1.
Modem
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A modem is a network hardware device that modulates one or more carrier wave signals to encode digital information for transmission and demodulates signals to decode the transmitted information. The goal is attempting to produce a signal that can be transmitted easily, modems can be used with any means of transmitting analog signals, from light emitting diodes to radio. Modems are generally classified by the amount of data they can send in a given unit of time, usually expressed in bits per second. Modems can also be classified by their rate, measured in baud. The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal. For example, the ITU V.21 standard used audio frequency shift keying with two frequencies, corresponding to two distinct symbols, to carry 300 bits per second using 300 baud. By contrast, the original ITU V.22 standard, which could transmit and receive four distinct symbols, news wire services in the 1920s used multiplex devices that satisfied the definition of a modem. However, the function was incidental to the multiplexing function, so they are not commonly included in the history of modems. S. SAGE modems were described by AT&Ts Bell Labs as conforming to their newly published Bell 101 dataset standard, while they ran on dedicated telephone lines, the devices at each end were no different from commercial acoustically coupled Bell 101,110 baud modems. The 201A and 201B Data-Phones were synchronous modems using two-bit-per-baud phase-shift keying, the famous Bell 103A dataset standard was also introduced by AT&T in 1962. It provided full-duplex service at 300 bit/s over normal phone lines, frequency-shift keying was used, with the call originator transmitting at 1,070 or 1,270 Hz and the answering modem transmitting at 2,025 or 2,225 Hz. The readily available 103A2 gave an important boost to the use of remote low-speed terminals such as the Teletype Model 33 ASR and KSR, AT&T reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C modems. For many years, the Bell System maintained a monopoly on the use of its phone lines, however, the seminal Hush-a-Phone v. FCC case of 1956 concluded it was within the FCCs jurisdiction to regulate the operation of the Bell System. The FCC found that as long as a device was not electronically attached to the system and this led to a number of devices that mechanically connected to the phone through a standard handset. Since most handsets were supplied by Western Electric and thus of a standard design and this type of connection was used for many devices, such as answering machines. Acoustically coupled Bell 103A-compatible 300 bit/s modems were common during the 1970s, well-known models included the Novation CAT and the Anderson-Jacobson, the latter spun off from an in-house project at Stanford Research Institute. An even lower-cost option was the Pennywhistle modem, designed to be built using parts from electronics scrap, in December 1972, Vadic introduced the VA3400, notable for full-duplex operation at 1,200 bit/s over the phone network. Like the 103A, it used different frequency bands for transmit, in November 1976, AT&T introduced the 212A modem to compete with Vadic
2.
Radio
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When radio waves strike an electrical conductor, the oscillating fields induce an alternating current in the conductor. The information in the waves can be extracted and transformed back into its original form, Radio systems need a transmitter to modulate some property of the energy produced to impress a signal on it, for example using amplitude modulation or angle modulation. Radio systems also need an antenna to convert electric currents into radio waves, an antenna can be used for both transmitting and receiving. The electrical resonance of tuned circuits in radios allow individual stations to be selected, the electromagnetic wave is intercepted by a tuned receiving antenna. Radio frequencies occupy the range from a 3 kHz to 300 GHz, a radio communication system sends signals by radio. The term radio is derived from the Latin word radius, meaning spoke of a wheel, beam of light, however, this invention would not be widely adopted. The switch to radio in place of wireless took place slowly and unevenly in the English-speaking world, the United States Navy would also play a role. Although its translation of the 1906 Berlin Convention used the terms wireless telegraph and wireless telegram, the term started to become preferred by the general public in the 1920s with the introduction of broadcasting. Radio systems used for communication have the following elements, with more than 100 years of development, each process is implemented by a wide range of methods, specialised for different communications purposes. Each system contains a transmitter, This consists of a source of electrical energy, the transmitter contains a system to modulate some property of the energy produced to impress a signal on it. This modulation might be as simple as turning the energy on and off, or altering more subtle such as amplitude, frequency, phase. Amplitude modulation of a carrier wave works by varying the strength of the signal in proportion to the information being sent. For example, changes in the strength can be used to reflect the sounds to be reproduced by a speaker. It was the used for the first audio radio transmissions. Frequency modulation varies the frequency of the carrier, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. FM has the capture effect whereby a receiver only receives the strongest signal, Digital data can be sent by shifting the carriers frequency among a set of discrete values, a technique known as frequency-shift keying. FM is commonly used at Very high frequency radio frequencies for high-fidelity broadcasts of music, analog TV sound is also broadcast using FM. Angle modulation alters the phase of the carrier wave to transmit a signal
3.
