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85-469: ADSL works by using spectrum above the band used by voice telephone calls . With a DSL filter , often called splitter , the frequency bands are isolated, permitting a single telephone line to be used for both ADSL service and telephone calls at the same time. ADSL is generally only installed for short distances from the telephone exchange (the last mile ), typically less than 4 kilometres (2.5 mi), but has been known to exceed 8 kilometres (5 mi) if

170-423: A call ), is a connection over a telephone network between the called party and the calling party . Telephone calls started in the late 19th century. As technology has improved, a majority of telephone calls are made over a cellular network through mobile phones or over the internet with Voice over IP . Telephone calls are typically used for real-time conversation between two or more parties, especially when

255-452: A [Private branch exchange|PBX]. In most cases a private network is connected to the public network in order to allow PBX users to dial the outside world. Incoming calls to a private network arrive at the PBX in two ways: either directly to a users phone using a DDI number or indirectly via a receptionist who will answer the call first and then manually put the caller through to the desired user on

340-451: A dedicated data line was installed. This way, the DSL signal is separated as close as possible to the central office and is not attenuated inside the customer's premises. However, this procedure was costly, and also caused problems with customers complaining about having to wait for the technician to perform the installation. So, many DSL providers started offering a "self-install" option, in which

425-565: A depth, usually 8 to 64, which describes how many Reed–Solomon codewords are accumulated before they are sent. As they can all be sent together, their forward error correction codes can be made more resilient. Interleaving adds latency as all the packets have to first be gathered (or replaced by empty packets) and they, of course, all take time to transmit. 8 frame interleaving adds 5 ms round-trip time , while 64 deep interleaving adds 25 ms. Other possible depths are 16 and 32. "Fastpath" connections have an interleaving depth of 1, that

510-422: A feature termed seamless rate adaptation (SRA), which can accommodate changes in total link capacity with less disruption to communications. Vendors may support the usage of higher frequencies as a proprietary extension to the standard. However, this requires matching vendor-supplied equipment on both ends of the line, and will likely result in crosstalk problems that affect other lines in the same bundle. There

595-400: 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

680-540: A given channel, which depends on its bandwidth and SNR. This relationship is described by the Shannon–Hartley theorem , which is a fundamental law of information theory. SNR can be calculated using different formulas depending on how the signal and noise are measured and defined. The most common way to express SNR is in decibels, which is a logarithmic scale that makes it easier to compare large or small values. Other definitions of SNR may use different factors or bases for

765-460: A justification for internet service providers to offer a more expensive service aimed at commercial users who host websites, and who therefore need a service which allows for as much data to be uploaded as downloaded. File sharing applications are an obvious exception to this situation. Secondly internet service providers, seeking to avoid overloading of their backbone connections, have traditionally tried to limit uses such as file sharing which generate

850-402: A lot of uploads. Currently, most ADSL communication is full-duplex . Full-duplex ADSL communication is usually achieved on a wire pair by either frequency-division duplex (FDD), echo-cancelling duplex (ECD) , or time-division duplex (TDD). FDD uses two separate frequency bands, referred to as the upstream and downstream bands. The upstream band is used for communication from the end user to

935-531: A low link capacity. The total maximum capacity derived from summing the bits-per-bin is reported by DSL modems and is sometimes termed sync rate . This will always be rather misleading: the true maximum link capacity for user data transfer rate will be significantly lower because extra data are transmitted that are termed protocol overhead , reduced figures for PPPoA connections of around 84–87 percent, at most, being common. In addition, some ISPs will have traffic policies that limit maximum transfer rates further in

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1020-537: A lower throughput rate or not at all; this reduces the maximum link capacity but allows the modem to maintain an adequate connection. The DSL modem will make a plan on how to exploit each of the bins, sometimes termed "bits per bin" allocation. Those bins that have a good signal-to-noise ratio (SNR) will be chosen to transmit signals chosen from a greater number of possible encoded values (this range of possibilities equating to more bits of data sent) in each main clock cycle. The number of possibilities must not be so large that

1105-454: A million times stronger. When the signal is constant or periodic and the noise is random, it is possible to enhance the SNR by averaging the measurements. In this case the noise goes down as the square root of the number of averaged samples. When a measurement is digitized, the number of bits used to represent the measurement determines the maximum possible signal-to-noise ratio. This is because

