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Heterodyne

A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called heterodyning, which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is used to shift signals from one frequency range into another, and is also involved in the processes of modulation and demodulation. The two input frequencies are combined in a nonlinear signal-processing device such as a vacuum tube, transistor, or diode, usually called a mixer.

In the most common application, two signals at frequencies $f_{1}$ and $f_{2}$ are mixed, creating two new signals, one at the sum of the two frequencies $f_{1} + f_{2}$, and the other at the difference between the two frequencies $f_{1} − f_{2}$. The new signal frequencies are called heterodynes. Typically, only one of the heterodynes is required and the other signal is filtered out of the output of the mixer. Heterodyne frequencies are related to the phenomenon of "beats" in acoustics.

A major application of the heterodyne process is in the superheterodyne radio receiver circuit, which is used in virtually all modern radio receivers.

History

In 1901, Reginald Fessenden demonstrated a direct-conversion heterodyne receiver or beat receiver as a method of making continuous wave radiotelegraphy signals audible. Fessenden's receiver did not see much application because of its local oscillator's stability problem. A stable yet inexpensive local oscillator was not available until Lee de Forest invented the triode vacuum tube oscillator. In a 1905 patent, Fessenden stated that the frequency stability of his local oscillator was one part per thousand.

In radio telegraphy, the characters of text messages are translated into the short duration dots and long duration dashes of Morse code that are broadcast as radio signals. Radio telegraphy was much like ordinary telegraphy. One of the problems was building high power transmitters with the technology of the day. Early transmitters were spark gap transmitters. A mechanical device would make sparks at a fixed but audible rate; the sparks would put energy into a resonant circuit that would then ring at the desired transmission frequency (which might be 100 kHz). This ringing would quickly decay, so the output of the transmitter would be a succession of damped waves. When these damped waves were received by a simple detector, the operator would hear an audible buzzing sound that could be transcribed back into alpha-numeric characters.

With the development of the arc converter radio transmitter in 1904, continuous wave (CW) modulation began to be used for radiotelegraphy. CW Morse code signals are not amplitude modulated, but rather consist of bursts of sinusoidal carrier frequency. When CW signals are received by an AM receiver, the operator does not hear a sound. The direct-conversion (heterodyne) detector was invented to make continuous wave radio-frequency signals audible.

The "heterodyne" or "beat" receiver has a local oscillator that produces a radio signal adjusted to be close in frequency to the incoming signal being received. When the two signals are mixed, a "beat" frequency equal to the difference between the two frequencies is created. Adjusting the local oscillator frequency correctly puts the beat frequency in the audio range, where it can be heard as a tone in the receiver's earphones whenever the transmitter signal is present. Thus the Morse code "dots" and "dashes" are audible as beeping sounds. This technique is still used in radio telegraphy, the local oscillator now being called the beat frequency oscillator or BFO. Fessenden coined the word heterodyne from the Greek roots hetero- "different", and dyn- "power" (cf. δύναμις or dunamis).

Superheterodyne receiver

An important and widely used application of the heterodyne technique is in the superheterodyne receiver (superhet). In the typical superhet, the incoming radio frequency signal from the antenna is mixed (heterodyned) with a signal from a local oscillator (LO) to produce a lower fixed frequency signal called the intermediate frequency (IF) signal. The IF signal is amplified and filtered and then applied to a detector that extracts the audio signal; the audio is ultimately sent to the receiver's loudspeaker.

The superheterodyne receiver has several advantages over previous receiver designs. One advantage is easier tuning; only the RF filter and the LO are tuned by the operator; the fixed-frequency IF is tuned ("aligned") at the factory and is not adjusted. In older designs such as the tuned radio frequency receiver (TRF), all of the receiver stages had to be simultaneously tuned. In addition, since the IF filters are fixed-tuned, the receiver's selectivity is the same across the receiver's entire frequency band. Another advantage is that the IF signal can be at a much lower frequency than the incoming radio signal, and that allows each stage of the IF amplifier to provide more gain. To first order, an amplifying device has a fixed gain-bandwidth product. If the device has a gain-bandwidth product of 60 MHz, then it can provide a voltage gain of 3 at an RF of 20 MHz or a voltage gain of 30 at an IF of 2 MHz. At a lower IF, it would take fewer gain devices to achieve the same gain. The regenerative radio receiver obtained more gain out of one gain device by using positive feedback, but it required careful adjustment by the operator; that adjustment also changed the selectivity of the regenerative receiver. The superheterodyne provides a large, stable gain and constant selectivity without troublesome adjustment.

The superior superheterodyne system replaced the earlier TRF and regenerative receiver designs, and since the 1930s most commercial radio receivers have been superheterodynes.

Applications
Heterodyning, also called frequency conversion, is used very widely in communications engineering to generate new frequencies and move information from one frequency channel to another. Besides its use in the superheterodyne circuit found in almost all radio and television receivers, it is used in radio transmitters, modems, satellite communications and set-top boxes, radar, radio telescopes, telemetry systems, cell phones, cable television converter boxes and headends, microwave relays, metal detectors, atomic clocks, and military electronic countermeasure (jamming) systems.

Up and down converters
In large scale telecommunication networks such as telephone network trunks, microwave relay networks, cable television systems, and communication satellite links, large bandwidth capacity links are shared by many individual communication channels by using heterodyning to move the frequency of the individual signals up to different frequencies, which share the channel. This is called frequency division multiplexing (FDM).

For example, a coaxial cable used by a cable television system can carry 500 television channels at the same time because each one is given a different frequency, so they do not interfere with one another. At the cable source or headend, electronic upconverters convert each incoming television channel to a new, higher frequency. They do this by mixing the television signal frequency, fCH with a local oscillator at a much higher frequency $f_{LO}$, creating a heterodyne at the sum $f_{CH} + f_{LO}$, which is added to the cable. At the consumer's home, the cable set top box has a downconverter that mixes the incoming signal at frequency $f_{CH} + f_{LO}$ with the same local oscillator frequency $f_{LO}$ creating the difference heterodyne frequency, converting the television channel back to its original frequency: $(f_{CH} + f_{LO}) − f_{LO} = f_{CH}$. Each channel is moved to a different higher frequency. The original lower basic frequency of the signal is called the baseband, while the higher channel it is moved to is called the passband.

Analog videotape recording
Many analog videotape systems rely on a downconverted color subcarrier to record color information in their limited bandwidth. These systems are referred to as "heterodyne systems" or "color-under systems". For instance, for NTSC video systems, the VHS (and S-VHS) recording system converts the color subcarrier from the NTSC standard 3.58 MHz to ~629 kHz. PAL VHS color subcarrier is similarly downconverted (but from 4.43 MHz). The now-obsolete 3/4" U-matic systems use a heterodyned ~688 kHz subcarrier for NTSC recordings (as does Sony's Betamax, which is at its basis a 1/2″ consumer version of U-matic), while PAL U-matic decks came in two mutually incompatible varieties, with different subcarrier frequencies, known as Hi-Band and Low-Band. Other videotape formats with heterodyne color systems include Video-8 and Hi8.

The heterodyne system in these cases is used to convert quadrature phase-encoded and amplitude modulated sine waves from the broadcast frequencies to frequencies recordable in less than 1 MHz bandwidth. On playback, the recorded color information is heterodyned back to the standard subcarrier frequencies for display on televisions and for interchange with other standard video equipment.

Some U-matic (3/4″) decks feature 7-pin mini-DIN connectors to allow dubbing of tapes without conversion, as do some industrial VHS, S-VHS, and Hi8 recorders.

Music synthesis
The theremin, an electronic musical instrument, traditionally uses the heterodyne principle to produce a variable audio frequency in response to the movement of the musician's hands in the vicinity of one or more antennae, which act as capacitor plates. The output of a fixed radio frequency oscillator is mixed with that of an oscillator whose frequency is affected by the variable capacitance between the antenna and the musician's hand as it is moved near the pitch control antenna. The difference between the two oscillator frequencies produces a tone in the audio range.

The ring modulator is a type of frequency mixer incorporated into some synthesizers or used as a stand-alone audio effect.

Optical heterodyning
Optical heterodyne detection (an area of active research) is an extension of the heterodyning technique to higher (visible) frequencies. Guerra (1995) first published the results of what he called a "form of optical heterodyning" in which light patterned by a 50 nm pitch grating illuminated a second grating of pitch 50 nm, with the gratings rotated with respect to each other by the angular amount needed to achieve magnification. Although the illuminating wavelength was 650 nm, the 50 nm grating was easily resolved. This showed a nearly 5-fold improvement over the Abbe resolution limit of 232 nm that should have been the smallest obtained for the numerical aperture and wavelength used. This super-resolution microscopic imaging through optical heterodyning later came to be know by many as "structured illumination microscopy".

In addition to super-resolution optical microscopy, optical heterodyning could greatly improve optical modulators, increasing the density of information carried by optical fibers. It is also being applied in the creation of more accurate atomic clocks based on directly measuring the frequency of a laser beam. See NIST subtopic 9.07.9-4.R for a description of research on one system to do this.

Since optical frequencies are far beyond the manipulation capacity of any feasible electronic circuit, all visible frequency photon detectors are inherently energy detectors not oscillating electric field detectors. However, since energy detection is inherently "square-law" detection, it intrinsically mixes any optical frequencies present on the detector. Thus, sensitive detection of specific optical frequencies necessitates optical heterodyne detection, in which two different (close by) wavelengths of light illuminate the detector so that the oscillating electrical output corresponds to the difference between their frequencies. This allows extremely narrow band detection (much narrower than any possible color filter can achieve) as well as precision measurements of phase and frequency of a light signal relative to a reference light source, as in a laser Doppler vibrometer.