Information
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In other words, it is the answer to a question of some kind. It is thus related to data and knowledge, as data represents values attributed to parameters, as it regards data, the informations existence is not necessarily coupled to an observer, while in the case of knowledge, the information requires a cognitive observer. At its most fundamental, information is any propagation of cause, Information can be encoded into various forms for transmission and interpretation. It can also be encrypted for safe storage and communication, the uncertainty of an event is measured by its probability of occurrence and is inversely proportional to that. The more uncertain an event, the information is required to resolve uncertainty of that event. The bit is a unit of information, but other units such as the nat may be used. Example, information in one fair coin flip, log2 =1 bit, the concept that information is the message has different meanings in different contexts. The English word was derived from the Latin stem of the nominative. Inform itself comes from the Latin verb informare, which means to give form, eidos can also be associated with thought, proposition, or even concept. The ancient Greek word for information is πληροφορία, which transliterates from πλήρης fully and it literally means fully bears or conveys fully. In modern Greek language the word Πληροφορία is still in use and has the same meaning as the word information in English. In addition to its meaning, the word Πληροφορία as a symbol has deep roots in Aristotles semiotic triangle. In this regard it can be interpreted to communicate information to the one decoding that specific type of sign, from the stance of information theory, information is taken as an ordered sequence of symbols from an alphabet, say an input alphabet χ, and an output alphabet ϒ. Information processing consists of a function that maps any input sequence from χ into an output sequence from ϒ. The mapping may be probabilistic or deterministic and it may have memory or be memoryless. Often information can be viewed as a type of input to an organism or system, inputs are of two kinds, some inputs are important to the function of the organism or system by themselves. In his book Sensory Ecology Dusenbery called these causal inputs, other inputs are important only because they are associated with causal inputs and can be used to predict the occurrence of a causal input at a later time. Some information is important because of association with information but eventually there must be a connection to a causal input
4.
Morse code
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Morse code is a method of transmitting text information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment. It is named for Samuel F. B, Morse, an inventor of the telegraph. Because many non-English natural languages use more than the 26 Roman letters, each Morse code symbol represents either a text character or a prosign and is represented by a unique sequence of dots and dashes. The duration of a dash is three times the duration of a dot, each dot or dash is followed by a short silence, equal to the dot duration. The letters of a word are separated by an equal to three dots, and the words are separated by a space equal to seven dots. The dot duration is the unit of time measurement in code transmission. To increase the speed of the communication, the code was designed so that the length of each character in Morse varies approximately inversely to its frequency of occurrence in English. Thus the most common letter in English, the letter E, has the shortest code, Morse code is used by some amateur radio operators, although knowledge of and proficiency with it is no longer required for licensing in most countries. Pilots and air controllers usually need only a cursory understanding. Aeronautical navigational aids, such as VORs and NDBs, constantly identify in Morse code, compared to voice, Morse code is less sensitive to poor signal conditions, yet still comprehensible to humans without a decoding device. Morse is, therefore, an alternative to synthesized speech for sending automated data to skilled listeners on voice channels. Many amateur radio repeaters, for example, identify with Morse, in an emergency, Morse code can be sent by improvised methods that can be easily keyed on and off, making it one of the simplest and most versatile methods of telecommunication. The most common signal is SOS or three dots, three dashes, and three dots, internationally recognized by treaty. Beginning in 1836, the American artist Samuel F. B, Morse, the American physicist Joseph Henry, and Alfred Vail developed an electrical telegraph system. This system sent pulses of current along wires which controlled an electromagnet that was located at the receiving end of the telegraph system. A code was needed to transmit natural language using only these pulses, around 1837, Morse, therefore, developed an early forerunner to the modern International Morse code. Around the same time, Carl Friedrich Gauss and Wilhelm Eduard Weber as well as Carl August von Steinheil had already used codes with varying lengths for their telegraphs. In 1837, William Cooke and Charles Wheatstone in England began using a telegraph that also used electromagnets in its receivers
5.
