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.
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
3.
Wavelength-division multiplexing
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This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity. This is purely convention because wavelength and frequency communicate the same information, a WDM system uses a multiplexer at the transmitter to join the several signals together, and a demultiplexer at the receiver to split them apart. With the right type of fiber it is possible to have a device that does both simultaneously, and can function as an optical add-drop multiplexer, the optical filtering devices used have conventionally been etalons. As there are three different WDM types, whereof one is called WDM, we would use the notation xWDM when discussing the technology as such. The concept was first published in 1978, and by 1980 WDM systems were being realized in the laboratory, the first WDM systems combined only two signals. Modern systems can handle 160 signals and can expand a basic 100 Gbit/s system over a single fiber pair to over 16 Tbit/s. A system of 320 channels in also present WDM systems are popular with telecommunications companies because they allow them to expand the capacity of the network without laying more fiber. By using WDM and optical amplifiers, they can accommodate several generations of development in their optical infrastructure without having to overhaul the backbone network. Capacity of a link can be expanded simply by upgrading the multiplexers and demultiplexers at each end. This is often done by use of optical-to-electrical-to-optical translation at the edge of the transport network. Most WDM systems operate on single-mode fiber optical cables, which have a diameter of 9 µm. Certain forms of WDM can also be used in multi-mode fiber cables which have diameters of 50 or 62.5 µm. Early WDM systems were expensive and complicated to run, however, recent standardization and better understanding of the dynamics of WDM systems have made WDM less expensive to deploy. Optical receivers, in contrast to laser sources, tend to be wideband devices, therefore, the demultiplexer must provide the wavelength selectivity of the receiver in the WDM system. WDM systems are divided into three different wavelength patterns, normal, coarse and dense, normal WDM uses the two normal wavelengths 1310 and 1550 on one fiber. Coarse WDM provides up to 16 channels across multiple transmission windows of silica fibers, dense wavelength division multiplexing uses the C-Band transmission window but with denser channel spacing. Channel plans vary, but a typical DWDM system would use 40 channels at 100 GHz spacing or 80 channels with 50 GHz spacing, some technologies are capable of 12.5 GHz spacing. New amplification options enable the extension of the wavelengths to the L-band
4.
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
5.
Spatial multiplexing
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Therefore, the space dimension is reused, or multiplexed, more than one time. If the transmitter is equipped with N t antennas and the receiver has N r antennas and this means that N s streams can be transmitted in parallel, ideally leading to an N s increase of the spectral efficiency. The practical multiplexing gain can be limited by spatial correlation, which means some of the parallel streams may have very weak channel gains. An often encountered problem in open loop spatial multiplexing is to guard against instance of high channel correlation, one such extension which is being considered for DVB-NGH systems is the so-called enhanced Spatial Multiplexing scheme. A closed-loop MIMO system utilizes Channel State Information at the transmitter, in most cases, only partial CSI is available at the transmitter because of the limitations of the feedback channel. A precoding matrix W is used to precode the symbols in the vector to enhance the performance, the column dimension N s of W can be selected smaller than N t which is useful if the system requires N s streams because of several reasons. Examples of the reasons are as follows, either the rank of the MIMO channel or the number of antennas is smaller than the number of transmit antennas. Space–time code Space–time trellis code WiMAX MIMO 3G MIMO
6.
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
7.
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
8.
