A finite-state machine or finite-state automaton, finite automaton, or a state machine, is a mathematical model of computation. It is an abstract machine that can be in one of a finite number of states at any given time; the FSM can change from one state to another in response to some external inputs. An FSM is defined by a list of its states, its initial state, the conditions for each transition. Finite state machines are of two types – deterministic finite state machines and non-deterministic finite state machines. A deterministic finite-state machine can be constructed equivalent to any non-deterministic one; the behavior of state machines can be observed in many devices in modern society that perform a predetermined sequence of actions depending on a sequence of events with which they are presented. Simple examples are vending machines, which dispense products when the proper combination of coins is deposited, whose sequence of stops is determined by the floors requested by riders, traffic lights, which change sequence when cars are waiting, combination locks, which require the input of combination numbers in the proper order.
The finite state machine has less computational power than some other models of computation such as the Turing machine. The computational power distinction means there are computational tasks that a Turing machine can do but a FSM cannot; this is because a FSM's memory is limited by the number of states it has. FSMs are studied in the more general field of automata theory. An example of a simple mechanism that can be modeled by a state machine is a turnstile. A turnstile, used to control access to subways and amusement park rides, is a gate with three rotating arms at waist height, one across the entryway; the arms are locked, blocking the entry, preventing patrons from passing through. Depositing a coin or token in a slot on the turnstile unlocks the arms, allowing a single customer to push through. After the customer passes through, the arms are locked again. Considered as a state machine, the turnstile has two possible states: Unlocked. There are two possible inputs that affect its state: pushing the arm.
In the locked state, pushing on the arm has no effect. Putting a coin in – that is, giving the machine a coin input – shifts the state from Locked to Unlocked. In the unlocked state, putting additional coins in has no effect. However, a customer pushing through the arms, giving a push input, shifts the state back to Locked; the turnstile state machine can be represented by a state transition table, showing for each possible state, the transitions between them and the outputs resulting from each input: The turnstile state machine can be represented by a directed graph called a state diagram. Each state is represented by a node. Edges show the transitions from one state to another; each arrow is labeled with the input. An input that doesn't cause a change of state is represented by a circular arrow returning to the original state; the arrow into the Locked node from the black dot indicates. A state is a description of the status of a system, waiting to execute a transition. A transition is a set of actions to be executed when a condition is fulfilled or when an event is received.
For example, when using an audio system to listen to the radio, receiving a "next" stimulus results in moving to the next station. When the system is in the "CD" state, the "next" stimulus results in moving to the next track. Identical stimuli trigger different actions depending on the current state. In some finite-state machine representations, it is possible to associate actions with a state: an entry action: performed when entering the state, an exit action: performed when exiting the state. Several state transition table types are used; the most common representation is shown below: the combination of current state and input shows the next state. The complete action's information is not directly described in the table and can only be added using footnotes. A FSM definition including the full actions information is possible using state tables; the Unified Modeling Language has a notation for describing state machines. UML state machines overcome the limitations of traditional finite state machines while retaining their main benefits.
UML state machines introduce the new concepts of hierarchically nested states and orthogonal regions, while extending the notion of actions. UML state machines have the characteristics of Moore machines, they support actions that depend on both the state of the system and the triggering event, as in Mealy machines, as well as entry and exit actions, which are associated with states rather than transitions, as in Moore machines. The Specification and Description Language is a standard from ITU that includes graphical symbols to describe actions in the transition: send an event receive an event start a timer cancel a timer start another concurrent state machine decisionSDL embeds basic data types called "Abstract Data Types", an action language, an execution semantic in order to make the finite state machine executable. There are a large number of variants to represent an FSM such as the one in figure 3. In addition to their use in modeling reactive systems
A mobile phone, cell phone, cellphone, or hand phone, sometimes shortened to mobile, cell or just phone, is a portable telephone that can make and receive calls over a radio frequency link while the user is moving within a telephone service area. The radio frequency link establishes a connection to the switching systems of a mobile phone operator, which provides access to the public switched telephone network. Modern mobile telephone services use a cellular network architecture, therefore, mobile telephones are called cellular telephones or cell phones, in North America. In addition to telephony, 2000s-era mobile phones support a variety of other services, such as text messaging, MMS, Internet access, short-range wireless communications, business applications, video games, digital photography. Mobile phones offering only those capabilities are known as feature phones; the first handheld mobile phone was demonstrated by John F. Mitchell and Martin Cooper of Motorola in 1973, using a handset weighing c. 2 kilograms.
