Misplaced Pages

#898101

52-468: Because the media gateway connects different types of networks, one of its main functions is to convert between different transmission and coding techniques. Media streaming functions such as echo cancellation , DTMF , and tone sender are also located in the media gateway. Media gateways are often controlled by a separate Media Gateway Controller which provides the call control and signaling functionality. Communication between media gateways and Call Agents

104-429: A fully functional AEC. Smart speakers and interactive voice response systems that accept speech for input use AEC while speech prompts are played to prevent the system's own speech recognition from falsely recognizing the echoed prompts and other output. Standard telephone lines use the same pair of wires to both send and receive audio, which results in a small amount of the outgoing signal being reflected back. This

156-474: A loudspeaker being reflected and recorded by a microphone, which can vary substantially over time) and line echo (electrical impulses caused by, e.g., coupling between the sending and receiving wires, impedance mismatches, electrical reflections, etc., which varies much less than acoustic echo). In practice, however, the same techniques are used to treat all types of echo, so an acoustic echo canceller can cancel line echo as well as acoustic echo. AEC in particular

208-713: A media streaming protocol, such as the Real-time Transport Protocol , (RTP), as well as a signaling protocol used in the VoIP system. Mobile access media gateways connect the radio access networks of a public land mobile network PLMN to a next-generation core network . 3GPP standards define the functionality of CS-MGW and IMS-MGW for UTRAN and GERAN based PLMNs . Echo cancellation Echo suppression and echo cancellation are methods used in telephony to improve voice quality by preventing echo from being created or removing it after it

260-416: A particular material occurs because the selected frequency of the light wave matches the frequency (or an integral multiple of the frequency) at which the particles of that material vibrate. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies (or portions of the spectrum) of infrared (IR) light. In optical fibers , attenuation

312-480: A tissue specimen as they have less chances of interacting with matter. This is mainly due to the photoelectric effect which states that "the probability of photoelectric absorption is approximately proportional to (Z/E) , where Z is the atomic number of the tissue atom and E is the photon energy. In context of this, an increase in photon energy (E) will result in a rapid decrease in the interaction with matter. Attenuation in fiber optics, also known as transmission loss,

364-423: Is "light scattering". Light scattering from the surfaces of objects is our primary mechanism of physical observation. Light scattering from many common surfaces can be modelled by reflectance. Light scattering depends on the wavelength of the light being scattered. Thus, limits to spatial scales of visibility arise, depending on the frequency of the incident lightwave and the physical dimension (or spatial scale) of

416-416: Is achieved by means of protocols such as MGCP or Megaco (H.248) or Session Initiation Protocol (SIP). Modern media gateways used with SIP are often stand-alone units with their own call and signaling control integrated and can function as independent, intelligent SIP end-points. Voice over Internet Protocol (VoIP) media gateways perform the conversion between Time-division multiplexing (TDM) voice to

468-683: Is already present. In addition to improving subjective audio quality, echo suppression increases the capacity achieved through silence suppression by preventing echo from traveling across a telecommunications network . Echo suppressors were developed in the 1950s in response to the first use of satellites for telecommunications. Echo suppression and cancellation methods are commonly called acoustic echo suppression ( AES ) and acoustic echo cancellation ( AEC ), and more rarely line echo cancellation ( LEC ). In some cases, these terms are more precise, as there are various types and causes of echo with unique characteristics, including acoustic echo (sounds from

520-405: Is an important consideration in weather radar , as raindrops absorb a part of the emitted beam that is more or less significant, depending on the wavelength used. Due to the damaging effects of high-energy photons, it is necessary to know how much energy is deposited in tissue during diagnostic treatments involving such radiation. In addition, gamma radiation is used in cancer treatments where it

572-443: Is an important factor limiting the transmission of a digital signal across large distances. Thus, much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal. Empirical research has shown that attenuation in optical fiber is caused primarily by both scattering and absorption. Attenuation in fiber optics can be quantified using the following equation: The propagation of light through

SECTION 10

#1732801583899

624-458: Is associated only with absorption and can be characterized with absorption coefficient only. Propagation through heterogeneous media requires taking into account scattering. Shortwave radiation emitted from the Sun have wavelengths in the visible spectrum of light that range from 360 nm (violet) to 750 nm (red). When the Sun's radiation reaches the sea surface, the shortwave radiation

676-473: Is attenuated by the water, and the intensity of light decreases exponentially with water depth. The intensity of light at depth can be calculated using the Beer-Lambert Law . In clear mid-ocean waters, visible light is absorbed most strongly at the longest wavelengths. Thus, red, orange, and yellow wavelengths are totally absorbed at shallower depths, while blue and violet wavelengths reach deeper in

728-484: Is commonly used to refer to echo cancelers in general, regardless of whether they were intended for acoustic echo, line echo, or both. Although echo suppressors and echo cancellers have similar goals—preventing a speaking individual from hearing an echo of their own voice—the methods they use are different: ITU standards G.168 and P.340 describe requirements and tests for echo cancellers in digital and PSTN applications, respectively. In telephony , echo