This phase sensitive detection has been applied for Doppler measurements of wind speed, and imaging through dense media. The high sensitivity against background light is especially useful for lidar.

In optical Kerr effect (OKE) spectroscopy, optical heterodyning of the OKE signal and a small part of the probe signal produces a mixed signal consisting of probe, heterodyne OKE-probe and homodyne OKE signal. The probe and homodyne OKE signals can be filtered out, leaving the heterodyne frequency signal for detection.

Heterodyne detection is often used in interferometry but usually confined to single point detection rather than widefield interferometry, however, widefield heterodyne interferometry is possible using a special camera. Using this technique which a reference signal extracted from a single pixel it is possible to build a highly stable widefield heterodyne interferometer by removing the piston phase component caused by microphonics or vibrations of the optical components or object.

Mathematical principle
Heterodyning is based on the trigonometric identity:

 * $$(\cos\theta_1) (\cos\theta_2) = \tfrac{1}{2}\cos(\theta_1 - \theta_2) + \tfrac{1}{2}\cos( \theta_1 + \theta_2)$$

The product on the left hand side represents the multiplication ("mixing") of a sine wave with another sine wave (both produced by cosine functions). The right hand side shows that the resulting signal is the sum of two sinusoidal terms, one at the sum of the two original frequencies, and one at the difference, which can be dealt with separately, since their (large) frequency difference makes it easy to cleanly filter out one signal's frequency, while leaving the other signal unchanged.

Using this trigonometric identity, the result of multiplying two cosine wave signals $$\ \cos\left(2 \pi f_1 t\right)\ $$ and $$\ \cos\left(2 \pi f_2 t\right)\ $$ at different frequencies $$\ f_1\ $$ and $$\ f_2\ $$ can be calculated:

 * $$\cos(2 \pi f_1 t) \cos(2 \pi f_2 t) = \tfrac{1}{2}\cos[2 \pi ( f_1 - f_2) t ] + \tfrac{1}{2}\cos[ 2 \pi ( f_1 + f_2) t ]$$

The result is the sum of two sinusoidal signals, one at the sum $f_{1} + f_{2}$ and one at the difference $f_{1} − f_{2}$ of the original frequencies.

Mixer
The two signals are combined in a device called a mixer. As seen in the previous section, an ideal mixer would be a device that multiplies the two signals. Some widely used mixer circuits, such as the Gilbert cell, operate in this way, but they are limited to lower frequencies. However, any nonlinear electronic component also multiplies signals applied to it, producing heterodyne frequencies in its output—so a variety of nonlinear components serve as mixers. A nonlinear component is one in which the output current or voltage is a nonlinear function of its input. Most circuit elements in communications circuits are designed to be linear. This means they obey the superposition principle; if $$\ F(v)\ $$ is the output of a linear element with an input of $$\ v\ $$:

 * $$\ F(v_1 + v_2) = F(v_1) + F(v_2)\ $$

So if two sine wave signals at frequencies $f_{1}$ and $f_{2}$ are applied to a linear device, the output is simply the sum of the outputs when the two signals are applied separately with no product terms. Thus, the function $$F$$ must be nonlinear to create mixer products. A perfect multiplier only produces mixer products at the sum and difference frequencies $(f_{1} ± f_{2})$, but more general nonlinear functions produce higher order mixer products: $n⋅f_{1} + m⋅f_{2}$ for integers $n$ and $m$. Some mixer designs, such as double-balanced mixers, suppress some high order undesired products, while other designs, such as harmonic mixers exploit high order differences.

Examples of nonlinear components that are used as mixers are vacuum tubes and transistors biased near cutoff (class C), and diodes. Ferromagnetic core inductors driven into saturation can also be used at lower frequencies. In nonlinear optics, crystals that have nonlinear characteristics are used to mix laser light beams to create optical heterodyne frequencies.

Output of a mixer
To demonstrate mathematically how a nonlinear component can multiply signals and generate heterodyne frequencies, the nonlinear function $$F$$ can be expanded in a power series (MacLaurin series):

 * $$\ F(v) = \alpha_1 v + \alpha_2 v^2 + \alpha_3 v^3 + \cdots\ $$

To simplify the math, the higher order terms above $α_{2}$ are indicated by an ellipsis ($$\ \cdots\ $$) and only the first terms are shown. Applying the two sine waves at frequencies $ω_{1} = 2\pif_{1}$ and $ω_{2} = 2\pif_{2}$ to this device:

 * $$\ v_\mathsf{out} = F\Bigl(A_1 \cos(\omega_1 t) + A_2 \cos(\omega_2 t) \Bigr)\ $$

 * $$\ v_\mathsf{out} = \alpha_1 \Bigl( A_1 \cos(\omega_1 t) + A_2 \cos(\omega_2 t) \Bigr) + \alpha_2 \Bigl( A_1 \cos(\omega_1 t) + A_2 \cos(\omega_2 t) \Bigr)^2 + \cdots\ $$

 * $$\ v_\mathsf{out} = \alpha_1 \Bigl( A_1 \cos(\omega_1 t) + A_2 \cos(\omega_2 t)\Bigr) + \alpha_2 \Bigl( A_1^2 \cos^2(\omega_1 t) + 2 A_1 A_2 \cos(\omega_1 t)\ \cos(\omega_2 t) + A_2^2 \cos^2(\omega_2 t) \Bigr) + \cdots\ $$

It can be seen that the second term above contains a product of the two sine waves. Simplifying with trigonometric identities:

\begin{align} v_\mathsf{out} = {} & \alpha_1 \Bigl( A_1 \cos(\omega_1 t) + A_2 \cos(\omega_2 t) \Bigr) \\ & {} + \alpha_2 \Bigl( \tfrac{1}{2} A_1^2 [ 1 + \cos(2 \omega_1 t) ] + A_1 A_2 [\cos(\omega_1 t - \omega_2 t ) + \cos (\omega_1 t + \omega_2 t) ] + \tfrac{1}{2} A_2^2 [ 1 + \cos(2 \omega_2 t) ] \Bigr) + \cdots \end{align} $$

Which leaves the two heterodyne frequencies as two among the many terms:
 * $$\ v_\mathsf{out} = \cdots + \alpha_2 A_1 A_2 \cos (\omega_1 - \omega_2 )t + \alpha_2 A_1 A_2 \cos (\omega_1 + \omega_2 ) t + \cdots\ $$

along with many other terms not shown.

Among many other frequencies, the output contains sinusoidal terms with frequencies at the sum $ω_{1} + ω_{2}$ and difference $ω_{1} − ω_{2}$ of the two original frequencies. It also contains terms at the original frequencies and terms at multiples of the original frequencies $2 ω_{1}$, $2 ω_{2}$ , $3 ω_{1}$ , $3 ω_{2}$ , etc., called harmonics. It also contains much more complicated terms at frequencies of $M ω_{1} + N ω_{2}$, called intermodulation products. These unwanted frequencies, along with the unwanted heterodyne frequency, must be removed from the mixer output by an electronic filter, to leave the desired heterodyne frequency.

title: Heterodyne


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Heuristic routing

Heuristic routing is a system used to describe how deliveries are made when problems in a network topology arise. Heuristic is an adjective used in relation to methods of learning, discovery, or problem solving. Routing is the process of selecting paths to specific destinations. Heuristic routing is used for traffic in the telecommunications networks and transport networks of the world.

Heuristic routing is achieved using specific algorithms to determine a better, although not always optimal, path to a destination. When an interruption in a network topology occurs, the software running on the networking electronics can calculate another route to the desired destination via an alternate available path.

According to : "The heuristic approach to problem solving consists of applying human intelligence, experience, common sense and certain rules of thumb (or heuristics) to develop an acceptable, but not necessarily an optimum, solution to a problem. Of course, determining what constitutes an acceptable solution is part of the task of deciding which approach to use; but broadly defined, an acceptable solution is one that is both reasonably good (close to optimum) and derived within reasonable effort, time, and cost constraints. Often the effort (manpower, computer, and other resources) required, the time limits on when the solution is needed, and the cost to compile, process, and analyze all the data required for deterministic or other complicated procedures preclude their usefulness or favor the faster, simpler heuristic approach. Thus, the heuristic approach is generally used when deterministic techniques or are not available, economical, or practical."

Heuristic routing allows a measure of route optimization in telecommunications networks based on recent empirical knowledge of the state of the network. Data, such as time delay, may be extracted from incoming messages, during specified periods and over different routes, and used to determine the optimum routing for transmitting data back to the sources.

IP routing
The IP routing protocols in use today are based on one of two algorithms: distance vector or link state. Distance vector algorithms broadcast routing information to all neighboring routers. Link state routing protocols build a topographical map of the entire network based on updates from neighbor routers, and then use the Dijkstra algorithm to compute the shortest path to each destination. Metrics used are based on the number of hops, delay, throughput, traffic, and reliability.

Distance vector algorithms

 * RIP uses number of hops, or gateways traversed, as its metric
 * IGRP uses bandwidth, delay, hop count, link reliability, load, and MTU
 * EIGRP uses the (DUAL) Diffusing Update Algorithm
 * BGP uses the distance vector algorithm

Link state algorithms

 * OSPF uses the Dijkstra algorithm.

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

title: Heuristic routing


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Hierarchical routing

Hierarchical routing is a method of routing in networks that is based on hierarchical addressing.

Background
Most Transmission Control Protocol/Internet Protocol (TCP/IP) routing is based on a two-level hierarchical routing in which an IP address is divided into a network portion and a host portion. Gateways use only the network portion until an IP datagram reaches a gateway that can deliver it directly. Additional levels of hierarchical routing are introduced by the addition of subnetworks.

