A telephone exchange is a telecommunications system used in the public switched telephone network or in large enterprises. An exchange consists of electronic components and in older systems human operators that interconnect telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers. In historical perspective, telecommunication terms have been used with different semantics over time; the term telephone exchange is used synonymously with central office, a Bell System term. A central office is defined as a building used to house the inside plant equipment of several telephone exchanges, each serving a certain geographical area; such an area has been referred to as the exchange. Central office locations may be identified in North America as wire centers, designating a facility from which a telephone obtains dial tone. For business and billing purposes, telephony carriers define rate centers, which in larger cities may be clusters of central offices, to define specified geographical locations for determining distance measurements.
In the United States and Canada, the Bell System established in the 1940s a uniform system of identifying central offices with a three-digit central office code, used as a prefix to subscriber telephone numbers. All central offices within a larger region aggregated by state, were assigned a common numbering plan area code. With the development of international and transoceanic telephone trunks driven by direct customer dialing, similar efforts of systematic organization of the telephone networks occurred in many countries in the mid-20th century. For corporate or enterprise use, a private telephone exchange is referred to as a private branch exchange, when it has connections to the public switched telephone network. A PBX is installed in enterprise facilities collocated with large office spaces or within an organizational campus to serve the local private telephone system and any private leased line circuits. Smaller installations might deploy a PBX or key telephone system in the office of a receptionist.
In the era of the electrical telegraph, post offices, railway stations, the more important governmental centers, stock exchanges few nationally distributed newspapers, the largest internationally important corporations and wealthy individuals were the principal users of such telegraphs. Despite the fact that telephone devices existed before the invention of the telephone exchange, their success and economical operation would have been impossible on the same schema and structure of the contemporary telegraph, as prior to the invention of the telephone exchange switchboard, early telephones were hardwired to and communicated with only a single other telephone. A telephone exchange is a telephone system located at service centers responsible for a small geographic area that provided the switching or interconnection of two or more individual subscriber lines for calls made between them, rather than requiring direct lines between subscriber stations; this made it possible for subscribers to call each other at businesses, or public spaces.
These made telephony an available and comfortable communication tool for everyday use, it gave the impetus for the creation of a whole new industrial sector. As with the invention of the telephone itself, the honor of "first telephone exchange" has several claimants. One of the first to propose a telephone exchange was Hungarian Tivadar Puskás in 1877 while he was working for Thomas Edison; the first experimental telephone exchange was based on the ideas of Puskás, it was built by the Bell Telephone Company in Boston in 1877. The world's first state-administered telephone exchange opened on November 12, 1877 in Friedrichsberg close to Berlin under the direction of Heinrich von Stephan. George W. Coy designed and built the first commercial US telephone exchange which opened in New Haven, Connecticut in January, 1878; the switchboard was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations. Charles Glidden is credited with establishing an exchange in Lowell, MA. with 50 subscribers in 1878.
In Europe other early telephone exchanges were based in London and Manchester, both of which opened under Bell patents in 1879. Belgium had its first International Bell exchange a year later. In 1887 Puskás introduced the multiplex switchboard.. Exchanges consisted of one to several hundred plug boards staffed by switchboard operators; each operator sat in front of a vertical panel containing banks of ¼-inch tip-ring-sleeve jacks, each of, the local termination of a subscriber's telephone line. In front of the jack panel lay a horizontal panel containing two rows of patch cords, each pair connected to a cord circuit; when a calling party lifted the receiver, the local loop current lit a signal lamp near the jack. The operator responded by inserting the rear cord into the subscriber's jack and switched her headset into the circuit to ask, "Number, please?" For a local call, the operator inserted the front cord of the pair into the called party's local jack and started the ringing cycle. For a long distance call, she plugged into a trunk circuit to connect to another operator in another bank of boards or at a remote central office.
