A dial tone (known in the British Isles as a dialling tone) is a telephony signal used to indicate that the telephone exchange is working and ready to accept a call. The tone stops when the first numeral is dialed, or if there is no response after going off-hook (a timeout), when it is usually followed by a special information tone.
When automated telephone systems were first being deployed, telephone companies noticed that customers were often confused by the apparent lack of response (before this, a telephone operator would answer), and would often assume the phone was not working. To avoid this, exchange systems would play a small buzzing sound into the line instead. Before modern electronic telephone switching systems came into use, dial tones were usually generated by electromechanical means; in the United States, the standard "city" dial tone consisted of a 600 Hz tone amplitude-modulated at 120 Hz. Some dial tones were simply adapted from 60 Hz AC line current. In the UK, the standard Post Office dial tone was 33 Hz: this was generated by a motor-driven ringing machine in most exchanges, and by a vibrating-reed generator in the smallest.
The modern dial tone varies between countries, being a "buzz" of two interfering tones (350 Hz and 440 Hz, as defined in the Precise Tone Plan) in the NANP (most of North America), and a constant single tone (425 Hz) in most of Europe. Modems, fax machines, and autodialers must be designed to recognise these so-called call-progress tones, as well as comply with differing standards and regulations.
Private or internal PBX or key phone system also have their own dial tone, sometimes the same as the external PSTN one, and sometimes different to remind users to dial a prefix for an outside line.
A "stuttered" or interrupted dial tone may mean that there is voice mail waiting, or may occur to confirm that a calling feature such as call forwarding has been activated.
Mobile phones do not have dial tones, as the user instead presses "send" to actually go off-hook and complete the call once the desired phone number is entered.
An experienced guitar player will be able to tune their instrument to a U.S. dial tone, since its two frequencies correspond to the standard concert pitch "A" of 440 Hz, and (approximately) the "F" below.
Showing posts with label Telephone. Show all posts
Showing posts with label Telephone. Show all posts
Monday, July 9, 2007
Telephone
The telephone is a telecommunications device which is used to transmit and receive sound (most commonly speech). Most telephones operate through transmission of electric signals over a complex telephone network which allows almost any phone user to communicate with almost any other.
[edit] Basic principle
The telephone handles two kinds of information: signals and voice, at different times on the same twisted pair of wires. The signaling equipment consists of a bell to alert the user of incoming calls, and a dial to enter the phone number for outgoing calls. A calling party wishing to speak to another telephone will pick up the handset, thus operating the switch hook, which puts the telephone into active state or off hook with a resistance short across the wires, causing current to flow. The telephone exchange detects the DC current, attaches a digit receiver, and sends dial tone to indicate readiness. The user pushes the number buttons, which are connected to a tone generator inside the dial, which generates DTMF tones. The exchange connects the line to the desired line and alerts that line.
When a phone is inactive (on hook), its bell, beeper, flasher or other alerting device is connected across the line through a capacitor. The inactive phone does not short the line, thus the exchange knows it is on hook and only the bell is electrically connected. When someone calls this phone, the telephone exchange applies a high voltage pulsating signal, which causes the sound mechanism to ring, beep or otherwise alert the called party. When that user picks up the handset, the switchhook disconnects the bell, connects the voice parts of the telephone, and puts a resistance short on the line, confirming that the phone has been answered and is active. Both lines being off hook, the signaling job is complete. The parties are connected together and may converse using the voice parts of their telephones.
The voice parts of the telephone are in the handset, and consist of a transmitter (often called microphone) and a receiver. The transmitter, powered from the line, puts out an electric current which varies in response to the acoustic pressure waves produced by the voice. The resulting variations in electric current are transmitted along the telephone line to the other phone, where they are fed into the coil of the receiver, which is a miniature loudspeaker. The varying electric current in the coil causes it to move back and forth, reproducing the acoustic pressure waves of the transmitter.
When a party "hangs up" (puts the handset on the cradle), DC current ceases to flow in that line, thus signaling to the exchange switch to disconnect the telephone call.