Description
Hierarchical routing is the procedure of arranging routers in a hierarchical manner. A good example would be to consider a corporate intranet. Most corporate intranets consist of a high speed backbone network. Connected to this backbone are routers which are in turn connected to a particular workgroup. These workgroups occupy a unique LAN. The reason this is a good arrangement is because even though there might be dozens of different workgroups, the span (maximum hop count to get from one host to any other host on the network) is 2. Even if the workgroups divided their LAN network into smaller partitions, the span could only increase to 4 in this particular example.

Considering alternative solutions with every router connected to every other router, or if every router was connected to 2 routers, shows the convenience of hierarchical routing. It decreases the complexity of network topology, increases routing efficiency, and causes much less congestion because of fewer routing advertisements. With hierarchical routing, only core routers connected to the backbone are aware of all routes. Routers that lie within a LAN only know about routes in the LAN. Unrecognized destinations are passed to the default route.

title: Hierarchical routing


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High-performance equipment

High-performance equipment describes telecommunications equipment that
 * (a) has the performance characteristics required for use in trunks or links,
 * (b) is designed primarily for use in global and tactical systems, and
 * (c) sufficiently withstands electromagnetic interference when operating in a variety of network or point-to-point circuits.

Note: Requirements for global and tactical high-performance equipment may differ.

title: High-performance equipment


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Hop

A hop is a type of jump.

Hop or hops may also refer to:

Arts and entertainment

 * Hop (film), a 2011 film
 * Hop! Channel, an Israeli TV channel
 * House of Payne, or HOP, an American sitcom
 * Lindy Hop, a swing dance of the 1920s and 1930s
 * Sock hop, an informal gathering which includes dancing
 * Hop Harrigan, a character in American comic books, radio serials and film serials from 1939 into the 1940s
 * Hop, a character from Pokémon Sword and Shield

People

 * Hop Bartlett, American baseball pitcher in the Negro leagues in 1924 and 1925
 * Hop Wilson (1921–1975), American Texas blues steel guitar player

Places

 * Hop River, Connecticut, United States
 * Hop Creek, South Dakota, United States
 * Hóp (Iceland), a lake
 * Hóp, a Viking settlement in Vinland

Plants

 * Humulus lupulus, the hop plant
 * Hops, its flower, used to prepare beer and other food

Science and medicine

 * HOP (gene), encoding the homeodomain-only protein
 * Hop (protein), the Hsp70-Hsp90 organizing protein
 * Hubble Origins Probe, or HOP, a proposed orbital telescope

Technology

 * Hop (networking), a portion of a route
 * Hop (software), a web broker and programming language
 * Hop (telecommunications)
 * Hindsight optimization, or Hop, an artificial intelligence technique
 * High Octet Preset, or HOP, a C1 control character
 * Spike (application), an email app formerly known as Hop

Transport

 * Air France Hop, a French airline
 * HOP card, a smart card used on public transit in Auckland, New Zealand
 * Hop Fastpass, a smart card used on public transit in Portland, Oregon, and Vancouver, Washington, United States
 * Heritage Operations Processing System, a tool for management of historic railways
 * Hope railway station (England), in England
 * Hope station (Arkansas), in Arkansas, United States
 * "The Hop", the brand for public transport in Sydney and New South Wales
 * "The Hop (streetcar)", streetcar system in Milwaukee

Other uses

 * Hop (unit), a small Korean unit of volume
 * Croatian Liberation Movement (Croatian: Hrvatski oslobodilački pokret)
 * Heritage of Pride, or HOP, the organizer of the annual gay pride march in New York City
 * Higher-Order Perl, or HOP, a Perl programming book
 * Hillsboro Hops, a minor league baseball team in the USA
 * Slang for Opium, heroin, or other narcotic or psychoactive drugs

title: Hop


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Horn

Horn may refer to:

Common uses

 * Horn (acoustic), a tapered sound guide
 * Horn antenna
 * Horn loudspeaker
 * Vehicle horn
 * Train horn
 * Horn (anatomy), a pointed, bony projection on the head of various animals
 * Horn (instrument), a family of musical instruments
 * French horn, often simply called a horn

Arts and entertainment

 * Horn (album), by Apink, 2022
 * Horn, a 2011 album by Pharaoh Overlord
 * "The Horn", a song by Super Furry Animals from the 2005 album Love Kraft
 * "The Horn", a comedy track from the 1978 album Derek and Clive Ad Nauseam
 * Horn (video game), 2012
 * The Horn (film), a 2020 Sri Lankan Sinhala sci-fi horror film

Places

 * Horn of Africa
 * Horn District, Austria
 * Horn, Austria
 * Horn (am Bodensee), Baden-Württemberg, Germany
 * Horn, Hamburg, Germany
 * Horn Island (Mississippi), United States
 * Horn, Nebraska, United States
 * Horn, Netherlands
 * Horn Island, Queensland, Australia
 * Horn, a ferry docking point on Randsfjorden, Norway
 * Horn, Rhineland-Palatinate, Germany
 * Horn River, Northwest Territories, Canada
 * Horn, Rutland, England
 * Horn (Schwarzbach), a river in eastern France and southwestern Germany
 * Horn, Sweden
 * Horn, Switzerland
 * The Horn (Antarctica), a hill on Eagle Island, Graham Land, Antarctica
 * The Horn (Mount Buffalo), a peak in Victoria, Australia
 * The Horn (New Hampshire), a peak in the United States

Other uses

 * Horn Cable Television, a television channel in Somalia
 * Horn (Chinese constellation), part of the European constellation Virgo
 * Horn (diacritic), in the Vietnamese alphabet
 * Horn (heraldry), common features in crests in Scandinavian and German heraldry
 * Horn (surname), including a list of people with the name
 * Horn, part of a saddle
 * Hörn, or Freyja, a Norse goddess
 * Pyramidal peak, sometimes called a glacial horn
 * The horn, a slang term for telephone

title: Horn


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Hotline

A hotline is a point-to-point communications link in which a call is automatically directed to the preselected destination without any additional action by the user when the end instrument goes off-hook. An example would be a phone that automatically connects to emergency services on picking up the receiver. Therefore, dedicated hotline phones do not need a rotary dial or keypad. A hotline can also be called an automatic signaling, ringdown, or off-hook service.

For crises and service
True hotlines cannot be used to originate calls other than to preselected destinations. However, in common or colloquial usage, a "hotline" often refers to a call center reachable by dialing a standard telephone number, or sometimes the phone numbers themselves.

This is especially the case with 24-hour, noncommercial numbers, such as police tip hotlines or suicide crisis hotlines, which are staffed around the clock and thereby give the appearance of real hotlines. Increasingly, however, the term is found being applied to any customer service telephone number.

Russia–United States
The most famous hotline between states is the Moscow–Washington hotline, also known as the "red telephone", although telephones have never been used in this capacity. This direct communications link was established on June 20, 1963, in the wake of the Cuban Missile Crisis, which convinced both sides of the need for better communications. It was the first time used by U.S. President John F. Kennedy on August 30, 1963 and utilized teletypewriter technology, later replaced by telecopier and then by electronic mail.

United Kingdom–United States
Already during World War II—two decades before the Washington–Moscow hotline was established—there was a hotline between No. 10 Downing Street and the Cabinet War Room bunker under the Treasury, Whitehall; with the White House in Washington, D.C. From 1943 to 1946, this link was made secure by using the very first voice encryption machine, called SIGSALY.

China–Russia
A hotline connection between Beijing and Moscow was used during the 1969 frontier confrontation between the two countries. The Chinese however refused the Russian peace attempts and ended the communications link. After a reconciliation between the former enemies, the hotline between China and Russia was revived in 1996.

France–Russia
On his visit to the Soviet Union in 1966, French President Charles de Gaulle announced that a hotline would be established between Paris and Moscow. The line was upgraded from a telex to a high-speed fax machine in 1989.

Russia–United Kingdom
A London–Moscow hotline was not formally established until a treaty of friendship between the two countries in 1992. An upgrade was announced when Foreign Secretary William Hague visited Moscow in 2011.

India–Pakistan
On 20 June 2004, both India and Pakistan agreed to extend a nuclear testing ban and to set up an Islamabad–New Delhi hotline between their foreign secretaries aimed at preventing misunderstandings that might lead to nuclear war. The hotline was set up with the assistance of United States military officers.

China–United States
The United States and China set up a defense hotline in 2008, but it has rarely been used in crises.

China–India
India and China announced a hotline for the foreign ministers of both countries while reiterating their commitment to strengthening ties and building "mutual political trust". As of August 2015 the hotline was yet to be made operational.

China–Japan
In February 2013, the Senkaku Islands dispute gave renewed impetus to a China–Japan hotline, which had been agreed to but due to rising tensions had not been established.

North and South Korea
Between North and South Korea there are over 40 direct phone lines, the first of which was opened in September 1971. Most of these hotlines run through the Panmunjeom Joint Security Area (JSA) and are maintained by the Red Cross. Since 1971, North Korea has deactivated the hotlines seven times, the last time in February 2016. After Kim Jong-un's New Years address, the border hotline was reopened on January 3, 2018.

India–United States
In August 2015 the hotline between the White House and New Delhi became operational. The decision of establishing this hotline was taken during Obama's visit to India in January 2015. This is the first hotline connecting an Indian Prime Minister to a head of state.

title: Hotline


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Hybrid (biology)

In biology, a hybrid is the offspring resulting from combining the qualities of two organisms of different varieties, subspecies, species or genera through sexual reproduction. Generally, it means that each cell has genetic material from two different organisms, whereas an individual where some cells are derived from a different organism is called a chimera. Hybrids are not always intermediates between their parents such as in blending inheritance (a now discredited theory in modern genetics by particulate inheritance), but can show hybrid vigor, sometimes growing larger or taller than either parent. The concept of a hybrid is interpreted differently in animal and plant breeding, where there is interest in the individual parentage. In genetics, attention is focused on the numbers of chromosomes. In taxonomy, a key question is how closely related the parent species are.

