Switching International Calls Via Submarine Cable
By J. Becker and R.D. Deming
Bell Laboratories Record, Volume 42, Number 7, July-August 1964 pages 234 to 239
(Scanned and OCR work performed by Tom Farley, http://www.privateline.com)
"Development of an automatic overseas switching system, compatible with switching systems of foreign countries, represents another step toward the attainment of worldwide customer dialing."
THE FIRST UNDERSEAS CABLE between Europe and the United States, designed for the transmission of voice frequencies, was put into operation in September, 1956 with facilities for 36 channels. Transmission was superior to the previously employed radio channels but the switching was still accomplished by the manual "ringdown" method. This method, in addition to requiring the services of an operator at each end of the overseas circuit, is more time consuming and prone to human error than automatic switching.
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"There are 100 overseas operator positions like this one at the Long Lines building in New York City, one of two gateway offices on the eastern seaboard set up to handle European calls."
As a result, development of an automatic overseas switching system, compatible with switching systems of foreign telephone administrations, was initiated. In July, 1960 TASI (Time Assignment Speech Interpolation) was introduced to the underseas cable. TASI effectively doubled the capacity of the cable by taking advantage of quiet pauses during telephone conversations. (RECORD, March, 1959). It also required the development of a signaling system to replace the standard steady tone system used by Bell Operating companies.
At present, all calls to Europe are "gated" through special overseas operators at White Plains or New York City. An important function of the gateway operator is to make the required translations to reconcile the differences between the telephone dials of the various national networks. (RECORD, July, 1961 ). When the use of letters is eliminated and "all number calling" becomes a reality, this particular problem will be eliminated. Since specialized gateway operators are required, it has been practical to place the special overseas equipment at a minimum number of gateway cities.
In the Bell System, the switching of international calls via the underseas cable is performed by a standard 4A or 4M toll switching system with the addition of the following major new circuits: (See the diagram on page 236)
1. Signal converter circuit
2. Two-way overseas trunk circuit
3. Overseas assistance operator trunk circuit
4. Overseas sender circuit
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The Overseas Signal Converter Circuit
To allow all countries to retain their respective national signaling systems, a signal converter must be located at each terminal of the underseas cable to provide conversion from TASI compatible signaling to that required for each individual national network. The converter used at the North American terminal will he described in a forthcoming RECORD article.
The Overseas Trunk Circuits
A trunk circuit is a control device used to associate a switching system with a transmission facility and to provide supervision of the connection thereafter. Three new trunk circuit designs were required for switching of international calls. The principal design was a two-way, four-wire, multifrequency pulsing type, employing E & M lead signaling. Two types of assistance operator trunk circuits were necessary, one for use at a two-wire switchboard and the other for use at a four-wire switchboard. These circuits provide access to an assistance operator in response to an incoming ring forward signal.
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"This block diagram illustrates the progress of an overseas call through a gateway office."
The two-way overseas trunk circuit is similar in many respects to its counterpart used in this country's national toll switching system with the following principal differences.
The existing four-wire intertoll trunk circuit is connected to a single frequency signaling unit by six leads. The signaling unit transmits tone in the idle state, and in response to signals from the trunk circuit, either connects or disconnects tone through to the transmission facility. The overseas trunk circuit is connected by 10 leads to a special type of signal converter which does not transmit tone in the idle condition. In response to trunk circuit signals, either or both of two frequencies are transmitted for short spurts, or under certain conditions until acknowledged by the distant signal converter. The four additional leads provide four new features which distinguish the overseas trunk from that used in this country's national network.
In the Bell System, many intertoll trunk circuits are provided with guard timing to provide a timed interval at the completion of an outgoing call to allow the release of far end equipment before allowing a trunk circuit to be reselected for a subsequent call. For overseas traffic, the guard timing responsibility has been transferred to the signal converter unit. To ensure that the signal converter at each terminal has restored to an idle state before allowing the trunk circuit to be reselected, the signal converter holds the trunk busy whenever it is in an off-normal state.
In the event a trunk circuit must be removed from service at the TASI terminal, a signal is transmitted to the trunk circuit through the signal converter. This signal makes the trunk appear busy to the common control equipment and lights a special lamp at the overseas toll testboard, indicating that the trunk has been made busy from the TASI terminal. If an established connection has been opened due to cable trouble, this signal will release the called customer to prevent his line being held out of service for an extended period.
A new signal is used on outgoing calls to inform the signal converter that an overseas sender has been connected through to the overseas trunk and is ready to outpulse. The signal converter requires this signal, in addition to a start dial signal from the distant terminal, before terminating the initial seizure signal. At the same instant the seizure signal is terminated, a signal is sent to the overseas sender to start outpulsing. This combination of signals is required to ensure that a prolonged quiet interval does not occur between the removal of seizure tone and the start of outpulsing. A prolonged quiet interval at this point could result in temporary loss of the TASI channel and possible mutilation of the address information.