[edit] History
Main articles: History of the telephone and Timeline of the telephone
Credit for inventing the electric telephone remains in dispute. Antonio Meucci, Johann Philipp Reis, Alexander Graham Bell, and Elisha Gray, among others, have all been credited with the invention.
The early history of the telephone is a confusing morass of claim and counterclaim, which was not clarified by the huge mass of lawsuits which hoped to resolve the patent claims of individuals. The Bell and Edison patents, however were forensically victorious and commercially decisive.
Further information: Invention of the telephone and Elisha Gray and Alexander Bell Controversy
[edit] Early development
The following is a brief summary of the legally supported history of the development of the telephone:
28 December 1871—Antonio Meucci files a patent caveat (n.3335) in the U.S. Patent Office titled "Sound Telegraph", describing communication of voice between two people by wire.
1874—Meucci, after having renewed the caveat for two years, fails to find the money to renew it. The caveat lapses.
6 April 1875—Bell's U.S. Patent 161,739 "Transmitters and Receivers for Electric Telegraphs" is granted. This uses multiple vibrating steel reeds in make-break circuits.
11 February 1876—Gray invents a liquid transmitter for use with a telephone but does not build one.
14 February 1876—Elisha Gray files a patent caveat for transmitting the human voice through a telegraphic circuit.
14 February 1876—Alexander Bell applies for the patent "Improvements in Telegraphy", for electromagnetic telephones using undulating currents.
19 February 1876—Gray is notified by the U.S. Patent Office of an interference between his caveat and Bell's patent application. Gray decides to abandon his caveat.
7 March 1876—Bell's U.S. patent 174,465 "Improvement in Telegraphy" is granted, covering "the method of, and apparatus for, transmitting vocal or other sounds telegraphically … by causing electrical undulations, similar in form to the vibrations of the air accompanying the said vocal or other sound."
30 January 1877—Bell's U.S. patent 186,787 is granted for an electro-magnetic telephone using permanent magnets, iron diaphragms, and a call bell.
27 April 1877—Edison files for a patent on a carbon (graphite) transmitter. The patent 474,230 was granted 3 May 1892, after a 15 year delay because of litigation. Edison was granted patent 222,390 for a carbon granules transmitter in 1879.
[edit] Early commercial instruments
Early telephones were technically diverse. Some used a liquid transmitter, which was dangerous, inconvenient, and soon went out of use. Some were dynamic: their diaphragm wriggled a coil of wire in the field of a permanent magnet or vice versa. This kind survived in small numbers through the 20th century in military and maritime applications where its ability to create its own electrical power was crucial. Most, however, used the Edison/Berliner carbon transmitter, which was much louder than the other kinds, even though it required an induction coil, actually acting as an impedance matching transformer to make it compatible to the impedance of the line. The Edison patents kept the Bell monopoly viable into the 20th century, by which time the network was more important than the instrument anyway.
Early telephones were locally powered, using a dynamic transmitter or else powering the transmitter with a local battery. One of the jobs of outside plant personnel was to visit each telephone periodically to inspect the battery. During the 20th century, "common battery" operation came to dominate, powered by "talk battery" from the telephone exchange over the same wires that carried the voice signals. Late in the century, wireless handsets brought a revival of local battery power.
Early telephones had one wire for both transmitting and receiving of audio, with ground return as used in telegraphs. The earliest dynamic telephones also had only one opening for sound, and the user alternately listened and spoke (rather, shouted) into the same hole. Sometimes the instruments were operated in pairs at each end, making conversation more convenient but were more expensive.
At first, the benefits of an exchange were not exploited. Telephones instead were leased in pairs to the subscriber, who had to arrange telegraph contractors to construct a line between them, for example between his home and his shop. Users who wanted the ability to speak to several different locations would need to obtain and set up three or four pairs of telephones. Western Union, already using telegraph exchanges, quickly extended the principle to its telephones in New York City and San Francisco, and Bell was not slow in appreciating the potential.