Wireless telegraphy
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Wireless telegraphy is the transmission of electric telegraphy signals without wires. Wireless telegraphy came to mean Morse code transmitted by radio waves, initially called Hertzian waves, the primitive spark radio transmitters used until World War 1 could not transmit audio. The code sounded like musical beeps in the earphone of the receiver, the receiving radio operator would translate the beeps back into text. The first practical wireless telegraphy transmitters and receivers were developed by Guglielmo Marconi beginning in 1895, by 1910 communication by Hertzian waves was universally referred to as radio, and the term wireless telegraphy has been largely replaced by the more modern term radiotelegraphy. The transmission of sound began to displace wireless telegraphy by the 1920s for many applications, continuous wave radiotelegraphy is regulated by the International Telecommunication Union as emission type A1A. Today, due to modern text transmission methods, Morse code radiotelegraphy for commercial use has become obsolete. On shipboard the computer and satellite linked GMDSS system has largely replaced Morse as a means of communication, Telegraphy is taught on a very limited basis by the military. A CW coastal station, KSM, still exists in California, run primarily as a museum by volunteers, Radio beacons, particularly in the aviation service, but also as placeholders for commercial ship-to-shore systems, also transmit Morse but at very slow speeds. The US Federal Communications Commission does still issue a lifetime commercial Radiotelegraph Operator License and this requires passing a simple written test on regulations, a more complex written exam on technology, and demonstrating Morse reception at 20 words per minute plain language and 16 wpm code groups. Wireless telegraphy is still used today by amateur radio hobbyists where it is commonly referred to as radio telegraphy, continuous wave. However its knowledge is not required to obtain any class of amateur license, a number of wireless electrical signaling schemes including electric currents through water and the ground were investigated for telegraphy before practical radio systems became available. The original telegraph used two wires between two stations to form an electrical circuit or loop. This led to speculation that it might be possible to both wires and therefore transmit telegraph signals through the ground without any wires connecting the stations. Other attempts were made to send the current through bodies of water, in order to span rivers. The first wireless voice telecommunication device, invented in 1880, was the photophone, both electrostatic and electromagnetic induction were used to develop wireless telegraph systems that saw limited commercial application. This system was successful technically but not economically, as there turned out to be little interest by train travelers in a telegraph service. During the Great Blizzard of 1888, this system was used to send, the disabled trains were able to maintain communications via their Edison induction wireless telegraph systems, perhaps the first successful use of wireless telegraphy to send distress calls. The most successful creator of an electromagnetic induction telegraph system was William Preece in the United Kingdom, beginning with tests across the Bristol Channel in 1892, Preece was able to telegraph across gaps of about 5 kilometres
6.