1190-445: A perfect input signal. If the input signal is already noisy (as is usually the case), the signal's noise may be larger than the quantization noise. Real analog-to-digital converters also have other sources of noise that further decrease the SNR compared to the theoretical maximum from the idealized quantization noise, including the intentional addition of dither . Although noise levels in a digital system can be expressed using SNR, it

1275-469: A process called "bitswap", where bins that have become noisier are only required to carry fewer bits and other channels will be chosen to be given a higher burden. The data transfer capacity the DSL modem therefore reports is determined by the total of the bits-per-bin allocations of all the bins combined. Higher signal-to-noise ratios and more bins being in use gives a higher total link capacity, while lower signal-to-noise ratios or fewer bins being used gives

1360-462: A quantity proportional to power, as shown below: The concepts of signal-to-noise ratio and dynamic range are closely related. Dynamic range measures the ratio between the strongest un- distorted signal on a channel and the minimum discernible signal, which for most purposes is the noise level. SNR measures the ratio between an arbitrary signal level (not necessarily the most powerful signal possible) and noise. Measuring signal-to-noise ratios requires

1445-650: A random variable ( S ) to random noise N is: S N R = E [ S 2 ] E [ N 2 ] , {\displaystyle \mathrm {SNR} ={\frac {\mathrm {E} [S^{2}]}{\mathrm {E} [N^{2}]}}\,,} where E refers to the expected value , which in this case is the mean square of N . If the signal is simply a constant value of s , this equation simplifies to: S N R = s 2 E [ N 2 ] . {\displaystyle \mathrm {SNR} ={\frac {s^{2}}{\mathrm {E} [N^{2}]}}\,.} If

1530-459: 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. A typical phone call using

1615-409: A ringer (A7), that remains connected to the phone line whenever the phone is " on hook " (i.e. the switch (A4) is open), and other components which are connected when the phone is " off hook ". The off-hook components include a transmitter (microphone, A2), a receiver (speaker, A1), and other circuits for dialing, filtering (A3), and amplification. To place a telephone call, the calling party picks up

1700-493: A suggested SNR margin to the customer's DSL modem when it initially connects, and the modem may make its bits-per-bin allocation plan accordingly. A high SNR margin will mean a reduced maximum throughput, but greater reliability and stability of the connection. A low SNR margin will mean high speeds, provided the noise level does not increase too much; otherwise, the connection will have to be dropped and renegotiated (resynced). ADSL2+ can better accommodate such circumstances, offering

1785-429: A telephone call has more than one called party it is referred to as a conference call . When two or more users of the network are sharing the same physical line, it is called a party line or Rural phone line. If the caller's wired phone is connected directly 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,

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1870-491: A telephone call, such as voice dialing . Voice over IP technology allows calls to be made through a PC , using a service like Skype . Other services, such as toll-free dial-around enable callers to initiate a telephone call through a third party without exchanging phone numbers. Originally, no phone calls could be made without first talking to the Switchboard operator . Using 21st century mobile phones does not require

1955-448: A telephone in his own home because it was something he invented by mistake and saw it as a distraction from his main studies. A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems , or facsimile transmission when they are using fax machines. The call may use land line , mobile phone , satellite phone or any combination thereof. When

2040-423: A traditional phone is placed by picking the phone handset up off the base and holding the handset so that the hearing end is next to the user's ear and the speaking end is within range of the mouth. The caller then rotary dials or presses buttons for the phone number needed to complete the call, and the call is routed to the phone which has that number. The second phone makes a ringing noise to alert its owner, while

2125-496: A traditional telephone call is placed, certain tones signify the progress and status of the telephone call: Cell phones generally do not use dial tones, because the technology used to transmit the dialed number is different from a landline. Unsolicited telephone calls are a modern nuisance. Common kinds of unwanted calls include prank calls , telemarketing calls, and obscene phone calls . Caller ID provides some protection against unwanted calls, but can still be turned off by

2210-400: A user will visit a number of web sites and will need to download the data that comprises the web pages from the site, images, text, sound files etc. but they will only upload a small amount of data, as the only uploaded data is that used for the purpose of verifying the receipt of the downloaded data (in very common TCP connections) or any data inputted by the user into forms etc. This provides