In 1979, Nippon Telegraph and Telephone launched the world's first cellular network in Japan. In 1983, the DynaTAC 8000x was the first commercially available handheld mobile phone. From 1983 to 2014, worldwide mobile phone subscriptions grew to over seven billion—enough to provide one for every person on Earth. In first quarter of 2016, the top smartphone developers worldwide were Samsung and Huawei, smartphone sales represented 78 percent of total mobile phone sales. For feature phones as of 2016, the largest were Samsung and Alcatel. A handheld mobile radio telephone service was envisioned in the early stages of radio engineering. In 1917, Finnish inventor Eric Tigerstedt filed a patent for a "pocket-size folding telephone with a thin carbon microphone". Early predecessors of cellular phones included analog radio communications from trains; the race to create portable telephone devices began after World War II, with developments taking place in many countries. The advances in mobile telephony have been traced in successive "generations", starting with the early zeroth-generation services, such as Bell System's Mobile Telephone Service and its successor, the Improved Mobile Telephone Service.
These 0G systems were not cellular, supported few simultaneous calls, were expensive. The first handheld cellular mobile phone was demonstrated by John F. Mitchell and Martin Cooper of Motorola in 1973, using a handset weighing 2 kilograms; the first commercial automated cellular network analog was launched in Japan by Nippon Telegraph and Telephone in 1979. This was followed in 1981 by the simultaneous launch of the Nordic Mobile Telephone system in Denmark, Finland and Sweden. Several other countries followed in the early to mid-1980s; these first-generation systems could support far more simultaneous calls but still used analog cellular technology. In 1983, the DynaTAC 8000x was the first commercially available handheld mobile phone. In 1991, the second-generation digital cellular technology was launched in Finland by Radiolinja on the GSM standard; this sparked competition in the sector as the new operators challenged the incumbent 1G network operators. Ten years in 2001, the third generation was launched in Japan by NTT DoCoMo on the WCDMA standard.
This was followed by 3.5G, 3G+ or turbo 3G enhancements based on the high-speed packet access family, allowing UMTS networks to have higher data transfer speeds and capacity. By 2009, it had become clear that, at some point, 3G networks would be overwhelmed by the growth of bandwidth-intensive applications, such as streaming media; the industry began looking to data-optimized fourth-generation technologies, with the promise of speed improvements up to ten-fold over existing 3G technologies. The first two commercially available technologies billed as 4G were the WiMAX standard, offered in North America by Sprint, the LTE standard, first offered in Scandinavia by TeliaSonera. 5G is a technology and term used in research papers and projects to denote the next major phase in mobile telecommunication standards beyond the 4G/IMT-Advanced standards. The term 5G is not used in any specification or official document yet made public by telecommunication companies or standardization bodies such as 3GPP, WiMAX Forum or ITU-R.
New standards beyond 4G are being developed by standardization bodies, but they are at this time seen as under the 4G umbrella, not for a new mobile generation. Smartphones have a number of distinguishing features; the International Telecommunication Union measures those with Internet connection, which it calls Active Mobile-Broadband subscriptions. In the developed world, smartphones have now overtaken the usage of earlier mobile systems. However, in the developing world, they account for around 50% of mobile telephony. Feature phone is a term used as a retronym to describe mobile phones which are limited in capabilities in contrast to a modern smartphone. Feature phones provide voice calling and text messaging functionality, in addition to basic multimedia and Internet capabilities, other services offered by the user's wireless service provider. A feature phone has additional functions over and above a basic mobile phone, only capable of voice calling and text messaging. Feature phones and basic mobile phones tend to use a proprietary, custom-designed software and user interface.
