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92-402: Many early technological inventions were developed here including automatic telephone panel and crossbar switches , the first experimental talking movies (1923), black-and-white and color TV , video telephones , radar , the vacuum tube , medical equipment, the development of the phonograph record and the first commercial broadcasts , including the first broadcast of a baseball game and

184-523: A Station Messaging Detail Record (SMDR). Virtual PBX systems or hosted PBX systems deliver PBX functionality as a service, available over the public switched telephone network (PSTN) or the Internet. Hosted PBXs are typically provided by a telephone company or service provider, using equipment located in the premises of a telephone exchange or the provider's data center. This means the customer does not need to buy or install PBX equipment. Generally,

276-464: A PBX from a hybrid key system are the amount, scope, and complexity of the features and facilities offered. A PBX is a telephone exchange or switching system that serves a private organization. A PBX permits the sharing of central office trunks between internally installed telephones, and provides intercommunication between those internal telephones within the organization without the use of external lines. The central office lines provide connections to

368-411: A central office trunk. Modern number-analysis systems permit users to dial internal and external telephone numbers without special codes to distinguish the intended destination. The term PBX originated when switchboard operators managed company switchboards manually using cord circuits. As automated electromechanical switches and later electronic switching systems gradually replaced the manual systems,

460-460: A comparatively small number of senders could handle a large amount of traffic, as each was only used for a short duration during call setup. This principle became known as common control , and was used in all subsequent switching systems. Revertive Pulsing (RP) was the primary signaling method used within and between panel switches. The selectors, once seized by the sender or another selector, would begin moving upwards under motor power. Each terminal

552-440: A conductive segment, it was grounded, thereby generating a pulse which was sent back to the sender for counting. When the sender counted the appropriate number of pulses, it cut the power to the solenoid in the terminating office, and caused the brush to stop at its current position. Calls from one panel office to another worked very similarly to calls within an office by use of revertive pulse signalling. The originating office used

644-467: A different name for each capability): Interfaces for connecting extensions to a PBX include: Interfaces for connecting PBXs to each other include: Interfaces for connecting PBXs to trunk lines include: Interfaces for collecting data from the PBX: A data record from a PBX or other telecommunication system that provides the statistics for a telephone call is usually termed a call detail record (CDR) or

736-514: A family of key telephone equipment, some of it compatible with Western Electric equipment, but it did not gain the widespread use enjoyed by Western Electric equipment. With the advent of LSI ICs , the same architecture could be implemented much less expensively than was possible using relays. In addition, it was possible to eliminate the many-wire cabling and replace it with much simpler cable similar to (or even identical to) that used by non-key systems. Electronic shared-control systems led quickly to

828-458: A few manual switchboards, Panel Call Indicator (PCI) signaling transmitted the called number to the "B" Board Machine Incoming operator, which lit lamps on the operator 's desk at the terminating manual office. The lamps illuminated digits on a display panel corresponding to the number dialed. The manual operator connected the call to the appropriate jack, and then repeated the process for the next incoming call. In areas with mostly manual switches,

920-526: A semi-mechanical method of switching, in which subscribers still used telephones without a dial, and operators answered calls and keyed the destination telephone number into the panel switch, which then completed the call automatically. These first panel-type exchanges were placed in service in Newark, New Jersey , on January 16, 1915 at the Mulberry central office serving 3640 subscribers, and on June 12 in

1012-418: A system often provides additional features for call handling. Business telephone systems are often broadly classified into key telephone systems and private branch exchanges, but many combinations (hybrid telephone systems) exist. A key telephone system was originally distinguished from a private branch exchange in that it did not require an operator or attendant at a switchboard to establish connections between

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1104-428: A total of 500 sets of terminals per frame. At the bottom, the frame had two electric motors to drive sixty selectors up and down by electromagnetically controlled clutches. As calls were completed through the system, selectors moved vertically over the sets of terminals until they reached the desired location, at which point the selector stopped its upward travel, and selections progressed to the next frame, until finally,

