that more than the theoretical percentage of originating calls are for the neighborhood of that office and can be completed in the multiple. This results in the determination of a factor by which the theoretical out-trunking can be multiplied to determine the probable real out-trunking. In most cases, the ratio of actual to theoretical out-trunking is 75 per cent, or approximately that. In special cases, it may be far from 75 per cent.
Trunk Efficiency.The capacities of trunks vary with their methods of operation and with the number of trunks in a group.For example, in the manual system where trunk operators in distant offices are instructed over call circuits and make disconnections in response to lamp signals, such an incoming trunk operator can complete from 250 to 500 connections per busy hour. The actual ability depends upon the number of distant offices served by that operator and upon the amount of work she has to perform on each call.
The number of messages which can be handled by one trunk in the busy hour will depend upon the number of trunks in the group and upon the system employed. It appears that the ability of trunks in this regard is higher in the automatic system than in the manual system. For the latter, Table XV gives representative facts.
TABLE XV
Messages per Trunk in Manual System
Number of Trunks in Group, Manual SystemMessages per Trunk per Busy Hour57109201240156018
Some of the reasons for the higher efficiencies of trunks in the automatic system are not well defined, but unquestionably exist. They have to do partly with the prompter answering observable in automatic systems. The operation of calling being simple, a called subscriber seems to fear that unless he answers promptly the calling party will disconnect and perhaps may call a competitor. The introduction of machine-ringing on automatic lines, where existing in competition with manual ringing on manual lines, seems to encourage subscribers to answer even more promptly. The length of conversation in automatic systems seems to be shorter than in manual systems. Still more important, disconnection in automatic systems is instantaneous during all hours, whereas in manual systems it is less prompt in the busiest and least busy hours than in the hours of intermediate congestion. The practical results of trunk efficiencies in automatic systems are given in Table XVI.
TABLE XVI
Messages per Trunk in Automatic System