By contrast, smartphones use a mobile operating system that shares common traits across devices. There are Orthodox Jewish religious re
Roaming is a wireless telecommunication term used with mobile devices. It refers to the mobile phone being used outside the range of its home network and connects to another available cell network. In more technical terms, roaming refers to the ability for a cellular customer to automatically make and receive voice calls and receive data, or access other services, including home data services, when travelling outside the geographical coverage area of the home network, by means of using a visited network. For example: should a subscriber travel beyond their cell phone company's transmitter range, their cell phone would automatically hop onto another phone company's service, if available; the process is supported by the Telecommunication processes of mobility management, authentication and accounting billing procedures. Roaming is divided into "SIM-based roaming" and "username/password-based roaming", whereby the technical term "roaming" encompasses roaming between networks of different network standards, e.g. WLAN or GSM.
Device equipment and functionality, such as SIM card capability and network interfaces, power management, determine the access possibilities. Using the example of WLAN/GSM roaming, the following scenarios can be differentiated: SIM-based: GSM subscriber roams onto a public WLAN operated by: their GSM operator, or another operator who has a roaming agreement with their GSM operator. Username/password based roaming: GSM subscriber roams onto a public WLAN operated by: their GSM operator, or another operator who has a roaming agreement with their GSM operator. Although these user/network scenarios focus on roaming from GSM network operator's networks roaming can be bi-directional, i.e. from public WLAN operators to GSM networks. Traditional roaming in networks of the same standard, e.g. from a WLAN to a WLAN or a GSM network to a GSM network, has been described above and is defined by the foreignness of the network based on the type of subscriber entry in the home subscriber register. In the case of session continuity, seamless access to these services across different access types is provided.
"Home network" refers to the network. "Visitor network" refers to the network a subscriber roams temporarily and is outside the bounds of the "home network". The legal roaming business aspects negotiated between the roaming partners for billing of the services obtained are stipulated in so called roaming agreements; the GSM Association broadly outlines the content of such roaming agreements in standardized form for its members. For the legal aspects of authentication and billing of the visiting subscriber, the roaming agreements can comprise minimal safety standards, as e.g. location update procedures or financial security or warranty procedures. The details of the roaming process differ among types of cellular networks, but in general, the process resembles the following: Location updating is the mechanism, used to determine the location of an MS in the idle state; when the mobile device is turned on or is transferred via a handover to the network, this new "visited" network sees the device, notices that it is not registered with its own system, attempts to identify its home network.
If there is no roaming agreement between the two networks, maintenance of service is impossible, service is denied by the visited network. The visited network contacts the home network and requests service information about the roaming device using the IMSI number. If successful, the visited network begins to maintain a temporary subscriber record for the device; the home network updates its information to indicate that the cell phone is on the visited network so that any information sent to that device can be routed. It occurs for example. Signaling process: The calling subscriber dials the mobile subscriber's MSISDN of the roaming cell phone. Based on the information contained in the MSISDN, the call is routed to the mobile network gateway MSC. It's done with an ISUP IAM message. To locate the MS, the GMSC sends to the HLR a MAP SRI message; the MAP SRI message contains the MSISDN number and with this MSISDN the HLR will obtain the IMSI. Because of past location updates, the HLR knows the VLR that serves the subscriber.