Species are reproductively isolated by strong barriers to hybridization, which include genetic and morphological differences, differing times of fertility, mating behaviors and cues, and physiological rejection of sperm cells or the developing embryo. Some act before fertilization and others after it. Similar barriers exist in plants, with differences in flowering times, pollen vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and the structure of the chromosomes. A few animal species and many plant species, however, are the result of hybrid speciation, including important crop plants such as wheat, where the number of chromosomes has been doubled.

A form of often intentional human-mediated hybridization is the crossing of wild and domesticated species. This is common in both traditional horticulture and modern agriculture; many commercially useful fruits, flowers, garden herbs, and trees have been produced by hybridization. One such flower, Oenothera lamarckiana, was central to early genetics research into mutationism and polyploidy. It is also more occasionally done in the livestock and pet trades; some well-known wild × domestic hybrids are beefalo and wolfdogs. Human selective breeding of domesticated animals and plants has also resulted in the development of distinct breeds (usually called cultivars in reference to plants); crossbreeds between them (without any wild stock) are sometimes also imprecisely referred to as "hybrids".

Hybrid humans existed in prehistory. For example, Neanderthals and anatomically modern humans are thought to have interbred as recently as 40,000 years ago.

Mythological hybrids appear in human culture in forms as diverse as the Minotaur, blends of animals, humans and mythical beasts such as centaurs and sphinxes, and the Nephilim of the Biblical apocrypha described as the wicked sons of fallen angels and attractive women.

In evolution
Hybridization between species plays an important role in evolution, though there is much debate about its significance. Roughly 25% of plants and 10% of animals are known to form hybrids with at least one other species. One example of an adaptive benefit to hybridization is that hybrid individuals can form a "bridge" transmitting potentially helpful genes from one species to another when the hybrid backcrosses with one of its parent species, a process called introgression. Hybrids can also cause speciation, either because the hybrids are genetically incompatible with their parents and not each other, or because the hybrids occupy a different niche than either parent.

Hybridization is a particularly common mechanism for speciation in plants, and is now known to be fundamental to the evolutionary history of plants. Plants frequently form polyploids, individuals with more than two copies of each chromosome. Whole genome doubling has occurred repeatedly in plant evolution. When two plant species hybridize, the hybrid may double its chromosome count by incorporating the entire nuclear genome of both parents, resulting in offspring that are reproductively incompatible with either parent because of different chromosome counts.

In conservation
Human impact on the environment has resulted in an increase in the interbreeding between regional species, and the proliferation of introduced species worldwide has also resulted in an increase in hybridization. This has been referred to as genetic pollution out of concern that it may threaten many species with extinction. Similarly, genetic erosion from monoculture in crop plants may be damaging the gene pools of many species for future breeding.

The conservation impacts of hybridization between species are highly debated. While hybridization could potentially threaten rare species or lineages by "swamping" the genetically "pure" individuals with hybrids, hybridization could also save a rare lineage from extinction by introducing genetic diversity. It has been proposed that hybridization could be a useful tool to conserve biodiversity by allowing organisms to adapt, and that efforts to preserve the separateness of a "pure" lineage could harm conservation by lowering the organisms' genetic diversity and adaptive potential, particularly in species with low populations. While endangered species are often protected by law, hybrids are often excluded from protection, resulting in challenges to conservation.

Etymology

The term hybrid is derived from Latin hybrida, used for crosses such as of a tame sow and a wild boar. The term came into popular use in English in the 19th century, though examples of its use have been found from the early 17th century. Conspicuous hybrids are popularly named with portmanteau words, starting in the 1920s with the breeding of tiger–lion hybrids (liger and tigon).

Animal and plant breeding
From the point of view of animal and plant breeders, there are several kinds of hybrid formed from crosses within a species, such as between different breeds. Single cross hybrids result from the cross between two true-breeding organisms which produces an F1 hybrid (first filial generation). The cross between two different homozygous lines produces an F1 hybrid that is heterozygous; having two alleles, one contributed by each parent and typically one is dominant and the other recessive. Typically, the F1 generation is also phenotypically homogeneous, producing offspring that are all similar to each other. Double cross hybrids result from the cross between two different F1 hybrids (i.e., there are four unrelated grandparents). Three-way cross hybrids result from the cross between an F1 hybrid and an inbred line. Triple cross hybrids result from the crossing of two different three-way cross hybrids. Top cross (or "topcross") hybrids result from the crossing of a top quality or pure-bred male and a lower quality female, intended to improve the quality of the offspring, on average.

Population hybrids result from the crossing of plants or animals in one population with those of another population. These include interspecific hybrids or crosses between different breeds. In biology, the result of crossing of two populations is called a synthetic population.

In horticulture, the term stable hybrid is used to describe an annual plant that, if grown and bred in a small monoculture free of external pollen (e.g., an air-filtered greenhouse) produces offspring that are "true to type" with respect to phenotype; i.e., a true-breeding organism.

Biogeography
Hybridization can occur in the hybrid zones where the geographical ranges of species, subspecies, or distinct genetic lineages overlap. For example, the butterfly Limenitis arthemis has two major subspecies in North America, L. a. arthemis (the white admiral) and L. a. astyanax (the red-spotted purple). The white admiral has a bright, white band on its wings, while the red-spotted purple has cooler blue-green shades. Hybridization occurs between a narrow area across New England, southern Ontario, and the Great Lakes, the "suture region". It is at these regions that the subspecies were formed. Other hybrid zones have formed between described species of plants and animals.

Genetics

From the point of view of genetics, several different kinds of hybrid can be distinguished. A genetic hybrid carries two different alleles of the same gene, where for instance one allele may code for a lighter coat colour than the other. A structural hybrid results from the fusion of gametes that have differing structure in at least one chromosome, as a result of structural abnormalities. A numerical hybrid results from the fusion of gametes having different haploid numbers of chromosomes. A permanent hybrid results when only the heterozygous genotype occurs, as in Oenothera lamarckiana, because all homozygous combinations are lethal. In the early history of genetics, Hugo de Vries supposed these were caused by mutation.

Genetic complementation
Genetic complementation is a hybridization test widely used in genetics to determine whether two separately isolated mutants that have the same (or similar) phenotype are defective in the same gene or in different genes (see Complementation (genetics) article). If a hybrid organism containing the genomes of two different mutant parental organisms displays a wild type phenotype, it is ordinarily considered that the two parental mutant organisms are defective in different genes. If the hybrid organism displays a distinctly mutant phenotype, the two mutant parental organisms are considered to be defective in the same gene. However, in some cases the hybrid organism may display a phenotype that is only weakly (or partially) wild-type, and this may reflect intragenic (interallelic) complementation.

Taxonomy
From the point of view of taxonomy, hybrids differ according to their parentage. Hybrids between different subspecies (such as between the Dog and Eurasian wolf) are called intra-specific hybrids. Interspecific hybrids are the offspring from interspecies mating; these sometimes result in hybrid speciation. Intergeneric hybrids result from matings between different genera, such as between sheep and goats. Interfamilial hybrids, such as between chickens and guineafowl or pheasants, are reliably described but extremely rare. Interordinal hybrids (between different orders) are few, but have been engineered between the sea urchin Strongylocentrotus purpuratus (female) and the sand dollar Dendraster excentricus (male).

Expression of parental traits

When two distinct types of organisms breed with each other, the resulting hybrids typically have intermediate traits (e.g., one plant parent has red flowers, the other has white, and the hybrid, pink flowers). Commonly, hybrids also combine traits seen only separately in one parent or the other (e.g., a bird hybrid might combine the yellow head of one parent with the orange belly of the other).

Mechanisms of reproductive isolation
Interspecific hybrids are bred by mating individuals from two species, normally from within the same genus. The offspring display traits and characteristics of both parents, but are often sterile, preventing gene flow between the species. Sterility is often attributed to the different number of chromosomes between the two species. For example, donkeys have 62 chromosomes, horses have 64 chromosomes, and mules or hinnies have 63 chromosomes. Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes, because differences in chromosome structure prevent appropriate pairing and segregation during meiosis, meiosis is disrupted, and viable sperm and eggs are not formed. However, fertility in female mules has been reported with a donkey as the father.

A variety of mechanisms limit the success of hybridization, including the large genetic difference between most species. Barriers include morphological differences, differing times of fertility, mating behaviors and cues, and physiological rejection of sperm cells or the developing embryo. Some act before fertilization; others after it.

In plants, some barriers to hybridization include blooming period differences, different pollinator vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and structural differences of the chromosomes.

Speciation

A few animal species are the result of hybridization. The Lonicera fly is a natural hybrid. The American red wolf appears to be a hybrid of the gray wolf and the coyote, although its taxonomic status has been a subject of controversy. The European edible frog is a semi-permanent hybrid between pool frogs and marsh frogs; its population requires the continued presence of at least one of the parent species. Cave paintings indicate that the European bison is a natural hybrid of the aurochs and the steppe bison.

Plant hybridization is more commonplace compared to animal hybridization. Many crop species are hybrids, including notably the polyploid wheats: some have four sets of chromosomes (tetraploid) or six (hexaploid), while other wheat species have (like most eukaryotic organisms) two sets (diploid), so hybridization events likely involved the doubling of chromosome sets, causing immediate genetic isolation.