1196-518: A variety of functionality and services in the 1A key telephone system developed in the Bell System in the 1930s. Key systems can be built using three principal architectures: electromechanical shared-control, electronic shared-control, or independent key sets. New installations of key telephone systems have become less common, as hybrid systems and private branch exchanges of comparable size have similar costs and greater functionality. Before

1288-477: Is a telephone system typically used in business environments, encompassing the range of technology from the key telephone system ( KTS ) to the private branch exchange ( PBX ). A business telephone system differs from an installation of several telephones with multiple central office (CO) lines in that the CO lines used are directly controllable in key telephone systems from multiple telephone stations, and that such

1380-530: Is a hosted PBX service that extends fixed-line PBX functionality to mobile devices such as cellular handsets, smartphones, and PDA phones by provisioning them as extensions. Mobile PBX services also can include fixed-line phones. Mobile PBX systems are different from other hosted PBX systems that simply forward data or calls to mobile phones by allowing the mobile phone itself, through the use of buttons, keys, and other input devices, to control PBX phone functions and to manage communications without having to call into

1472-426: Is being phased out by most major telecommunication carriers throughout Europe in favor of all-IP networks, with some expecting complete migration by 2025. Originally having started as an organization's manual switchboard or attendant console operated by a telephone operator or just simply the operator , PBXs have evolved into VoIP centers that are hosted by the operators or even manufacturers. Even though VoIP

1564-452: Is considered by many people as the future of telephony, the circuit switched network remains the core of communications, and the existing PBX systems are competitive in services with modern IP systems. Five distinct scenarios exist: For the option to call from the IP network to the circuit-switched PSTN ( SS7 / ISUP ), the hosted solutions include interconnecting media gateways. Historically,

1656-523: Is now isolated from the rest of the former railroad viaduct, which is now the High Line elevated park. Panel switch The Panel Machine Switching System is a type of automatic telephone exchange for urban service that was used in the Bell System in the United States for seven decades. The first semi-mechanical types of this design were installed in 1915 in Newark, New Jersey , and

1748-506: Is possible to get hosted PBX services that include feature sets from minimal functionality to advanced feature combinations. In addition to the features available from premises-based PBX systems, hosted PBX: The ongoing migration of most major telecommunication carriers to IP-based networks, coupled with the rise in Cloud Communications has resulted in a significant rise in the uptake of hosted PBX solutions. A mobile PBX

1840-632: The New York Philharmonic with Arturo Toscanini conducting. It also served as the headquarters for the company from 1925 to the early 1960s, after which the headquarters moved to Murray Hill, New Jersey . The site was also the home for part of the Manhattan Project during World War II. After two years of renovations by Richard Meier , the building was reopened in 1970 as Westbeth Artists Community for low- to middle-income artists . In addition to affordable artist housing,

1932-817: The Waverly central office, which had 6480 lines. Panel development continued throughout the rest of the 1910s and in the 1920s in the United States. A third system in Newark ( Branch Brook ) followed in April 1917 for testing automatic call distribution. The first fully machine-switching Panel systems using common control principles were the Douglas and Tyler exchanges in Omaha, Nebraska, completed in December 1921. Subscribers were issued new telephones with dials, that permitted

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2024-558: The Western Electric Laboratories focused on different technologies, using competitive development to stimulate invention and increase product quality, a concept that had been successful at AT&T previously in transmitter design. One group continued existing work that yielded the Rotary system , while the second group developed a system that was based on linear movement of switch components, which became known as

2116-422: The hybrid designation. A hybrid system typically has some call appearance buttons that directly correspond to individual lines and/or stations, but may also support direct dialing to extensions or outside lines without selecting a line appearance. The modern key system is usually fully digital, although analog variants persist and some systems implement VOIP services. Effectively, the aspects that distinguish