The HLR will send to the VLR a MAP PRN message to obtain the MSRN of the roaming cell phone. Like that the HLR will be able to route the call to the correct MSC. With the IMSI contained in the MAP PRN message, the VLR assigns a temporary number known as the mobile station roaming number to the roaming cell phone; this MSRN number is sent back to the HLR in a MAP RIA message. Now with the MSRN number, the GMSC knows; the call is made using ISUP signaling between the GMSC and the visited MSC. The GMSC will generate an ISUP IAM message with the MSRN as the called party number; when the MSC of the visitor network receives the IAM, it recognizes the MSRN and knows the IMSI for which the MSRN was allocated. The MSC returns the MSRN to the pool for future use on another call. Afterwards, the MSC sends to the VLR a MAP SI message to request informati
A backbone is a part of computer network that interconnects various pieces of network, providing a path for the exchange of information between different LANs or subnetworks. A backbone can tie together diverse networks in the same building, in different buildings in a campus environment, or over wide areas; the backbone's capacity is greater than the networks connected to it. A large corporation that has many locations may have a backbone network that ties all of the locations together, for example, if a server cluster needs to be accessed by different departments of a company that are located at different geographical locations; the pieces of the network connections that bring these departments together is mentioned as network backbone. Network congestion is taken into consideration while designing backbones. One example of a backbone network is the Internet backbone; the theory, design principles, first instantiation of the backbone network came from the telephone core network, when traffic was purely voice.
The core network was the central part of a telecommunications network that provided various services to customers who were connected by the access network. One of the main functions was to route telephone calls across the PSTN; the term referred to the high capacity communication facilities that connect primary nodes. A core network provided paths for the exchange of information between different sub-networks. In the United States, local exchange core networks were linked by several competing interexchange networks. Core networks had a mesh topology that provided any-to-any connections among devices on the network. Many main service providers would have their own core/backbone networks; some large enterprises have their own core/backbone network, which are connected to the public networks. Core networks provided the following functionality: Aggregation: The highest level of aggregation in a service provider network; the next level in the hierarchy under the core nodes is the distribution networks and the edge networks.
Customer-premises equipment do not connect to the core networks of a large service provider. Authentication: The function to decide whether the user requesting a service from the telecom network is authorized to do so within this network or not. Call Control/Switching: call control or switching functionality decides the future course of call based on the call signalling processing. E.g. switching functionality may decide based on the "called number" that the call be routed towards a subscriber within this operator's network or with number portability more prevalent to another operator's network. Charging: This functionality of the collation and processing of charging data generated by various network nodes. Two common types of charging mechanisms found in present-day networks are prepaid charging and postpaid charging. See Automatic Message Accounting Service Invocation: Core network performs the task of service invocation for its subscribers. Service invocation may happen based on some explicit action by user or implicitly.
It's important to note however that service "execution" may or may not be a core network functionality as third party network/nodes may take part in actual service execution. Gateways: Gateways shall be present in the core network to access other networks. Gateway functionality is dependent on the type of network. Physically, one or more of these logical functionalities may exist in a given core network node. Besides above mentioned functionalities, the following formed part of a telecommunications core network: O&M: Operations & Maintenance centre or Operations Support Systems to configure and provision the core network nodes. Number of subscribers, peak hour call rate, nature of services, geographical preferences are some of the factors which impact the configuration. Network statistics collection, alarm monitoring and logging of various network nodes actions happens in the O&M centre; these stats and traces form important tools for a network operator to monitor the network health and performance and improvise on the same.
Subscriber Database: Core network hosts the subscribers database. Subscriber database is accessed by core network nodes for functions like authentication, service invocation etc. A distributed backbone is a backbone network that consists of a number of connectivity devices connected to a series of central connectivity devices, such as hubs, switches, or routers, in a hierarchy; this kind of topology allows for simple expansion and limited capital outlay for growth, because more layers of devices can be added to existing layers. In a distributed backbone network, all of the devices that access the backbone share the transmission media, as every device connected to this network is sent all transmissions placed on that network. Distributed backbones, in all practicality, are in use by all large-scale networks. Applications in enterprise-wide scenarios confined to a single building are practical, as certain connectivity devices can be assigned to certain floors or departments; each floor or department possesses a LAN and a wiring closet with that workgroup's main hub or router connected to a bus-style network using backbone cabling.