Hybridization may be important in speciation in some plant groups. However, homoploid hybrid speciation (not increasing the number of sets of chromosomes) may be rare: by 1997, only eight natural examples had been fully described. Experimental studies suggest that hybridization offers a rapid route to speciation, a prediction confirmed by the fact that early generation hybrids and ancient hybrid species have matching genomes, meaning that once hybridization has occurred, the new hybrid genome can remain stable.

Many hybrid zones are known where the ranges of two species meet, and hybrids are continually produced in great numbers. These hybrid zones are useful as biological model systems for studying the mechanisms of speciation. Recently DNA analysis of a bear shot by a hunter in the Northwest Territories confirmed the existence of naturally occurring and fertile grizzly–polar bear hybrids.

Hybrid vigour

Hybridization between reproductively isolated species often results in hybrid offspring with lower fitness than either parental. However, hybrids are not, as might be expected, always intermediate between their parents (as if there were blending inheritance), but are sometimes stronger or perform better than either parental lineage or variety, a phenomenon called heterosis, hybrid vigour, or heterozygote advantage. This is most common with plant hybrids. A transgressive phenotype is a phenotype that displays more extreme characteristics than either of the parent lines. Plant breeders use several techniques to produce hybrids, including line breeding and the formation of complex hybrids. An economically important example is hybrid maize (corn), which provides a considerable seed yield advantage over open pollinated varieties. Hybrid seed dominates the commercial maize seed market in the United States, Canada and many other major maize-producing countries.

In a hybrid, any trait that falls outside the range of parental variation (and is thus not simply intermediate between its parents) is considered heterotic. Positive heterosis produces more robust hybrids, they might be stronger or bigger; while the term negative heterosis refers to weaker or smaller hybrids. Heterosis is common in both animal and plant hybrids. For example, hybrids between a lion and a tigress ("ligers") are much larger than either of the two progenitors, while "tigons" (lioness × tiger) are smaller. Similarly, the hybrids between the common pheasant (Phasianus colchicus) and domestic fowl (Gallus gallus) are larger than either of their parents, as are those produced between the common pheasant and hen golden pheasant (Chrysolophus pictus). Spurs are absent in hybrids of the former type, although present in both parents.

Anthropogenic hybridization
Hybridization is greatly influenced by human impact on the environment, through effects such as habitat fragmentation and species introductions. Such impacts make it difficult to conserve the genetics of populations undergoing introgressive hybridization. Humans have introduced species worldwide to environments for a long time, both intentionally for purposes such as biological control, and unintentionally, as with accidental escapes of individuals. Introductions can drastically affect populations, including through hybridization.

Management

There is a kind of continuum with three semi-distinct categories dealing with anthropogenic hybridization: hybridization without introgression, hybridization with widespread introgression (backcrossing with one of the parent species), and hybrid swarms (highly variable populations with much interbreeding as well as backcrossing with the parent species). Depending on where a population falls along this continuum, the management plans for that population will change. Hybridization is currently an area of great discussion within wildlife management and habitat management. Global climate change is creating other changes such as difference in population distributions which are indirect causes for an increase in anthropogenic hybridization.

Conservationists disagree on when is the proper time to give up on a population that is becoming a hybrid swarm, or to try and save the still existing pure individuals. Once a population becomes a complete mixture, the goal becomes to conserve those hybrids to avoid their loss. Conservationists treat each case on its merits, depending on detecting hybrids within the population. It is nearly impossible to formulate a uniform hybridization policy, because hybridization can occur beneficially when it occurs "naturally", and when hybrid swarms are the only remaining evidence of prior species, they need to be conserved as well.

Genetic mixing and extinction
Regionally developed ecotypes can be threatened with extinction when new alleles or genes are introduced that alter that ecotype. This is sometimes called genetic mixing. Hybridization and introgression, which can happen in natural and hybrid populations, of new genetic material can lead to the replacement of local genotypes if the hybrids are more fit and have breeding advantages over the indigenous ecotype or species. These hybridization events can result from the introduction of non-native genotypes by humans or through habitat modification, bringing previously isolated species into contact. Genetic mixing can be especially detrimental for rare species in isolated habitats, ultimately affecting the population to such a degree that none of the originally genetically distinct population remains.

Effect on biodiversity and food security

In agriculture and animal husbandry, the Green Revolution's use of conventional hybridization increased yields by breeding "high-yielding varieties". The replacement of locally indigenous breeds, compounded with unintentional cross-pollination and crossbreeding (genetic mixing), has reduced the gene pools of various wild and indigenous breeds resulting in the loss of genetic diversity. Since the indigenous breeds are often well-adapted to local extremes in climate and have immunity to local pathogens, this can be a significant genetic erosion of the gene pool for future breeding. Therefore, commercial plant geneticists strive to breed "widely adapted" cultivars to counteract this tendency.

Mammals
Familiar examples of equid hybrids are the mule, a cross between a female horse and a male donkey, and the hinny, a cross between a female donkey and a male horse. Pairs of complementary types like the mule and hinny are called reciprocal hybrids. Polar bears and brown bears are another case of a hybridizing species pairs, and introgression among non-sister species of bears appears to have shaped the Ursidae family tree. Among many other mammal crosses are hybrid camels, crosses between a bactrian camel and a dromedary. There are many examples of felid hybrids, including the liger. The oldest-known animal hybrid bred by humans is the kunga equid hybrid produced as a draft animal and status symbol 4,500 years ago to ancient Syria.

The first known instance of hybrid speciation in marine mammals was discovered in 2014. The clymene dolphin (Stenella clymene) is a hybrid of two Atlantic species, the spinner and striped dolphins. In 2019, scientists confirmed that a skull found 30 years earlier was a hybrid between the beluga whale and narwhal, dubbed the narluga.

Birds
Hybridization between species is common in birds. Hybrid birds are purposefully bred by humans, but hybridization is also common in the wild. Waterfowl have a particularly high incidence of hybridization, with at least 60% of species known to produce hybrids with another species. Among ducks, mallards widely hybridize with many other species, and the genetic relationships between ducks are further complicated by the widespread gene flow between wild and domestic mallards.

One of the most common interspecific hybrids in geese occurs between Greylag and Canada geese (Anser anser x Branta canadensis). One potential mechanism for the occurrence of hybrids in these geese is interspecific nest parasitism, where an egg is laid in the nest of another species to be raised by non-biological parents. The chick imprints upon and eventually seeks a mate among the species that raised it, instead of the species of its biological parents.

Cagebird breeders sometimes breed bird hybrids known as mules between species of finch, such as goldfinch × canary.

Amphibians
Among amphibians, Japanese giant salamanders and Chinese giant salamanders have created hybrids that threaten the survival of Japanese giant salamanders because of competition for similar resources in Japan.

Fish
Among fish, a group of about fifty natural hybrids between Australian blacktip shark and the larger common blacktip shark was found by Australia's eastern coast in 2012.

Russian sturgeon and American paddlefish were hybridized in captivity when sperm from the paddlefish and eggs from the sturgeon were combined, unexpectedly resulting in viable offspring. This hybrid is called a sturddlefish.

Cephalochordates
The two genera Asymmetron and Branchiostoma are able to produce viable hybrid offspring, even if none have lived into adulthood so far, despite the parents' common ancestor living tens of millions of years ago.

Insects
Among insects, so-called killer bees were accidentally created during an attempt to breed a strain of bees that would both produce more honey and be better adapted to tropical conditions. It was done by crossing a European honey bee and an African bee.

The Colias eurytheme and C. philodice butterflies have retained enough genetic compatibility to produce viable hybrid offspring. Hybrid speciation may have produced the diverse Heliconius butterflies, but that is disputed.

The two closely related harvester ant species Pogonomyrmex barbatus and Pogonomyrmex rugosus have evolved to depend on hybridization. When a queen fertilizes her eggs with sperm from males of her own species, the offspring is always new queens. And when she fertilizes the eggs with sperm from males of the other species, the offspring is always sterile worker ants (and because ants are haplodiploid, unfertilized eggs become males). Without mating with males of the other species, the queens are unable to produce workers, and will fail to establish a colony of their own.

In plants

Plant species hybridize more readily than animal species, and the resulting hybrids are fertile more often. Many plant species are the result of hybridization, combined with polyploidy, which duplicates the chromosomes. Chromosome duplication allows orderly meiosis and so viable seed can be produced.

Plant hybrids are generally given names that include an "×" (not in italics), such as Platanus × acerifolia for the London plane, a natural hybrid of P. orientalis (oriental plane) and P. occidentalis (American sycamore). The parent's names may be kept in their entirety, as seen in Prunus persica × Prunus americana, with the female parent's name given first, or if not known, the parent's names given alphabetically.

Plant species that are genetically compatible may not hybridize in nature for various reasons, including geographical isolation, differences in flowering period, or differences in pollinators. Species that are brought together by humans in gardens may hybridize naturally, or hybridization can be facilitated by human efforts, such as altered flowering period or artificial pollination. Hybrids are sometimes created by humans to produce improved plants that have some of the characteristics of each of the parent species. Much work is now being done with hybrids between crops and their wild relatives to improve disease resistance or climate resilience for both agricultural and horticultural crops.

Some crop plants are hybrids from different genera (intergeneric hybrids), such as Triticale, × Triticosecale, a wheat–rye hybrid. Most modern and ancient wheat breeds are themselves hybrids; bread wheat, Triticum aestivum, is a hexaploid hybrid of three wild grasses. Several commercial fruits including loganberry (Rubus × loganobaccus) and grapefruit (Citrus × paradisi) are hybrids, as are garden herbs such as peppermint (Mentha × piperita), and trees such as the London plane (Platanus × acerifolia). Among many natural plant hybrids is Iris albicans, a sterile hybrid that spreads by rhizome division, and Oenothera lamarckiana, a flower that was the subject of important experiments by Hugo de Vries that produced an understanding of polyploidy.