2295-399: A very wide dynamic range , signals are often expressed using the logarithmic decibel scale. Based upon the definition of decibel, signal and noise may be expressed in decibels (dB) as and In a similar manner, SNR may be expressed in decibels as Using the definition of SNR Using the quotient rule for logarithms Substituting the definitions of SNR, signal, and noise in decibels into

2380-468: Is a direct relationship between the number of channels available and the throughput capacity of the ADSL connection. The exact data capacity per channel depends on the modulation method used. ADSL initially existed in two versions (similar to VDSL ), namely CAP and DMT. CAP was the de facto standard for ADSL deployments up until 1996, deployed in 90 percent of ADSL installations at the time. However, DMT

2465-406: Is a greater risk case of error should future signal-to-noise ratios deteriorate to the point where the bits-per-bin allocations chosen are too high to cope with the greater noise present. This conservatism, involving a choice of using fewer bits per bin as a safeguard against future noise increases, is reported as the signal-to-noise ratio margin or SNR margin . The telephone exchange can indicate

2550-564: Is a measure used in science and engineering that compares the level of a desired signal to the level of background noise . SNR is defined as the ratio of signal power to noise power , often expressed in decibels . A ratio higher than 1:1 (greater than 0 dB) indicates more signal than noise. SNR is an important parameter that affects the performance and quality of systems that process or transmit signals, such as communication systems , audio systems , radar systems , imaging systems , and data acquisition systems. A high SNR means that

2635-403: Is a uniformly distributed random signal with a peak-to-peak amplitude of one quantization level, making the amplitude ratio 2 /1. The formula is then: This relationship is the origin of statements like " 16-bit audio has a dynamic range of 96 dB". Each extra quantization bit increases the dynamic range by roughly 6 dB. Assuming a full-scale sine wave signal (that is, the quantizer

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2720-433: Is employed to characterize sensitivity of imaging systems; see Signal-to-noise ratio (imaging) . Related measures are the " contrast ratio " and the " contrast-to-noise ratio ". Channel signal-to-noise ratio is given by where W is the bandwidth and k a {\displaystyle k_{a}} is modulation index Output signal-to-noise ratio (of AM receiver) is given by Channel signal-to-noise ratio

2805-399: Is given by Output signal-to-noise ratio is given by All real measurements are disturbed by noise. This includes electronic noise , but can also include external events that affect the measured phenomenon — wind, vibrations, the gravitational attraction of the moon, variations of temperature, variations of humidity, etc., depending on what is measured and of the sensitivity of the device. It

2890-503: Is in many places the most common type offered to home users. On the technical side, there is likely to be more crosstalk from other circuits at the DSLAM end (where the wires from many local loops are close to each other) than at the customer premises. Thus the upload signal is weakest at the noisiest part of the local loop, while the download signal is strongest at the noisiest part of the local loop. It therefore makes technical sense to have

2975-407: Is more common to use E b /N o , the energy per bit per noise power spectral density. The modulation error ratio (MER) is a measure of the SNR in a digitally modulated signal. For n -bit integers with equal distance between quantization levels ( uniform quantization ) the dynamic range (DR) is also determined. Assuming a uniform distribution of input signal values, the quantization noise

3060-647: Is more susceptible to electromagnetic interference and has more attenuation than solid twisted-pair copper wires typically wired to telephone jacks. These effects are especially significant where the customer's phone line is more than 4 km from the DSLAM in the telephone exchange, which causes the signal levels to be lower relative to any local noise and attenuation. This will have the effect of reducing speeds or causing connection failures. FTTx usually has no surge problems, for example, caused by lightning . ADSL defines three "Transmission protocol-specific transmission convergence (TPS-TC)" layers: In home installation,

3145-411: Is often possible to reduce the noise by controlling the environment. Internal electronic noise of measurement systems can be reduced through the use of low-noise amplifiers . When the characteristics of the noise are known and are different from the signal, it is possible to use a filter to reduce the noise. For example, a lock-in amplifier can extract a narrow bandwidth signal from broadband noise