Signalling began in an appropriately primitive manner. The user alerted the other end, or the exchange operator, by whistling into the transmitter. Exchange operation soon resulted in telephones being equipped with a bell, first operated over a second wire and later with the same wire using a condenser. Telephones connected to the earliest Strowger automatic exchanges had seven wires, one for the knife switch, one for each telegraph key, one for the bell, one for the push button and two for speaking.
Rural and other telephones that were not on a common battery exchange had a "magneto" or hand cranked generator to produce a high voltage alternating signal to ring the bells of other telephones on the line and to alert the operator.
In the 1890s a new smaller style of telephone was introduced, packaged in three parts. The transmitter stood on a stand, known as a "candlestick" for its shape. When not in use, the receiver hung on a hook with a switch in it, known as a "switchhook." Previous telephones required the user to operate a separate switch to connect either the voice or the bell. With the new kind, the user was less likely to leave the phone "off the hook". In phones connected to magneto exchanges, the bell, induction coil, battery and magneto were in a separate "bell box." In phones connected to common battery exchanges, the bell box was installed under a desk, or other out of the way place, since it did not need a battery or magneto.
Cradle designs were also used at this time, having a handle with the receiver and transmitter attached, separate from the cradle base that housed the magneto crank and other parts. They were larger than the "candlestick" and more popular.
Disadvantages of single wire operation such as crosstalk and hum from nearby AC power wires had already led to the use of twisted pairs and, for long distance telephones, four-wire circuits. Users at the beginning of the 20th century did not place long distance calls from their own telephones but made an appointment to use a special sound proofed long distance telephone booth furnished with the latest technology.
What turned out to be the most popular and longest lasting physical style of telephone was introduced in the early 20th century, including Bell's Model 102. A carbon granule transmitter and electromagnetic receiver were united in a single molded plastic handle, which when not in use sat in a cradle in the base unit. The circuit diagram of the Model 102 shows the direct connection of the receiver to the line, while the transmitter was induction coupled, with energy supplied by a local battery. The coupling transformer, battery, and ringer were in a separate enclosure. The dial switch in the base interrupted the line current by repeatedly but very briefly disconnecting the line 1-10 times for each digit, and the hook switch (in the center of the circuit diagram) permanently disconnected the line and the transmitter battery while the handset was on the cradle.
After the 1930s, the base also enclosed the bell and induction coil, obviating the old separate bell box. Power was supplied to each subscriber line by central office batteries instead of a local battery, which required periodic service. For the next half century, the network behind the telephone became progressively larger and much more efficient, but after the dial was added the instrument itself changed little until touch tone replaced the dial in the 1960s.
[edit] Digital Telephony
Main article: Digital Telephony
The Public Switched Telephone Network (PSTN) has gradually evolved towards digital telephony which has improved the capacity and quality of the network. End-to-end analog telephone networks were first modified in the early 1960s by upgrading transmission networks with T1 carrier systems. Later technologies such as SONET and fiber optic transmission methods further advanced digital transmission. Although analog carrier systems existed, digital transmission made it possible to significantly increase the number of channels multiplexed on a single transmission medium. While today the end instrument remains analog, the analog signals reaching the aggregation point (Serving Area Interface (SAI) or the central office (CO) ) are typically converted to digital signals. Digital loop carriers (DLC) are often used, placing the digital network ever closer to the customer premises, relegating the analog local loop to legacy status.
[edit] IP telephony
A WiFi-based VoIP phoneInternet Protocol (IP) telephony (also known as internet telephony) is a service based on Voice over IP (VoIP), a disruptive technology that is rapidly gaining ground against traditional telephone network technologies. In Japan and South Korea up to 10% of subscribers, as of January 2005, have switched to this digital telephone service. A January 2005 Newsweek article suggested that internet telephony may be "the next big thing." [1]
As of 2006 many VoIP companies offer service to consumers and businesses.
IP telephony uses a broadband internet connection to transmit conversations as data packets. In addition to replacing POTS, IP telephony is also competing with mobile phone networks by offering free or lower cost connections via WiFi hotspots. VoIP is also used on private wireless networks which may or may not have a connection to the outside telephone network.