Sterility in a non-polyploid hybrid is often a result of chromosome number; if parents are of differing chromosome pair number, the offspring will have an odd number of chromosomes, which leaves them unable to produce chromosomally balanced gametes. While that is undesirable in a crop such as wheat, for which growing a crop that produces no seeds would be pointless, it is an attractive attribute in some fruits. Triploid bananas and watermelons are intentionally bred because they produce no seeds and are also parthenocarpic.

In fungi
Hybridization between fungal species is common and well established, particularly in yeast. Yeast hybrids are widely found and used in human-related activities, such as brewing and winemaking. The production of lager beers for instance are known to be carried out by the yeast Saccharomyces pastorianus, a cryotolerant hybrid between Saccharomyces cerevisiae and Saccharomyces eubayanus, which allows fermentation at low temperatures.

In humans

There is evidence of hybridization between modern humans and other species of the genus Homo. In 2010, the Neanderthal genome project showed that 1–4% of DNA from all people living today, apart from most Sub-Saharan Africans, is of Neanderthal heritage. Analyzing the genomes of 600 Europeans and East Asians found that combining them covered 20% of the Neanderthal genome that is in the modern human population. Ancient human populations lived and interbred with Neanderthals, Denisovans, and at least one other extinct Homo species. Thus, Neanderthal and Denisovan DNA has been incorporated into human DNA by introgression.

In 1998, a complete prehistorical skeleton found in Portugal, the Lapedo child, had features of both anatomically modern humans and Neanderthals. Some ancient human skulls with especially large nasal cavities and unusually shaped braincases represent human-Neanderthal hybrids. A 37,000- to 42,000-year-old human jawbone found in Romania's Oase cave contains traces of Neanderthal ancestry from only four to six generations earlier. All genes from Neanderthals in the current human population are descended from Neanderthal fathers and human mothers.

Mythology

Folk tales and myths sometimes contain mythological hybrids; the Minotaur was the offspring of a human, Pasiphaë, and a white bull. More often, they are composites of the physical attributes of two or more kinds of animals, mythical beasts, and humans, with no suggestion that they are the result of interbreeding, as in the centaur (man/horse), chimera (goat/lion/snake), hippocamp (fish/horse), and sphinx (woman/lion). The Old Testament mentions a first generation of half-human hybrid giants, the Nephilim, while the apocryphal Book of Enoch describes the Nephilim as the wicked sons of fallen angels and attractive women.

title: Hybrid (biology)


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Hybrid balance

In telecommunications, a hybrid balance is an expression of the degree of electrical symmetry between two impedances connected to two conjugate sides of a hybrid coil or resistance hybrid. It is usually expressed in dB.

If the respective impedances of the branches of the hybrid that are connected to the conjugate sides of the hybrid are known, hybrid balance may be computed by the formula for return loss.

title: Hybrid balance


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Hybrid transformer

A hybrid transformer (also known as a bridge transformer, hybrid coil, or just hybrid) is a type of directional coupler which is designed to be configured as a circuit having four ports that are conjugate in pairs, implemented using one or more transformers. It is a particular case of the more general concept of a hybrid coupler.

A signal arriving at one port is divided equally between the two adjacent ports but does not appear at the opposite port. In the schematic diagram, the signal into W splits between X and Z, and no signal passes to Y. Similarly, signals into X split to W and Y with none to Z, etc.

Correct operation requires matched characteristic impedance at all four ports. Forms of hybrid other than transformer coils are possible; any format of directional coupler can be designed to be a hybrid. These formats include transmission lines and waveguides.

Motivation
The primary use of a voiceband hybrid transformer is to convert between 2-wire and 4-wire operation in sequential sections of a communications circuit, for example in a four-wire terminating set. Such conversion was necessary when repeaters were introduced in a 2-wire circuit, a frequent practice at early 20th century telephony. Without hybrids, the output of one amplifier feeds directly into the input of the other, resulting in uncontrollable feedback oscillation (upper diagram). By using hybrids, the outputs and inputs are isolated, resulting in correct 2-wire repeater operation. Late in the century, this practice became rare but hybrids continued in use in line cards.

Implementations
Hybrids are realized using transformers. Two versions of transformer hybrids were used, the single transformer version providing unbalanced outputs with one end grounded, and the double transformer version providing balanced ports.

Single transformer
For use in 2-wire repeaters, the single transformer version suffices, since amplifiers in the repeaters have grounded inputs and outputs. X, Y, and Z share a common ground. As shown at left, signal into W, the 2-wire port, will appear at X and Z. But since Y is bridged from center of coil to center of X and Z, no signal appears. Signal into X will appear at W and Y. But signal at Z is the difference of what appears at Y and, through the transformer coil, at W, which is zero. Similar reasoning proves both pairs, W & Y, X & Z, are conjugates.

Double transformer
When both the 2-wire and the 4-wire circuits must be balanced, double transformer hybrids are used, as shown at right. Signal into port W splits between X and Z, but due to reversed connection to the windings, cancel at port Y. Signal into port X goes to W and Y. But due to reversed connection to ports W and Y, Z gets no signal. Thus the pairs, W & Y, X & Z, are conjugates.

Applications
Telephone hybrids are used in telephone exchanges to convert the 4-wire appearance to the 2-wire last mile connection to the subscriber's telephone. A different kind of hybrid is used in telephone handsets to convert the four wires of the transmitter (earpiece) and receiver (microphone) to the 2-wire line connection. This kind of hybrid is more commonly called an "induction coil" due to its derivation from high-voltage induction coils. It does not produce a high voltage, but like the high-voltage variety, it is a step-up transformer in order to impedance match the low-impedance carbon button transmitter to the higher impedance parts of the system. The simple induction coil later evolved into a form of hybrid as a sidetone reduction measure, or volume of microphone output that was fed back to the earpiece. Without this, the phone user's own voice would be louder in the earpiece than the other party's.  Today, the transformer version of the hybrid has been replaced by resistor networks and compact IC versions, which use integrated circuit electronics to do the job of the hybrid coil.

Radio-frequency hybrids are used to split radio signals, including television. The splitter divides the antenna signal to feed multiple receivers.

title: Hybrid transformer


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Hydroxyl ion absorption

Hydroxyl ion absorption is the absorption in optical fibers of electromagnetic radiation, including the near-infrared, due to the presence of trapped hydroxyl ions remaining from water as a contaminant.

The hydroxyl (OH−) ion, can penetrate glass during or after product fabrication, resulting in significant attenuation of discrete optical wavelengths, e.g., centred at 1.383 μm, used for communications via optical fibres.

title: Hydroxyl ion absorption


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Identifier

An identifier is a name that identifies (that is, labels the identity of) either a unique object or a unique class of objects, where the "object" or class may be an idea, physical countable object (or class thereof), or physical noncountable substance (or class thereof). The abbreviation ID (or Id) often refers to identity, identification (the process of identifying), or an identifier (that is, an instance of identification). An identifier may be a word, number, letter, symbol, or any combination of those.

The words, numbers, letters, or symbols may follow an encoding system (wherein letters, digits, words, or symbols stand for [represent] ideas or longer names) or they may simply be arbitrary. When an identifier follows an encoding system, it is often referred to as a code or id code. For instance the ISO/IEC 11179 metadata registry standard defines a code as system of valid symbols that substitute for longer values in contrast to identifiers without symbolic meaning. Identifiers that do not follow any encoding scheme are often said to be arbitrary Ids; they are arbitrarily assigned and have no greater meaning. (Sometimes identifiers are called "codes" even when they are actually arbitrary, whether because the speaker believes that they have deeper meaning or simply because they are speaking casually and imprecisely.)

The unique identifier (UID) is an identifier that refers to only one instance—only one particular object in the universe. A part number is an identifier, but it is not a unique identifier—for that, a serial number is needed, to identify each instance of the part design. Thus the identifier "Model T" identifies the class (model) of automobiles that Ford's Model T comprises; whereas the unique identifier "Model T Serial Number 159,862" identifies one specific member of that class—that is, one particular Model T car, owned by one specific person.

The concepts of name and identifier are denotatively equal, and the terms are thus denotatively synonymous; but they are not always connotatively synonymous, because code names and Id numbers are often connotatively distinguished from names in the sense of traditional natural language naming. For example, both "Jamie Zawinski" and "Netscape employee number 20" are identifiers for the same specific human being; but normal English-language connotation may consider "Jamie Zawinski" a "name" and not an "identifier", whereas it considers "Netscape employee number 20" an "identifier" but not a "name." This is an emic indistinction rather than an etic one.

Metadata
In metadata, an identifier is a language-independent label, sign or token that uniquely identifies an object within an identification scheme. The suffix "identifier" is also used as a representation term when naming a data element.

ID codes may inherently carry metadata along with them. For example, when you know that the food package in front of you has the identifier "2011-09-25T15:42Z-MFR5-P02-243-45", you not only have that data, you also have the metadata that tells you that it was packaged on September 25, 2011, at 3:42pm UTC, manufactured by Licensed Vendor Number 5, at the Peoria, IL, USA plant, in Building 2, and was the 243rd package off the line in that shift, and was inspected by Inspector Number 45.

Arbitrary identifiers might lack metadata. For example, if a food package just says 100054678214, its ID may not tell anything except identity—no date, manufacturer name, production sequence rank, or inspector number. In some cases, arbitrary identifiers such as sequential serial numbers leak information (i.e. the German tank problem). Opaque identifiers—identifiers designed to avoid leaking even that small amount of information—include "really opaque pointers" and Version 4 UUIDs.