3230-411: Is one packet is sent at a time. This has a low latency, usually around 10 ms (interleaving adds to it, this is not greater than interleaved) but it is extremely prone to errors, as any burst of noise can take out the entire packet and so require it all to be retransmitted. Such a burst on a large interleaved packet only blanks part of the packet, it can be recovered from error correction information in

3315-407: Is only an approximation since E ⁡ [ X 2 ] = σ 2 + μ 2 {\displaystyle \operatorname {E} \left[X^{2}\right]=\sigma ^{2}+\mu ^{2}} . It is commonly used in image processing , where the SNR of an image is usually calculated as the ratio of the mean pixel value to the standard deviation of

3400-437: Is rarely done. It is usually much easier to install filters at each telephone jack that is in use. DSL signals may be degraded by older telephone lines, surge protectors, poorly designed microfilters, repetitive electrical impulse noise , and by long telephone extension cords. Telephone extension cords are typically made with small-gauge, multi-strand copper conductors which do not maintain a noise-reducing pair twist. Such cable

3485-412: Is usually not included while measuring power or energy of a signal. This may cause some confusion among readers, but the resistance factor is not significant for typical operations performed in signal processing, or for computing power ratios. For most cases, the power of a signal would be considered to be simply An alternative definition of SNR is as the reciprocal of the coefficient of variation , i.e.,

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3570-481: The ADSL modem tests each of the bins to determine the signal-to-noise ratio at each bin's frequency. Distance from the telephone exchange , cable characteristics, interference from AM radio stations , and local interference and electrical noise at the modem's location can adversely affect the signal-to-noise ratio at particular frequencies. Bins for frequencies exhibiting a reduced signal-to-noise ratio will be used at

3655-425: The DSLAM transmit at a higher bit rate than does the modem on the customer end. Since the typical home user in fact does prefer a higher download speed, the telephone companies chose to make a virtue out of necessity, hence ADSL. The marketing reasons for an asymmetric connection are that, firstly, most users of internet traffic will require less data to be uploaded than downloaded. For example, in normal web browsing,

3740-726: The PBX. Most telephone calls through the PSTN are set up using ISUP signalling messages or one of its variants between telephone exchanges to establish the end to end connection. Calls through PBX networks are set up using QSIG , DPNSS or variants. Some types of calls are not charged, such as local calls (and internal calls) dialed directly by a telephone subscriber in Canada , the United States , Hong Kong , United Kingdom , Ireland or New Zealand (Residential subscribers only). In most other areas, all telephone calls are charged

3825-446: The above equation results in an important formula for calculating the signal to noise ratio in decibels, when the signal and noise are also in decibels: In the above formula, P is measured in units of power, such as watts (W) or milliwatts (mW), and the signal-to-noise ratio is a pure number. However, when the signal and noise are measured in volts (V) or amperes (A), which are measures of amplitude, they must first be squared to obtain

3910-509: The calling party is usually given a tone to indicate they should begin dialing the desired number. In some (now very rare) cases, if the calling party cannot dial calls directly, they will be connected to an operator who places the call for them. Calls may be placed through a public network (such as the Public Switched Telephone Network ) provided by a commercial telephone company or a private network called

3995-571: The calling party. Even where end-user Caller ID is not available, calls are still logged, both in billing records at the originating telco and via automatic number identification , so the perpetrator's phone number can still be discovered in many cases. However, this does not provide complete protection: harassers can use payphones, in some cases, automatic number identification itself can be spoofed or blocked, and mobile telephone abusers can (at some cost) use "throwaway" phones or SIMs. Signal-to-noise ratio Signal-to-noise ratio ( SNR or S/N )

4080-404: The customer's premises to avoid interference between the DSL, voice services, and any other connections to the line (for example intruder alarms). This is desirable for the voice service and essential for a reliable ADSL connection. In the early days of DSL, installation required a technician to visit the premises. A splitter or microfilter was installed near the demarcation point , from which

4165-497: The customer's premises. Commonly, microfilters are only low-pass filters, so beyond them only low frequencies (voice signals) can pass. In the data section, a microfilter is not used because digital devices that are intended to extract data from the DSL signal will, themselves, filter out low frequencies. Voice telephone devices will pick up the entire spectrum so high frequencies, including the ADSL signal, will be "heard" as noise in telephone terminals, and will affect and often degrade