Telephone operating companies
In some countries, many telephone operating companies (commonly abbreviated to telco in American English) are in competition to provide telephone services. Some of them are included in the following list. However, the list only includes facilities based providers and not companies which lease services from facilities based providers in order to serve their customers.
[edit] Basic principle
The telephone handles two kinds of information: signals and voice, at different times on the same twisted pair of wires. The signaling equipment consists of a bell to alert the user of incoming calls, and a dial to enter the phone number for outgoing calls. A calling party wishing to speak to another telephone will pick up the handset, thus operating the switch hook, which puts the telephone into active state or off hook with a resistance short across the wires, causing current to flow. The telephone exchange detects the DC current, attaches a digit receiver, and sends dial tone to indicate readiness. The user pushes the number buttons, which are connected to a tone generator inside the dial, which generates DTMF tones. The exchange connects the line to the desired line and alerts that line.
When a phone is inactive (on hook), its bell, beeper, flasher or other alerting device is connected across the line through a capacitor. The inactive phone does not short the line, thus the exchange knows it is on hook and only the bell is electrically connected. When someone calls this phone, the telephone exchange applies a high voltage pulsating signal, which causes the sound mechanism to ring, beep or otherwise alert the called party. When that user picks up the handset, the switchhook disconnects the bell, connects the voice parts of the telephone, and puts a resistance short on the line, confirming that the phone has been answered and is active. Both lines being off hook, the signaling job is complete. The parties are connected together and may converse using the voice parts of their telephones.
The voice parts of the telephone are in the handset, and consist of a transmitter (often called microphone) and a receiver. The transmitter, powered from the line, puts out an electric current which varies in response to the acoustic pressure waves produced by the voice. The resulting variations in electric current are transmitted along the telephone line to the other phone, where they are fed into the coil of the receiver, which is a miniature loudspeaker. The varying electric current in the coil causes it to move back and forth, reproducing the acoustic pressure waves of the transmitter.
When a party "hangs up" (puts the handset on the cradle), DC current ceases to flow in that line, thus signaling to the exchange switch to disconnect the telephone call.
[edit] History
Main articles: History of the telephone and Timeline of the telephone
Credit for inventing the electric telephone remains in dispute. Antonio Meucci, Johann Philipp Reis, Alexander Graham Bell, and Elisha Gray, among others, have all been credited with the invention.
The early history of the telephone is a confusing morass of claim and counterclaim, which was not clarified by the huge mass of lawsuits which hoped to resolve the patent claims of individuals. The Bell and Edison patents, however were forensically victorious and commercially decisive.
Further information: Invention of the telephone and Elisha Gray and Alexander Bell Controversy
[edit] Early development
The following is a brief summary of the legally supported history of the development of the telephone:
28 December 1871—Antonio Meucci files a patent caveat (n.3335) in the U.S. Patent Office titled "Sound Telegraph", describing communication of voice between two people by wire.
1874—Meucci, after having renewed the caveat for two years, fails to find the money to renew it. The caveat lapses.
6 April 1875—Bell's U.S. Patent 161,739 "Transmitters and Receivers for Electric Telegraphs" is granted. This uses multiple vibrating steel reeds in make-break circuits.
11 February 1876—Gray invents a liquid transmitter for use with a telephone but does not build one.
14 February 1876—Elisha Gray files a patent caveat for transmitting the human voice through a telegraphic circuit.
14 February 1876—Alexander Bell applies for the patent "Improvements in Telegraphy", for electromagnetic telephones using undulating currents.
19 February 1876—Gray is notified by the U.S. Patent Office of an interference between his caveat and Bell's patent application. Gray decides to abandon his caveat.
7 March 1876—Bell's U.S. patent 174,465 "Improvement in Telegraphy" is granted, covering "the method of, and apparatus for, transmitting vocal or other sounds telegraphically … by causing electrical undulations, similar in form to the vibrations of the air accompanying the said vocal or other sound."
30 January 1877—Bell's U.S. patent 186,787 is granted for an electro-magnetic telephone using permanent magnets, iron diaphragms, and a call bell.