In computer science
In computer science, identifiers (IDs) are lexical tokens that name entities. Identifiers are used extensively in virtually all information processing systems. Identifying entities makes it possible to refer to them, which is essential for any kind of symbolic processing.

In computer languages
In computer languages, identifiers are tokens (also called symbols) which name language entities. Some of the kinds of entities an identifier might denote include variables, types, labels, subroutines, and packages.

Identifiers (IDs) versus Unique identifiers (UIDs)
A resource may carry multiple identifiers. Typical examples are:
 * One person with multiple names, nicknames, and forms of address (titles, salutations)
 * For example: One specific person may be identified by all of the following identifiers: Jane Smith; Jane Elizabeth Meredith Smith; Jane E. M. Smith; Jane E. Smith; Janie Smith; Janie; Little Janie (as opposed to her mother or sister or cousin, Big Janie); Aunt Jane; Auntie Janie; Mom; Grandmom; Nana; Kelly's mother; Billy's grandmother; Ms. Smith; Dr. Smith; Jane E. Smith, PhD; and Fuzzy (her jocular nickname at work).
 * One document with multiple versions
 * One substance with multiple names (for example, CAS index names versus IUPAC names; INN generic drug names versus USAN generic drug names versus brand names)

The inverse is also possible, where multiple resources are represented with the same identifier (discussed below).

Implicit context and namespace conflicts
Many codes and nomenclatural systems originate within a small namespace. Over the years, some of them bleed into larger namespaces (as people interact in ways they formerly had not, e.g., cross-border trade, scientific collaboration, military alliance, and general cultural interconnection or assimilation). When such dissemination happens, the limitations of the original naming convention, which had formerly been latent and moot, become painfully apparent, often necessitating retronymy, synonymity, translation/transcoding, and so on. Such limitations generally accompany the shift away from the original context to the broader one. Typically the system shows implicit context (context was formerly assumed, and narrow), lack of capacity (e.g., low number of possible IDs, reflecting the outmoded narrow context), lack of extensibility (no features defined and reserved against future needs), and lack of specificity and disambiguating capability (related to the context shift, where longstanding uniqueness encounters novel nonuniqueness). Within computer science, this problem is called naming collision. The story of the origination and expansion of the CODEN system provides a good case example in a recent-decades, technical-nomenclature context. The capitalization variations seen with specific designators reveals an instance of this problem occurring in natural languages, where the proper noun/common noun distinction (and its complications) must be dealt with. A universe in which every object had a UID would not need any namespaces, which is to say that it would constitute one gigantic namespace; but human minds could never keep track of, or semantically interrelate, so many UIDs.

title: Identifier


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Image antenna

In telecommunications and antenna design, an image antenna is an electrical mirror-image of an antenna element formed by the radio waves reflecting from a conductive surface called a ground plane, such as the surface of the earth. It is used as a geometrical technique in calculating the radiation pattern of the antenna.

When a radio antenna is mounted near a conductive surface such as the Earth or a flat metal plate or screen, the radio waves directed toward the surface reflect off it. The radiation received at a distant point is the sum of two contributions: the waves that travel directly from the antenna to the point, and the waves that reach the point after reflecting off the ground plane. Because of the reflection, these second waves appear to come from a second antenna behind the plane, just as a visible object in front of a flat mirror forms a virtual image that seems to lie behind the mirror. The radiation pattern of the antenna is exactly the same as it would be if the ground plane were replaced by a mirror image of the antenna, located an equal distance behind the plane. This second apparent source of radio waves is the image antenna.

The image antenna is used in calculating electric field vectors, magnetic field vectors, and electromagnetic fields emanating from the real antenna, particularly in the vicinity of the antenna and along the ground. Each charge and current in the real antenna has its counterpart in the image, and may also be considered as a source of radiation.

To form an image of the antenna, the ground plane need not be grounded to the Earth. Many antenna types, such as reflective array antennas, use flat surfaces of metal or metal screen to reflect radio waves from the antenna elements, and these can be analyzed using image antennas. If there is more than one reflective surface in the antenna, as in a corner reflector antenna, each surface forms its own image of the antenna elements. In order to form an image, the ground plane surface must generally have dimensions of at least a quarter-wavelength of the radio waves used.

title: Image antenna


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Improved-definition television

Improved-definition television (IDTV) or enhanced-quality television transmitters and receivers exceed the performance requirements of the NTSC standard, while remaining within the general parameters of NTSC emissions standards.

IDTV improvements may be made at the television transmitter or receiver. Improvements include enhancements in encoding, digital filtering, scan interpolation, interlaced line scanning, and ghost cancellation.

IDTV improvements must allow the TV signal to be transmitted and received in the standard 4:3 aspect ratio.

The only relevant implementation of IDTV for NTSC-based broadcasts before the introduction of full-digital TV distribution (DTV) was the Japanese Clear-Vision. In European countries, PALplus and MAC had a similar role. The more commonly used term for advanced display technology before the advent of high-definition television (HDTV) was enhanced-definition television (EDTV), used for instance for plasma TV sets with a 16:9 aspect ratio in the early 2000s.

title: Improved-definition television


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Index-matching material

In optics, an index-matching material is a substance, usually a liquid, cement (adhesive), or gel, which has an index of refraction that closely approximates that of another object (such as a lens, material, fiber-optic, etc.).

When two substances with the same index are in contact, light passes from one to the other with neither reflection nor refraction. As such, they are used for various purposes in science, engineering, and art.

For example, in a popular home experiment, a glass rod is made almost invisible by immersing it in an index-matched transparent fluid such as mineral spirits.

In microscopy
In light microscopy, oil immersion is a technique used to increase the resolution of a microscope. This is achieved by immersing both the objective lens and the specimen in a transparent oil of high refractive index, thereby increasing the numerical aperture of the objective lens.

Immersion oils are transparent oils that have specific optical and viscosity characteristics necessary for use in microscopy. Typical oils used have an index of refraction around 1.515. An oil immersion objective is an objective lens specially designed to be used in this way. The index of the oil is typically chosen to match the index of the microscope lens glass, and of the cover slip.

For more details, see the main article, oil immersion. Some microscopes also use other index-matching materials besides oil; see water immersion objective and solid immersion lens.

In fiber optics
In fiber optics and telecommunications, an index-matching material may be used in conjunction with pairs of mated connectors or with mechanical splices to reduce signal reflected in the guided mode (known as return loss) (see Optical fiber connector). Without the use of an index-matching material, Fresnel reflections will occur at the smooth end faces of a fiber unless there is no fiber-air interface or other significant mismatch in refractive index. These reflections may be as high as −14 dB (i.e., 14 dB below the optical power of the incident signal). When the reflected signal returns to the transmitting end, it may be reflected again and return to the receiving end at a level that is 28 dB plus twice the fiber loss below the direct signal. The reflected signal will also be delayed by twice the delay time introduced by the fiber. The twice-reflected, delayed signal superimposed on the direct signal may noticeably degrade an analog baseband intensity-modulated video signal. Conversely, for digital transmission, the reflected signal will often have no practical effect on the detected signal seen at the decision point of the digital optical receiver except in marginal cases where bit-error ratio is significant. However, certain digital transmitters such as those employing a Distributed Feedback Laser may be affected by back reflection and then fall outside specifications such as Side Mode Suppression Ratio, potentially degrading system bit error ratio, so networking standards intended for DFB lasers may specify a back-reflection tolerance such as −10 dB for transmitters so that they remain within specification even without index matching. This back-reflection tolerance might be achieved using an optical isolator or by way of reduced coupling efficiency.

For some applications, instead of standard polished connectors (e.g. FC/PC), angle polished connectors (e.g. FC/APC) may be used, whereby the non-perpendicular polish angle greatly reduces the ratio of reflected signal launched into the guided mode even in the case of a fiber-air interface.

In experimental fluid dynamics
Index matching is used in liquid-liquid and liquid-solid (Multiphase flow) experimental systems to minimise the distortions that occur in these systems, this is particularly important for systems with many interfaces which become optically inaccessible. Matching the refractive index minimises reflection, refraction, diffraction and rotations that occurs at the interfaces allowing access to regions that would otherwise be inaccessible to optical measurements. This is particularly important for advanced optical measurements like Laser-induced fluorescence, Particle image velocimetry and Particle tracking velocimetry to name a few.

In art conservation
If a sculpture is broken into several pieces, art conservators may reattach the pieces using an adhesive such as Paraloid B-72 or epoxy. If the sculpture is made of a transparent or semitransparent material (such as glass), the seam where the pieces are attached will usually be much less noticeable if the refractive index of the adhesive matches the refractive index of the surrounding object. Therefore, art conservators may measure the index of objects and then use an index-matched adhesive. Similarly, losses (missing sections) in transparent or semitransparent objects are often filled using an index-matched material.

In optical component adhesives
Certain optical components, such as a Wollaston prism or Nicol prism, are made of multiple transparent pieces that are directly attached to each other. The adhesive is usually index-matched to the pieces. Historically, Canada balsam was used in this application, but it is now more common to use epoxy or other synthetic adhesives.

title: Index-matching material


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Inductive coupling

In electrical engineering, two conductors are said to be inductively coupled or magnetically coupled when they are configured in a way such that change in current through one wire induces a voltage across the ends of the other wire through electromagnetic induction. A changing current through the first wire creates a changing magnetic field around it by Ampere's circuital law. The changing magnetic field induces an electromotive force (EMF) voltage in the second wire by Faraday's law of induction. The amount of inductive coupling between two conductors is measured by their mutual inductance.