Spark gap
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A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the voltage of the gas within the gap, a spark forms, ionizing the gas. An electric current then flows until the path of ionized gas is broken or the current reduces below a value called the holding current. This usually happens when the drops, but in some cases occurs when the heated gas rises, stretching out. Usually, the action of ionizing the gas is violent and disruptive, often leading to sound, light, Spark gaps were used historically in early electrical equipment, such as spark gap radio transmitters, electrostatic machines, and X-ray machines. The light emitted by a spark does not come from the current of electrons itself, when electrons collide with molecules of air in the gap, they excite their orbital electrons to higher energy levels. When these excited electrons fall back to their energy levels. It is impossible for a spark to form in a vacuum. Without intervening matter capable of electromagnetic transitions, the spark will be invisible, Spark gaps are essential to the functioning of a number of electronic devices. A spark plug uses a gap to initiate combustion. The heat of the trail, but more importantly, UV radiation and hot free electrons ignite a fuel-air mixture inside an internal combustion engine, or a burner in a furnace, oven. The more UV radiation is produced and successfully spread into the combustion chamber, Spark gaps are frequently used to prevent voltage surges from damaging equipment. Spark gaps are used in switches, large power transformers, in power plants. Such switches are constructed with a large, remote-operated switching blade with a hinge as one contact, if the blade is opened, a spark may keep the connection between blade and spring conducting. Here, a Jacobs ladder on top of the switch will pull the arc apart, one might also find small Jacobs ladders mounted on top of ceramic insulators of high-voltage pylons. These are sometimes called horn gaps, if a spark should ever manage to jump over the insulator and give rise to an arc, it will be extinguished. Smaller spark gaps are used to protect sensitive electrical or electronic equipment from high-voltage surges. In sophisticated versions of these devices, a spark gap breaks down during an abnormal voltage surge, safely shunting the surge to ground
7.
Amplitude modulation
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Amplitude modulation is a modulation technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. In amplitude modulation, the amplitude of the wave is varied in proportion to the waveform being transmitted. That waveform may, for instance, correspond to the sounds to be reproduced by a loudspeaker and this technique contrasts with frequency modulation, in which the frequency of the carrier signal is varied, and phase modulation, in which its phase is varied. AM was the earliest modulation method used to transmit voice by radio and it was developed during the first two decades of the 20th century beginning with Roberto Landell De Moura and Reginald Fessendens radiotelephone experiments in 1900. It remains in use today in many forms of communication, for example it is used in two way radios, VHF aircraft radio, Citizens Band Radio, and in computer modems. AM is often used to refer to mediumwave AM radio broadcasting, when it reaches its destination, the information signal is extracted from the modulated carrier by demodulation. In amplitude modulation, the amplitude or strength of the oscillations is what is varied. For example, in AM radio communication, a continuous wave signal has its amplitude modulated by an audio waveform before transmission. The audio waveform modifies the amplitude of the wave and determines the envelope of the waveform. In the frequency domain, amplitude modulation produces a signal with power concentrated at the carrier frequency, each sideband is equal in bandwidth to that of the modulating signal, and is a mirror image of the other. Standard AM is thus sometimes called double-sideband amplitude modulation to distinguish it more sophisticated modulation methods also based on AM. One disadvantage of all amplitude modulation techniques is that the receiver amplifies and detects noise, increasing the received signal to noise ratio, say, by a factor of 10, thus would require increasing the transmitter power by a factor of 10. For this reason AM broadcast is not favored for music and high fidelity broadcasting, another disadvantage of AM is that it is inefficient in power usage, at least two-thirds of the power is concentrated in the carrier signal. The carrier signal contains none of the information being transmitted. However its presence provides a means of demodulation using envelope detection, providing a frequency. The receiver may regenerate a copy of the frequency from a greatly reduced pilot carrier to use in the demodulation process. Even with the carrier totally eliminated in double-sideband suppressed-carrier transmission, carrier regeneration is possible using a Costas phase-locked loop and this doesnt work however for single-sideband suppressed-carrier transmission, leading to the characteristic Donald Duck sound from such receivers when slightly detuned. Single sideband is used widely in amateur radio and other voice communications both due to its power efficiency and bandwidth efficiency
8.
