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Survey of Telephone Switching
Chapter 11

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The Bell System serves upwards of 46 million telephones and averages 1,69 million local and long distance conversations a day. In dealing with quantities such as these, it does not take much imagination to appreciate that the accounting and billing problems are great. To carry on such a business the collection of earned revenue becomes most essential, and before this revenue can be collected bills must be prepared and presented to the customer. It is in the preparation of the data that is to be presented on the bills that Automatic Message Accounting (AMA) concerns itself.

Telephone companies have unique problems in preparing bills. They sell tailor-made service items to millions of customers. Each service item consists of putting temporarily at the customer's disposal, an impressive physical plant, and a skilled human organization to enable him to communicate at will with his family, friends or business associates, whether they be just around the corner or thousands of miles away. A very large proportion of these service items are of truly low price. Although the Bell System is a multibillion dollar concern, its revenue is derived primarily from service items, a great many of which cost only 10 cents or even less. There are, of course, items that bring 15, 20, 25 cents or higher, but even these fall in a group which can be classified as low priced service items. What runs up the system's total annual revenue into hundreds of millions of dollars is the enormous volume of these low priced service items. Therein lies the problems of recording, accounting for, and billing the customer.

Historical Background

Within the past few years, the Automatic Message Accounting system has been placed in commercial service by the Bell System in a number of metropolitan areas. These systems record in the central offices all the data required to charge for subscriber-dialed telephone messages, both local and toll, and mechanically process this record in accounting centers. Both the recording and processing arrangements employ many novel circuit and apparatus components. The use of the system will permit wide expansion of direct subscriber dialing to nearby and more remote points, with resultant increased speed, and convenience to telephone customers. Although AMA is the first system to carry out automatically both recording and accounting for toll calls, it is not the first step in this direction but is rather the culmination of a long line of developments.

During the early years of the telephone, subscribers were charged exclusively on a flat monthly or yearly basis but since the end of the last century, message rate service has been available in most of the larger cities. Initially the record of such calls was in the form of tickets prepared by operators. This method was later supplemented by the use of a small electro-magnetic counter, called a message register, associated with each message rate line and operated once for each call made. As the metropolitan areas grew larger and subscribers began to call regularly beyond their local areas, zone registration was adopted. It provides circuits that, on each call beyond the local area, operate the message register the proper number of times to represent the cost of the call. Thus if the charge on a call was 20 cents and each message register operation represented a charge of five cents, the register would be operated four times for this particular call.

Although zone registration is an economical method of charging for short calls, it does not leave any record of the details of the various calls. For calls requiring more than five message register operations, it has generally been felt desirable to have a record not only of the point to which the call was placed but of the day and time it was made. To secure such a record, and at the same time to obtain the economies and increased speed possible from automatic operation, an automatic ticketing arrangement was developed some years ago for step-by-step areas. With this system a ticket is automatically printed for each chargeable call, and thus all essential information pertaining to the call is permanently available.

The printing of a toll ticket, is only part of the work of charging for calls. Before subscribers can be billed for their calls, the tickets must be brought together and sorted out according to subscribers, computed, and totaled, and then the bill must be prepared, This work is extremely laborious and represents an appreciable item of expense.

It was recognized for some time that the entire process of recording calls and preparing the customer's bills could be done mechanically. One of the early suggestions was to provide an automatic accounting system that would supplement the automatic ticketing system, using the automatically prepared tickets as the basic information. Problems encountered, however, indicated that it would be desirable to record the information pertaining to the calls in a different manner.

In the automatic message accounting system, the information pertaining to all calls requiring a charge is perforated in code on an oil impregnated paper tape three inches wide. A specimen of the section of the tape after perforation is shown in Figure 11-1. 'There is space for twenty-eight holes across the tape, which is used for recording six digits, each representing a single item of information. Adjacent rows are about one-tenth inch apart, and either four, six, or seven rows of information are required per call. The items of perforated information are automatically read and interpreted at the accounting center at a rate of over 80 digits (15-1/2 reader cycles) per second.

