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Emphasis (telecommunications)

In signal processing, pre-emphasis is a technique to protect against anticipated noise and loss. The idea is to boost (and hence distort) the frequency range that is most susceptible to noise and loss beforehand, so that after a noisy and lossy process (transmission over cable, tape recording...) more information can be recovered from that frequency range. Removal of the distortion caused by pre-emphasis is called de-emphasis, making the output accurately reproduce the original input.

Emphasis is commonly used in many places ranging from FM broadcasting (preemphasis improvement) and vinyl (e.g. LP) records to PCI Express. For example, high-frequency signal components may be emphasized to produce a more equal modulation index for a transmitted frequency spectrum, and therefore a better signal-to-noise ratio for the entire frequency range.

In audio signals
In processing electronic audio signals, pre-emphasis refers to a system process designed to increase (within a frequency band) the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation distortion or saturation of recording media in subsequent parts of the system. The mirror operation is called de-emphasis, and the system as a whole is called emphasis.

Pre-emphasis is achieved with a pre-emphasis network which is essentially a calibrated filter. The frequency response is decided by special time constants. The cutoff frequency can be calculated from that value.

Pre-emphasis is commonly used in telecommunications, digital audio recording, record cutting, in FM broadcasting transmissions, and in displaying the spectrograms of speech signals. One example of this is the RIAA equalization curve on 33 rpm and 45 rpm vinyl records. Another is the Dolby noise-reduction system as used with magnetic tape.

Pre-emphasis is employed in frequency modulation or phase modulation transmitters to equalize the modulating signal drive power in terms of deviation ratio. The receiver demodulation process includes a reciprocal network, called a de-emphasis network, to restore the original signal power distribution.

De-emphasis
In telecommunication, de-emphasis is the complement of pre-emphasis, in the antinoise system called emphasis. De-emphasis is a system process designed to decrease, (within a band of frequencies), the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation distortion or saturation of recording media in subsequent parts of the system.

Special time constants dictate the frequency response curve, from which one can calculate the cutoff frequency.

Red Book audio
Although rarely used, there exists the capability for standardized emphasis in Red Book CD mastering. As CD players were originally implemented with affordable 14-bit converters, a specification for pre-emphasis was included to compensate for quantization noise. After economies of scale eventually allowed full 16 bits, quantization noise became less of a concern, but emphasis remained an option. The pre-emphasis is described as a first-order filter with a gain of 10 dB (at 20 dB/decade) and time constants 50 μs and 15 μs.

In digital transmission
In serial data transmission, emphasis is used to improve signal quality at the output of a communication channel. In transmitting signals at high data rates, the transmission medium may introduce distortions, so emphasis is used to distort the transmitted signal to correct for this distortion. When done properly this produces a received signal that more closely resembles the original or desired signal, allowing the use of higher data rates or producing fewer bit errors. Most real world channels have loss that increases with frequency (effectively a low pass filter), so emphasis needs to invert this effect (functioning as a high pass filter). This makes emphasis a form of equalization, implemented at the transmit side of the channel.

Emphasis can be implemented either by boosting high frequencies (pre-emphasis, increasing the amplitude of transition bits) or attenuating low frequencies (de-emphasis, reducing the amplitude of non-transition bits). Both have the same net effect of producing a flatter system frequency response; de-emphasis is typically more convenient to do in real circuits since it only requires attenuation rather than amplification. Well-known serial data standards such as PCI Express, SATA and SAS require transmitted signals to use de-emphasis.

One common implementation of emphasis in real SERDES is a 3-tap feed-forward equalizer (FFE): rather than driving the output pin with the desired output voltage directly, the actual output voltage is a weighted sum of the desired bit value (main cursor), the previous bit (post cursor), and the next bit to be transmitted (pre cursor). The main cursor coefficient controls the nominal amplitude of the bit and is always positive (as a negative coefficient would invert the bit value). The pre cursor coefficient removes ISI at the receiver caused by bits which have not yet arrived (e.g. fields coupling across meanders in a delay-matched trace) and is typically zero or a very small negative value, as this is often not a major contribution to total ISI. The post cursor coefficient removes ISI at the receiver caused by the immediately preceding bit and is typically a larger negative value, with lossier channels requiring a larger tap value. Higher numbers of taps are possible but increase circuit complexity and tend to result in diminishing returns so are not commonly used.

The effects of emphasis on a signal can be clearly seen in the eye pattern. In the following demonstration, we consider a 10.3125 Gbps PRBS-31 test pattern with NRZ modulation, typical for testing 10-Gigabit Ethernet. The channel has an insertion loss of roughly 2 dB at the fundamental, 3 dB at the 2nd harmonic, and 4 dB at the 3rd. The goal is to achieve a well-equalized channel response in which the eye is maximally open without excessive overshoot. Excessive equalization can worsen jitter, increase overshoot, and result in a less open eye than a properly equalized signal.

title: Emphasis (telecommunications)


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Encode

Encode or encoding may refer to:

 * APL (programming language) dyadic Encode function and its symbol ⊤
 * Binary encoding
 * Binary-to-text encoding
 * Character encoding
 * Coding region of a gene
 * Encoding, or code
 * Encoding (memory)
 * Encoding (semiotics)
 * ENCODE (Encyclopedia of DNA Elements)
 * MPEG encoding
 * Semantics encoding
 * Text encoding — see character encoding applied to textual data
 * Video encoding

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Encode


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End distortion

In start-stop teletypewriter operation, end distortion refers to the shifting of the end of all marking pulses, except the stop pulse, from their proper positions in relation to the beginning of the next start pulse.

Shifting of the end of the stop pulse is a deviation in character time and rate rather than an end distortion.

Spacing end distortion is the termination of marking pulses before the proper time. Marking end distortion is the continuation of marking pulses past the proper time.

The magnitude of the distortion is expressed as a percentage of an ideal pulse length.

title: End distortion


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End-of-Transmission character

In telecommunication, an End-of-Transmission character (EOT) is a transmission control character. Its intended use is to indicate the conclusion of a transmission that may have included one or more texts and any associated message headings.

An EOT is often used to initiate other functions, such as releasing circuits, disconnecting terminals, or placing receive terminals in a standby condition. Its most common use today is to cause a Unix terminal driver to signal end of file and thus exit programs that are awaiting input.

In ASCII and Unicode, the character is encoded at. It can be referred to as, ^D in caret notation. Unicode provides the character for when EOT needs to be displayed graphically. In addition, can also be used as a graphic representation of EOT; it is defined in Unicode as "symbol for End of Transmission".

Meaning in Unix
The EOT character in Unix is different from the Control-Z in DOS. The DOS Control-Z byte is actually sent and/or placed in files to indicate where the text ends. In contrast, the Control-D causes the Unix terminal driver to signal the EOF condition, which is not a character, while the byte has no special meaning if actually read or written from a file or terminal.

In Unix, the end-of-file character (by default EOT) causes the terminal driver to make available all characters in its input buffer immediately; normally the driver would collect characters until it sees an end-of-line character. If the input buffer is empty (because no characters have been typed since the last end-of-line or end-of-file), a program reading from the terminal reads a count of zero bytes. In Unix, such a condition is understood as having reached the end of the file.

This can be demonstrated with the cat program on Unix-like operating systems such as Linux: Run the cat command with no arguments, so it accepts its input from the keyboard and prints output to the screen. Type a few characters without pressing, then type. The characters typed to that point are sent to cat, which then writes them to the screen. If is typed without typing any characters first, the input stream is terminated and the program ends. An actual EOT is obtained by typing then.

If the terminal driver is in "raw" mode, it no longer interprets control characters, and the EOT character is sent unchanged to the program, which is free to interpret it any way it likes. A program may then decide to handle the EOT byte as an indication that it should end the text; this would then be similar to how is handled by DOS programs.

Usage in mainframe computer system communications protocols
The EOT character is used in legacy communications protocols by mainframe computer manufacturers such as IBM, Burroughs Corporation, and the BUNCH. Terminal transmission control protocols such as IBM 3270 Poll/Select, or Burroughs TD830 Contention Mode protocol use the EOT character to terminate a communications sequence between two cooperating stations (such as a host multiplexer or Input/Output terminal).