4250-448: The exchange counts to decode each digit of the telephone number. If the called party's line is available, the terminating exchange applies an intermittent alternating current (AC) ringing signal of 40 to 90 volts to alert the called party of the incoming call. If the called party's line is in use, however, the exchange returns a busy signal to the calling party. If the called party's line is in use but subscribes to call waiting service,

4335-449: The exchange sends an intermittent audible tone to the called party to indicate another call. The electromechanical ringer of a telephone (A7) is connected to the line through a capacitor (A6), which blocks direct current and passes the alternating current of the ringing power. The telephone draws no current when it is on hook, while a DC voltage is continually applied to the line. Exchange circuitry (D2) can send an alternating current down

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4420-433: The incoming speaker signal and the outgoing microphone signal from interfering with each other. This is accomplished through a hybrid coil (A3). The incoming audio signal passes through a resistor (A8) and the primary winding of the coil (A3) which passes it to the speaker (A1). Since the current path A8 – A3 has a far lower impedance than the microphone (A2), virtually all of the incoming signal passes through it and bypasses

4505-440: The line is not in use. When a party initiates a call to this line, the exchange sends the ringing signal. When the called party picks up the handset, they actuate a double-circuit switchhook (not shown) which may simultaneously disconnect the alerting device and connect the audio circuitry to the line. This, in turn, draws direct current through the line, confirming that the called phone is now active. The exchange circuitry turns off

4590-416: The line to activate the ringer and announce an incoming call. In manual service exchange areas, before dial service was installed, telephones had hand-cranked magneto generators to generate a ringing voltage back to the exchange or any other telephone on the same line. When a landline telephone is inactive (on hook), the circuitry at the telephone exchange detects the absence of direct current to indicate that

4675-447: The line. Once a line has had DSL enabled, the DSL signal is present on all telephone wiring in the building, causing attenuation and echo. A way to circumvent this is to go back to the original model, and install one filter upstream from all telephone jacks in the building, except for the jack to which the DSL modem will be connected. Since this requires wiring changes by the customer, and may not work on some household telephone wiring, it

4760-419: The logarithm, depending on the context and application. One definition of signal-to-noise ratio is the ratio of the power of a signal (meaningful input) to the power of background noise (meaningless or unwanted input): where P is average power. Both signal and noise power must be measured at the same or equivalent points in a system, and within the same system bandwidth . The signal-to-noise ratio of

4845-450: The microphone. At the same time the DC voltage across the line causes a DC current which is split between the resistor-coil (A8-A3) branch and the microphone-coil (A2-A3) branch. The DC current through the resistor-coil branch has no effect on the incoming audio signal. But the DC current passing through the microphone is turned into AC (in response to voice sounds) which then passes through only

4930-401: The minimum possible noise level is the error caused by the quantization of the signal, sometimes called quantization noise . This noise level is non-linear and signal-dependent; different calculations exist for different signal models. Quantization noise is modeled as an analog error signal summed with the signal before quantization ("additive noise"). This theoretical maximum SNR assumes

5015-458: The networks beyond the exchange, and traffic congestion on the Internet, heavy loading on servers and slowness or inefficiency in customers' computers may all contribute to reductions below the maximum attainable. When a wireless access point is used, low or unstable wireless signal quality can also cause reduction or fluctuation of actual speed. In fixed-rate mode, the sync rate is predefined by

5100-422: The noise has expected value of zero, as is common, the denominator is its variance , the square of its standard deviation σ N . The signal and the noise must be measured the same way, for example as voltages across the same impedance . Their root mean squares can alternatively be used according to: where A is root mean square (RMS) amplitude (for example, RMS voltage). Because many signals have

5185-504: The noise level to 1 (0 dB) and measuring how far the signal 'stands out'. In physics, the average power of an AC signal is defined as the average value of voltage times current; for resistive (non- reactive ) circuits, where voltage and current are in phase, this is equivalent to the product of the rms voltage and current: But in signal processing and communication, one usually assumes that R = 1 Ω {\displaystyle R=1\Omega } so that factor