27 April 1877—Edison files for a patent on a carbon (graphite) transmitter. The patent 474,230 was granted 3 May 1892, after a 15 year delay because of litigation. Edison was granted patent 222,390 for a carbon granules transmitter in 1879.
[edit] Early commercial instruments
Early telephones were technically diverse. Some used a liquid transmitter, which was dangerous, inconvenient, and soon went out of use. Some were dynamic: their diaphragm wriggled a coil of wire in the field of a permanent magnet or vice versa. This kind survived in small numbers through the 20th century in military and maritime applications where its ability to create its own electrical power was crucial. Most, however, used the Edison/Berliner carbon transmitter, which was much louder than the other kinds, even though it required an induction coil, actually acting as an impedance matching transformer to make it compatible to the impedance of the line. The Edison patents kept the Bell monopoly viable into the 20th century, by which time the network was more important than the instrument anyway.
Early telephones were locally powered, using a dynamic transmitter or else powering the transmitter with a local battery. One of the jobs of outside plant personnel was to visit each telephone periodically to inspect the battery. During the 20th century, "common battery" operation came to dominate, powered by "talk battery" from the telephone exchange over the same wires that carried the voice signals. Late in the century, wireless handsets brought a revival of local battery power.
Early telephones had one wire for both transmitting and receiving of audio, with ground return as used in telegraphs. The earliest dynamic telephones also had only one opening for sound, and the user alternately listened and spoke (rather, shouted) into the same hole. Sometimes the instruments were operated in pairs at each end, making conversation more convenient but were more expensive.
At first, the benefits of an exchange were not exploited. Telephones instead were leased in pairs to the subscriber, who had to arrange telegraph contractors to construct a line between them, for example between his home and his shop. Users who wanted the ability to speak to several different locations would need to obtain and set up three or four pairs of telephones. Western Union, already using telegraph exchanges, quickly extended the principle to its telephones in New York City and San Francisco, and Bell was not slow in appreciating the potential.
Signalling began in an appropriately primitive manner. The user alerted the other end, or the exchange operator, by whistling into the transmitter. Exchange operation soon resulted in telephones being equipped with a bell, first operated over a second wire and later with the same wire using a condenser. Telephones connected to the earliest Strowger automatic exchanges had seven wires, one for the knife switch, one for each telegraph key, one for the bell, one for the push button and two for speaking.
Rural and other telephones that were not on a common battery exchange had a "magneto" or hand cranked generator to produce a high voltage alternating signal to ring the bells of other telephones on the line and to alert the operator.
In the 1890s a new smaller style of telephone was introduced, packaged in three parts. The transmitter stood on a stand, known as a "candlestick" for its shape. When not in use, the receiver hung on a hook with a switch in it, known as a "switchhook." Previous telephones required the user to operate a separate switch to connect either the voice or the bell. With the new kind, the user was less likely to leave the phone "off the hook". In phones connected to magneto exchanges, the bell, induction coil, battery and magneto were in a separate "bell box." In phones connected to common battery exchanges, the bell box was installed under a desk, or other out of the way place, since it did not need a battery or magneto.
Cradle designs were also used at this time, having a handle with the receiver and transmitter attached, separate from the cradle base that housed the magneto crank and other parts. They were larger than the "candlestick" and more popular.
Disadvantages of single wire operation such as crosstalk and hum from nearby AC power wires had already led to the use of twisted pairs and, for long distance telephones, four-wire circuits. Users at the beginning of the 20th century did not place long distance calls from their own telephones but made an appointment to use a special sound proofed long distance telephone booth furnished with the latest technology.
What turned out to be the most popular and longest lasting physical style of telephone was introduced in the early 20th century, including Bell's Model 102. A carbon granule transmitter and electromagnetic receiver were united in a single molded plastic handle, which when not in use sat in a cradle in the base unit. The circuit diagram of the Model 102 shows the direct connection of the receiver to the line, while the transmitter was induction coupled, with energy supplied by a local battery. The coupling transformer, battery, and ringer were in a separate enclosure. The dial switch in the base interrupted the line current by repeatedly but very briefly disconnecting the line 1-10 times for each digit, and the hook switch (in the center of the circuit diagram) permanently disconnected the line and the transmitter battery while the handset was on the cradle.