2208-452: The panel bank . As work continued, many subassemblies were shared, and the two switches only distinguished themselves in the switching mechanisms. By 1910, the design of the Rotary system had progressed farther and internal trials employed it at Western Electric as a private branch exchange (PBX). However, by 1912, the company had decided that the panel system showed better promise to solve

2300-436: The public switched telephone network (PSTN) and the concentration aspect of a PBX permits the shared use of these lines between all stations in the organization. Its intercommunication ability allows two or more stations to directly connect while not using the public switched telephone network. This method reduces the number of lines needed from the organization to the public switched telephone network. Each device connected to

2392-430: The telephone dial , the more sophisticated Panel switch had senders , which registered and stored the digits that the customer dialed, and then translated the received digits into numbers appropriate to drive the selectors to their desired position: District Brush, District Group, Office Brush, Office Group, Incoming Brush, Incoming Group, Final Brush, Final Tens, Final Units. The use of senders provided advantages over

2484-486: The voice over IP PBX, or IP PBX . The other trend involved the idea of focusing on core competence . PBX services had always been hard to arrange for smaller companies, and many companies realized that handling their own telephony was not their core competence. These considerations gave rise to the concept of the hosted PBX. In wireline telephony, the original hosted PBX was the Centrex service provided by telcos since

2576-473: The "A" board, or outgoing operator, to request the number from the caller, connect to an idle trunk to the distant exchange, and relay the desired number to the B Board Manual Incoming Call operator, who keyed it to the Panel machine for setting up the incoming and final frames to the called telephone number. The panel switch is an example of a power drive system, in that it used 1/16 horsepower motors to drive

2668-607: The 1960s; later competitive offerings evolved into the modern competitive local exchange carrier . In voice-over IP, hosted solutions are easier to implement as the PBX may be located at and managed by any telephone service provider, connecting to the individual extensions via the Internet. The upstream provider no longer needs to run direct, local leased lines to the served premises. Many manufacturers provided manually operated private branch exchange systems in various sizes and features; examples are pictured here: A PBX system often includes

2760-474: The 1970s. The last Panel switch, located in the Bigelow central office in Newark, was decommissioned by 1983. When a subscriber removes the receiver (earpiece) from the hookswitch of a telephone, the local loop circuit to the central office is closed. This causes the flow of current through the loop and a line relay, which causes the relay to operate, starting a selector in the line finder frame to hunt for

2852-516: The 1980s when the arrival of electronic key systems with their easier installation and greater features signaled the end of electromechanical key systems. Two lesser-known key systems were used at airports for air traffic control communications, the 102 and 302 key systems. These were uniquely designed for communications between the air traffic control tower and radar approach control ( RAPCON ) or ground control approach (GCA) and included radio line connections. Automatic Electric Company also produced

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2944-410: The 1990s led to new types of PBX systems. One was the massive growth of data networks and increased public understanding of packet switching . Companies needed packet-switched networks for data, so using them for telephone calls proved tempting, and the availability of the Internet as a global delivery system made packet-switched communications even more attractive. These factors led to the development of

3036-557: The 21st century, the distinction between key systems and PBX systems has become increasingly blurred. Early electronic key systems used dedicated handsets which displayed and allowed access to all connected PSTN lines and stations. The modern key system now supports SIP , ISDN , analog handsets (in addition to its own proprietary handsets – usually digital) as well as a raft of features more traditionally found on larger PBX systems. Their support for both analog and digital signaling, and of some PBX functionality gives rise to

3128-638: The Call Annunciator signaling system was used to avoid installing lamp panels at every operator station. The Call Annunciator used speech recorded on strips of photographic film to verbally announce the called number to the answering operator. PCI signaling continued to be used for tandem purposes, decades after its original need had disappeared. In the 1950s, auxiliary senders were added for storing more than eight digits, and sending by multi-frequency (MF) signaling for direct distance dialing (DDD). Calls from manual offices to panel offices required