In 1918, the average time to complete the connection for a long-distance call was 15 minutes. Early manual switchboards required the operator to operate listening keys and ringing keys, but by the late 1910s and 1920s, advances in switchboard technology led to features which allowed the call to be automatic
X.25 is an ITU-T standard protocol suite for packet-switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, leased lines, plain old telephone service connections, or ISDN connections as physical links. X.25 was defined by the International Telegraph and Telephone Consultative Committee in a series of drafts and finalized in a publication known as The Orange Book in 1976. X.25 networks were popular during the 1980s with telecommunications companies and in financial transaction systems such as automated teller machines. However, most uses have moved to Internet Protocol systems instead. X.25 is still used and available in niche applications such as Retronet that allows vintage computers to use the internet. X.25 is one of the oldest packet-switched services available. It was developed before the OSI Reference Model; the protocol suite is designed as three conceptual layers, which correspond to the lower three layers of the seven-layer OSI model.
It supports functionality not found in the OSI network layer. X.25 was developed in the ITU-T Study Group VII based upon a number of emerging data network projects. Various updates and additions were worked into the standard recorded in the ITU series of technical books describing the telecommunication systems; these books were published every fourth year with different-colored covers. The X.25 specification is only part of the larger set of X-Series specifications on public data networks. The public data network was the common name given to the international collection of X.25 providers. Their combined network had large global coverage into the 1990s. Publicly accessible X.25 networks were set up in most countries during the 1970s and 1980s, to lower the cost of accessing various online services. Beginning in the early 1990s, in North America, use of X.25 networks started to be replaced by Frame Relay services offered by national telephone companies. Most systems that required X.25 now use TCP/IP, however it is possible to transport X.25 over TCP/IP when necessary.
X.25 networks are still in use throughout the world. A variant called AX.25 is used by amateur packet radio. Racal Paknet, now known as Widanet, is still in operation in many regions of the world, running on an X.25 protocol base. In some countries, like the Netherlands or Germany, it is possible to use a stripped version of X.25 via the D-channel of an ISDN-2 connection for low-volume applications such as point-of-sale terminals. Additionally X.25 is still under heavy use in the aeronautical business though a transition to modern protocols like X.400 is without option as X.25 hardware becomes rare and costly. As as March 2006, the United States National Airspace Data Interchange Network has used X.25 to interconnect remote airfields with Air Route Traffic Control Centers. France was one of the last remaining countries where commercial end-user service based on X.25 operated. Known as Minitel it was based on Videotex, itself running on X.25. In 2002, Minitel had about 9 million users, in 2011, it still accounted for about 2 million users in France when France Télécom announced it would shut down the service by 30 June 2012.
As planned, service was terminated 30 June 2012. There were 800,000 terminals still in operation at the time; the general concept of the X. 25 was to create a global packet-switched network. Much of the X.25 system is a description of the rigorous error correction needed to achieve this, as well as more efficient sharing of capital-intensive physical resources. The X. 25 specification defines only the interface between an X. 25 network. X.75, a protocol similar to X.25, defines the interface between two X.25 networks to allow connections to traverse two or more networks. X.25 does not specify how the network operates internally – many X.25 network implementations used something similar to X.25 or X.75 internally, but others used quite different protocols internally. The ISO protocol equivalent to X.25, ISO 8208, is compatible with X.25, but additionally includes provision for two X.25 DTEs to be directly connected to each other with no network in between. By separating the Packet-Layer Protocol, ISO 8208 permits operation over additional networks such as ISO 8802 LLC2 and the OSI data link layer.
X.25 defined three basic protocol levels or architectural layers. In the original specifications these were referred to as levels and had a level number, whereas all ITU-T X.25 recommendations and ISO 8208 standards released after 1984 refer to them as layers. The layer numbers were dropped to avoid confusion with the OSI Model layers. Physical layer: This layer specifies the physical, electrical and procedural characteristics to control the physical link between a DTE and a DCE. Common implementations use X. 21, EIA-449 or other serial protocols. Data link layer: The data link layer consists of the link access procedure for data interchange on the link between a DTE and a DCE. In its implementation, the Link Access Procedure, Balanced is a data link protocol that manages a communication session and controls the packet framing, it is a bit-oriented protocol that provides orderly delivery. Packet layer: This layer defined a packet-layer protocol for exchanging control and user data packets to form a packet-switching network based on virtual calls, acco
Business telephone system
A business telephone system is a multiline telephone system used in business environments, encompassing systems ranging from the small key telephone system to the large private branch exchange. A business telephone system differs from an installation of several telephones with multiple central office lines in that the CO lines used are directly controllable in key telephone systems from multiple telephone stations, that such a system provides additional features related to call handling. Business telephone systems are broadly classified into key telephone systems, private branch exchanges, but many hybrid systems exist. A key telephone system was distinguished from a private branch exchange in that it did not require an operator or attendant at the switchboard to establish connections between the central office trunks and stations, or between stations. Technologically, private branch exchanges share lineage with central office telephone systems, 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 defined by arrangements with individual line selection buttons for each available telephone line; the earliest systems were known as wiring plans and consisted of telephone sets, keys and wiring. Key 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 wiring plans evolved into modular hardware building blocks with 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 cost and greater functionality.