A single Poll (ask the station for data) or Select (send data to the station) operation will include two round-trip send-reply operations between the polling station and the station being polled, the final operation being transmission of a single EOT character to the initiating station.

title: End-of-Transmission character


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Endurability

In telecommunication, endurability is the property of a system, subsystem, equipment, or process that enables it to continue to function within specified performance limits for an extended period of time, usually months, despite a severe natural or man-made disturbance, such as a nuclear attack, or a loss of external logistic or utility support.

Endurability is not compromised by temporary failures when the local capability exists to restore and maintain the system, subsystem, equipment, or process to an acceptable performance level.

title: Endurability


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Enhanced service

Enhanced service is service offered over commercial carrier transmission facilities used in interstate communications, that employs computer processing applications that act on the format, content, code, protocol, or similar aspects of the subscriber's transmitted information; provides the subscriber with additional, different, or restructured information; or involves subscriber interaction with stored information.

title: Enhanced service


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Epoch

In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.

The moment of epoch is usually decided by congruity, or by following conventions understood from the epoch in question. The epoch moment or date is usually defined from a specific, clear event of change, an epoch event. In a more gradual change, a deciding moment is chosen when the epoch criterion was reached.

Pre-modern eras

 * The Yoruba calendar (Kọ́jọ́dá) uses 8042 BC as the epoch, regarded as the year of the creation of Ile-Ife by the god Obatala, also regarded as the creation of the earth.
 * Anno Mundi (years since the creation of the world) is used in the Byzantine calendar (5509 BC).
 * Anno Mundi (years since the creation of the world) as used in the Hebrew calendar (3761 BC).
 * The Mesoamerican Long Count Calendar uses the creation of the fourth world in 3114 BC.
 * Olympiads, the ancient Greek era of four-year periods between Olympic Games, beginning in 776 BC.
 * Ab urbe condita ("from the foundation of the city"), used to some extent by Roman calendars of the Roman imperial period (753 BC).
 * Buddhist calendars tend to use the epoch of 544 BC (date of Buddha's parinirvana).
 * The term Hindu calendar may refer to a number of traditional Indian calendars. A notable example of a Hindu epoch is the Vikram Samvat (58 BC), also used in modern times as the national calendars of Nepal and Bangladesh.
 * The Julian and Gregorian calendars use as epoch the Incarnation of Jesus as calculated in the 6th century by Dionysius Exiguus. (Subsequent research has shown that this moment is about four years after the best estimate for the date of birth of Jesus.) This epoch was applied retrospectively to the Julian calendar, long after its original creation by Julius Caesar.
 * The epoch of the Islamic calendar is the Hijra (AD 622). The year count in this calendar shifts relative to the solar year count, as the calendar is purely lunar: its year consists of 12 lunations and is thus ten or eleven days shorter than a solar year. This calendar denotes "lunar years" as Anno Hegiræ ([since] the year of the Hijra) or AH. This calendar is used in Sunni Islam and related sects.
 * The epoch of the official Iranian calendar is also the Hijra, but it is a solar calendar; each year begins at the Northern spring equinox. This calendar is used in Shia Islam and related sects.

Modern eras

 * The Bahá'í calendar is dated from the vernal equinox of the year the Báb proclaimed his religion (AD 1844). Years are grouped in Váḥids of 19 years, and Kull-i-Shay of 361 (19×19) years.
 * In Thailand in 1888 King Chulalongkorn decreed a National Thai Era dating from the founding of Bangkok on April 6, 1782. In 1912, New Year's Day was shifted to April 1. In 1941, Prime Minister Phibunsongkhram decided to count the years since 543 BC. This is the Thai solar calendar using the Thai Buddhist Era. Except for this era, it is the Gregorian calendar.
 * In the French Republican Calendar, a calendar used by the French government for about twelve years from late 1793, the epoch was the beginning of the "Republican Era", September 22, 1792 (the day the French First Republic was proclaimed, one day after the Convention abolished the Ancien Regime).
 * The Indian national calendar, introduced in 1957, follows the Saka era (AD 78).
 * The Minguo calendar used by officials of Taiwan and its predecessor dates from January 1, 1912, the first year after the Xinhai Revolution, which overthrew the Qing Empire.
 * North Korea uses a system that starts in 1912 (= Juche 1), the year of the birth of its founder Kim Il-Sung.
 * The Fascist Era dates to Mussolini's March on Rome in 1922, and was in use only in countries under hegemony of the Fascist regime of Benito Mussolini. It has been defunct since the fall of the Italian Social Republic in 1945.
 * In the scientific Before Present system of numbering years for purposes of radiocarbon dating, the reference date is January 1, 1950 (though the specific date January 1 is quite unnecessary, as radiocarbon dating has limited precision).
 * Different branches of Freemasonry have selected different years to date their documents according to a Masonic era, such as the Anno Lucis (A.L.).
 * The Holocene calendar uses 10,000 BC as the epoch, the beginning of the Holocene epoch on the geological time scale.

Regnal eras
The official Japanese system numbers years from the accession of the current emperor, regarding the calendar year during which the accession occurred as the first year. A similar system existed in China before 1912, being based on the accession year of the emperor (1911 was thus the third year of the Xuantong period). With the establishment of the Republic of China in 1912, the republican era was introduced. It is still very common in Taiwan to date events via the republican era. The People's Republic of China adopted the common era calendar in 1949 (the 38th year of the Chinese Republic).

Other applications
An epoch in computing is the time at which the representation is zero. For example, Unix time is represented as the number of seconds since 00:00:00 UTC on 1 January 1970, not counting leap seconds.

An epoch in astronomy is a reference time used for consistency in calculation of positions and orbits. A common astronomical epoch is J2000, which is noon on January 1, 2000, Terrestrial Time.

An epoch in Geochronology is a period of time, typically in the order of tens of millions of years. The current epoch is the Holocene.

title: Epoch


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Equivalent noise resistance

In telecommunication, an equivalent noise resistance is a quantitative representation in resistance units of the spectral density of a noise-voltage generator, given by $$R_n = \frac {\pi W_n}{k T_0} $$ where $$W_n$$ is the spectral density, $$k$$ is the Boltzmann constant, $$T_0$$ is the standard noise temperature (290 K), so $$kT_0 = 4.00 \times 10^{-21}\,[Ws]$$.

Note: The equivalent noise resistance in terms of the mean-square noise-generator voltage, e2, within a frequency increment, Δ f, is given by

 * $$R_n = \frac{e^2}{4 k T_0\,\Delta f}.$$

title: Equivalent noise resistance


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Equivalent pulse code modulation noise

In telecommunication, equivalent pulse code modulation (PCM) noise is the amount of noise power on a frequency-division multiplexing (FDM) or wire communication channel necessary to approximate the same judgment of speech quality created by quantization noise in a PCM channel.

 * Note 1: The speech quality judgment is based on comparative tests.

 * Note 2: Generally, 33.5 dBrnC ±2.5 dB is considered the approximate equivalent PCM noise of a 7-bit PCM system.

title: Equivalent pulse code modulation noise


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Error

An error (from the Latin errāre, meaning 'to wander' ) is an inaccurate or incorrect action, thought, or judgement.

In statistics, "error" refers to the difference between the value which has been computed and the correct value. An error could result in failure or in a deviation from the intended performance or behavior.

Human behavior
One reference differentiates between "error" and "mistake" as follows: "An 'error' is a deviation from accuracy or correctness. A 'mistake' is an error caused by a fault: the fault being misjudgment, carelessness, or forgetfulness. Now, say that I run a stop sign because I was in a hurry, and wasn't concentrating, and the police stop me, that is a mistake. If, however, I try to park in an area with conflicting signs, and I get a ticket because I was incorrect on my interpretation of what the signs meant, that would be an error. The first time it would be an error. The second time it would be a mistake since I should have known better."

In human behavior the norms or expectations for behavior or its consequences can be derived from the intention of the actor or from the expectations of other individuals or from a social grouping or from social norms. (See deviance.) Gaffes and faux pas can be labels for certain instances of this kind of error. More serious departures from social norms carry labels such as misbehavior and labels from the legal system, such as misdemeanor and crime. Departures from norms connected to religion can have other labels, such as sin.