The coupling between two wires can be increased by winding them into coils and placing them close together on a common axis, so the magnetic field of one coil passes through the other coil. Coupling can also be increased by a magnetic core of a ferromagnetic material like iron or ferrite in the coils, which increases the magnetic flux. The two coils may be physically contained in a single unit, as in the primary and secondary windings of a transformer, or may be separated. Coupling may be intentional or unintentional. Unintentional inductive coupling can cause signals from one circuit to be induced into a nearby circuit, this is called cross-talk, and is a form of electromagnetic interference.

An inductively coupled transponder consists of a solid state transceiver chip connected to a large coil that functions as an antenna. When brought within the oscillating magnetic field of a reader unit, the transceiver is powered up by energy inductively coupled into its antenna and transfers data back to the reader unit inductively.

Magnetic coupling between two magnets can also be used to mechanically transfer power without contact, as in the magnetic gear.

Uses
Inductive coupling is widely used throughout electrical technology; examples include:
 * Electric motors and generators
 * Inductive charging products
 * Induction cookers and induction heating systems
 * Induction loop communication systems
 * Metal detectors
 * Transformers
 * Wireless power transfer
 * Testing:
 * Radio-frequency identification
 * Presence of voltage

Low-frequency induction
Low-frequency induction can be a dangerous form of inductive coupling when it happens inadvertently. For example, if a long-distance metal pipeline is installed along a right of way in parallel with a high-voltage power line, the power line can induce current on the pipe. Since the pipe is a conductor, insulated by its protective coating from the earth, it acts as a secondary winding for a long, drawn out transformer whose primary winding is the power line. Voltages induced on the pipe are then a hazard to people operating valves or otherwise touching metal parts of the metal pipeline.

Reducing low-frequency magnetic fields may be necessary when dealing with electronics, as sensitive circuits in close proximity to an instrument with a power transformer could begin to display 60Hz pickup. Twisted wires are an effective way of reducing the interference as signals induced in the successive twists cancel. Using magnetic shielding is also an effective way of reducing unwanted inductive coupling, though moving the source of the magnetic field away from sensitive electronics is the simplest solution if possible.

Although induced currents can be harmful, they can also be helpful. Electrical distribution line engineers use inductive coupling to tap power for cameras on towers and at substations that allow remote monitoring of the facilities. Using this they can watch from anywhere and not need to worry about changing camera batteries or solar panel maintenance.

title: Inductive coupling


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Information-bearer channel

In telecommunications, an information-bearer channel is one of:

 * 1) A communication channel capable of transmitting all the information required for communication, such as user data, synchronizing sequences, and control signals. The information-bearer channel may operate at a higher data rate than that required for user data alone.
 * 2) A basic communications channel with the necessary bandwidth but without enhanced or value-added services.

title: Information-bearer channel


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Information-transfer transaction

A transaction is a change of state, an information-transfer transaction is a transaction in which one of the following changes occurs: content, ownership, location, format, etc. An information-transfer transaction usually consists of three consecutive phases called the access phase, the information transfer phase, and the disengagement phase. Examples of these consecutive phases are the copying and transporting of information. Once a transaction occurs there are also costs to consider, which are associated with that certain transaction. When it comes to the transfer of information some transaction costs include time and means (money).

History of Information-transfer transactions
There are many social systems and devices that have contributed to information-transfer transactions; starting from people writing letters using postal systems to emailing using information technology. Two main examples of information-transfer transactions technology development is the copying and transportation of information.

History of Copying
Copying is the process of duplicating information with the change of location or format of the original information. The transfer transaction of information through copying has been going on for ages and there has been many advances in technology to decrease the time it takes to make copies of said information. The art of copying started with people having to write a copy out by hand, then the printing press, all the way to digital copying with ICTs. These developments lead to quicker information-transfer transactions in the form of distributing copies of original information to others through a changes of location or format.

History of Transporting
Transporting is the movement of information with the change of location or ownership of the original information. The transfer transaction of information through transporting has been going on for ages and there has been social and technological developments to decrease the time it takes for information to change ownership or location. The transportation of information started with people sending letters by foot, then by horse, the public and international postal service, all the way down to technology networks. It is these developments which led to the ability to send information further and quicker through information-transfer transactions.

Transaction Costs
Every time a transaction occurs, there are always costs to be considered. In the case of information-transfer transactions, one most consider the costs of time and means (money). Both of these costs are corollated with one another in that to decrease one, you must increase the other. For example, say Person #1 sends a letter through the mail, while Person #2 sends letters through email. For Person #1 to send their letter they had to buy paper, means of writing, envelopes, stamps, etc., while Person #2 needed to buy a source of electricity, internet, computer technology, etc. to send an email. It seems like Person #1 has lower transaction costs then Person #2 in terms of means; however, when you look at both information-transfer transactions in terms of time that is a different story. For Person #1 although they had little costs in sending their letter, the time it takes for the transfer of that letter is about 3+ days, while Person #2's transfer through email happens in less than minutes, but they endured high mean costs. Therefore, for information-transfer transaction times to decrease, the costs of means have to increase and vice versa.

Telecommunication
In telecommunication, an information-transfer transaction is a coordinated sequence of user and telecommunications system actions that cause information present at a source user to become present at a destination user.

title: Information-transfer transaction


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Input

Input may refer to:

Computing

 * Input (computer science), the act of entering data into a computer or data processing system
 * Information, any data entered into a computer or data processing system
 * Input device
 * Input method
 * Input port (disambiguation)
 * Input/output (I/O), in computing

Other

 * Input (talk show)
 * Input (typeface)
 * International Public Television Screening Conference (INPUT), an international public television organization
 * Input (online magazine), an online technology and culture magazine owned by Bustle Digital Group

title: Input


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Insertion gain

In telecommunication, insertion gain is the gain resulting from the insertion of a device in a transmission line, expressed as the ratio of the signal power delivered to that part of the line following the device to the signal power delivered to that same part before insertion. Gains less than unity indicate insertion loss. Incident power is made of two part, the reflection from the device and the power absorbed by the device.

Insertion gain is usually expressed in dB.

title: Insertion gain


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Insertion loss

In telecommunications, insertion loss is the loss of signal power resulting from the insertion of a device in a transmission line or optical fiber and is usually expressed in decibels (dB).

If the power transmitted to the load before insertion is PT and the power received by the load after insertion is PR, then the insertion loss in decibels is given by,

 * $$IL(\mathrm{dB}) = 10 \log_{10} {P_\mathrm T \over P_\mathrm R}$$

Electronic filters
Insertion loss is a figure of merit for an electronic filter and this data is generally specified with a filter. Insertion loss is defined as a ratio of the signal level in a test configuration without the filter installed ($$\left\vert V_1 \right\vert$$) to the signal level with the filter installed ($$\left\vert V_2 \right\vert$$). This ratio is described in decibels by the following equation:

 * $$\mbox{Insertion loss (dB)} = 10 \log_{10} {\frac{ \left\vert V_1 \right\vert ^2 }{ \left\vert V_2 \right\vert ^2 }} = 20 \log_{10} { \frac{\left\vert V_1 \right\vert}{\left\vert V_2 \right\vert}}$$

For passive filters, $$\left\vert V_2 \right\vert$$ will be smaller than $$\left\vert V_1 \right\vert$$. In this case, the insertion loss is positive and measures how much smaller the signal is after adding the filter.

Link with scattering parameters
In case the two measurement ports use the same reference impedance, the insertion loss ($$IL$$) is defined as:

 * $$ IL = -20\log_{10}\left|S_{21}\right| \,\text{dB}$$.

Here $$S_{21}$$ is one of the scattering parameters. Insertion loss is the extra loss produced by the introduction of the DUT between the 2 reference planes of the measurement. The extra loss can be introduced by intrinsic loss in the DUT and/or mismatch. In case of extra loss the insertion loss is defined to be positive.

title: Insertion loss


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Inside plant

In telecommunication, the term inside plant has the following meanings:

 * All the cabling and equipment installed in a telecommunications facility, including the main distribution frame (MDF) and all the equipment extending inward therefrom, such as PABX or central office equipment, MDF heat coil protectors, and grounding systems.
 * In radio and radar systems, all communications-electronics (C-E) equipment that is installed in buildings.

Around the turn of the 21st century, DSLAMs became an important part of telephone company inside plant. Inside plant will also have distribution frames and other equipment including passive optical network (name depends on the Service Provider).

Power
A typical power system for a switching office in an inside plant consists of the elements listed below: For safety and reliability reasons, it is desirable that all telecommunications loads be DC powered with minimal AC-powered devices used. Telcordia GR-513, Power Requirements in Telecommunications Plant (LSSGR Section 13), contains detailed industry requirements for using power in an inside plant.
 * AC power system
 * AC input switch gear
 * Standby AC plant (where appropriate)
 * AC distribution system (essential and non-essential loads)
 * AC backup systems for uninterruptible loads and protected loads
 * DC power system
 * Rectifiers and plant controls
 * Storage batteries
 * DC distribution system.

Both integrated and isolated bonding networks as per Telcordia GR-295, Mesh and Isolated Bonding Networks: Definition and Application to Telephone Central Offices, are a technically viable means to ground and bond the equipment in a safe and effective manner. However, the integrated or mesh bonding schemes are preferred over isolated bonding networks because of the added costs and efforts required to manage, control, and maintain the isolation for the equipment, particularly during equipment upgrade and modifications to the plant. This preference is based on a pragmatic desire for lower costs and ease of management, and to simplify operations during plant modifications/upgrades.

For a comprehensive analysis of the energy efficiency and environmental soundness of a power system, one should ideally consider a wider range of factors than strict energy conversion AC-to-DC power efficiency. These environmental factors cover a wider vision known as Industrial Ecology within which each manufacturing step of the products need to be considered from a Design for Environment (DfE) factors standpoint.

title: Inside plant