Before the advent of large-scale integrated circuits, key systems were composed of electromechanical components as were larger telephone switching systems. The systems marketed in North America as the 1A, 6A, 1A1 and the 1A2 Key System are typical examples and sold for many decades; the 1A family of Western Electric Company key telephone units were introduced in the late 1930s and remained in use to the 1950s. 1A equipment required at least two KTUs per line. The telephone instrument used by 1A systems was the WECo 300-series telephone. Introduced in 1953, 1A1 key systems simplified wiring with a single KTU for both line and station termination, increased 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 installed as the dial intercom in a 1A1 or 1A2 key system.
The 6A systems were complex and expensive, never became popular. The advent of 1A2 technology in the 1964 simplified key system maintenance; these continued to be used throughout 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 or ground control approach, included radio line connections. Automatic Electric Company produced 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 that used by non-key systems.
Electronic shared-control systems led to the modern hybrid telephone system, as the features of PBX and key system 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: Answering machine functions Automatic call accounting Caller ID Remote supervision of the entire system Selection of signaling sounds Speed dialing Station-specific limitations Features could be added or modified 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 allowed smaller systems to distribute the control into individual telephone sets that don't require any single shared control unit; these systems are used with a few telephone sets and it is more difficult to keep the feature set in synchrony between the various sets. Into the 21st century, the distinction between key systems and PBX systems has become 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 it
A residential area is a land used in which housing predominates, as opposed to industrial and commercial areas. Housing may vary between, through, residential areas; these include multi-family residential, or mobile homes. Zoning for residential use may permit some services or work opportunities or may exclude business and industry, it may only permit low density uses. Residential zoning includes a smaller FAR than business, commercial or industrial/manufacturing zoning; the area may be small. In certain residential areas rural, large tracts of land may have no services whatever, thus residents seeking services must use a motor vehicle or other transport, so the need for transport has resulted in land development following existing or planned transport infrastructure such as rail and road. Development patterns may be regulated by restrictive covenants contained in the deeds to the properties in the development, may result from or be reinforced by zoning. Restrictive covenants are not changed when the agreement of all property owners is required.
The area so restricted may be small. Residential areas may be subcategorized in the concentric zone model and other schemes of urban geography. Residential development is real estate development for residential purposes; some such developments are called a subdivision, when the land is divided into lots with houses constructed on each lot. Such developments became common during the late nineteenth century in the form of streetcar suburbs. In previous centuries, residential development was of two kinds. Rich people bought a townlot, hired an architect and/or contractor, built a bespoke / customized house or mansion for their family. Poor urban people lived in tenements built for rental. Single-family houses were built on speculation, for future sale to residents not yet identified; when cities and the middle class expanded and mortgage loans became commonplace, a method, rare became commonplace to serve the expanding demand for home ownership. Post–World War II economic expansion in major cities of the United States New York City and Los Angeles produced a demand for thousands of new homes, met by speculative building.
Its large-scale practitioners disliked the term "property speculator" and coined the new name "residential development" for their activity. Entire farms and ranches were subdivided and developed with one individual or company controlling all aspects of entitlement, land development and housing. Communities like Levittown, Long Island or Lakewood south of Los Angeles saw new homes sold at unprecedented rates—more than one a day. Many techniques which had made the automobile affordable made housing affordable: standardization of design and small, repetitive assembly tasks, a smooth flow of capital. Mass production resulted in a similar uniformity of product, a more comfortable lifestyle than cramped apartments in the cities. With the advent of government-backed mortgages, it could be cheaper to own a house in a new residential development than to rent; as with other products, continual refinements appeared. Curving streets, greenbelt parks, neighborhood pools, community entry monumentation appeared.