Another advantage of using a distributed backbone is the ability for network administrator to segregate workgroups for ease of management. There is the possibility of single points of failure, referring to connectivity devices high in the series hierarchy; the distributed backbo
Telecommunication is the transmission of signs, messages, writings and sounds or information of any nature by wire, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology, it is transmitted either electrically over physical media, such as cables, or via electromagnetic radiation. Such transmission paths are divided into communication channels which afford the advantages of multiplexing. Since the Latin term communicatio is considered the social process of information exchange, the term telecommunications is used in its plural form because it involves many different technologies. Early means of communicating over a distance included visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, optical heliographs. Other examples of pre-modern long-distance communication included audio messages such as coded drumbeats, lung-blown horns, loud whistles. 20th- and 21st-century technologies for long-distance communication involve electrical and electromagnetic technologies, such as telegraph and teleprinter, radio, microwave transmission, fiber optics, communications satellites.
A revolution in wireless communication began in the first decade of the 20th century with the pioneering developments in radio communications by Guglielmo Marconi, who won the Nobel Prize in Physics in 1909, other notable pioneering inventors and developers in the field of electrical and electronic telecommunications. These included Charles Wheatstone and Samuel Morse, Alexander Graham Bell, Edwin Armstrong and Lee de Forest, as well as Vladimir K. Zworykin, John Logie Baird and Philo Farnsworth; the word telecommunication is a compound of the Greek prefix tele, meaning distant, far off, or afar, the Latin communicare, meaning to share. Its modern use is adapted from the French, because its written use was recorded in 1904 by the French engineer and novelist Édouard Estaunié. Communication was first used as an English word in the late 14th century, it comes from Old French comunicacion, from Latin communicationem, noun of action from past participle stem of communicare "to share, divide out.
Homing pigeons have been used throughout history by different cultures. Pigeon post had Persian roots, was used by the Romans to aid their military. Frontinus said; the Greeks conveyed the names of the victors at the Olympic Games to various cities using homing pigeons. In the early 19th century, the Dutch government used the system in Sumatra, and in 1849, Paul Julius Reuter started a pigeon service to fly stock prices between Aachen and Brussels, a service that operated for a year until the gap in the telegraph link was closed. In the Middle Ages, chains of beacons were used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could only pass a single bit of information, so the meaning of the message such as "the enemy has been sighted" had to be agreed upon in advance. One notable instance of their use was during the Spanish Armada, when a beacon chain relayed a signal from Plymouth to London. In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system between Lille and Paris.
However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres. As a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880. On 25 July 1837 the first commercial electrical telegraph was demonstrated by English inventor Sir William Fothergill Cooke, English scientist Sir Charles Wheatstone. Both inventors viewed their device as "an improvement to the electromagnetic telegraph" not as a new device. Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837, his code was an important advance over Wheatstone's signaling method. The first transatlantic telegraph cable was completed on 27 July 1866, allowing transatlantic telecommunication for the first time; the conventional telephone was invented independently by Alexander Bell and Elisha Gray in 1876. Antonio Meucci invented the first device that allowed the electrical transmission of voice over a line in 1849.
However Meucci's device was of little practical value because it relied upon the electrophonic effect and thus required users to place the receiver in their mouth to "hear" what was being said. The first commercial telephone services were set-up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven and London. Starting in 1894, Italian inventor Guglielmo Marconi began developing a wireless communication using the newly discovered phenomenon of radio waves, showing by 1901 that they could be transmitted across the Atlantic Ocean; this was the start of wireless telegraphy by radio. Voice and music had little early success. World War I accelerated the development of radio for military communications. After the war, commercial radio AM broadcasting began in the 1920s and became an important mass medium for entertainment and news. World War II again accelerated development of radio for the wartime purposes of aircraft and land communication, radio navigation and radar. Development of stereo FM broadcasting of radio
Internet protocol suite
The Internet protocol suite is the conceptual model and set of communications protocols used in the Internet and similar computer networks. It is known as TCP/IP because the foundational protocols in the suite are the Transmission Control Protocol and the Internet Protocol, it is known as the Department of Defense model because the development of the networking method was funded by the United States Department of Defense through DARPA. The Internet protocol suite provides end-to-end data communication specifying how data should be packetized, transmitted and received; this functionality is organized into four abstraction layers, which classify all related protocols according to the scope of networking involved. From lowest to highest, the layers are the link layer, containing communication methods for data that remains within a single network segment; the technical standards underlying the Internet protocol suite and its constituent protocols are maintained by the Internet Engineering Task Force.