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5270-434: The noise standard deviation σ {\displaystyle \sigma } does not change between the two states. The Rose criterion (named after Albert Rose ) states that an SNR of at least 5 is needed to be able to distinguish image features with certainty. An SNR less than 5 means less than 100% certainty in identifying image details. Yet another alternative, very specific, and distinct definition of SNR

5355-481: The operator and the DSL modem chooses a bits-per-bin allocation that yields an approximately equal error rate in each bin. In variable-rate mode, the bits-per-bin are chosen to maximize the sync rate, subject to a tolerable error risk. These choices can either be conservative, where the modem chooses to allocate fewer bits per bin than it possibly could, a choice that makes for a slower connection, or less conservative in which more bits per bin are chosen in which case there

5440-500: The originally laid wire gauge allows for further distribution. At the telephone exchange, the line generally terminates at a digital subscriber line access multiplexer (DSLAM) where another frequency splitter separates the voice band signal for the conventional phone network . Data carried by the ADSL are typically routed over the telephone company 's data network and eventually reach a conventional Internet Protocol network. There are both technical and marketing reasons why ADSL

5525-429: The parties cannot meet in person. A telephone call historically was between two live people. It has progressed to also include a live person and a recorded message, or a live person with an AI generated message. The term "call" is now broadly used for other connections over a network when you are using your voice to communicate (as opposed to typing text), including audio calls and video calls . The first telephone call

5610-402: The pixel values over a given neighborhood. Sometimes SNR is defined as the square of the alternative definition above, in which case it is equivalent to the more common definition : This definition is closely related to the sensitivity index or d ' , when assuming that the signal has two states separated by signal amplitude μ {\displaystyle \mu } , and

5695-464: The prevalent transport protocol is ATM. On top of ATM, there are multiple possibilities of additional layers of protocols (two of them are abbreviated in a simplified manner as " PPPoA " or " PPPoE "), with TCP / IP providing the connection to the Internet . Telephone call A telephone call or telephone conversation (or telcon ), also known as a phone call or voice call (or simply

5780-415: The provider provided equipment and instructions to the customer. Instead of separating the DSL signal at the demarcation point, the DSL signal is filtered at each telephone outlet by use of a low-pass filter for voice and a high-pass filter for data, usually enclosed in what is known as a microfilter . This microfilter can be plugged by an end user into any telephone jack: it does not require any rewiring at

5865-455: The ratio of mean to standard deviation of a signal or measurement: where μ {\displaystyle \mu } is the signal mean or expected value and σ {\displaystyle \sigma } is the standard deviation of the noise, or an estimate thereof. Notice that such an alternative definition is only useful for variables that are always non-negative (such as photon counts and luminance ), and it

5950-412: The receiver might incorrectly decode which one was intended in the presence of noise. Noisy bins may only be required to carry as few as two bits, a choice from only one of four possible patterns, or only one bit per bin in the case of ADSL2+, and very noisy bins are not used at all. If the pattern of noise versus frequencies heard in the bins changes, the DSL modem can alter the bits-per-bin allocations, in

6035-440: The rest of the packet. A "fastpath" connection will result in extremely high latency on a poor line, as each packet will take many retries. ADSL deployment on an existing plain old telephone service (POTS) telephone line presents some problems because the DSL is within a frequency band that might interact unfavorably with existing equipment connected to the line. It is therefore necessary to install appropriate frequency filters at

6120-658: The ring signal, and both telephones are now active and connected through the exchange. The parties may now converse as long as both phones remain off hook. When a party hangs up, placing the handset back on the cradle or hook, direct current ceases in that line, signaling the exchange to disconnect the call. Calls to parties beyond the local exchange are carried over trunk lines which establish connections between exchanges. In modern telephone networks, fiber-optic cable and digital technology are often employed in such connections. Satellite technology may be used for communication over very long distances. In most landline telephones,

6205-433: The selection of a representative or reference signal. In audio engineering , the reference signal is usually a sine wave at a standardized nominal or alignment level , such as 1 kHz at +4 dBu (1.228 V RMS ). SNR is usually taken to indicate an average signal-to-noise ratio, as it is possible that instantaneous signal-to-noise ratios will be considerably different. The concept can be understood as normalizing