Packet switching
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Packet switching increases network efficiency and robustness, and enables technological convergence of many applications operating on the same network. Packets are composed of a header and payload, Information in the header is used by networking hardware to direct the packet to its destination where the payload is extracted and used by application software. This concept contrasted and contradicted then-established principles of pre-allocation of network bandwidth, the new concept found little resonance among network implementers until the independent work of British computer scientist Donald Davies at the National Physical Laboratory in the late 1960s. Packet mode communication may be implemented with or without intermediate forwarding nodes, in case of a shared physical medium, the packets may be delivered according to a multiple access scheme. In the late 1950s, the US Air Force established a wide area network for the Semi-Automatic Ground Environment radar defense system and they sought a system that might survive a nuclear attack to enable a response, thus diminishing the attractiveness of the first strike advantage by enemies. Report P-2626 described a general architecture for a large-scale, distributed, Barans work was known to Robert Taylor and J. C. R. Licklider at the Information Processing Technology Office, who advocated wide area networks, starting in 1965, Donald Davies at the National Physical Laboratory, UK, independently developed the same message routing methodology as developed by Baran. He called it packet switching, a more accessible name than Barans and he gave a talk on the proposal in 1966, after which a person from the Ministry of Defence told him about Barans work. A member of Davies team met Lawrence Roberts at the 1967 ACM Symposium on Operating System Principles, Davies had chosen some of the same parameters for his original network design as did Baran, such as a packet size of 1024 bits. In 1966, Davies proposed that a network should be built at the laboratory to serve the needs of NPL, the NPL Data Communications Network entered service in 1970. In 1974, Vint Cerf and Bob Kahn published the specifications for Transmission Control Protocol, Packet switching may be classified into connectionless packet switching, also known as datagram switching, and connection-oriented packet switching, also known as virtual circuit switching. Examples of connectionless protocols are Ethernet, Internet Protocol, and the User Datagram Protocol, connection-oriented protocols include X.25, Frame Relay, Multiprotocol Label Switching, and the Transmission Control Protocol. In connectionless mode each packet includes complete addressing information, the packets are routed individually, sometimes resulting in different paths and out-of-order delivery. Each packet is labeled with an address, source address. It may also be labeled with the number of the packet. At the destination, the original message/data is reassembled in the correct order, connection-oriented transmission requires a setup phase in each involved node before any packet is transferred to establish the parameters of communication. The packets include a connection identifier rather than address information and are negotiated between endpoints so that they are delivered in order and with error checking, the signaling protocols used allow the application to specify its requirements and discover link parameters. Acceptable values for service parameters may be negotiated, routing a packet requires the node to look up the connection id in a table
9.
Phase-shift keying
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Phase-shift keying is a digital modulation scheme that conveys data by changing the phase of a reference signal. The modulation occurs by varying the sine and cosine inputs at a precise time and it is widely used for wireless LANs, RFID and Bluetooth communication. Any digital modulation scheme uses a number of distinct signals to represent digital data. PSK uses a number of phases, each assigned a unique pattern of binary digits. Usually, each phase encodes a number of bits. Each pattern of bits forms the symbol that is represented by the particular phase and this requires the receiver to be able to compare the phase of the received signal to a reference signal — such a system is termed coherent. Alternatively, instead of operating with respect to a constant reference wave, changes in phase of a single broadcast waveform can be considered the significant items. In this system, the demodulator determines the changes in the phase of the signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying, in exchange, it produces more erroneous demodulation. In the case of PSK, the phase is changed to represent the data signal, a convenient method to represent PSK schemes is on a constellation diagram. This shows the points in the plane where, in this context. Such a representation on perpendicular axes lends itself to straightforward implementation, the amplitude of each point along the in-phase axis is used to modulate a cosine wave and the amplitude along the quadrature axis to modulate a sine wave. By convention, in-phase modulates cosine and quadrature modulates sine, in PSK, the constellation points chosen are usually positioned with uniform angular spacing around a circle. This gives maximum phase-separation between adjacent points and thus the best immunity to corruption and they are positioned on a circle so that they can all be transmitted with the same energy. In this way, the moduli of the numbers they represent will be the same. Two common examples are binary phase-shift keying which uses two phases, and quadrature phase-shift keying which uses four phases, although any number of phases may be used. Since the data to be conveyed are usually binary, the PSK scheme is designed with the number of constellation points being a power of 2. It is a form of the complementary Gaussian error function
10.
Quadrature amplitude modulation
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Quadrature amplitude modulation is both an analog and a digital modulation scheme. The modulated waves are summed, and the waveform is a combination of both phase-shift keying and amplitude-shift keying, or, in the analog case, of phase modulation. In the digital QAM case, a number of at least two phases and at least two amplitudes are used. PSK modulators are often designed using the QAM principle, but are not considered as QAM since the amplitude of the carrier signal is constant. QAM is used extensively as a scheme for digital telecommunication systems. Arbitrarily high spectral efficiencies can be achieved with QAM by setting a suitable size, limited only by the noise level. QAM is being used in optical systems as bit rates increase, QAM16. Like all modulation schemes, QAM conveys data by changing some aspect of a signal, or the carrier wave. In the case of QAM, the amplitude of two waves of the frequency, 90° out-of-phase with each other are changed to represent the data signal. Amplitude modulating two carriers in quadrature can be viewed as both amplitude modulating and phase modulating a single carrier. Phase modulation and phase-shift keying can be regarded as a case of QAM. This can also be extended to frequency modulation and frequency-shift keying, at the receiver, these two modulating signals can be demodulated using a coherent demodulator. Such a receiver multiplies the received signal separately with both a cosine and sine signal to produce the received estimates of I and Q respectively, because of the orthogonality property of the carrier signals, it is possible to detect the modulating signals independently. This filtered signal is unaffected by Q, showing that the component can be received independently of the quadrature component. Similarly, we may multiply s by a wave and then low-pass filter to extract Q. Analog QAM suffers from the problem as Single-sideband modulation, the exact phase of the carrier is required for correct demodulation at the receiver. If the demodulating phase is even a little off, it results in crosstalk between the modulated signals and this issue of carrier synchronization at the receiver must be handled somehow in QAM systems. The coherent demodulator needs to be exactly in phase with the received signal and this is achieved typically by transmitting a burst subcarrier or a Pilot signal
11.