780-407: Is determining the response characteristics of the filter to be applied to the far-end signal such that it resembles the resultant near-end echo. The filter is essentially a model of speaker, microphone and the room's acoustical attributes. Echo cancellers must be adaptive because the characteristics of the near-end's speaker and microphone are generally not known in advance. The acoustical attributes of

832-427: Is important to know how much energy will be deposited in healthy and in tumorous tissue. In computer graphics attenuation defines the local or global influence of light sources and force fields. In CT imaging , attenuation describes the density or darkness of the image. Attenuation is an important consideration in the modern world of wireless telecommunications . Attenuation limits the range of radio signals and

884-411: Is in ultrasound physics. Attenuation in ultrasound is the reduction in amplitude of the ultrasound beam as a function of distance through the imaging medium. Accounting for attenuation effects in ultrasound is important because a reduced signal amplitude can affect the quality of the image produced. By knowing the attenuation that an ultrasound beam experiences traveling through a medium, one can adjust

936-433: Is measured as echo return loss (ERL). This is the ratio, expressed in decibels , of the original signal and its echo. High values mean the echo is very weak, while low values mean the echo is very strong. Negative indicate the echo is stronger than the original signal, which if left unchecked would cause audio feedback . The performance of an echo canceller is measured in echo return loss enhancement (ERLE), which

988-458: Is the amount of additional signal loss applied by the echo canceller. Most echo cancellers are able to apply 18 to 35 dB ERLE. The total signal loss of the echo (ACOM) is the sum of the ERL and ERLE. Sources of echo are found in everyday surroundings such as: In some of these cases, sound from the loudspeaker enters the microphone almost unaltered. The difficulties in canceling echo stem from

1040-467: Is the gradual loss of flux intensity through a medium . For instance, dark glasses attenuate sunlight , lead attenuates X-rays , and water and air attenuate both light and sound at variable attenuation rates. Hearing protectors help reduce acoustic flux from flowing into the ears. This phenomenon is called acoustic attenuation and is measured in decibels (dBs). In electrical engineering and telecommunications , attenuation affects

1092-401: Is the output power at the far end of this cable. Attenuation in a coaxial cable is a function of the materials and the construction. The beam of X-ray is attenuated when photons are absorbed when the x-ray beam passes through the tissue. Interaction with matter varies between high energy photons and low energy photons. Photons travelling at higher energy are more capable of travelling through

SECTION 20

#1732801583899

1144-423: Is the rate at which the signal light decreases in intensity. For this reason, glass fiber (which has a low attenuation) is used for long-distance fiber optic cables; plastic fiber has a higher attenuation and, hence, shorter range. There also exist optical attenuators that decrease the signal in a fiber optic cable intentionally. Attenuation of light is also important in physical oceanography . This same effect

1196-408: Is the reduction in intensity of the light beam (or signal) with respect to distance travelled through a transmission medium. Attenuation coefficients in fiber optics usually use units of dB/km through the medium due to the relatively high quality of transparency of modern optical transmission. The medium is typically a fiber of silica glass that confines the incident light beam to the inside. Attenuation

1248-406: Is the reflected copy of one's voice heard some time later. If the delay is fairly significant (more than a few hundred milliseconds), it is considered annoying. If the delay is very small (tens of milliseconds or less ), the phenomenon is called sidetone . If the delay is slightly longer, around 50 milliseconds, humans cannot hear the echo as a distinct sound, but instead hear a chorus effect . In

1300-466: Is useful for people talking on the phone, as it provides a signal to the speaker that their voice is making it through the system. However, this reflected signal causes problems for a modem, which is unable to distinguish between a signal from the remote modem and the echo of its own signal. For this reason, earlier dial-up modems split the signal frequencies, so that the devices on either end used different tones, allowing each one to ignore any signals in

1352-403: Is usually measured in units of decibels per unit length of medium (dB/cm, dB/km, etc.) and is represented by the attenuation coefficient of the medium in question. Attenuation also occurs in earthquakes ; when the seismic waves move farther away from the hypocenter , they grow smaller as they are attenuated by the ground . One area of research in which attenuation plays a prominent role,

1404-464: The propagation of waves and signals in electrical circuits , in optical fibers , and in air. Electrical attenuators and optical attenuators are commonly manufactured components in this field. In many cases, attenuation is an exponential function of the path length through the medium. In optics and in chemical spectroscopy , this is known as the Beer–Lambert law . In engineering, attenuation

1456-464: The water column . Because the blue and violet wavelengths are absorbed least compared to the other wavelengths, open-ocean waters appear deep blue to the eye. Near the shore, coastal water contains more phytoplankton than the very clear mid-ocean waters. Chlorophyll -a pigments in the phytoplankton absorb light, and the plants themselves scatter light, making coastal waters less clear than mid-ocean waters. Chlorophyll-a absorbs light most strongly in

1508-404: The adaptive filter is much reduced compared to voice echo cancelling because the transmit signal is a digital bit stream. Instead of a multiplication and an addition operation for every tap in the filter, only the addition is required. A RAM lookup table based echo cancelling scheme eliminates even the addition operation by simply addressing a memory with a truncated transmit bit stream to obtain