Language
An individual language user's deviations from standard language norms in grammar, pronunciation and orthography are sometimes referred to as errors. However, in light of the role of language usage in everyday social class distinctions, many feel that linguistics should restrain itself from such prescriptivist judgments to avoid reinforcing dominant class value claims about what linguistic forms should and should not be used. One may distinguish various kinds of linguistic errors – some, such as aphasia or speech disorders, where the user is unable to say what they intend to, are generally considered errors, while cases where natural, intended speech is non-standard (as in vernacular dialects), are considered legitimate speech in scholarly linguistics, but might be considered errors in prescriptivist contexts. See also Error analysis (linguistics).

Gaffe
A gaffe is usually made in a social environment and may come from saying something that may be true but inappropriate. It may also be an erroneous attempt to reveal a truth. Gaffes can be malapropisms, grammatical errors or other verbal and gestural weaknesses or revelations through body language. Actually revealing factual or social truth through words or body language, however, can commonly result in embarrassment or, when the gaffe has negative connotations, friction between people involved.

Philosophers and psychologists interested in the nature of the gaffe include Sigmund Freud (Freudian slip) and Gilles Deleuze. Deleuze, in his The Logic of Sense, places the gaffe in a developmental process that can culminate in stuttering.

Sportswriters and journalists commonly use "gaffe" to refer to any kind of mistake, e.g. a dropped ball (baseball error) by a player in a baseball game.

Science and engineering

In statistics, an error (or residual) is not a "mistake" but rather a difference between a computed, estimated, or measured value and the accepted true, specified, or theoretically correct value.

In science and engineering in general, an error is defined as a difference between the desired and actual performance or behavior of a system or object. This definition is the basis of operation for many types of control systems, in which error is defined as the difference between a set point and the process value. An example of this would be the thermostat in a home heating system – the operation of the heating equipment is controlled by the difference (the error) between the thermostat setting and the sensed air temperature. Another approach is related to considering a scientific hypothesis as true or false, giving birth to two types of errors: Type 1 and Type 2. The first one is when a true hypothesis is considered false, while the second is the reverse (a false one is considered true).

Engineers seek to design devices, machines and systems and in such a way as to mitigate or preferably avoid the effects of error, whether unintentional or not. Such errors in a system can be latent design errors that may go unnoticed for years, until the right set of circumstances arises that cause them to become active. Other errors in engineered systems can arise due to human error, which includes cognitive bias. Human factors engineering is often applied to designs in an attempt to minimize this type of error by making systems more forgiving or error-tolerant.

(In computational mechanics, when solving a system such as Ax = b there is a distinction between the "error" – the inaccuracy in x – and residual – the inaccuracy in Ax.)

A notable result of Engineering and Scientific errors that occurred in history is the Chernobyl disaster of 1986, which caused a nuclear meltdown in the City of Chernobyl in present-day Ukraine, and is used as a case study in many Engineering/Science research

Numerical analysis
Numerical analysis provides a variety of techniques to represent (store) and compute approximations to mathematical numerical values. Errors arise from a trade-off between efficiency (space and computation time) and precision, which is limited anyway, since (using common floating-point arithmetic) only a finite amount of values can be represented exactly. The discrepancy between the exact mathematical value and the stored/computed value is called the approximation error.

Cybernetics
In applying corrections to the trajectory or course being steered, cybernetics can be seen as the most general approach to error and its correction for the achievement of any goal. The term was suggested by Norbert Wiener to describe a new science of control and information in the animal and the machine. Wiener's early work was on noise.

The cybernetician Gordon Pask held that the error that drives a servomechanism can be seen as a difference between a pair of analogous concepts in a servomechanism: the current state and the goal state. Later he suggested error can also be seen as an innovation or a contradiction depending on the context and perspective of interacting (observer) participants. The founder of management cybernetics, Stafford Beer, applied these ideas most notably in his viable system model.

Biology
In biology, an error is said to occur when perfect fidelity is lost in the copying of information. For example, in an asexually reproducing species, an error (or mutation) has occurred for each DNA nucleotide that differs between the child and the parent. Many of these mutations can be harmful, but unlike other types of errors, some are neutral or even beneficial. Mutations are an important force driving evolution. Mutations that make organisms more adapted to their environment increase in the population through natural selection as organisms with favorable mutations have more offspring.

Philately
In philately, an error refers to a postage stamp or piece of postal stationery that exhibits a printing or production mistake that differentiates it from a normal specimen or from the intended result. Examples are stamps printed in the wrong color or missing one or more colors, printed with a vignette inverted in relation to its frame, produced without any perforations on one or more sides when the normal stamps are perforated, or printed on the wrong type of paper. Legitimate errors must always be produced and sold unintentionally. Such errors may or may not be scarce or rare. A design error may refer to a mistake in the design of the stamp, such as a mislabeled subject, even if there are no printing or production mistakes.

Law
In appellate review, error typically refers to mistakes made by a trial court or some other court of first instance in applying the law in a particular legal case. This may involve such mistakes as improper admission of evidence, inappropriate instructions to the jury, or applying the wrong standard of proof.

Stock market
A stock market error is a stock market transaction that was done due to an error, due to human failure or computer errors.

Governmental policy
Within United States government intelligence agencies, such as Central Intelligence Agency agencies, error refers to intelligence error, as previous assumptions that used to exist at a senior intelligence level within senior intelligence agencies, but has since been disproven, and is sometimes eventually listed as unclassified, and therefore more available to the public and citizenry of the United States. The Freedom of information act provides American citizenry with a means to read intelligence reports that were mired in error. Per United States Central Intelligence Agency's website (as of August, 2008) intelligence error is described as:

"Intelligence errors are factual inaccuracies in analysis resulting from poor or missing data; intelligence failure is systemic organizational surprise resulting from incorrect, missing, discarded, or inadequate hypotheses."

Numismatics
In numismatics, an error refers to a coin or medal that has a minting mistake, similar to errors found in philately. Because the U.S. Bureau of the Mint keeps a careful eye on all potential errors, errors on U.S. coins are very few and usually very scarce. Examples of numismatic errors: extra metal attached to a coin, a clipped coin caused by the coin stamp machine stamping a second coin too early, double stamping of a coin. A coin that has been overdated, e.g. 1942/41, is also considered an error.

Linguistics
In applied linguistics, an error is an unintended deviation from the immanent rules of a language variety made by a second language learner. Such errors result from the learner's lack of knowledge of the correct rules of the target language variety. A significant distinction is generally made between errors (systematic deviations) and mistakes (speech performance errors) which are not treated the same from a linguistic viewpoint. The study of learners' errors has been the main area of investigation by linguists in the history of second-language acquisition research.

Medicine
A medical error is a preventable adverse effect of care ("iatrogenesis"), whether or not it is evident or harmful to the patient. This might include an inaccurate or incomplete diagnosis or treatment of a disease, injury, syndrome, behavior, infection, or other ailment.

The word error in medicine is used as a label for nearly all of the clinical incidents that harm patients. Medical errors are often described as human errors in healthcare. Whether the label is a medical error or human error, one definition used in medicine says that it occurs when a healthcare provider chooses an inappropriate method of care, improperly executes an appropriate method of care, or reads the wrong CT scan. It has been said that the definition should be the subject of more debate. For instance, studies of hand hygiene compliance of physicians in an ICU show that compliance varied from 19% to 85%. The deaths that result from infections caught as a result of treatment providers improperly executing an appropriate method of care by not complying with known safety standards for hand hygiene are difficult to regard as innocent accidents or mistakes.

There are many types of medical error, from minor to major, and causality is often poorly determined.

There are many taxonomies for classifying medical errors.

title: Error


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Burst error

In telecommunication, a burst error or error burst is a contiguous sequence of symbols, received over a communication channel, such that the first and last symbols are in error and there exists no contiguous subsequence of m correctly received symbols within the error burst. The integer parameter m is referred to as the guard band of the error burst. The last symbol in a burst and the first symbol in the following burst are accordingly separated by m correct symbols or more. The parameter m should be specified when describing an error burst.