The recording machines are installed in cabinets like those shown in Figure 11-2. They are associated with the outgoing trunks in the No. 5 crossbar and No. 1 step-by-step systems and with the district junctors in the No. I crossbar system. One recorder serves 100 trunks or district junctors. Together with their associated equipment, they are installed in the individual telephone central offices. Each day at about 3 a. m. the tapes in all the recorders are automatically prepared for cutting by perforating a readily recognized pattern to indicate the section where the tape is to be manually cut. After cutting they are transported to the accounting center for processing. This accounting center may handle the tapes from many central offices and may be remote from any of them.

Since each tape from a recorder includes the information for all calls handled by a group of 100 trunks or district junctors, calls from a particular subscriber, may be distributed over a number of tapes, and the information for any one particular call, although on a single tape, will not usually be on adjacent lines of the tape. Certain of the information is recorded as the call is dialed or shortly thereafter, but the time the called subscriber answers, which is the beginning of the charge period, is somewhat later, and in the meantime information relating to other calls may have been recorded on the same tape. The time the conversation is completed will in general be considerably later, and thus there will be information pertaining to many calls between the beginning and the ending of any particular call. Each tape, moreover, will include information on calls that are charged for in entirely different ways. For calls that are billed in bulk, it is not necessary to record the office and number of the called subscriber, since the duration of the call and other billing information provided in the call record is sufficient to determine the charge. This is the type of call that requires only four lines on the tape, while six or seven lines are required when information pertaining to the called subscriber must be recorded.

In the accounting centers, the work in general consists of automatically assembling the information pertaining to each call, computing the conversation time for each call, sorting as to type of call-that is, toll calls, message unit calls that are bulk billed, and miscellaneous types (straddle and observing)- and grouping the message unit calls according to the subscriber and then producing a printed or punched card output.

Central Office Features - Crossbar

A simple block diagram showing the various circuits associated with an AMA call in a No. 5 crossbar office is given in Figure 11-3. The digits dialed by the subscriber are temporarily stored in the originating register and then transferred to the marker as for a non-AMA call. The marker through the line and trunk link frames, prepares a path to an idle AMA trunk. It also transfers to an outgoing sender the information needed for completing the call and for making the AMA record. This includes the calling subscriber's line location and party identity, the number of the recorder serving the selected trunk, and a digit known as the message billing index. This digit is determined from the class of service of the calling subscriber together with the office to which his call is directed, and constitutes the basis for determining the charging rate on bulk billed calls where the called number is not recorded on the AMA tape. The sender now signals the transverter connector for connection to a transverter, and transfers to it all the foregoing information.

From the message billing index, the transverter determines whether the call is to be bulk billed on a message-unit basis or detail billed to include the called number. For a bulk billed call a two line entry is adequate, a detail billed home area call or foreign area call with code compression requires four lines, and a foreign area call without code compression requires five lines. The transverter then summons the translator and obtains from it the directory number corresponding to the calling subscriber's line and party assignments. The transverter also causes the call identity indexer to furnish the call identity index to the recorder. This completes the information required for the initial entry. Primed with this information, the transverter engages the recorder assigned to the outgoing trunk selected and causes it to operate the proper perforator magnets one line at a time. The actual perforation of the tape is accomplished by a perforator which is individual to the recorder circuits. This initial entry is normally recorded while the switching circuits are completing the connection to the called subscriber's line in a distant office.

The arrangement of the information on the central office tape for the more common entries is shown in Figure 11-4. Six digits are recorded in each line in positions designated A to F from left to right. For the A digit, one of the three punch positions is perforated to designate digits 0, 1, or 2, while for the digits B to F, two out of five punch positions are perforated to designate digits 0 to 9. The first few digits of each line comprise the entry index, and indicate the type of information the line contains.

At a central office, the lines are perforated in the order from top to bottom of Figure 11-4.
At the accounting center, on the other hand, the tapes are read in the opposite direction.

At the start of conversation, the call identity indexer, under control of the trunk, brings the recorder in on the connection a second time to perforate the time of answer. At the end of conversation,
the process is repeated to enter the time of disconnect.