The Internet protocol suite predates the OSI model, a more comprehensive reference framework for general networking systems. The Internet protocol suite resulted from research and development conducted by the Defense Advanced Research Projects Agency in the late 1960s. After initiating the pioneering ARPANET in 1969, DARPA started work on a number of other data transmission technologies. In 1972, Robert E. Kahn joined the DARPA Information Processing Technology Office, where he worked on both satellite packet networks and ground-based radio packet networks, recognized the value of being able to communicate across both. In the spring of 1973, Vinton Cerf, who helped develop the existing ARPANET Network Control Program protocol, joined Kahn to work on open-architecture interconnection models with the goal of designing the next protocol generation for the ARPANET. By the summer of 1973, Kahn and Cerf had worked out a fundamental reformulation, in which the differences between local network protocols were hidden by using a common internetwork protocol, instead of the network being responsible for reliability, as in the ARPANET, this function was delegated to the hosts.
Cerf credits Hubert Zimmermann and Louis Pouzin, designer of the CYCLADES network, with important influences on this design. The protocol was implemented as the Transmission Control Program, first published in 1974; the TCP managed both datagram transmissions and routing, but as the protocol grew, other researchers recommended a division of functionality into protocol layers. Advocates included Jonathan Postel of the University of Southern California's Information Sciences Institute, who edited the Request for Comments, the technical and strategic document series that has both documented and catalyzed Internet development. Postel stated, "We are screwing up in our design of Internet protocols by violating the principle of layering." Encapsulation of different mechanisms was intended to create an environment where the upper layers could access only what was needed from the lower layers. A monolithic design would lead to scalability issues; the Transmission Control Program was split into two distinct protocols, the Transmission Control Protocol and the Internet Protocol.
The design of the network included the recognition that it should provide only the functions of efficiently transmitting and routing traffic between end nodes and that all other intelligence should be located at the edge of the network, in the end nodes. This design is known as the end-to-end principle. Using this design, it became possible to connect any network to the ARPANET, irrespective of the local characteristics, thereby solving Kahn's initial internetworking problem. One popular expression is that TCP/IP, the eventual product of Cerf and Kahn's work, can run over "two tin cans and a string." Years as a joke, the IP over Avian Carriers formal protocol specification was created and tested. A computer called, it forwards network packets forth between them. A router was called gateway, but the term was changed to avoid confusion with other types of gateways. From 1973 to 1974, Cerf's networking research group at Stanford worked out details of the idea, resulting in the first TCP specification.
A significant technical influence was the early networking work at Xerox PARC, which produced the PARC Universal Packet protocol suite, much of which existed around that time. DARPA contracted with BBN Technologies, Stanford University, the University College London to develop operational versions of the protocol on different hardware platforms. Four versions were developed: TCP v1, TCP v2, TCP v3 and IP v3, TCP/IP v4; the last protocol is still in use today. In 1975, a two-network TCP/IP communications test was performed between Stanford and University College London. In November 1977, a three-network TCP/IP test was conducted between sites in the US, the UK, Norway. Several other TCP/IP prototypes were developed at multiple research centers between 1978 and 1983. In March 1982, the US Department of Defense declared TCP/IP as the standard for all military computer networking. In the same year, Peter T. Kirstein's research group at University College London adopted the protocol; the migration of the ARPANET to TCP/IP was completed on flag day January 1, 1983, when the new protocols were permanently activated.