Channel model
The Gilbert–Elliott model is a simple channel model introduced by Edgar Gilbert and E. O. Elliott that is widely used for describing burst error patterns in transmission channels and enables simulations of the digital error performance of communications links. It is based on a Markov chain with two states G (for good or gap) and B (for bad or burst). In state G the probability of transmitting a bit correctly is k and in state B it is h. Usually, it is assumed that k = 1. Gilbert provided equations for deriving the other three parameters (G and B state transition probabilities and h) from a given success/failure sequence. In his example, the sequence was too short to correctly find h (a negative probability was found) and so Gilbert assumed that h = 0.5.

title: Burst error


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Escape character

In computing and telecommunication, an escape character is a character that invokes an alternative interpretation on the following characters in a character sequence. An escape character is a particular case of metacharacters. Generally, the judgement of whether something is an escape character or not depends on the context.

In the telecommunications field, escape characters are used to indicate that the following characters are encoded differently. This is used to alter control characters that would otherwise be noticed and acted on by the underlying telecommunications hardware, such as illegal characters. In this context, the use of escape characters is often referred to as quoting.

Definition
An escape character may not have its own meaning, so all escape sequences are of two or more characters.

Escape characters are part of the syntax for many programming languages, data formats, and communication protocols. For a given alphabet an escape character's purpose is to start character sequences (so named escape sequences), which have to be interpreted differently from the same characters occurring without the prefixed escape character.

The functions of escape sequences include:

 * To encode a syntactic entity, such as device commands or special data, which cannot be directly represented by the alphabet.
 * To represent characters, referred to as character quoting, which cannot be typed in the current context, or would have an undesired interpretation. In this case, an escape sequence is a digraph consisting of an escape character itself and a "quoted" character.

Control character
Generally, an escape character is not a particular case of (device) control characters, nor vice versa. If we define control characters as non-graphic, or as having a special meaning for an output device (e.g. printer or text terminal) then any escape character for this device is a control one. But escape characters used in programming (such as the backslash, "\") are graphic, hence are not control characters. Conversely most (but not all) of the ASCII "control characters" have some control function in isolation, therefore they are not escape characters.

In many programming languages, an escape character also forms some escape sequences which are referred to as control characters. For example, line break has an escape sequence of \n.

JavaScript
JavaScript uses the \ (backslash) as an escape character for:
 * \' single quote
 * \" double quote
 * \\ backslash
 * \n new line
 * \r carriage return
 * \t tab
 * \b backspace
 * \f form feed
 * \v vertical tab (Internet Explorer 9 and older treats '\v as 'v instead of a vertical tab ('\x0B). If cross-browser compatibility is a concern, use \x0B instead of \v.)
 * \0 null character (U+0000 NULL) (only if the next character is not a decimal digit; else it is an octal escape sequence)
 * \xFF character represented by the hexadecimal byte "FF"

The \v and \0 escapes are not allowed in JSON strings. Example code:

ASCII escape character
The ASCII "escape" character (octal: \033, hexadecimal: \x1B, or, in decimal, 27, also represented by the sequences ^[ or \e) is used in many output devices to start a series of characters called a control sequence or escape sequence. Typically, the escape character was sent first in such a sequence to alert the device that the following characters were to be interpreted as a control sequence rather than as plain characters, then one or more characters would follow to specify some detailed action, after which the device would go back to interpreting characters normally. For example, the sequence of ^[, followed by the printable characters [2;10H, would cause a Digital Equipment Corporation (DEC) VT102 terminal to move its cursor to the 10th cell of the 2nd line of the screen. This was later developed into ANSI escape codes covered by the ANSI X3.64 standard. The escape character also starts each command sequence in the Hewlett-Packard Printer Command Language.

An early reference to the term "escape character" is found in Bob Bemer's IBM technical publications, who is credited with inventing this mechanism during his work on the ASCII character set.

The Escape key is usually found on standard PC keyboards. However, it is commonly absent from keyboards for PDAs and other devices not designed primarily for ASCII communications. The DEC VT220 series was one of the few popular keyboards that did not have a dedicated Esc key, instead of using one of the keys above the main keypad. In user interfaces of the 1970s–1980s it was not uncommon to use this key as an escape character, but in modern desktop computers, such use is dropped. Sometimes the key was identified with AltMode (for alternative mode). Even with no dedicated key, the escape character code could be generated by typing [ while simultaneously holding down Ctrl.

Programming and data formats
Many modern programming languages specify the double-quote character (") as a delimiter for a string literal. The backslash (\) escape character typically provides two ways to include double-quotes inside a string literal, either by modifying the meaning of the double-quote character embedded in the string (\" becomes "), or by modifying the meaning of a sequence of characters including the hexadecimal value of a double-quote character (\x22 becomes ").

C, C++, Java, and Ruby all allow exactly the same two backslash escape styles. The PostScript language and Microsoft Rich Text Format also use backslash escapes. The quoted-printable encoding uses the equals sign as an escape character.

URL and URI use %-escapes to quote characters with a special meaning, as for non-ASCII characters. The ampersand (&) character may be considered as an escape character in SGML and derived formats such as HTML and XML.

Some programming languages also provide other ways to represent special characters in literals, without requiring an escape character (see e.g. delimiter collision).

Communication protocols
The Point-to-Point Protocol (PPP) uses the 0x7D octet (\175, or ASCII: {{code| } }}) as an escape character. The octet immediately following should be XORed by 0x20 before being passed to a higher level protocol. This is applied to both 0x7D itself and the control character 0x7E (which is used in PPP to mark the beginning and end of a frame) when those octets need to be transmitted by a higher level protocol encapsulated by PPP, as well as other octets negotiated when the link is established. That is, when a higher level protocol wishes to transmit 0x7D, it is transmitted as the sequence 0x7D 0x5D, and 0x7E is transmitted as 0x7D 0x5E.

Bourne shell
In Bourne shell (sh), the asterisk (*) and question mark (?) characters are wildcard characters expanded via globbing. Without a preceding escape character, an * will expand to the names of all files in the working directory that do not start with a period if and only if there are such files, otherwise * remains unexpanded. So to refer to a file literally called "*", the shell must be told not to interpret it in this way, by preceding it with a backslash (\). This modifies the interpretation of the asterisk (*). Compare:

Windows Command Prompt
The Windows command-line interpreter uses a caret character (^) to escape reserved characters that have special meanings (in particular: &,, , <, >, ^). The DOS command-line interpreter, though it has similar syntax, does not support this.

For example, on the Windows Command Prompt, this will result in a syntax error. whereas this will output the string:

Windows PowerShell
In Windows, the backslash is used as a path separator; therefore, it generally cannot be used as an escape character. PowerShell uses backtick ( ` ) instead.

For example, the following command:

Others

 * Quoted-printable, which encodes 8-bit data into 7-bit data of limited line lengths, uses the equals sign (=) as an escape character.

title: Escape character


text:
Essential service (telecommunications)

In telecommunication, an essential service (critical service) is a network-provided service feature in which a priority dial tone is furnished. Essential service is typically provided to fewer than 10% of network users, and recommended for use in conjunction with NS/EP telecommunications services.

title: Essential service (telecommunications)


text:
Exchange

Exchange or exchanged may refer to:

Film and television

 * Exchange (film), or Deep Trap, 2015 South Korean psychological thriller
 * Exchanged (film), 2019 Peruvian fantasy comedy
 * Exchange (TV program), 2021 South Korean dating reality show

Gaming

 * Exchange (chess), closely related captures of pieces of both players in chess
 * The exchange (chess), an exchange of a minor piece for a rook

Music

 * Exchange, a new-age jazz band of Steve Sexton and Gerald O'Brien, and their 1992 self-titled album
 * Exchange (EP), by Against All Authority and The Criminals, 1999
 * "Exchange" (song), by Bryson Tiller, 2015
 * "Exchange" and "(Exchange)", songs on Massive Attack's 1998 album Mezzanine (album)

Business and economics

 * Bureau de change, or currency exchange
 * Cryptocurrency exchange, to trade cryptocurrencies or digital currencies
 * Exchange (economics), in a market economy
 * Exchange (organized market), where securities etc are bought and sold

Places

 * Exchange, Indiana, U.S.
 * Exchange, Missouri, U.S.
 * Exchange, Pennsylvania, U.S.
 * Exchange, West Virginia, U.S.
 * Exchange Alley, in London, United Kingdom
 * Exchange District, a historic area in Winnipeg, Manitoba, Canada

Science and technology

 * .exchange, an ICANN-era generic Internet top-level domain
 * Microsoft Exchange Server, mail and calendar software
 * Telephone exchange, connecting telephone calls

Other uses

 * USS Exchange (1862), an American Civil War steamer

title: Exchange


text:
Exempted addressee

In telecommunication, an exempted addressee is an organization, activity, or person included in the collective address group of a message and deemed by the message originator as having no need for the information in the message.