The answer and disconnect entries show only the number of minutes past an hour and do not specify the hour. The hour information is obtained from the master timer and at the turn of each new hour is automatically recorded on the tape of each recorder.

The principal function of the master timer is to keep a running record of time in steps of six seconds. This record includes the month, day, hour, tens and units minutes digits, and the tenths of a minute digit. A running record of the time within each hour is delivered to each recorder where it is always available for the recording of answer and disconnect entries.

In No. 1 crossbar, the basic equipment and circuit features are similar but some of the details are different because of differences in the detailed circuit operations of the two systems. The AMA tapes are identical in the two types of offices, as indeed they would have to be to avoid the need for dissimilar handling at the accounting center. Figure 11-5 shows a block diagram of the principal circuits involved in AMA recording in a No. 1 crossbar office. ,In this system the subscriber's dialing is registered directly in the sender instead of initially in a register as in No. 5. Also the transmission circuit is a district junctor instead of a trunk. In both systems the sender seizes a transverter via a transverter connector, and the operations of the AMA circuits in recording the data on the tape are the same.

Centralized Automatic Message Accounting (CAMA)

Extended customer dialing as provided for by the original AMA equipment had certain limitations. For instance, there are many offices in areas where the dialable traffic to charge points is relatively light and the installation of AMA equipment would not be economical. Likewise, in offices actually equipped with AMA the equipment for the automatic identification of the calling customer for charge purposes recognized only individual and two-party customers. Four-party and multiparty customers must therefore place their charge calls through an operator. There are also the existing dial offices of the panel type where to design local AMA equipment to function with this equipment would prove rather costly, and even if available, might well be prohibited by the lack of floor space in the existing office. Centralized Automatic Message Ac.counting (CAMA) therefore, was developed to care for these subscribers not served by local AMA. It locates the recording equipment in a central spot and serves a number of central offices.

Broad plans have been prepared for CAMA using No. 5, 4A, and step-by-step as the centralized switching systems, and specific developments for its use with crossbar tandem offices have been completed. The first system of this type was put into service in Washington, D.C., in November, 1953, to serve the District of Columbia and nearby points in Maryland and Virginia.

CAMA at crossbar tandem offices was originally equipped to handle only calls originating in panel, No. I crossbar, and No. 5 crossbar central offices. All these offices can use both PCI - panel call indicator - and revertive outpulsing, but, since the number of digits that can be transmitted by revertive pulsing is restricted, the CAMA equipment has been designed to receive PCI. It will outpulse, however, with revertive, PCI, dial or multifrequency pulsing as needed. At present CAMA is designed to handle calls originating from a maximum of 200 central offices. In addition to the above PCI arrangement for panel and crossbar offices, crossbar tandem has been redesigned to handle CAMA calls from step-by-step offices on a dial pulse basis.

The operation of CAMA for the PCI arrangement, as illustrated in Figure 11-6, is basically quite similar to that of local AMA. In both cases, a sender transmits the required information about a call from the trunk to a transverter. In addition, the transverter receives the billing information from a billing indexer which compares the calling and called codes and determines the charge. It then forwards all its information to a recorder. The trunks also inform the recorder through a call identity indexer when a call is answered and when it is disconnected. The recorder then directs a perforator to include all this information on a punched tape for use by the accounting center in compiling customers' bills.

Many of the pieces of equipment indicated in Figure 11-6, including the transverter, call identity indexer, the recorder, the perforator, and the master timer, are similar to those used in local AMA. In addition to the work of local AMA, however, CAMA has certain other problems to solve. In the local equipment, one marker group operates into the AMA unit which must then be able to identify as many as ten calling office designations. CAMA has the additional problems of identifying one of a possible 20 recorder groups with a maximum of 200 offices. These offices may have widely differing rate treatments even though the actual charge on a particular call as indicated by the billing indexer is one of only nine. Two central offices, for example, may have identical rates on calls to 25 other offices but different rates on calls to the 26th and the CAMA equipment must provide for the various combinations.