Frequency
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Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as frequency, which emphasizes the contrast to spatial frequency. The period is the duration of time of one cycle in a repeating event, for example, if a newborn babys heart beats at a frequency of 120 times a minute, its period—the time interval between beats—is half a second. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as vibrations, audio signals, radio waves. For cyclical processes, such as rotation, oscillations, or waves, in physics and engineering disciplines, such as optics, acoustics, and radio, frequency is usually denoted by a Latin letter f or by the Greek letter ν or ν. For a simple motion, the relation between the frequency and the period T is given by f =1 T. The SI unit of frequency is the hertz, named after the German physicist Heinrich Hertz, a previous name for this unit was cycles per second. The SI unit for period is the second, a traditional unit of measure used with rotating mechanical devices is revolutions per minute, abbreviated r/min or rpm. As a matter of convenience, longer and slower waves, such as ocean surface waves, short and fast waves, like audio and radio, are usually described by their frequency instead of period. Spatial frequency is analogous to temporal frequency, but the axis is replaced by one or more spatial displacement axes. Y = sin = sin d θ d x = k Wavenumber, in the case of more than one spatial dimension, wavenumber is a vector quantity. For periodic waves in nondispersive media, frequency has a relationship to the wavelength. Even in dispersive media, the frequency f of a wave is equal to the phase velocity v of the wave divided by the wavelength λ of the wave. In the special case of electromagnetic waves moving through a vacuum, then v = c, where c is the speed of light in a vacuum, and this expression becomes, f = c λ. When waves from a monochrome source travel from one medium to another, their remains the same—only their wavelength. For example, if 71 events occur within 15 seconds the frequency is, the latter method introduces a random error into the count of between zero and one count, so on average half a count. This is called gating error and causes an error in the calculated frequency of Δf = 1/, or a fractional error of Δf / f = 1/ where Tm is the timing interval. This error decreases with frequency, so it is a problem at low frequencies where the number of counts N is small, an older method of measuring the frequency of rotating or vibrating objects is to use a stroboscope
9.
Frequency modulation
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In telecommunications and signal processing, frequency modulation is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. This contrasts with amplitude modulation, in which the amplitude of the wave varies. This modulation technique is known as frequency-shift keying, FSK is widely used in modems and fax modems, and can also be used to send Morse code. Frequency modulation is used for FM radio broadcasting. For this reason, most music is broadcast over FM radio, frequency modulation has a close relationship with phase modulation, phase modulation is often used as an intermediate step to achieve frequency modulation. Mathematically both of these are considered a case of quadrature amplitude modulation. While most of the energy of the signal is contained within fc ± fΔ, the frequency spectrum of an actual FM signal has components extending infinitely, although their amplitude decreases and higher-order components are often neglected in practical design problems. Mathematically, a baseband modulated signal may be approximated by a continuous wave signal with a frequency fm. This method is also named as Single-tone Modulation. As in other systems, the modulation index indicates by how much the modulated variable varies around its unmodulated level. e. The maximum deviation of the frequency from the carrier frequency. For a sine wave modulation, the index is seen to be the ratio of the peak frequency deviation of the carrier wave to the frequency of the modulating sine wave. If h ≪1, the modulation is called narrowband FM, sometimes modulation index h<0.3 rad is considered as Narrowband FM otherwise Wideband FM. In the case of digital modulation, the carrier f c is never transmitted, rather, one of two frequencies is transmitted, either f c + Δ f or f c − Δ f, depending on the binary state 0 or 1 of the modulation signal. If h ≫1, the modulation is called wideband FM, if the frequency deviation is held constant and the modulation frequency increased, the spacing between spectra increases. The carrier and sideband amplitudes are illustrated for different modulation indices of FM signals, for particular values of the modulation index, the carrier amplitude becomes zero and all the signal power is in the sidebands. Since the sidebands are on sides of the carrier, their count is doubled, and then multiplied by the modulating frequency to find the bandwidth. For example,3 kHz deviation modulated by a 2.2 kHz audio tone produces an index of 1.36. Suppose that we limit ourselves to only those sidebands that have an amplitude of at least 0.01
10.