3220-573: The PBX, such as a telephone, a fax machine , or a computer modem, is referred to as an extension and has a designated extension telephone number that may or may not be mapped automatically to the numbering plan of the central office and the telephone number block allocated to the PBX. Initially, PBX systems offered the primary advantage of cost savings for internal phone calls: handling the circuit switching locally reduced charges for telephone service via central-office lines. As PBX systems gained popularity, they began to feature services not available in

3312-407: The alarm condition, staff could inspect the sender and its associated selectors, and resolve whatever trouble occurred before returning the sender and selectors back to service. When the sender's job was complete, it connected the talk path from the originating to the terminating side, and dropped out of the call. At this time, the sender was available to handle another subscriber's call. In this way,

3404-408: The call. As better technology became available, Panel senders were upgraded to the all-relay type. These were more reliable, and in addition, replaced the translator equipment with decoders, which also operated entirely with relays, rather than with motor-driven apparatus, which yielded faster call completion, and required less maintenance. Another important improvement involved a fundamental change in

3496-497: The called subscriber's line was busy, the final selector sent a busy signal back to the called party to alert them that the caller was on the phone and could not accept their call. As in the Strowger system, each central office could address up to 10,000 numbered lines (0000 to 9999), requiring four digits for each subscriber station. The panel system was designed to connect calls in a local metropolitan calling area. Each office

3588-541: The called subscriber's line was reached. In c. 1906, AT&T organized two research groups for solving the unique challenges in switching telephone traffic in the large urban centers in the Bell System . Large cities had a complex infrastructure of manual switching that prevented complete ad hoc conversion to mechanical switching, but more favorable economics was anticipated from conversion to mechanical operation. No satisfactory methods existed for interconnecting manual systems with machines for switching. The two groups at

3680-672: The central office trunks and stations, or between stations. Technologically, private branch exchanges share lineage with central office telephone systems, and in larger or more complex systems, may rival a central office system in capacity and features. With a key telephone system, a station user could control the connections directly using line buttons, which indicated the status of lines with built-in lamps. Key telephone systems are primarily defined by arrangements with individual line selection buttons for each available telephone line. The earliest systems were known as wiring plans and simply consisted of telephone sets, keys, lamps, and wiring. Key

3772-556: The complex contains a theatre, an art gallery, and a synagogue. It was declared a National Historic Landmark in 1975. The complex was listed a second time on the National Register in 2009, for its high-profile and successful example of adaptive reuse of the property. The southern viaduct section of the West Side Line railroad passed underneath the building at first floor level. This segment remains in place but

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3864-402: The connection to the terminating office was established, the sender used the last four (or five) digits of the telephone number to reach the called party. It did so by converting the digits into specific locations on the remaining incoming and final frames. After the connection was established all the way to the final frame, the called party's line was tested for busy. If the line was not busy,

3956-472: The control (and features) into individual telephone sets that don't require any single shared control unit. Such systems were dubbed KSU-less ; the first such phone was introduced in 1975 with the Com Key 416. Generally, these systems are used with relatively few telephone sets and it is often more difficult to keep the feature set (such as speed-dialing numbers) in synchrony between the various sets. Into

4048-466: The decoder or translator could direct the sender to their location as needed. Additionally, because the sender stored the telephone number dialed by the subscriber, and then controlled the selectors itself, there was no need for the subscriber's dial to have a direct-control relationship to the selectors themselves. This allowed the selectors to hunt at their own speed, over large groups of terminals, and allowed for smooth, motor controlled motion, rather than

4140-465: The development of large-scale integrated circuits , key systems typically consisted of electromechanical components, such as relays , as were larger telephone switching systems. The systems marketed in North America as the 1A, 1A1, 1A2 Key System , and the 6A, are typical examples and were sold for many decades. The Western Electric 1A family of key telephone units (KTUs) was introduced in

4232-407: The electrical logic of the switching system. The Panel originally shipped in a ground cut-off (GCO) configuration, wherein the cut-off relay had ground potential on one side of its winding at all times. A busy line condition was indicated by -48 volt battery applied to the other side of the cut-off relay winding, and thus at the sleeve lead. This would be detected by the final selector as it hunted over