Identification of the calling customer, is necessary for proper billing in any automatic system. The types of identification circuits used in local AMA and automatic ticketing are impracticable for use in this application. In the early CAMA installation, therefore, identification of the calling customer by an operator will be used. Shortly after the train of pulses representing a called number begins to enter it, a sender in the equipment will signal for an operator position. This signal is timed so that, on the average, an operator will be available just at the end of the pulsing. If the office load is high and an operator is not immediately available, however, the customer will hear a ringing signal to assure him that the call will be handled. When an operator is cut into the connection, a tone tells her to request the calling number. She will key the number into a register in the CAMA sender circuit, and with this information, the final output of the system will be punched tape record identical to that produced in the local AMA office.

Active development work directed toward automatic identification of the calling subscriber on one and two-party lines is now in progress. When perfected, this equipment will reduce the load on the CAMA switchboard, which will then be used primarily for the identification of subscribers of multiparty lines. The new equipment has been designed with provisions to include automatic identification and it is expected that automatic and operator identification will operate side by side. The operator will then handle calls from very small offices and from four party or multiparty lines. Automatic party line identification of four or more parties, when developed, will further reduce. the load on the operator.

Step-by-Step Automatic Ticketing

Automatic ticketing equipment provides facilities in No. 1 step-by-step offices to permit subscribers to dial certain calls that would otherwise be completed and ticketed by operators. It prints a ticket automatically with the calling subscriber's number, elapsed conversation time, rate of charge, called office and number and other pertinent information required for commercial, traffic and plant use. A general schematic of the automatic ticketing equipment is shown in Figure 11-7. Automatically ticketed calls are restricted to individual and 2-party non-coin lines.

Access to an automatic ticketing trunk may be obtained from the levels of the first, second or third selectors either directly or through rotary out trunk switches. The trunk contains the message ticketer which will print the ticket from which the bill is made. Upon seizure, the trunk connects to a sender and registers the first digit dialed after trunk seizure. This is done to prevent loss or mutilation of a digit while the sender is being connected. The sender seizes an identifier, registers the remaining digits dialed and coordinates subsequent functioning of the ticketing equipment.

The identifier has two general functions to perform on an automatically ticketed call. First to reconstruct the called office code (1, 2 or 3 digits were used in reaching the trunk) and give the sender the necessary information for completing selections to the called office; secondly, to identify the calling subscriber's number and class, required by the sender to complete its ticketing functions.

To determine the called office code (Figure 11-8) the identifier, under control of the sender, is connected to the trunk through an identifier trunk connector. The identifier records the digit registered in the trunk and the code connector to be used. It then seizes the proper code connector to determine the selector level dialed to seize the ticketing trunk. Several different selector levels may lead to the same ticketing trunk, however the path employed represents a specific series of digits dialed. Associated with each selector level appearance of the trunk and that portion of ROTS bank on which it appears is a 'IC" lead which is extended to a code connector. When the identifier operates the proper code connector it obtains access to all the 'IC" leads associated with the trunk involved. A D.C. path is thus set up from the identifier through the identifier trunk connector, through the trunk circuit, through the 'IC" lead as determined by the selector level or ROTS through the code connector back to the identifier where it is associated with the trunk digit and sender first digit to operate the particular route relay in the identifier corresponding to the complete called office code. The route relay provides the sender with the necessary routing information to complete the call.

To obtain the calling subscriber's number and class (Figure 11-9) the identifier sends an A-C signal of a specified frequency over the sleeve of the ticketing trunk through the selector, line finder and subscribers line cross-connection to a network associated with the subscriber's connector terminal. These networks are located on the thousand number frame and are cross-connected for presentation to the identifier through tip, ring or denied service coils, and group connector relays on the common number and class frame. The subscriber's number is determined by searching first through the thousands groups, then the hundreds, tens and units, each search narrowing the number to a unit of ten. Each step in the identification results in an associated registration leaving a complete record in the identifier of the office unit, thousands, hundreds, tens and units. The class of call is determined in a similar manner by searching for the same tone on the class network and coils. Since each identifier has its own specific frequency for the A-C signal simultaneous identification may be made by all identifiers. This information is also passed to the sender, and the identifier and associated connections are then released.