After the 1930s, the base also enclosed the bell and induction coil, obviating the old separate bell box. Power was supplied to each subscriber line by central office batteries instead of a local battery, which required periodic service. For the next half century, the network behind the telephone became progressively larger and much more efficient, but after the dial was added the instrument itself changed little until touch tone replaced the dial in the 1960s.
[edit] Digital Telephony
Main article: Digital Telephony
The Public Switched Telephone Network (PSTN) has gradually evolved towards digital telephony which has improved the capacity and quality of the network. End-to-end analog telephone networks were first modified in the early 1960s by upgrading transmission networks with T1 carrier systems. Later technologies such as SONET and fiber optic transmission methods further advanced digital transmission. Although analog carrier systems existed, digital transmission made it possible to significantly increase the number of channels multiplexed on a single transmission medium. While today the end instrument remains analog, the analog signals reaching the aggregation point (Serving Area Interface (SAI) or the central office (CO) ) are typically converted to digital signals. Digital loop carriers (DLC) are often used, placing the digital network ever closer to the customer premises, relegating the analog local loop to legacy status.
[edit] IP telephony
A WiFi-based VoIP phoneInternet Protocol (IP) telephony (also known as internet telephony) is a service based on Voice over IP (VoIP), a disruptive technology that is rapidly gaining ground against traditional telephone network technologies. In Japan and South Korea up to 10% of subscribers, as of January 2005, have switched to this digital telephone service. A January 2005 Newsweek article suggested that internet telephony may be "the next big thing." [1]
As of 2006 many VoIP companies offer service to consumers and businesses.
IP telephony uses a broadband internet connection to transmit conversations as data packets. In addition to replacing POTS, IP telephony is also competing with mobile phone networks by offering free or lower cost connections via WiFi hotspots. VoIP is also used on private wireless networks which may or may not have a connection to the outside telephone network.
Telephone operating companies
In some countries, many telephone operating companies (commonly abbreviated to telco in American English) are in competition to provide telephone services. Some of them are included in the following list. However, the list only includes facilities based providers and not companies which lease services from facilities based providers in order to serve their customers.
Telephone Call
A telephone call is a connection over a telephone network between the calling party and the called party.
Information transmission
A telephone call may consist of an ordinary voice transmission using a telephone, a data transmission when the calling party and called party are using modems, or a facsimile transmission when the two parties are using fax machines. Where a telephone call has more than one called party it is referred to as a conference call.
Calls are usually placed through a network (such as the Public Switched Telephone Network) provided by a commercial telephone company. If the caller's wireline phone is directly connected to the calling party, when the caller takes their telephone off-hook, the calling party's phone will ring. This is called a hot line or ringdown. Otherwise, the calling party is usually given a tone to indicate they should begin dialing the desired number. In some (now very rare) cases, the calling party cannot dial calls directly, and is connected to an operator who places the call for them.
Most telephone calls in the world are set up using ISUP messages or one of its variants between telephone exchanges to establish the end to end connection.
[edit] Costs
Some types of calls are not charged, such as local calls dialed directly by a telephone subscriber in Canada, the United States, Hong Kong, or New Zealand (Residential subscribers only). In most other areas, all telephone calls are charged a fee for the connection. Fees depend on the provider of the service, the type of service being used (a call placed from a landline or wired telephone will have one rate, and a call placed from a mobile telephone will have a different rate) and the distance between the calling and the called parties. In most circumstances, the calling party pays this fee. However, in some circumstances such as a reverse charge or collect call, the called party pays the cost of the call. In some circumstances, the caller pays a flat rate charge for the telephone connection and does not pay any additional charge for all calls made. Telecommunication liberalization has been established in several countries to allows customers to keep their local phone provider and use an alternate provider for a certain call in order to save money.