4324-414: The electrical requirements for signaling over the newly established connection, and the rate at which the subscriber should be billed, should the call successfully complete. On the district or office selectors themselves, idle outgoing trunks were picked by the "sleeve test" method. After being directed by the sender to the correct group of terminals corresponding to the outgoing trunks to the called office,

4416-479: The expense of full-fledged PBX systems has put them out of reach of small businesses and individuals. However, since the 1990s many small, consumer-grade, and consumer-size PBXs have become available. These systems are not comparable in size, robustness, or flexibility to commercial-grade PBXs, but still provide many features. The first consumer PBX systems used analog ( POTS ) telephone lines, typically supporting four private analog and one public analog line. They were

4508-573: The features available. As the 1A1 systems became commonplace, requirements for intercom features grew. The original intercom KTUs, WECo Model 207, were wired for a single talk link, that is, a single conversation on the intercom at a time. The WECo 6A dial intercom system provided two talk links and was often installed as the dial intercom in a 1A1 or 1A2 key system. The 6A systems were complex, troublesome, and expensive, and never became popular. The advent of 1A2 technology in 1964 simplified key system setup and maintenance. These continued to be used throughout

4600-522: The following: Since the advent of Internet telephony ( Voice over IP ) technologies, PBX development has tended toward the IP PBX , which uses the Internet Protocol to carry calls. Most modern PBXs support VoIP. ISDN PBX systems also replaced some traditional PBXs in the 1990s, as ISDN offers features such as conference calling , call forwarding, and programmable caller ID. As of 2015, ISDN

4692-496: The form of either a wooden, switchboard-like desk, a wheeled cart, known as a "Tea Wagon", or a small box-type test set that could be carried to the apparatus that required testing. The central test location in the office was known as the "OGT Desk", or "Trouble Desk", and took the form of a large wooden desk with lamps, jacks, keys, cords, and a voltmeter. This desk served as the central point for analysis and trouble resolution. Other test apparatus included frame-mounted equipment that

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4784-466: The incoming selector circuit sent ringing voltage forward to the called party's line and waited for the called party to answer their telephone. If the called party answered, supervision signals were sent backwards through the sender, and to the district frame, which established a talking path between both subscribers, and charged the calling party for the call. At this time, the sender was released, and could be used again in service of an entirely new call. If

4876-693: The large-city problem, and delegated the use of the Rotary system for use in Europe to satisfy the growing demand and competition from other vendors there, under the management and manufacture by the International Western Electric Company in Belgium . After a trial installation as a PBX within Western Electric in 1913, Panel system planning commenced with design and construction of field trial central offices using

4968-422: The last four digits of the telephone number , had a limit of 10,000 phone numbers. In some of the urban areas where Panel was used, even a single square mile might have three or five times that many telephone subscribers. Thus the incoming selectors of several separate switching entities shared floor space and staff, but required separate incoming trunk groups from distant offices. Sometimes an Office Selector Tandem

5060-415: The last were retired in the same city in 1983. The Panel switch was named for its tall panels which consisted of layered strips of terminals. Between each strip was placed an insulating layer, which kept each metal strip electrically isolated from the ones above and below. These terminals were arranged in banks , five of which occupied an average selector frame. Each bank contained 100 sets of terminals, for

5152-470: The late 1930s and remained in use until the 1950s. 1A equipment was primitive and required at least two KTUs per line; one for line termination and one for station (telephone instrument) termination. The telephone instrument commonly used by 1A systems was the WECo 300/400-series telephone . Introduced in 1953, 1A1 key systems simplified wiring with a single KTU for both line and station termination, and increased

5244-470: The line finder had a fourth wire known as the "M" lead. This enabled the District circuit to send metering pulses to control the subscriber's message register. The introduction of direct distance dialing (DDD) in the 1950s required the addition of automatic number identification equipment for centralized automatic message accounting . The terminating section of the office was fixed to the structure of