The sender now has complete information for. making selections in the distant office and for ticketing the call. These functions of the sender are independent of each other and in general are performed concurrently.

The sender, to control the printing of a ticket, connects itself to the message ticketer in the trunk through the sender trunk connector. The information stored in the sender plus the month, day and hour provided by a day and hour circuit allows the sender to print the entire ticket with the exception of the conversation time. The conversation time is under control of a timer in the trunk and is printed on the ticket at the termination of the call. After completing the connection to the called office and printing its portion of the ticket the sender and its associated equipment are released.


The tickets produced by the automatic ticketing equipment have recorded charging data equivalent to that written by an operator and require manual processing by the accounting department. To reduce this manual operation and to provide for foreign area customer dialing, an AMA system for step-by-step offices has been developed.

The flexibility of this arrangement is such that it can be introduced with substantial savings in offices in which the existing automatic ticketing equipment is modified and with somewhat greater savings if installed in new offices. The following are some of the attractive features of the system:

(a) It permits mixing detail billed short haul and long haul toll calls with bulk billed message unit business without affecting the relative cheap handling of the bulk billed calls.

(b) Trunks can be routed directly to the terminating step-by-step office or through tandem or toll offices as traffic designates without changes in equipment in these offices.

(c) It uses the same accounting center equipment as the No. I and No. 5 crossbar.

(d) The automatic ticketing equipment can be readily converted a small part at a time to the new arrangement while continuing to carry traffic.

(e) Nationwide customer dialing can readily be accommodated.

Figure 11-10 is a block diagram of AMA equipment for step-by-step offices. The operation of this equipment is a combination of automatic ticketing and crossbar AMA. The called code is reconstructed, the calling subscriber's line number, the called number and routing are handled the same as in automatic ticketing. The recording of the call on the tape instead of a ticket follows in general the crossbar AMA.

After the central office tape is produced it is processed at the AMA center in a manner identical to those received from crossbar offices.

AMA Center

Although the charging information for telephone calls is recorded in the central offices, all the work of telephone message pricing, billing, and bookkeeping for the Bell System is concentrated in the accounting centers (Figure 11-11), their related punched card toll units and revenue accounting offices.

In addition to its magnitude, this message accounting job has, of course, very exacting requirements of accuracy and promptness. Perhaps the most challenging factor in message accounting is, however, the need for the highest efficiency, since the accounting costs must add but a negligible amount to the low price of billable telephone messages.

These basic characteristics of the message accounting job had a controlling influence on the design of the machinery for the AMA center. The original message data for the AMA centers system are recorded in central offices as patterns of perforations on paper tapes. These tapes are then sent periodically to the accounting center where machinery performs the various data-processing tasks.

Processing Order

The processing technique is based on putting the central office data sequentially through different types of machines. The machines as originally designed for the accounting center served a single purpose. Later some ' of the machines were combined thus reducing the amount of intermediate tape handling. For simplicity our description of the tape processing will be based on the single purpose machines.

Figure 11-12 shows the machine and arrangements for the early accounting centers. It has practical limitations on printing toll calls requiring detail billing and so limits the foreign areas that customers could dial. The processing under this arrangement consists of assembling, computing, sorting, summarizing for message unit tapes, and printing. In each of these stages the operation consists of feeding tape into a machine in the proper order, collecting the output tape, storing them when necessary and arranging them for input to the next stage. In the last or printer stage the output is in printed form. After the printer the customer's bill is completed in the Revenue Accounting Office.

A machine known as the Tape-to-Card Converter has been developed and introduced as shown in Figure 11-13. It removes the limitation on detail billing and makes possible full foreign area.customer dialing. When it is used to process toll messages the computer output toll tape is fed directly to the converter and a punched card is produced for each message. The card for each subscriber's monthly message unit usage can also be obtained by processing the message unit summary tape through the converter. These cards are then processed by commercial business machines, including the printing of toll statements and the computation of the subscriber's message unit charges for the billing month.