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The signal converter, in response to an incoming ring forward signal from a European operator, signals the overseas trunk circuit to establish a connection to an assistance operator trunk circuit. When an assistance operator trunk circuit is bridged to the two-way overseas trunk, a lamp at the switchboard alerts an assistance operator to connect to the assistance trunk with a set of switchboard cords. After the required assistance has been given, the assistance operator, by removing the cords, disconnects the assistance trunk from the overseas trunk. While attached, the assistance operator aids the European gateway operator in the interpretation of tones and announcements. If requested, the assistance operator may initiate a new ring forward toward the national network. The assistance operator does not have the ability to complete calls. Assistance trunks are provided in sets of two to serve groups of overseas trunks varying in number depending upon traffic considerations.
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"Miss Agnes Cleary, an overseas operator for the Long Lines Dept. in New York City, handles an outgoing call to an European city."
The Overseas Sender
A key circuit in crossbar switching systems is the incoming sender. The primary sender functions are (1) to receive and store the incoming digits, (2) to call in the decoder and marker at the appropriate time for establishing the connection to an outgoing trunk to the desired destination, and (3) to outpulse the required digits in the proper form. Normally, this circuit is connected to a call as long as the call is being processed in the crossbar office. The overseas sender performs these functions under different considerations depending on whether the call was originated by the Bell System network or the overseas network. A class mark from the trunk informs the sender of the direction in which the call is progressing.
A prime consideration in the design of all overseas switching equipment has been minimum usage of the submarine cable to establish a connection. To this end, an outgoing call never reaches the cable until the gateway operator has pulsed the complete called number, and both the sender and common control equipment have verified its format. When the common control equipment has established a path to the overseas trunk circuit, the sender makes the linkage and trunk continuity tests while waiting for a European sender or register to be attached. In the usual crossbar network, these tests are made sequentially. Once the far end sender has returned a proceed to send signal, it becomes the responsibility of the originating sender to complete its functions and to ensure the continuous attachment of the TASI channel. This is accomplished by starting to send the address information within 60 to 100 milliseconds after reception of the start dial signal by the sender. The 60 millisecond minimum requirement allows the converter adequate time to restore the transmission path while the 100 millisecond maximum assures the uninterrupted TASI channel association with the trunk. The increase in pulsing speed to 10 digits-per-second over the Bell System standard of about 7 digits per-second also reduces the cable holding time.
The digits of the address are sent over the regular transmission path and consist of the standard Bell System 2 of 6 multifrequency tones but include the three previously unused combinations. These new combinations are only used on outgoing calls to the cable and have been named (1) code "eleven," (2) code "twelve," and (3) transit signal.
In the European countries, which require these codes, they have the following significance. Code "eleven" is a request for an inward or information operator. The Bell System equivalent on operator dialed calls is the digits 121 for an inward operator, and the digits 131 for an information operator. Code "twelve" is a request for a leave-word operator, the Bell System's equivalent being the digits 11 followed by the leaveword operator's number. European gateway switching machines will use the transit signal as an indication that the call is to be routed to another country instead of to their national network. At the present time, there is no North American equivalent to this signal. However, the Bell System senders have been arranged to send the transit signal, followed by the country code when the European network requires this type of indirect routing.
Another signal that has significance in international switching is the language digit. This digit is always sent directly before the called number and is only useful when the services of a European operator are required. The previously mentioned European grouping of the information operator and the inward operator avoids the necessity of having multilanguage operators at all information centers. By agreement, the Bell System is not required to furnish bilingual operators.
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Other steps that have been taken to reduce cable holding time include shortened trouble timing consistent with good service. Additional information is recorded on trouble conditions and the automatic holding of equipment from service for trouble investigation has been practically eliminated. In addition, senders and other units, used in common, have been liberally engineered to reduce the probability of delay.
Outgoing Overseas Call
A customer in the United States desiring to place a call to a European country or Australia calls a local long distance operator who establishes a connection to the appropriate gateway city. (See the diagram on page 236). This gateway operator obtains calling and called number information directly from the calling customer, and by observing lamp signals, determines if an overseas circuit is available. The customer is then informed of when the connection can be established and is requested to hang up. Under all circumstances, the gateway operator calls back the calling customer to maintain proper supervision of the entire connection. If a circuit is available, the customer is called back immediately and the overseas connection is set up in the following manner:
The gateway operator selects a switchboard tandem trunk having access to overseas sender circuits and, upon receipt of a sender attached signal, keys into the sender the called number, preceded by an arbitrary three-digit country access code, and followed by a start signal. Upon receipt of the start signal, the overseas sender attaches to common control equipment which performs the following functions:
1. Selection of an idle two-way overseas trunk.
2. Selection of a channel connecting the overseas trunk to the switchboard tandem trunk.
3. Transfer of information to the overseas sender controlling the form in which the address information will be transmitted overseas. (See the diagram [below, ed.] on page 239.)