5336-634: The modern hybrid telephone system, as the features of PBXs and key systems quickly merged. One of the most recognized such systems is the AT&;T Merlin . Additionally, these more modern systems allowed a diverse set of features including: Features could be added or modified simply using software, allowing easy customization of these systems. The stations were easier to maintain than the previous electromechanical key systems, as they used efficient LEDs instead of incandescent light bulbs for line status indication. LSI also allowed smaller systems to distribute

5428-419: The next selector frame. In the case of the final frame, the last selection would result in connection to an individual's phone line and would begin ringing. As the selectors were driven upwards by the motors, brushes attached to the vertical selector rods wiped over commutators at the top of the frame. These commutators contained alternating segments serving as insulators or conductors. When the brush passed over

5520-407: The originating side, and one on the terminating side. The line circuit consisted of a line relay on the originating side to indicate that a customer had gone off-hook , and a cutoff relay to keep the line relay from interfering with an established connection. The cutoff relay was controlled by a sleeve lead that, as with the multiple switchboard, could be activated by either the originating section or

5612-445: The outgoing line automatically. The telephone sets connected to a PBX do not normally have special keys for central-office line control, but it is not uncommon for key systems to be connected to a PBX to extend its services. A PBX, in contrast to a key system, employs an organizational numbering plan for its stations. In addition, a dial plan determines whether additional digit sequences must be prefixed when dialing to obtain access to

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5704-412: The previous direct control systems, because they allowed the office code of the telephone number to be decoupled from the actual location on the switching fabric. Thus, an office code (for example, "722") had no direct relationship to the physical layout of the trunks on the district and office frames. By the usage of translation, the trunks could be located arbitrarily on the physical frames themselves, and

5796-578: The provider actually hosts the PBX and the phone handsets are connected to it through an internet connection. Open source projects have provided PBX-style features since the 1990s. These projects provide flexibility, features, and programmability. Functionally, the PBX performs four main call processing duties: In addition to these basic functions, PBXs offer many other calling features and capabilities, with different manufacturers providing different features in an effort to differentiate their products. Common capabilities include (manufacturers may have

5888-406: The public network, such as hunt groups , call forwarding, and extension dialing. From the 1960s, a simulated PBX, known as Centrex , provided similar features from the central telephone exchange . A PBX differs from a key telephone system (KTS) in that users of a key system manually select their own outgoing lines on special telephone sets that control buttons for this purpose, while PBXs select

5980-475: The same protocol, but inserted a compensating resistance during pulsing so its sender encountered the same resistance for all trunks. This is in contrast to more modern forms of forward pulsing, where the originating equipment will directly outpulse to the terminating side the information it needs to connect the call. Later systems maintained compatibility with revertive pulsing, even as more advanced signaling methods were developed. The Number One Crossbar , which

6072-405: The selector continued moving upward through a number of terminals, checking for one with an un-grounded sleeve lead, then selecting and grounding it. If all the trunks were busy, the selector hunted to the end of the group, and finally sent back an "all circuits busy" tone . There was no provision for alternate routing as in earlier manual systems and later more sophisticated mechanical ones. Once

6164-413: The selector passed would send a pulse of ground potential along the circuit, back to the sender. The sender counted each pulse, and when the correct terminal was reached, the sender then signalled the selector to disengage the upward drive clutch and stop on the appropriate terminal as determined by the sender and decoder. The selector then either began its next selection operation, or extended the circuit to

6256-410: The selectors vertically to hunt for the desired connection, and back down again when the call was completed. In contrast, Strowger or crossbar systems used individual electromagnets for operation, and in their case the power available from an electromagnet limits the maximum size of the switch element it can move. With Panel having no such restriction, its dimensions were determined solely by the needs of

6348-431: The sender required either six or seven digits in order to complete the call. As the subscriber dialed, relays in the sender counted and stored the digits for later usage. As soon as the two, or three digits of the office code were dialed and stored, the sender performed a lookup against a translator (early-type) or decoder (later-type). The translator or decoder took the two or three digits as input, and returned data to