The operations performed in any of these stages are controlled by manually setting the control panel switches of the accounting sender machine involved. The leading end of the tape contains identification information which should correspond to the setting of this control panel. The machine checks the tape identification against this setting before the operation can proceed.

The tapes as received from the central office carry the billing data for calls in the order that the data became available. The data for a particular call are interspersed with corresponding data on other calls. The assembler collects all the recorded data on each call so that subsequent processing can deal with complete information per call.

Assembling is basically a two-stage sorting process. The fragments of information belonging to a particular call are identified by a two-digit call identity index number and their time sequence on the central office tape. The assembler (Figure 11-14) sorts the fragments in accordance with these call identity indexes without disturbing the chronological order of the calls. The equipment is contained in a relay cabinet, a reader cabinet and five perforator cabinets for ten perforators.

After the calls have been assembled, the required computations can be conveniently carried out. This is done by the computer (Figure 11-15) into which are fed the output tapes of the assembler. For toll or detail-billed messages the computer calculates the "chargeable interval" in minutes. For local or bulk-billed messages, the computer evaluates the chargeable time into "message units", in accordance with the applicable rate structure. Uncompleted calls are automatically discarded. The computer perforates separate output tapes for the local or bulk-billed messages and for the toll or detail-billed messages. It also perforates other miscellaneous output tapes for calls whose segregation is desired for various reasons. The equipment is contained in four relay cabinets, a reader cabinet and seven perforator cabinets for fourteen perforators.

The assembling and computing operation have now been combined into a single machine known as the assembler-computer (Figure 11-16). The output tape from this machine is the same as that of the computer except the, straddle tape. This tape when processed by the AMA printer results in a different form of the printed straddle page. The assembler-computer can be used for additions and replacements in existing centers and requires only one pass of the central office tape through the reader to complete all the assembling and computing functions. The equipment is contained in eleven relay cabinets, a reader cabinet, and seven perforator cabinets for fourteen perforators. The readers and perforators are the high-speed type (25 reader cycles per second). This machine has a capacity Equivalent to about three assemblers and three computers.

The computed messages are next arranged by the sorter (Figure 11-17) according to the telephone numbers for the calling customers. On the output tapes of this machine the calling customers' telephone numbers appear in ascending order, and all the messages originated by the same customer are perforated adjacent to each other. Sorting is a four-stage process, one stage of sorting being required for each of the four digits of a telephone number. The equipment is contained in a relay cabinet, a reader cabinet and five perforator cabinets for ten perforators.

After all the local messages chargeable to a customer have thus been brought together, it is necessary to summarize their message unit values for bulk billing. This is accomplished by the summarizer, (Figure 11-18) which perforates on its output tape the total number of message units chargeable to each . customer for a designated period. It is contained in two relay cabinets, a reader cabinet and a perforator cabinet, for one perforator.

When the tape-to-card converter is used the four sorting stages and printer employed for toll calls, and the printer for message unit tapes, are eliminated. The tape-to-card converter (Figure 11-19) takes over the output from the computer or summarizer. This machine converts the information on toll and observing tapes from the computer and assembler-computer into IBM cards, one card per call. It also converts the information on summary tapes into cards, one card per summary. The toll cards are processed in a punched card machine system where they are sorted by called place, calculated and punched with rate and called place information, sorted by calling line number and finally used in a tabulator for printing the customer toll statement.

The printer (Figure 11-20) presents the output of the AMA tapes in a form readily understandable by human beings. It can be used to print message unit summary, toll slips, straddle records, intermediate round summaries and (where FACD calls are not involved) observing records. It is contained in three or four relay cabinets, one reader cabinet and one printer cabinet.

The printer-comparer-scanner (Figure 11-21) is a combined accounting and maintenance machine. As its name implies it can be used for printing, for comparing tapes and for scanning or searching for specific lines on an AMA tape.

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