Exempted addressees may be explicitly excluded from the collective address group for the particular message to which the exemption applies.

title: Exempted addressee


text:
Extinction ratio

In telecommunications, extinction ratio (re) is the ratio of two optical power levels of a digital signal generated by an optical source, e.g., a laser diode. The extinction ratio may be expressed as a fraction, in dB, or as a percentage. It may be given by
 * $$r_e = \frac{P_1}{P_0},$$

where P1 is the optical power level generated when the light source is on, and P0 is the power level generated when the light source is off.

The polarization extinction ratio (PER) is the ratio of optical powers of perpendicular polarizations, usually called TE (transverse electric) and TM (transverse magnetic). In telecommunications, the PER is used to characterize the degree of polarization in a polarization-maintaining device or fiber. For coherent transmitter and receiver, the PER is a key parameter, since X polarization and Y polarization are coded with different signals.

title: Extinction ratio


text:
Eye pattern

In telecommunication, an eye pattern, also known as an eye diagram, is an oscilloscope display in which a digital signal from a receiver is repetitively sampled and applied to the vertical input (y-axis), while the data rate is used to trigger the horizontal sweep (x-axis). It is so called because, for several types of coding, the pattern looks like a series of eyes between a pair of rails. It is a tool for the evaluation of the combined effects of channel noise, dispersion and intersymbol interference on the performance of a baseband pulse-transmission system. The technique was first used with the WWII SIGSALY secure speech transmission system.

From a mathematical perspective, an eye pattern is a visualization of the probability density function (PDF) of the signal, modulo the unit interval (UI). In other words, it shows the probability of the signal being at each possible voltage across the duration of the UI. Typically a color ramp is applied to the PDF in order to make small brightness differences easier to visualize.

Several system performance measurements can be derived by analyzing the display. If the signals are too long, too short, poorly synchronized with the system clock, too high, too low, too noisy, or too slow to change, or have too much undershoot or overshoot, this can be observed from the eye diagram. An open eye pattern corresponds to minimal signal distortion. Distortion of the signal waveform due to intersymbol interference and noise appears as closure of the eye pattern.

Source data
The first step of computing an eye pattern is normally to obtain the waveform being analyzed in a quantized form. This may be done by measuring an actual electrical system with an oscilloscope of sufficient bandwidth, or by creating synthetic data with a circuit simulator in order to evaluate the signal integrity of a proposed design. A combination of the two approaches may be used as well: simulating the effects of an arbitrary circuit or transmission line on a measured signal, perhaps to determine whether a signal will still be intelligible after passing through a long cable. Interpolation may also be applied at this time in order to increase the number of samples per unit interval (UI) and produce a smooth, gap-free plot which is more visually appealing and easier to understand.

Slicing
Next, the position of each sample within the UI must be determined. There are several methods for doing this depending on the characteristics of the signal and the capabilities of the oscilloscope and software in use. This step is critically important for accurate visualization of jitter in the eye.

Triggering
A very simple method of slicing is to set the oscilloscope display to be slightly more than one UI wide, trigger on both rising and falling edges in the signal, and enable display persistence so that all measured waveforms "stack" into a single plot. This has the advantage of being possible on almost any oscilloscope (even fully analog ones) and can provide decent visualization of noise and overall signal shape, but completely destroys the jitter content of the signal since the instrument's trigger re-synchronizes the plot to each UI. The only jitter visible with this method is that of the oscilloscope itself, as well as extremely high frequency jitter (frequencies with period less than the UI).

Fixed rate
A simple way to have the eye pattern display jitter in the signal is to estimate the symbol rate of the signal (perhaps by counting the average number of zero crossings in a known window of time) and acquiring many UIs in a single oscilloscope capture. The first zero crossing in the capture is located and declared to be the start of the first UI, and the remainder of the waveform is divided into chunks one UI long.

This approach can work adequately for stable signals in which the symbol rate remains exactly the same over time, however inaccuracies in the system mean that some drift is inevitable so it is rarely used in practice. In some protocols, such as SATA, the symbol rate is intentionally varied by use of spread spectrum clocking, so assuming a fixed rate will lead to the eye grossly exaggerating the actual jitter present on the signal. (While spread spectrum modulation on a clock is technically jitter in the strict sense, receivers for these systems are designed to track the modulation. The only jitter of interest to a signal integrity engineer is jitter much faster than the modulation rate, which the receiver cannot track effectively.)

Reference clock
With some protocols, such as HDMI, a reference clock is supplied along with the signal, either at the symbol rate or at a lower (but synchronized) frequency from which a symbol clock can be reconstructed. Since the actual receiver in the system uses the reference clock to sample the data, using this clock to determine UI boundaries allows the eye pattern to faithfully display the signal as the receiver sees it: only jitter between the signal and the reference clock is displayed.

Clock recovery
Most high speed serial signals, such as PCIe, DisplayPort, and most variants of Ethernet, use a line code which is intended to allow easy clock recovery by means of a PLL. Since this is how the actual receiver works, the most accurate way to slice data for the eye pattern is to implement a PLL with the same characteristics in software. Correct PLL configuration allows for the eye to conceal the effects of spread spectrum clocking and other long-term variation in the symbol rate which do not contribute to errors at the receiver, while still displaying higher frequency jitter.

Integration
The samples are then accumulated into a two-dimensional histogram, with the X axis representing time within the UI and the Y axis representing voltage. This is then normalized by dividing the value in each histogram bin by the value in the largest bin. Tone mapping, logarithmic scaling, or other mathematical transformations may be applied in order to emphasize different portions of the distribution, and a color gradient is applied to the final eye for display.

Large amounts of data may be needed to provide an accurate representation of the signal; tens to hundreds of millions of UIs are frequently used for a single eye pattern. In the example below, the eye using twelve thousand UIs only shows the basic shape of the eye, while the eye using eight million UIs shows far more nuance on the rising and falling edges.

Modulation
Each form of baseband modulation produces an eye pattern with a unique appearance.

NRZ
The eye pattern of a NRZ signal should consist of two clearly distinct levels with smooth transitions between them.

MLT-3
The eye pattern of a MLT-3 signal should consist of three clearly distinct levels (nominally -1, 0, +1 from bottom to top). The 0 level should be located at zero volts and the overall shape should be symmetric about the horizontal axis. The +1 and -1 states should have equal amplitude. There should be smooth transitions from the 0 state to the +1 and -1 states, however there should be no direct transitions from the -1 to +1 state (which would indicate the signal is PAM-3 rather than MLT-3).

PAM
The eye pattern of a PAM signal should consist of N clearly distinct levels (depending on the PAM order, for example PAM-4 should have four levels and PAM-3 should have three). The overall shape should be symmetric about the horizontal axis and the spacing of all levels should be uniform.

Channel effects
Many properties of a channel can be seen in the eye pattern.

Emphasis
Emphasis applied to a signal produces an additional level for each value of the signal which is higher (for pre-emphasis) or lower (for de-emphasis) than the nominal value.

The eye pattern for a signal with emphasis may be mistaken for that of a PAM signal at first glance, however closer inspection reveals some key differences. Most notably, an emphasized signal has a limited set of legal transitions:

 * Strong state to corresponding weak state (1-1 or 0-0 bit pattern)
 * Strong state to opposite strong state (second transition of a 1-0-1 or 0-1-0 bit pattern)
 * Weak state to opposite strong state (second transition of a 1-1-0 or 0-0-1 bit pattern)

An emphasized signal will never transition from a weak state to the corresponding strong state, a weak state to another weak state, or remain in the same strong state for more than one UI. A PAM signal also normally has equally spaced levels while emphasized levels are normally closer to the nominal signal level.

High-Frequency Loss
Loss of printed circuit board traces and cables increases with frequency due to dielectric loss, which causes the channel to behave as a low-pass filter. The effect of this is an increase in signal rise/fall time. If the data rate is high enough or the channel is lossy enough, the signal may not even reach its full value during a fast 0-1-0 or 1-0-1 transition, and only stabilize after a run of several identical bits. This results in vertical closure of the eye.