6440-440: The sender that contained the parameters for connecting to the called central office. After the sender received the data provided by the translator or decoder, the sender used this information to guide the district selector and office selector to the location of the terminals that would connect the caller to the central office where the terminating line was located. The sender also stored and utilized other information pertaining to

6532-411: The sender to direct the final selector not only to the correct terminal, but to ring the correct subscriber's line on that terminal. The panel system supported individual, 2-party, and 4-party lines. Similar to the divided-multiple telephone switchboard , the panel system was divided into an originating section and a terminating section. The subscriber's line had two appearances in a local office: one on

6624-499: The service is provided by a lease agreement and the provider can, in some configurations, use the same switching equipment to service multiple hosted PBX customers. The first hosted PBX services were feature-rich compared to most premises-based systems of the time. Some PBX functions, such as follow-me calling, appeared in a hosted service before they became available in hardware PBX equipment. Since its introduction, updates and new offerings have moved feature sets in both directions. It

6716-612: The size of a small cigar box . In Europe, these systems for analog phones were followed by consumer-grade PBXs for ISDN . Using small PBXs for ISDN is a logical step since the ISDN basic rate interface provides two logical phone lines (via two ISDN B channels) that can be used in parallel. With the adoption of VoIP by consumers, consumer VoIP PBXs have appeared, with PBX functions becoming simple additional software features of consumer-grade routers and switches. Additionally, many telecommunications providers now offer hosted PBX systems where

6808-487: The staccato, momentary motion of the step-by-step system. The sender also provided fault detection. As it was responsible for driving the selectors to their destinations, it was able to detect errors (known as trouble ) and alert central office staff of the problem by lighting a lamp at the appropriate panel. In addition to lighting a lamp, the sender held itself and the selectors that were under its control out of service, which prevented their use by other callers. Upon noting

6900-698: The subscriber to place local calls without operator assistance. This installation was followed by the first installations in the eastern region in the Sherwood and Syracuse-2 central offices in Paterson, New Jersey , in May and July 1922, respectively. The storied Pennsylvania exchange in New York City was cut-over to service in October 1922. Most Panel installations were replaced by modern systems during

6992-473: The switch, and the design of the exchange. The driving electric motor can be made as large as is necessary to move the switch elements. Thus, most calls required only about half as many stages as in earlier systems. Motors used on panel frames were capable of operating on alternating (AC) or direct current (DC), however they could only be started with DC. In the event of an AC power failure the motor would switch to its DC windings, and continue running until AC power

7084-410: The telephone number corresponded to the station number, which pointed to the physical location of the subscriber's telephone on the final frame of the called office. For instance, a telephone number may be listed as PA2-5678, where PA2 (722) is the office code and 5678 is the station number. In areas that served party lines , the system accepted an additional digit for party identification. This allowed

7176-407: The terminal of the subscriber's line. Simultaneously, a sender is selected, which provides dial tone to the caller once the line is found. The line finder then operates a cutoff relay, which prevents that telephone from being called, should another subscriber happen to dial the number. Dial tone confirms to the subscriber that the system is ready for dialing. Depending on the local numbering system,

7268-402: The terminals. Starting in 1929, all newer panel systems were deployed as battery cut-off (BCO) systems. In this revision, the presence of ground and -48V was reversed. Battery was constantly applied to one side of the cut-off relay, and the presence of ground on the other side of the winding indicated the line was busy. This change necessitated a fundamental change in the design of the system, and

7360-446: The terminating. On the terminating end, the line circuit was connected to a final selector, which was used in call completion. Thus, when a call was completed to a subscriber, the final selector circuit connected to the desired line, and then performed a sleeve (busy) test. If the line was not busy, the final selector operated the cut-off relay via the sleeve lead, and proceeded to ring the called subscriber. Supervision ( line signaling )