[edit] Tones
A typical phone call is placed by picking the phone handset up off the base and holding the handset so that the hearing end is next to the users ear and the speaking end is within range of the mouth. Headsets are becoming more and more common, especially in car headsets, thus changing the way that people are conducting telephone calls in modern times. Preceding, during, and after a telephone call is placed, certain tones signify the progress and status of the telephone call:
a dial tone signifying that the call is ready to be placed
either:
a ringing tone signifying that the calling party has yet to answer the telephone
a busy signal (or engaged tone) signifying that the calling party's telephone is being used in a telephone call to another person (or is "off the hook" though no number has been dialled, ie the customer does not want to be disturbed)
a busy signal (or overflow busy tone) signifying that there is congestion in the telephone network, or possibly that the calling subscriber has delayed too long in dialling all the necessary digits
status tones such as STD notification tones (to inform the caller that the telephone call is being trunk dialled at a greater cost to the calling party), minute minder beeps (to inform the caller of the relative duration of the telephone call on calls that are charged on a time basis), and others
a tone (sometimes the busy signal) to signify that the called party has hung up.
tones used by earlier inband telephone switching systems were simulated by a Black box or a Blue box used by "phone phreaks" to illegally make or receive free trunk/toll calls, see Blue box.
Information transmission
A telephone call may consist of an ordinary voice transmission using a telephone, a data transmission when the calling party and called party are using modems, or a facsimile transmission when the two parties are using fax machines. Where a telephone call has more than one called party it is referred to as a conference call.
Calls are usually placed through a network (such as the Public Switched Telephone Network) provided by a commercial telephone company. If the caller's wireline phone is directly connected to the calling party, when the caller takes their telephone off-hook, the calling party's phone will ring. This is called a hot line or ringdown. Otherwise, the calling party is usually given a tone to indicate they should begin dialing the desired number. In some (now very rare) cases, the calling party cannot dial calls directly, and is connected to an operator who places the call for them.
Most telephone calls in the world are set up using ISUP messages or one of its variants between telephone exchanges to establish the end to end connection.
[edit] Costs
Some types of calls are not charged, such as local calls dialed directly by a telephone subscriber in Canada, the United States, Hong Kong, or New Zealand (Residential subscribers only). In most other areas, all telephone calls are charged a fee for the connection. Fees depend on the provider of the service, the type of service being used (a call placed from a landline or wired telephone will have one rate, and a call placed from a mobile telephone will have a different rate) and the distance between the calling and the called parties. In most circumstances, the calling party pays this fee. However, in some circumstances such as a reverse charge or collect call, the called party pays the cost of the call. In some circumstances, the caller pays a flat rate charge for the telephone connection and does not pay any additional charge for all calls made. Telecommunication liberalization has been established in several countries to allows customers to keep their local phone provider and use an alternate provider for a certain call in order to save money.
[edit] Tones
A typical phone call is placed by picking the phone handset up off the base and holding the handset so that the hearing end is next to the users ear and the speaking end is within range of the mouth. Headsets are becoming more and more common, especially in car headsets, thus changing the way that people are conducting telephone calls in modern times. Preceding, during, and after a telephone call is placed, certain tones signify the progress and status of the telephone call:
a dial tone signifying that the call is ready to be placed
either:
a ringing tone signifying that the calling party has yet to answer the telephone
a busy signal (or engaged tone) signifying that the calling party's telephone is being used in a telephone call to another person (or is "off the hook" though no number has been dialled, ie the customer does not want to be disturbed)
a busy signal (or overflow busy tone) signifying that there is congestion in the telephone network, or possibly that the calling subscriber has delayed too long in dialling all the necessary digits
status tones such as STD notification tones (to inform the caller that the telephone call is being trunk dialled at a greater cost to the calling party), minute minder beeps (to inform the caller of the relative duration of the telephone call on calls that are charged on a time basis), and others
a tone (sometimes the busy signal) to signify that the called party has hung up.
tones used by earlier inband telephone switching systems were simulated by a Black box or a Blue box used by "phone phreaks" to illegally make or receive free trunk/toll calls, see Blue box.
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