The image below shows a 1.25 Gbit/s NRZ signal after passing through a lossy channel - an RG-188 coaxial cable approximately 12 feet (3.65 meters) in length. This channel has loss increasing in a fairly linear fashion from 0.1 dB at DC to 9 dB at 6 GHz.

The top and bottom "rails" of the eye show the final voltage the signal reaches after several consecutive bits with the same value. Since the channel has minimal loss at DC, the maximum signal amplitude is largely unaffected. Looking at the rising edge of the signal (a 0-1 pattern) we can see that the signal starts to level off around -300 ps, but continues to rise slowly over the duration of the UI. At around +300 ps, the signal either begins falling again (a 0-1-0 pattern) or continues rising slowly (an 0-1-1 pattern).

As high frequency losses increase the overall shape of the eye gradually degrades into a sinusoid (once higher frequency harmonics of the data has been eliminated, all that remains is the fundamental) and decreases in amplitude.

Impedance Mismatches
Stubs, impedance mismatches, and other defects in a transmission line can cause reflections visible as defects in the edges of the signal. Reflections with a delay greater than one UI often render the eye completely unreadable due to inter-symbol interference (ISI), however those with a shorter delay can be easily seen in the shape of the eye.

In the image below, a roughly one inch (25.4 mm) open circuited stub is present in the line, causing an initial low-impedance effect (reduced amplitude) followed by a positive reflection from the end of the stub with a delay of about 320 ps or 0.4 UIs. This can be clearly seen as a "step" in the rising edge in which the signal rises to a fraction of the full value, levels off for the round trip delay of the stub, then rises to its full value when the reflection arrives.

In the image below, an additional three inches of cable is added to the end of the same stub. The same "step" is present but is now four times as long, producing reflections at about 1280 ps or 1.6 UI. This produces extreme ISI (since the reflection of each UI arrives during the subsequent UI) which completely closes the eye.

Measurements
There are many measurements that can be obtained from an eye diagram:

Amplitude measurements Time measurements
 * Eye amplitude
 * Eye crossing amplitude
 * Eye crossing percentage
 * Eye height
 * Eye level
 * Eye signal-to-noise ratio
 * Quality factor
 * Vertical eye opening
 * Deterministic jitter
 * Eye crossing time
 * Eye delay
 * Eye fall time
 * Eye rise time
 * Eye width
 * Horizontal eye opening
 * Peak-to-peak jitter
 * Random jitter
 * RMS jitter
 * CRC jitter
 * Total jitter

title: Eye pattern


text:
Telecommunications facility

In telecommunications, a facility is defined by Federal Standard 1037C as:
 * 1) A fixed, mobile, or transportable structure, including (a) all installed electrical and electronic wiring, cabling, and equipment and (b) all supporting structures, such as utility, ground network, and electrical supporting structures.
 * 2) A network-provided service to users or the network operating administration.
 * 3) A transmission pathway and associated equipment.
 * 4) In a protocol applicable to a data unit, such as a block or frame, an additional item of information or a constraint encoded within the protocol to provide the required control.
 * 5) A real property entity consisting of one or more of the following: a building, a structure, a utility system, pavement, and underlying land.

Global Telecom Facility
Telecommunications facility, is where a service can be offered. A location where a Incumbent Local Exchange Carriers (ILECs) have their Hardware to process Telecom services. A phone call made to Jamaica can not be processed by a Telecom Facility in Canada or USA. If a phone number series belongs to say operator or carriers in Jamaica then only that operator knows if that phone number is valid and further if that phone number is on net to place a valid phone call. So all carriers in the world get the signal from the Telecom Facilities of the Digicel in Jamaica.

In Canada
Telecommunications facility, is same as Airport Facility. A location where a Incumbent Local Exchange Carriers (ILECs) have their Hardware to process Telecom services. Under Canadian federal and Québécois provincial law, a telecommunications facility, for the purposes of determining whether GST applies, is defined by §123(1) of the GST Act to be "any facility, apparatus, or other thing (including any wire, cable, radio, optical, or other electromagnetic system, or any similar technical system or any part thereof) that is used or is capable of being used for telecommunications". This is a very broad definition that includes a wide range of things from satellites and earth stations, to telephones and fax machines.

title: Telecommunications facility


text:
Facsimile converter

In telecommunication, the term facsimile converter has the following meanings:

1. In a facsimile receiver, a device that changes the signal modulation from frequency-shift keying (FSK) to amplitude modulation (AM).

2. In a facsimile transmitter, a device that changes the signal modulation from amplitude modulation (AM) to frequency-shift keying (FSK).

title: Facsimile converter


text:
Fade margin

In telecommunication, the term fade margin (fading margin) has the following meanings:

 * A design allowance that provides for sufficient system gain or sensitivity to accommodate expected fading, for the purpose of ensuring that the required quality of service is maintained.
 * The amount by which a received signal level may be reduced without causing system performance to fall below a specified threshold value. It is mainly used to describe a communication system such as satellite, for example a system like globalstar operates at 25-35 dB Fade margin.

title: Fade margin


text:
Fading distribution

'''Fading distribution is the probability distribution of the value of signal fading, relative to a specified reference level.

In the case of phase interference fading, the time distribution of the instantaneous field strength usually approximates a Rayleigh distribution when several signal components of equal amplitude are present.

The field strength is usually measured in volts per meter.

The fading distribution may also be measured in terms of power level, where the unit of measure is usually watts per square meter and the expression is in decibels.

title: Fading distribution


text:
Fail-safe

In engineering, a fail-safe is a design feature or practice that, in the event of a failure of the design feature, inherently responds in a way that will cause minimal or no harm to other equipment, to the environment or to people. Unlike inherent safety to a particular hazard, a system being "fail-safe" does not mean that failure is naturally inconsequential, but rather that the system's design prevents or mitigates unsafe consequences of the system's failure. If and when a "fail-safe" system fails, it remains at least as safe as it was before the failure. Since many types of failure are possible, failure mode and effects analysis is used to examine failure situations and recommend safety design and procedures.

Some systems can never be made fail-safe, as continuous availability is needed. Redundancy, fault tolerance, or contingency plans are used for these situations (e.g. multiple independently controlled and fuel-fed engines).

Mechanical or physical
Examples include:
 * Roller-shutter fire doors that are activated by building alarm systems or local smoke detectors must close automatically when signaled regardless of power. In case of power outage the coiling fire door does not need to close, but must be capable of automatic closing when given a signal from the building alarm systems or smoke detectors. A temperature-sensitive fusible link may be employed to hold the fire doors open against gravity or a closing spring. In case of fire, the link melts and releases the doors, and they close.
 * Some airport baggage carts require that the person hold down a given cart's handbrake switch at all times; if the handbrake switch is released, the brake will activate, and assuming that all other portions of the braking system are working properly, the cart will stop. The handbrake-holding requirement thus both operates according to the principles of "fail-safety" and contributes to (but does not necessarily ensure) the fail-security of the system. This is an example of a dead man's switch.
 * Lawnmowers and snow blowers have a hand-closed lever that must be held down at all times. If it is released, it stops the blade's or rotor's rotation. This also functions as a dead man's switch.
 * Air brakes on railway trains and air brakes on trucks. The brakes are held in the "off" position by air pressure created in the brake system. Should a brake line split, or a carriage become uncoupled, the air pressure will be lost and the brakes applied, by springs in the case of trucks, or by a local air reservoir in trains. It is impossible to drive a truck with a serious leak in the air brake system. (Trucks may also employ wig wags to indicate low air pressure.)
 * Motorized gates – In case of power outage the gate can be pushed open by hand with no crank or key required. However, as this would allow virtually anyone to go through the gate, a fail-secure design is used: In a power outage, the gate can only be opened by a hand crank that is usually kept in a safe area or under lock and key. When such a gate provides vehicle access to homes, a fail-safe design is used, where the door opens to allow fire department access.
 * Safety valves – Various devices that operate with fluids use fuses or safety valves as fail-safe mechanisms.
 * A railway semaphore signal is specially designed so that, should the cable controlling the signal break, the arm returns to the "danger" position, preventing any trains passing the inoperative signal.
 * Isolation valves, and control valves, that are used for example in systems containing hazardous substances, can be designed to close upon loss of power, for example by spring force. This is known as fail-closed upon loss of power.
 * An elevator has brakes that are held off brake pads by the tension of the elevator cable. If the cable breaks, tension is lost and the brakes latch on the rails in the shaft, so that the elevator cabin does not fall.
 * Vehicle air conditioning – Defrost controls require vacuum for diverter damper operation for all functions except defrost. If vacuum fails, defrost is still available.