7452-427: The terms private automatic branch exchange (PABX) and private manual branch exchange (PMBX) differentiated them. Solid-state digital systems were sometimes referred to as electronic private automatic branch exchanges (EPABX) . As of 2021 , the term PBX is by far the most widely recognized. The abbreviation now applies to all types of complex, in-house telephony switching systems. Two significant developments during

7544-510: The top of the frame to compensate for their mass. Later line finders used 10 brushes and rearranged the layout to accommodate 400 lines per line finder frame. This increased capacity while eliminating the need for compensating equipment. Western Electric estimated that the design changes of 1925 to 1927 accounted for a 60% reduction in overall costs for the Panel system. The following table presents early major panel system upgrades: Business telephone system A business telephone system

7636-416: Was a Bell System term of art for a customer-controlled switching system such as the line buttons on the phones associated with such systems. The electrical components that allow for the selection of lines and features such as hold and intercom are housed in a panel or cabinet, called the key service unit or key system unit (KSU). The wiring plans evolved into modular hardware building blocks with

7728-418: Was assigned a two- or three-digit office code, called an office code , which indicated to the system the central office in which the desired party was located. Callers dialed the office code followed by the station number. In larger cities, such as New York City, dialing required a three-digit office code, and in less-populated cities, such as Seattle, WA and Omaha, NE, a two-digit code. The remaining digits of

7820-424: Was not supervised by the district circuit, and only returned to normal once the called party hung up. Some District frames were equipped with the more complex supervisory and timing circuits required to generate coin collect and return signals for handling calls from payphones . Many of the urban and commercial areas where Panel was first used had message rate service rather than flat rate calling. For this reason

7912-470: Was restored. Because of its relative complexity compared to direct control systems, the Panel system incorporated many new types of testing apparatus. At the time of its design, it was decided that maintenance should be done on a preventative basis, and regular testing of the equipment would be used to identify faults before they became severe enough to affect subscribers. To this end, multiple types of test equipment were provided. Test equipment generally took

8004-405: Was supplied by a District circuit, similar to the cord circuit that plugged into a line jack on a switchboard. The District circuit supervised the calling party, and when the calling party went on-hook , it released the ground on the sleeve lead, thus releasing all selectors except the final, which returned down to their start position to make ready for further traffic. The final selector circuit

8096-401: Was the first successor to the Panel system also used this method of signaling exclusively, until later upgrades introduced newer signaling such as Multi-frequency signaling . Panel was initially installed in cities where many stations still used manual (non-dial) service. For compatibility with manual offices, two types of signaling were supported. In areas with mostly machine switches and only

8188-565: Was undertaken for many reasons. One of the most notable was that GCO offices were more prone to fire. The line finder was also improved during the system's lifetime. Originally, the line finder frame had a capacity of 300 lines each, and used 15 brushes (vertical hunting segments) on each rod. This was intended to reduce hunting time as there were more brushes hunting over a shorter distance. As these line finders went into service, however, it became evident that 15 brushes on each vertical selector rod were quite heavy, and needed springs and pulleys at

8280-411: Was upgraded as new features became available or necessary. Starting in the mid-1920s, such upgrades improved the initial design. Major attention was initially focused on improving the sender. Early two- and three-digit type senders stored dialed digits on rotary selector switches. The senders employed translators to convert the dialed digits into the appropriate brush and group selections needed to complete

8372-497: Was used to distribute incoming traffic among the offices. This was a Panel office with no senders or other common control equipment; just one stage of selectors and accepting only the Office Brush and Office Group parameters. Panel Sender Tandems were also used when their greater capabilities were worth their additional cost. While the Strowger ( step-by-step ) switch moved under direct control of dial pulses that came from

8464-404: Was used to routine commonly used circuits within the office. These included an automatic routine sender test frame , and an automatic routine selector test frame . When testing was to be done manually by a switchman, he or she used a Tea Wagon, which was wheeled to the apparatus to be tested, and plugged into jacks that were provided for this purpose. Throughout its service time, the Panel system

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