Diverse floor plans with differing room counts, multiple elevations appeared. Developers remained competitive with each other on everything, including location, community amenities, kitchen appliance packages, price. Today, a typical residential development in the United States might include traffic calming features, such as a winding street, dead-end road, or looped road lined with homes. Suburban developments help form the stereotypical image of a "suburban America," and are associated with the American middle-class. Most offer homes in a narrow range of age, price and features, thus potential residents having different needs, wishes or resources must look elsewhere; some residential developments are gated communities. Criticisms of residential developments may include: They do not mesh well with the greater community; some are isolated, with only one entrance, or otherwise connected with the rest of the community in few ways. Being commuter towns, they serve no more purpose for the greater community than other specialized settlements do, thus require residents to go to the greater community for commercial or other purposes.
Whereas mixed-use developments provide for commerce and other activities, thus residents need not go as to the greater community. The dictionary definition of residential at Wiktionary Meadowbrook symbol of postwar housing boom - Pantagraph Residential Property Valuations
Integrated Services Digital Network
Integrated Services Digital Network is a set of communication standards for simultaneous digital transmission of voice, video and other network services over the traditional circuits of the public switched telephone network. It was first defined in 1988 in the CCITT red book. Prior to ISDN, the telephone system was viewed as a way to transport voice, with some special services available for data; the key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. The ISDN standards define several kinds of access interfaces, such as Basic Rate Interface, Primary Rate Interface, Narrowband ISDN, Broadband ISDN. ISDN is a circuit-switched telephone network system, which provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better voice quality than an analog phone can provide, it offers circuit-switched connections, packet-switched connections, in increments of 64 kilobit/s.
In some countries, ISDN found major market application for Internet access, in which ISDN provides a maximum of 128 kbit/s bandwidth in both upstream and downstream directions. Channel bonding can achieve a greater data rate. ISDN is employed as data-link and physical layers in the context of the OSI model. In common use, ISDN is limited to usage to Q.931 and related protocols, which are a set of signaling protocols establishing and breaking circuit-switched connections, for advanced calling features for the user. They were introduced in 1986. In a videoconference, ISDN provides simultaneous voice and text transmission between individual desktop videoconferencing systems and group videoconferencing systems. Integrated services refers to ISDN's ability to deliver at minimum two simultaneous connections, in any combination of data, voice and fax, over a single line. Multiple devices can be attached to the line, used as needed; that means an ISDN line can take care of what were expected to be most people's complete communications needs at a much higher transmission rate, without forcing the purchase of multiple analog phone lines.
It refers to integrated switching and transmission in that telephone switching and carrier wave transmission are integrated rather than separate as in earlier technology. The entry level interface to ISDN is the Basic Rate Interface, a 128 kbit/s service delivered over a pair of standard telephone copper wires; the 144 kbit/s overall payload rate is divided into two 64 kbit/s bearer channels and one 16 kbit/s signaling channel. This is sometimes referred to as 2B+D; the interface specifies the following network interfaces: The U interface is a two-wire interface between the exchange and a network terminating unit, the demarcation point in non-North American networks. The T interface is a serial interface between a computing device and a terminal adapter, the digital equivalent of a modem; the S interface is a four-wire bus. The R interface defines the point between a non-ISDN device and a terminal adapter which provides translation to and from such a device. BRI-ISDN is popular in Europe but is much less common in North America.
It is common in Japan — where it is known as INS64. The other ISDN access available is the Primary Rate Interface, carried over T-carrier with 24 time slots in North America, over E-carrier with 32 channels in most other countries; each channel provides transmission at a 64 kbit/s data rate. With the E1 carrier, the available channels are divided into 30 bearer channels, one data channel, one timing and alarm channel; this scheme is referred to as 30B+2D. In North America, PRI service is delivered via T1 carriers with only one data channel referred to as 23B+D, a total data rate of 1544 kbit/s. Non-Facility Associated Signalling allows two or more PRI circuits to be controlled by a single D channel, sometimes called 23B+D + n*24B. D-channel backup allows for a second D channel in case the primary fails. NFAS is used on a Digital Signal 3. PRI-ISDN is popular throughout the world for connecting private branch exchanges to the public switched telephone network. Though many network professionals use the term ISDN to refer to the lower-bandwidth BRI circuit, in North America BRI is uncommon whilst PRI circuits serving PBXs are commonplace.