6290-444: The service in fax, dataphones and modems. From the point of view of DSL devices, any acceptance of their signal by POTS devices mean that there is a degradation of the DSL signal to the devices, and this is the central reason why these filters are required. A side effect of the move to the self-install model is that the DSL signal can be degraded, especially if more than 5 voiceband (that is, POTS telephone-like) devices are connected to

6375-442: The signal is clear and easy to detect or interpret, while a low SNR means that the signal is corrupted or obscured by noise and may be difficult to distinguish or recover. SNR can be improved by various methods, such as increasing the signal strength, reducing the noise level, filtering out unwanted noise, or using error correction techniques. SNR also determines the maximum possible amount of data that can be transmitted reliably over

6460-590: The telephone central office. The downstream band is used for communicating from the central office to the end user. With commonly deployed ADSL over POTS (Annex A), the band from 26.075  kHz to 137.825 kHz is used for upstream communication, while 138–1104 kHz is used for downstream communication. Under the usual discrete multitone modulation (DMT) scheme, each of these is further divided into smaller frequency channels of 4.3125 kHz. These frequency channels are sometimes termed bins . During initial training to optimize transmission quality and speed,

6545-480: The telephone exchange. The exchange detects this current, attaches a digit receiver circuit to the line, and sends dial tone to indicate its readiness. On a modern push-button telephone , the caller then presses the number keys to send the telephone number of the destination, the called party . The keys control a tone generator circuit (not shown) that sends DTMF tones to the exchange. A rotary-dial telephone uses pulse dialing (A5), sending electrical pulses, that

6630-425: The telephone line to the local exchange then on to the other phone (via the local exchange or via a larger network), where it passes through the coil of the receiver (A3). The varying current in the coil produces a corresponding movement of the receiver's diaphragm, reproducing the original sound waves present at the transmitter. Along with the microphone and speaker, additional circuitry is incorporated to prevent

6715-409: The telephone's handset, thereby operating a lever that closes the hook switch (A4). This powers the telephone by connecting the transmission hybrid transformer, as well as the transmitter (microphone) and receiver (speaker) to the line. In this off-hook state, the telephone circuitry has a low resistance of typically less than 300 ohms , which causes the flow of direct current (DC) in the line (C) from

6800-411: The transmitter and receiver (microphone and speaker) are located in the handset, although in a speakerphone these components may be located in the base or in a separate enclosure. Powered by the line, the microphone (A2) produces a modulated electric current which varies its frequency and amplitude in response to the sound waves arriving at its diaphragm . The resulting current is transmitted along

6885-495: The upper branch of the coil's (A3) primary winding, which has far fewer turns than the lower primary winding. This causes a small portion of the microphone output to be fed back to the speaker, while the rest of the AC goes out through the phone line. A lineman's handset is a telephone designed for testing the telephone network and may be attached directly to aerial lines and other infrastructure components. Preceding, during, and after

6970-405: The use of an operator to complete a phone call. The use of headsets is becoming more common for placing or receiving a call. Headsets can either come with a cord or be wireless . A special number can be dialed for operator assistance , which may be different for local vs. long-distance or international calls. The landline telephone contains a switchhook (A4) and an alerting device, usually

7055-440: The user of the first phone hears a ringing noise in its earpiece. If the second phone is picked up, then the operators of the two units are able to talk to one another through them. If the phone is not picked up, the operator of the first phone continues to hear a ringing noise until they hang up their own phone. In addition to the traditional method of placing a telephone call, new technologies allow different methods for initiating

7140-455: Was chosen for the first ITU-T ADSL standards, G.992.1 and G.992.2 (also called G.dmt and G.lite respectively). Therefore, all modern installations of ADSL are based on the DMT modulation scheme. ISPs (but users rarely, apart from Australia where it's the default) have the option to use interleaving of packets to counter the effects of burst noise on the telephone line. An interleaved line has

7225-421: Was made on March 10, 1876, by Alexander Graham Bell . Bell demonstrated his ability to "talk with electricity" by transmitting a call to his assistant, Thomas Watson. The first words transmitted were "Mr Watson, come here. I want to see you." This event has been called Bell's "greatest success", as it demonstrated the first successful use of the telephone. Although it was his greatest success, he refused to have

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