Electrical or electronic
Examples include:
 * Many devices are protected from short circuit by fuses, circuit breakers, or current limiting circuits. The electrical interruption under overload conditions will prevent damage or destruction of wiring or circuit devices due to overheating.
 * Avionics using redundant systems to perform the same computation using three different systems. Different results indicate a fault in the system.
 * Drive-by-wire and fly-by-wire controls such as an Accelerator Position Sensor typically have two potentiometers which read in opposite directions, such that moving the control will result in one reading becoming higher, and the other generally equally lower. Mismatches between the two readings indicates a fault in the system, and the ECU can often deduce which of the two readings is faulty.
 * Traffic light controllers use a Conflict Monitor Unit to detect faults or conflicting signals and switch an intersection to an all flashing error signal, rather than displaying potentially dangerous conflicting signals, e.g. showing green in all directions.
 * The automatic protection of programs and/or processing systems when a computer hardware or software failure is detected in a computer system. A classic example is a watchdog timer. See Fail-safe (computer).
 * A control operation or function that prevents improper system functioning or catastrophic degradation in the event of circuit malfunction or operator error; for example, the failsafe track circuit used to control railway block signals. The fact that a flashing amber is more permissive than a solid amber on many railway lines is a sign of a failsafe, as the relay, if not working, will revert to a more restrictive setting.
 * The iron pellet ballast on the Bathyscaphe is dropped to allow the submarine to ascend. The ballast is held in place by electromagnets. If electrical power fails, the ballast is released, and the submarine then ascends to safety.
 * Many nuclear reactor designs have neutron-absorbing control rods suspended by electromagnets. If the power fails, they drop under gravity into the core and shut down the chain reaction in seconds by absorbing the neutrons needed for fission to continue.
 * In industrial automation, alarm circuits are usually "normally closed". This ensures that in case of a wire break the alarm will be triggered. If the circuit were normally open, a wire failure would go undetected, while blocking actual alarm signals.
 * Analog sensors and modulating actuators can usually be installed and wired such that the circuit failure results in an out-of-bound reading – see current loop. For example, a potentiometer indicating pedal position might only travel from 20% to 80% of its full range, such that a cable break or short results in a 0% or 100% reading.
 * In control systems, critically important signals can be carried by a complementary pair of wires ( and ). Only states where the two signals are opposite (one is high, the other low) are valid. If both are high or both are low the control system knows that something is wrong with the sensor or connecting wiring. Simple failure modes (dead sensor, cut or unplugged wires) are thereby detected. An example would be a control system reading both the normally open (NO) and normally closed (NC) poles of a SPDT selector switch against common, and checking them for coherency before reacting to the input.
 * In HVAC control systems, actuators that control dampers and valves may be fail-safe, for example, to prevent coils from freezing or rooms from overheating. Older pneumatic actuators were inherently fail-safe because if the air pressure against the internal diaphragm failed, the built-in spring would push the actuator to its home position – of course the home position needed to be the "safe" position. Newer electrical and electronic actuators need additional components (springs or capacitors) to automatically drive the actuator to home position upon loss of electrical power.
 * Programmable logic controllers (PLCs). To make a PLC fail-safe the system does not require energization to stop the drives associated. For example, usually, an emergency stop is a normally closed contact. In the event of a power failure this would remove the power directly from the coil and also the PLC input. Hence, a fail-safe system.
 * If a voltage regulator fails, it can destroy connected equipment. A crowbar (circuit) prevents damage by short-circuiting the power supply as soon as it detects overvoltage.

Procedural safety
As well as physical devices and systems fail-safe procedures can be created so that if a procedure is not carried out or carried out incorrectly no dangerous action results. For example:
 * Spacecraft trajectory - During early Apollo program missions to the Moon, the spacecraft was put on a free return trajectory — if the engines had failed at lunar orbit insertion, the craft would have safely coasted back to Earth.
 * The pilot of an aircraft landing on an aircraft carrier increases the throttle to full power at touchdown. If the arresting wires fail to capture the aircraft, it is able to take off again; this is an example of fail-safe practice.
 * In railway signalling signals which are not in active use for a train are required to be kept in the 'danger' position. The default position of every controlled absolute signal is therefore "danger", and therefore a positive action — setting signals to "clear" — is required before a train may pass. This practice also ensures that, in case of a fault in the signalling system, an incapacitated signalman, or the unexpected entry of a train, that a train will never be shown an erroneous "clear" signal.
 * Railroad engineers are instructed that a railway signal showing a confusing, contradictory or unfamiliar aspect (for example a colour light signal that has suffered an electrical failure and is showing no light at all) must be treated as showing "danger". In this way, the driver contributes to the fail-safety of the system.

Other terminology
Fail-safe (foolproof) devices are also known as poka-yoke devices. Poka-yoke, a Japanese term, was coined by Shigeo Shingo, a quality expert. "Safe to fail" refers to civil engineering designs such as the Room for the River project in Netherlands and the Thames Estuary 2100 Plan which incorporate flexible adaptation strategies or climate change adaptation which provide for, and limit, damage, should severe events such as 500-year floods occur.

Fail safe and fail secure
Fail-safe and fail-secure are distinct concepts. Fail-safe means that a device will not endanger lives or property when it fails. Fail-secure, also called fail-closed, means that access or data will not fall into the wrong hands in a security failure. Sometimes the approaches suggest opposite solutions. For example, if a building catches fire, fail-safe systems would unlock doors to ensure quick escape and allow firefighters inside, while fail-secure would lock doors to prevent unauthorized access to the building.

The opposite of fail-closed is called fail-open.

Fail active operational
Fail active operational can be installed on systems that have a high degree of redundancy so that a single failure of any part of the system can be tolerated (fail active operational) and a second failure can be detected – at which point the system will turn itself off (uncouple, fail passive). One way of accomplishing this is to have three identical systems installed, and a control logic which detects discrepancies. An example for this are many aircraft systems, among them inertial navigation systems and pitot tubes.

Failsafe point
During the Cold War, "failsafe point" was the term used for the point of no return for American Strategic Air Command nuclear bombers, just outside Soviet airspace. In the event of receiving an attack order, the bombers were required to linger at the failsafe point and wait for a second confirming order; until one was received, they would not arm their bombs or proceed further. The design was to prevent any single failure of the American command system causing nuclear war. This sense of the term entered the American popular lexicon with the publishing of the 1962 novel Fail-Safe. (Other nuclear war command control systems have used the opposite scheme, fail-deadly, which requires continuous or regular proof that an enemy first-strike attack has not occurred to prevent the launching of a nuclear strike.)

title: Fail-safe


text:
Fall time

In electronics, fall time (pulse decay time) $$ t_f$$ is the time taken for the amplitude of a pulse to decrease (fall) from a specified value (usually 90% of the peak value exclusive of overshoot or undershoot) to another specified value (usually 10% of the maximum value exclusive of overshoot or undershoot).

Limits on undershoot and oscillation (also known as ringing and hunting) are sometimes additionally stated when specifying fall time limits.

title: Fall time


text:
Fast packet switching

In telecommunications, fast packet switching is a variant of packet switching that increases the throughput by eliminating overhead associated with flow control and error correction functions, which are either offloaded to upper layer networking protocols or removed altogether. ATM and Frame Relay are two major implementations of fast packet switching.

title: Fast packet switching


text:
Fault

Fault commonly refers to:
 * Fault (geology), planar rock fractures showing evidence of relative movement
 * Fault (law), blameworthiness or responsibility

Fault(s) may also refer to:

Arts, entertainment, and media

 * "Fault", a song by Taproot from Welcome
 * Faults (film), 2014

Science and technology

 * Fault (computing), also called a trap or an exception, a type of interrupt in software or operating systems
 * Fault (technology), an abnormal condition or defect that may lead to a failure
 * Electrical fault, an abnormal current

Sport and competition

 * Fault (breeding), an undesirable aspect of structure or appearance of an animal
 * Fault, in pickleball, any infringement of the rules by a player
 * Fault, in show jumping, a penalty
 * Fault, in tennis jargon, a serve that fails to place a tennis ball in the correct area of play

title: Fault


text:
Fault management

In network management, fault management is the set of functions that detect, isolate, and correct malfunctions in a telecommunications network, compensate for environmental changes, and include maintaining and examining error logs, accepting and acting on error detection notifications, tracing and identifying faults, carrying out sequences of diagnostics tests, correcting faults, reporting error conditions, and localizing and tracing faults by examining and manipulating database information.