The bearer channel is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz with G.711 encoding. B-channels can be used to carry data, since they are nothing more than digital channels; each one of these channels is known as a DS0. Most B channels can carry a 64 kbit/s signal, but some were limited to 56K because they traveled over RBS lines; this has since become less so. X.25 can be carried over the B or D channels of a BRI line, over the B channels of a PRI line. X.25 over the D channel is used at many point-of-sale terminals because it eliminates the modem setup, because it connects to the central system over a B channel, thereby eliminating the need for modems and making much better use of the central system's telephone lines. X.25 was part of an ISDN protocol
Synchronization is the coordination of events to operate a system in unison. The conductor of an orchestra keeps the orchestra synchronized or in time. Systems that operate with all parts in synchrony are said to be synchronous or in sync—and those that are not are asynchronous. Today, time synchronization can occur between systems around the world through satellite navigation signals. Time-keeping and synchronization of clocks has been a critical problem in long-distance ocean navigation. Before radio navigation and satellite-based navigation, navigators required accurate time in conjunction with astronomical observations to determine how far east or west their vessel traveled; the invention of an accurate marine chronometer revolutionized marine navigation. By the end of the 19th century, important ports provided time signals in the form of a signal gun, flag, or dropping time ball so that mariners could check their chronometers for error. Synchronization was important in the operation of 19th century railways, these being the first major means of transport fast enough for differences in local time between adjacent towns to be noticeable.
Each line handled the problem by synchronizing all its stations to headquarters as a standard railroad time. In some territories, sharing of single railroad tracks was controlled by the timetable; the need for strict timekeeping led the companies to settle on one standard, civil authorities abandoned local mean solar time in favor of that standard. In electrical engineering terms, for digital logic and data transfer, a synchronous circuit requires a clock signal. However, the use of the word "clock" in this sense is different from the typical sense of a clock as a device that keeps track of time-of-day. In a different sense, electronic systems are sometimes synchronized to make events at points far apart appear simultaneous or near-simultaneous from a certain perspective. Timekeeping technologies such as the GPS satellites and Network Time Protocol provide real-time access to a close approximation to the UTC timescale and are used for many terrestrial synchronization applications of this kind.
Synchronization is an important concept in the following fields: Computer science Cryptography Multimedia Music Neuroscience Photography Physics Synthesizers Telecommunication Synchronization of multiple interacting dynamical systems can occur when the systems are autonomous oscillators. For instance, integrate-and-fire oscillators with either two-way or one-way coupling can synchronize when the strength of the coupling is greater than the differences among the free-running natural oscillator frequencies. Poincare phase oscillators are model systems that can interact and synchronize within random or regular networks. In the case of global synchronization of phase oscillators, an abrupt transition from unsynchronized to full synchronization takes place when the coupling strength exceeds a critical threshold; this is known as the Kuramoto model phase transition. Synchronization is an emergent property that occurs in a broad range of dynamical systems, including neural signaling, the beating of the heart and the synchronization of fire-fly light waves.
Synchronization of movement is defined as similar movements between two or more people who are temporally aligned. This is different to mimicry. Muscular bonding is the idea; this sparked some of the first research into movement synchronization and its effects on human emotion. In groups, synchronization of movement has been shown to increase conformity and trust however more research on group synchronization is needed to determine its effects on the group as a whole and on individuals within a group. In dyads, groups of two people, synchronization has been demonstrated to increase affiliation, self-esteem and altruistic behaviour and increase rapport. During arguments, synchrony between the arguing pair has been noted to decrease, however it is not clear whether this is due to the change in emotion or other factors. There is evidence to show that movement synchronization requires other people to cause its beneficial effects, as the effect on affiliation does not occur when one of the dyad is synchronizing their movements to something outside the dyad.