Line code
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In telecommunication, a line code is a code chosen for use within a communications system for transmitting a digital signal down a line. Line coding is used for digital data transport. Some line codes are digital baseband modulation or digital baseband transmission methods, Line coding consists of representing the digital signal to be transported, by a waveform that is appropriate for the specific properties of the physical channel. The pattern of voltage, current or photons used to represent the data on a transmission link is called line encoding. The common types of line encoding are unipolar, polar, bipolar, after line coding, the signal is put through a physical channel, either a transmission medium or data storage medium. Sometimes the characteristics of two very different-seeming channels are similar enough that the line code is used for them. The most common physical channels are, the signal can directly be put on a transmission line. The line-coded signal undergoes further pulse shaping and then modulated to create an RF signal that can be sent through free space, the line-coded signal can be used to turn on and off a light source in free-space optical communication, most commonly used in an infrared remote control. The line-coded signal can be printed on paper to create a bar code, the line-coded signal can be converted to magnetized spots on a hard drive or tape drive. The line-coded signal can be converted to pits on an optical disc, some of the more common or binary line codes include, Each line code has advantages and disadvantages. The running disparity is the total of the disparity of all previously transmitted words. Unfortunately, most long-distance communication channels cannot reliably transport a DC component, the DC component is also called the disparity, the bias, or the DC coefficient. The simplest possible line code, unipolar, gives too many errors on such systems, most line codes eliminate the DC component – such codes are called DC-balanced, zero-DC, or DC-free. There are three ways of eliminating the DC component, Use a constant-weight code, for example, Manchester code and Interleaved 2 of 5. In other words, the transmitter has to make sure that every word that averages to a negative level is paired with another code word that averages to a positive level. Therefore, it must keep track of the running DC buildup, the receiver is designed so that either code word of the pair decodes to the same data bits. For example, AMI, 8B10B, 4B3T, etc, for example, the scrambler specified in RFC2615 for 64b/66b encoding. This is important if the signal must pass through a transformer or a transmission line
12.
Amplitude-shift keying
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Amplitude-shift keying is a form of amplitude modulation that represents digital data as variations in the amplitude of a carrier wave. In an ASK system, the binary symbol 1 is represented by transmitting a carrier wave. If the signal value is 1 then the signal will be transmitted, otherwise. Any digital modulation scheme uses a number of distinct signals to represent digital data. ASK uses a number of amplitudes, each assigned a unique pattern of binary digits. Usually, each amplitude encodes a number of bits. Each pattern of bits forms the symbol that is represented by the particular amplitude, frequency and phase of the carrier are kept constant. Like AM, an ASK is also linear and sensitive to noise, distortions, propagation conditions on different routes in PSTN. Both ASK modulation and demodulation processes are relatively inexpensive, the ASK technique is also commonly used to transmit digital data over optical fiber. For LED transmitters, binary 1 is represented by a pulse of light. Laser transmitters normally have a bias current that causes the device to emit a low light level. This low level represents binary 0, while a higher-amplitude lightwave represents binary 1, the simplest and most common form of ASK operates as a switch, using the presence of a carrier wave to indicate a binary one and its absence to indicate a binary zero. For instance, an encoding scheme can represent two bits with each shift in amplitude, an eight-level scheme can represent three bits, and so on. These forms of amplitude-shift keying require a high ratio for their recovery. ASK system can be divided into three blocks, the first one represents the transmitter, the second one is a linear model of the effects of the channel, the third one shows the structure of the receiver. These impulses are sent to the filter ht to be sent through the channel, in other words, for each symbol a different carrier wave is sent with the relative amplitude. After the A/D conversion the signal z can be expressed in the form, z = n r + v g + ∑ n ≠ k v g In this relationship, the second term represents the symbol to be extracted. The others are unwanted, the first one is the effect of noise and it is shown in cyan for just one of them
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This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".
This article incorporates