When a fault or event occurs, a network component will often send a notification to the network operator using a protocol such as SNMP. An alarm is a persistent indication of a fault that clears only when the triggering condition has been resolved. A current list of problems occurring on the network component is often kept in the form of an active alarm list such as is defined in RFC 3877, the Alarm MIB. A list of cleared faults is also maintained by most network management systems.

Fault management systems may use complex filtering systems to assign alarms to severity levels. These can range in severity from debug to emergency, as in the syslog protocol. Alternatively, they could use the ITU X.733 Alarm Reporting Function's perceived severity field. This takes on values of cleared, indeterminate, critical, major, minor or warning. Note that the latest version of the syslog protocol draft under development within the IETF includes a mapping between these two different sets of severities. It is considered good practice to send a notification not only when a problem has occurred, but also when it has been resolved. The latter notification would have a severity of clear.

A fault management console allows a network administrator or system operator to monitor events from multiple systems and perform actions based on this information. Ideally, a fault management system should be able to correctly identify events and automatically take action, either launching a program or script to take corrective action, or activating notification software that allows a human to take proper intervention (i.e. send e-mail or SMS text to a mobile phone). Some notification systems also have escalation rules that will notify a chain of individuals based on availability and severity of alarm.

Types
There are two primary ways to perform fault management - these are active and passive. Passive fault management is done by collecting alarms from devices (normally via SNMP traps) when something happens in the devices. In this mode, the fault management system only knows if a device it is monitoring is intelligent enough to generate an error and report it to the management tool. However, if the device being monitored fails completely or locks up, it won't throw an alarm and the problem will not be detected. Active fault management addresses this issue by actively monitoring devices via tools such as ping to determine if the device is active and responding. If the device stops responding, active monitoring will throw an alarm showing the device as unavailable and allows for the proactive correction of the problem.

Fault management includes any tools or procedure for testing, diagnosing or repairing the network when a failure occurs.

title: Fault management


text:
FCC (disambiguation)

The FCC, or Federal Communications Commission, is an independent agency of the United States government.

FCC may also refer to:

Christianity

 * Federated Colored Catholics
 * Free Church of Scotland (disambiguation)
 * Franciscan Clarist Congregation

Education

 * Faujdarhat Cadet College, in Bangladesh
 * Federal City College, now merged into the University of the District of Columbia
 * Felpham Community College, in England
 * Florida Christian College, now Johnson University Florida, in Kissimmee, Florida, United States
 * Forman Christian College, in Lahore, Pakistan
 * Frederick Community College, in Maryland, United States
 * Fresno City College, in Fresno, California, United States
 * Footscray City College, a high school in Melbourne, Australia
 * freeCodeCamp, a non-profit online course which teaches computer programming

Science and technology

 * Face-centered cubic, a crystal system
 * Female cosmetic coalitions, a theory about the emergence of art, ritual and symbolic culture
 * Fibrocystic change
 * Flash column chromatography
 * Fluid catalytic cracking
 * Food Chemicals Codex
 * Future Circular Collider, a proposed particle collider
 * A hypothetical 3-in-one vaccine, for the flu, common cold, and COVID-19.

Sport

 * Chilean Cycling Federation (Spanish: Federación Ciclista de Chile)
 * Colombian Cycling Federation (Spanish: Federación Colombiana de Ciclismo)
 * Cuban Cycling Federation (Spanish: Federación Cubana de Ciclismo)
 * FC Carl Zeiss Jena, a German association football club
 * FC Chartres, a French association football club
 * FC Cincinnati, an American soccer club who currently play in MLS
 * FC Cincinnati (2016–18), predecessor to the above, which played in the league now known as the USL Championship
 * FIFA Confederations Cup, international association football tournament for men's national teams organised by FIFA
 * Four Continents Championships, a figure skating competition

Transport

 * Ferrocarril Central, a former Peruvian railway company
 * Ferrocarrils de la Generalitat de Catalunya, Railway operator in Catalonia, Spain
 * Ferrocarriles de Cuba, the National Railways of Cuba
 * First Capital Connect, a defunct United Kingdom train operator

Other uses

 * Fairfield Community Connection, an American bulletin board system
 * Falling Creek Camp, in North Carolina, United States
 * Farm Credit Canada, formerly the Farm Credit Corporation, a Canadian crown corporation
 * Flying Cunts of Chaos, an American rock band and former backing band of Serj Tankian
 * FCC Environment, a British waste management firm
 * "FCC Song", by Eric Idle
 * Federal Correctional Complex; see List of U.S. federal prisons
 * Five Country Conference, a conference of the immigration authorities for English speaking countries
 * Fomento de Construcciones y Contratas, a Spanish construction firm
 * Foreign Correspondents' Club, a group of clubs for correspondents and journalists
 * Foreign Correspondents' Club, Hong Kong
 * Seoul Foreign Correspondents’ Club
 * Civil Forum for Change (FCC in French), an alliance of civil society groups created in Algeria in 2019
 * The Future Cities Catapult, which in April 2019 became the Connected Places Catapult

title: FCC (disambiguation)


text:
FCC registration program

In telecommunication, FCC registration program is the Federal Communications Commission (FCC) program and associated directives intended to assure that all connected terminal equipment and protective circuitry will not harm the public switched telephone network or certain private line services.

Note 1: The FCC registration program requires the registering of terminal equipment and protective circuitry in accordance with Subpart C of part 68, Title 47 of the Code of Federal Regulations. This includes the assignment of identification numbers to the equipment and the testing of the equipment.

Note 2: The FCC registration program contains no requirement that accepted terminal equipment be compatible with, or function with, the network.

title: FCC registration program


text:
Feed

Feed or The Feed may refer to:

Animal foodstuffs

 * Animal feed, food given to domestic animals in the course of animal husbandry
 * Fodder, foodstuffs manufactured for animal consumption
 * Forage, foodstuffs that animals gather themselves, such as by grazing
 * Compound feed, foodstuffs that are blended from various raw materials and additives

Comedy

 * A straight man who 'feeds' lines to the funny man in a comic dialogue

Film

 * Feed (2005 film), a 2005 film directed by Brett Leonard
 * Feed (2017 film), a 2017 film directed by Tommy Bertelsen

Literature

 * Feed (Anderson novel), a 2002 novel by M. T. Anderson
 * Feed (Grant novel), a 2010 novel by Seanan McGuire under the name "Mira Grant"

Music

 * "Feed Us", 2007 song by Serj Tankian from Elect the Dead
 * "Feed", 2022 song by Demi Lovato from Holy Fvck

Online media

 * Feed Magazine, one of the earliest e-zines that relied entirely on its original online content
 * "The Feed", video game news and blogs, published by G4 Media, an NBCUniversal subsidiary
 * Web feed or news feed, a data format used for providing users with frequently updated content
 * Feed (Facebook), a web feed on the social networking site

Television

 * The Feed (Australian TV series), an Australian news TV series
 * The Feed (British TV series), a 2019 psychological thriller drama television series
 * The Feed, a recurring segment on the American TV series Attack of the Show!

Video games

 * In video game terminology, to die repeatedly

Computing and telecommunications

 * Antenna feed, the components of an antenna which feed the radio waves to the rest of the antenna structure
 * Data feed, a mechanism for users to receive updates from data sources
 * Web feed
 * feed URI scheme, a non-standard URI scheme designed to facilitate subscription to web feeds
 * Relay (disambiguation), any of several technologies for forwarding messages between stations
 * Feed, a broadcasting signal sent from one station to another, or to or from a central facility, intended for retransmission

Science and technology

 * The liquid input into a filtration is called the feed
 * Speeds and feeds

title: Feed