Single-sideband modulation
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Amplitude modulation produces an output signal that has twice the bandwidth of the original baseband signal. Single-sideband modulation avoids this bandwidth doubling, and the power wasted on a carrier, at the cost of increased device complexity, radio transmitters work by mixing a radio frequency signal of a specific frequency, the carrier wave, with the signal to be broadcast. That is, the signal has a spectrum with twice the bandwidth of the original input signal. In conventional AM radio, this signal is sent to the radio frequency amplifier. Due to the nature of the process, the quality of the resulting signal can be defined by the difference between the maximum and minimum signal energy. Normally the maximum signal energy will be the carrier itself, perhaps twice as powerful as the mixed signals, SSB takes advantage of the fact that the entire original signal is encoded in either one of these sidebands. It is not necessary to broadcast the entire mixed signal, a receiver can extract the entire signal from either the upper or lower sideband. This means that the amplifier can be used more efficiently. A transmitter can choose to only the upper or lower sideband. By doing so, the amplifier only has to work effectively on one half the bandwidth, as a result, SSB transmissions use the available amplifier energy more efficiently, providing longer-range transmission with little or no additional cost. The first U. S. patent for SSB modulation was applied for on December 1,1915 by John Renshaw Carson, the U. S. Navy experimented with SSB over its radio circuits before World War I. SSB first entered service on January 7,1927 on the longwave transatlantic public radiotelephone circuit between New York and London. The high power SSB transmitters were located at Rocky Point, New York and Rugby, the receivers were in very quiet locations in Houlton, Maine and Cupar Scotland. SSB was also used over long distance lines, as part of a technique known as frequency-division multiplexing. FDM was pioneered by telephone companies in the 1930s and this enabled many voice channels to be sent down a single physical circuit, for example in L-carrier. SSB allowed channels to be spaced just 4,000 Hz apart, amateur radio operators began serious experimentation with SSB after World War II. The Strategic Air Command established SSB as the standard for its aircraft in 1957. It has become a de facto standard for voice radio transmissions since then
11.
Spread spectrum
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This is a technique in which a telecommunication signal is transmitted on a bandwidth considerably larger than the frequency content of the original information. Frequency hopping is a modulation technique used in spread spectrum signal transmission. This technique decreases the potential interference to other receivers while achieving privacy, spread spectrum generally makes use of a sequential noise-like signal structure to spread the normally narrowband information signal over a relatively wideband band of frequencies. The receiver correlates the received signals to retrieve the information signal. Frequency-hopping spread spectrum, direct-sequence spread spectrum, time-hopping spread spectrum, chirp spread spectrum, ultra-wideband is another modulation technique that accomplishes the same purpose, based on transmitting short duration pulses. Wireless standard IEEE802.11 uses either FHSS or DSSS in its radio interface, techniques known since the 1940s and used in military communication systems since the 1950s spread a radio signal over a wide frequency range several magnitudes higher than minimum requirement. DS is good at resisting continuous-time narrowband jamming, while FH is better at resisting pulse jamming, by contrast, in narrowband systems where the signal bandwidth is low, the received signal quality will be severely lowered if the jamming power happens to be concentrated on the signal bandwidth. Multiple access capability, known as code-division multiple access or code-division multiplexing and their approach was unique in that frequency coordination was done with paper player piano rolls - a novel approach which was never put into practice. A synchronous digital system is one that is driven by a signal and. In fact, a clock signal would have all its energy concentrated at a single frequency. It has become a technique to gain regulatory approval because it requires only simple equipment modification. It is even more popular in portable electronics devices because of faster clock speeds, since these devices are designed to be lightweight and inexpensive, traditional passive, electronic measures to reduce EMI, such as capacitors or metal shielding, are not viable. Active EMI reduction techniques such as spread-spectrum clocking are needed in these cases, however, spread-spectrum clocking, like other kinds of dynamic frequency change, can also create challenges for designers. Principal among these is clock/data misalignment, or clock skew, note that this method does not reduce total radiated energy, and therefore systems are not necessarily less likely to cause interference. Spreading energy over a larger bandwidth effectively reduces electrical and magnetic readings within narrow bandwidths, typical measuring receivers used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 kHz wide. If the system under test were to all its energy in a narrow bandwidth. Distributing this same energy into a larger bandwidth prevents systems from putting enough energy into any one narrowband to exceed the statutory limits, FCC certification testing is often completed with the spread-spectrum function enabled in order to reduce the measured emissions to within acceptable legal limits. However, the spread-spectrum functionality may be disabled by the user in some cases and this might be considered a loophole, but is generally overlooked as long as spread-spectrum is enabled by default