In 1985, the Internet Advisory Board held a three-day TCP/
Dual-tone multi-frequency signaling
Dual-tone multi-frequency signaling is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed in the Bell System in the United States, became known under the trademark Touch-Tone for use in push-button telephones supplied to telephone customers, starting in 1963. DTMF is standardized as ITU-T Recommendation Q.23. It is known in the UK as MF4; the Touch-Tone system using a telephone keypad replaced the use of rotary dial and has become the industry standard for landline and mobile service. Other multi-frequency systems are used for internal signaling within the telephone network. Prior to the development of DTMF, telephone numbers were dialed by users with a loop-disconnect signaling, more known as pulse dialing in the U. S, it functions by interrupting the current in the local loop between the telephone exchange and the calling party's telephone at a precise rate with a switch in the telephone, operated by the rotary dial as it spins back to its rest position after having been rotated to each desired number.
The exchange equipment responds to the dial pulses either directly by operating relays, or by storing the number in a digit register recording the dialed number. The physical distance for which this type of dialing was possible was restricted by electrical distortions and was only possible on direct metallic links between end points of a line. Placing calls over longer distances required either operator assistance or provision of special subscriber trunk dialing equipment. Operators used an earlier type of multi-frequency signaling. Multi-frequency signaling is a group of signaling methods that use a mixture of two pure tone sounds. Various MF signaling protocols were devised by the Bell System and CCITT; the earliest of these were for in-band signaling between switching centers, where long-distance telephone operators used a 16-digit keypad to input the next portion of the destination telephone number in order to contact the next downstream long-distance telephone operator. This semi-automated signaling and switching proved successful in both cost effectiveness.
Based on this prior success with using MF by specialists to establish long-distance telephone calls, dual-tone multi-frequency signaling was developed for end-user signaling without the assistance of operators. The DTMF system uses a set of eight audio frequencies transmitted in pairs to represent 16 signals, represented by the ten digits, the letters A to D, the symbols # and *; as the signals are audible tones in the voice frequency range, they can be transmitted through electrical repeaters and amplifiers, over radio and microwave links, thus eliminating the need for intermediate operators on long-distance circuits. AT&T described the product as "a method for pushbutton signaling from customer stations using the voice transmission path." In order to prevent consumer telephones from interfering with the MF-based routing and switching between telephone switching centers, DTMF frequencies differ from all of the pre-existing MF signaling protocols between switching centers: MF/R1, R2, CCS4, CCS5, others that were replaced by SS7 digital signaling.
DTMF was known throughout the Bell System by the trademark Touch-Tone. The term was first used by AT&T in commerce on July 5, 1960 and was introduced to the public on November 18, 1963, when the first push-button telephone was made available to the public, it was a registered trademark by AT&T from September 4, 1962 to March 13, 1984. It is standardized by ITU-T Recommendation Q.23. In the UK, it is known as MF4. Other vendors of compatible telephone equipment called the Touch-Tone feature tone dialing or DTMF, or used their other trade names such as Digitone by Northern Electric Company in Canada; as a method of in-band signaling, DTMF signals were used by cable television broadcasters to indicate the start and stop times of local commercial insertion points during station breaks for the benefit of cable companies. Until out-of-band signaling equipment was developed in the 1990s, unacknowledged DTMF tone sequences could be heard during the commercial breaks of cable channels in the United States and elsewhere.
Terrestrial television stations used DTMF tones to control remote transmitters. In IP telephony, DTMF signals can be delivered as either in-band or out-of-band tones, or as a part of signaling protocols, as long as both endpoints agree on a common approach to adopt; the engineers had envisioned telephones being used to access computers and automated response systems. They consulted with companies to determine the requirements; this led to the addition of the number sign and asterisk or "star" keys as well as a group of keys for menu selection: A, B, C and D. In the end, the lettered keys were dropped from most phones, it was many years before the two symbol keys became used for vertical service codes such as *67 in the United States of America and Canada to suppress caller ID. Public payphones that accept credit cards use these additional codes to send the information from the magnetic strip; the AUTOVON telephone system of the United States Armed Forces used these signals to assert certain privilege and priority levels when placing telephone calls.
Precedence is still a feature of military telephone networks. For example, entering 93 before a number is a priority call. Present-day uses of the A, B, C and D signals on telephone networks are few, are exclusive to network control. For example, the A key is used