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"The chart above illustrates typical changes in code format as an overseas call progresses."
When the two-way overseas trunk is selected by the common control equipment, a seizure signal is transmitted to set up receiving equipment at the distant end. A steady seizure signal is maintained by the signal converter unit until both the receiving and sending devices signal readiness to transfer the address information. At this time, the seizure signal is terminated and the sender outpulses to the distant receiver and subsequently releases from the connection.
The gateway operator, by sending a ring forward signal, can establish a connection with an assistance operator in the called country. A language digit, sent as part of the address information, signals that the assistance operator should be able to speak English. The assistance operator, by being bridged onto the existing connection, can provide aid in the interpretation of tones and announcements foreign to the gateway operator.
The gateway operator assumes the billing responsibility, is responsive to switchhook flashing from either customer, and releases the connection at the completion of the call.
Incoming Overseas Call
A two-way overseas trunk, upon receipt of an incoming seizure signal, connects to an overseas sender circuit. The sender, when ready to receive pulsing, passes a wink "start dial" signal via the overseas trunk circuit to the signal converter. Shortly thereafter, pulsing is received and connection established to our national network in a conventional manner. Upon receipt of a ring forward signal, an assistance operator trunk circuit is bridged to the established connection and an assistance operator alerted. The assistance operator in the United States will speak English only and will give assistance relating to tone and announcement interpretation. If the ring forward signal was intended for an inward operator, the assistance operator can generate a new ring forward signal before releasing from the connection. The entire connection is under control of the European gateway operator who will disconnect at the completion of the call.
Future Development
In the near future, operator dialing of international calls will be extended to many new areas in the Pacific and the Far East. New submarine cables are currently being laid to expand the existing Pacific network. When these new circuits are placed in service, the United States will have to provide transit switching, requiring considerable design modification to pass through many types of signals currently absorbed in our own system. Echo suppressors, used to improve transmission over long circuits, produce an undesirable effect if too many are used in tandem. A method must be found for controlling the maximum number used on a particular connection. To attain the most effective use of present overseas facilities, constant effort will be made to reduce the amount of time required to set up overseas connections.
It was estimated that the time saved by converting from ringdown operation to operator dialing would provide the equivalent of four additional circuits per 72-circuit TASI group. Early reports indicate that this estimate has been greatly exceeded. In addition, new cables are under consideration to handle the increased customer usage that generally follows improved service.
The attainment of worldwide customer dialing is dependent on a number of factors. Dial differences, numbering plans, and call progress signals are some of the more important such considerations. The trend, however, is clear. Worldwide dialing is a feasible proposition. The date it is achieved will depend on the speed with which the necessary international agreements are reached.
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Switching International Calls Via Submarine Cable
By J. Becker and R.D. Deming
Bell Laboratories Record, Volume 42, Number 7, July-August 1964 pages 234 to 239
(Scanned and OCR work performed by Tom Farley, http://www.privateline.com)
The Authors
Roy D. Deming started his Bell system with the Long Lines Department of the A.T.&T. company in 1927 in New York. Mr. Deming's work at Long lines was primarily concerned with maintenance and analysis problems. During this time, he attended Cooper Union and Columbia University. In 1951, Mr. Deming was transferred to the Switching Systems Development Department at Bell Laboratories Mr. Deming, currently associated with the Columbus Laboratories, was borne in Burke, N.Y., and now resides with his family in Worthington, Ohio.
Joseph Becker, co-author of "Switching International Calls Via Submarine Cable" in this issue, was born in New York City He joined the New York Telephone Company in 1948 after two years service with the Navy which included electronics training. Mr. Becker joined Bell Laboratories in 1961 and was later transferred to the Columbus Laboratory. At the laboratories he has been concerned with overseas trunk circuits, a cost reduction program for intertoll circuits, the Emergency Reporting System, and various trunk circuit services improvement items.
What was it like to be an international operator for AT&T before The Breakup? This internal Bell System newsletter gives you some idea (3 megs in .pdf) (internal link) It's from 1980 and I think Ronald Briggs submitted it. I thank him and everyone else who e-mails in these important contributions. A little tidbit from the newsletter: Back in 1980 connecting to Colombia from the States was a real problem. Only a radio link existed, not a hardwired cable connection. Very few calls, therefore, could go back and forth. Spelling was also a problem. Despite the Bell System's resources, they being the largest corporation on Earth, AT&T spelled Colombia "Columbia" at least three times. Sigh.