This is known as interpersonal synchrony. There has been dispute regarding the true effect of synchrony in these studies. Research in this area detailing the positive effects of synchrony, have attributed this to synchrony alone. Indeed, the Reinforcement of Cooperation Model suggests that perception of synchrony leads to reinforcement that cooperation is occurring, which leads to the pro-social effects of synchrony. More research is required to separate the effect of intentionality from the beneficial effect of synchrony. Film synchronization of image and sound in sound film. Synchronization is important in fields such as digital telephony and digital audio where streams of sam
The human voice consists of sound made by a human being using the vocal tract, such as talking, laughing, screaming, etc. The human voice frequency is a part of human sound production in which the vocal folds are the primary sound source. Speaking, the mechanism for generating the human voice can be subdivided into three parts; the lung, the "pump" must produce adequate air pressure to vibrate vocal folds. The vocal folds vibrate to use airflow from the lungs to create audible pulses that form the laryngeal sound source; the muscles of the larynx adjust the length and tension of the vocal folds to ‘fine-tune’ pitch and tone. The articulators articulate and filter the sound emanating from the larynx and to some degree can interact with the laryngeal airflow to strengthen it or weaken it as a sound source; the vocal folds, in combination with the articulators, are capable of producing intricate arrays of sound. The tone of voice may be modulated to suggest emotions such as anger, fear, happiness or sadness.
The human voice is used to express emotion, can reveal the age and sex of the speaker. Singers use the human voice as an instrument for creating music. Adult men and women have different sizes of vocal fold. Adult male voices are lower-pitched and have larger folds; the male vocal folds, are between 17 25 mm in length. The female vocal folds are between 17.5 mm in length. The folds are within the larynx, they are attached at the back to the arytenoids cartilages, at the front to the thyroid cartilage. They have no outer edge as they blend into the side of the breathing tube while their inner edges or "margins" are free to vibrate, they have a three layer construction of an epithelium, vocal ligament muscle, which can shorten and bulge the folds. They are pearly white in color. Above both sides of the vocal cord is the vestibular fold or false vocal cord, which has a small sac between its two folds; the difference in vocal folds size between men and women means that they have differently pitched voices.
Additionally, genetics causes variances amongst the same sex, with men's and women's singing voices being categorized into types. For example, among men, there are bass, baritone and countertenor, among women, mezzo-soprano and soprano. There are additional categories for operatic voices; this is not the only source of difference between male and female voice. Men speaking, have a larger vocal tract, which gives the resultant voice a lower-sounding timbre; this is independent of the vocal folds themselves. Human spoken language makes use of the ability of all people in a given society to dynamically modulate certain parameters of the laryngeal voice source in a consistent manner; the most important communicative, or phonetic, parameters are the voice pitch and the degree of separation of the vocal folds, referred to as vocal fold adduction or abduction. The ability to vary the ab/adduction of the vocal folds has a strong genetic component, since vocal fold adduction has a life-preserving function in keeping food from passing into the lungs, in addition to the covering action of the epiglottis.
The muscles that control this action are among the fastest in the body. Children can learn to use this action during speech at an early age, as they learn to speak the difference between utterances such as "apa" as "aba". Enough, they can learn to do this well before the age of two by listening only to the voices of adults around them who have voices much different from their own, though the laryngeal movements causing these phonetic differentiations are deep in the throat and not visible to them. If an abductory movement or adductory movement is strong enough, the vibrations of the vocal folds will stop. If the gesture is abductory and is part of a speech sound, the sound will be called voiceless. However, voiceless speech sounds are sometimes better identified as containing an abductory gesture if the gesture was not strong enough to stop the vocal folds from vibrating; this anomalous feature of voiceless speech sounds is better understood if it is realized that it is the change in the spectral qualities of the voice as abduction proceeds, the primary acoustic attribute that the listener attends to when identifying a voiceless speech sound, not the presence or absence of voice.
An adductory gesture is identified by the change in voice spectral energy it produces. Thus, a speech sound having an adductory gesture may be referred to as a "glottal stop" if the vocal fold vibrations do not stop. Other aspects of the voice, such as variations in the regularity of vibration, are used for communication, are important for the trained voice user to master, but are more used