1560-428: The alteration of the original sound by the ambient space. These changes can include certain frequencies being absorbed by soft furnishings and reflection of different frequencies at varying strength. Implementing AEC requires engineering expertise and a fast processor, usually in the form of a digital signal processor (DSP), this cost in processing capability may come at a premium, however, many embedded systems do have

1612-409: The assessment of possible strong groundshaking. A seismic wave loses energy as it propagates through the earth ( seismic attenuation ). This phenomenon is tied into the dispersion of the seismic energy with the distance. There are two types of dissipated energy: In porous fluid—saturated sedimentary rocks such as sandstones , intrinsic attenuation of seismic waves is primarily caused by

Media gateway - Misplaced Pages Continue

1664-449: The connection is completed, they send their signals into the phone lines as normal, but also into the delay line. When their signal is reflected back, it is mixed with the inverted signal from the delay line, which cancels out the echo. This allowed both modems to use the full spectrum available, doubling the possible speed. Echo cancellation is also applied by many telcos to the line itself and can cause data corruption rather than improving

1716-499: The core of an optical fiber is based on total internal reflection of the lightwave. Rough and irregular surfaces, even at the molecular level of the glass, can cause light rays to be reflected in many random directions. This type of reflection is referred to as "diffuse reflection", and it is typically characterized by wide variety of reflection angles. Most objects that can be seen with the naked eye are visible due to diffuse reflection. Another term commonly used for this type of reflection

1768-415: The earlier days of telecommunications, echo suppression was used to reduce the objectionable nature of echos to human users. One person speaks while the other listens, and they speak back and forth. An echo suppressor attempts to determine which is the primary direction and allows that channel to go forward. In the reverse channel, it places attenuation to block or suppress any signal on the assumption that

1820-696: The echo estimate. Echo cancellation is now commonly implemented with Digital Signal Processor (DSP) techniques. Some modems use separate incoming and outgoing frequencies or allocate separate time slots for transmitting and receiving to eliminate the need for echo cancellation. Higher frequencies beyond the original design limits of telephone cables suffer significant attenuation distortion due to bridge taps and incomplete impedance matching . Deep, narrow frequency gaps which cannot be remedied by echo cancellation often result. These are detected and mapped out during connection negotiation. Attenuation In physics , attenuation (in some contexts, extinction )

1872-478: The echo from the talker's signal, and subtracts that estimate from the return path. This technique requires an adaptive filter to generate a signal accurate enough to effectively cancel the echo, where the echo can differ from the original due to various kinds of degradation along the way. Since invention at AT&T Bell Labs echo cancellation algorithms have been improved and honed. Like all echo cancelling processes, these first algorithms were designed to anticipate

1924-409: The frequency range it was using for transmission. However, this diminished the amount of bandwidth available to both sides. Echo cancellation mitigated this problem. During the call setup and negotiation period, both modems send a series of unique tones and then listen for them to return through the phone system. They measure the total delay time, then configure a delay line for that same period. Once

1976-430: The input signal amplitude to compensate for any loss of energy at the desired imaging depth. Wave equations which take acoustic attenuation into account can be written on a fractional derivative form. In homogeneous media, the main physical properties contributing to sound attenuation are viscosity and thermal conductivity. Attenuation coefficients are used to quantify different media according to how strongly

2028-440: The internal surfaces or interfaces are in the form of grain boundaries that separate tiny regions of crystalline order. It has recently been shown that, when the size of the scattering center (or grain boundary) is reduced below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent. This phenomenon has given rise to the production of transparent ceramic materials. Likewise,

2080-499: The most ideal locations for the occurrence of light scattering. This same phenomenon is seen as one of the limiting factors in the transparency of IR missile domes. In addition to light scattering, attenuation or signal loss can also occur due to selective absorption of specific wavelengths, in a manner similar to that responsible for the appearance of color. Primary material considerations include both electrons and molecules as follows: The selective absorption of infrared (IR) light by

2132-576: The near-end's room are also not generally known in advance, and may change (e.g., if the microphone is moved relative to the speaker, or if individuals walk around the room causing changes in the acoustic reflections). By using the far-end signal as the stimulus, modern systems use an adaptive filter and can converge from providing no cancellation to 55 dB of cancellation in around 200 ms. Echo cancellation alone may be insufficient in many applications. Echo cancellation and suppression can work in conjunction to achieve acceptable performance. Echo

Media gateway - Misplaced Pages Continue

2184-463: The path. The primary causes of attenuation in matter are the photoelectric effect , Compton scattering , and, for photon energies of above 1.022 MeV, pair production . The attenuation of RF cables is defined by: where P 1 {\displaystyle P_{1}} is the input power into a 100 m long cable terminated with the nominal value of its characteristic impedance, and P 2 {\displaystyle P_{2}}

2236-445: The relationship is quadratic ). Attenuation coefficients vary widely for different media. In biomedical ultrasound imaging however, biological materials and water are the most commonly used media. The attenuation coefficients of common biological materials at a frequency of 1 MHz are listed below: There are two general ways of acoustic energy losses: absorption and scattering . Ultrasound propagation through homogeneous media

2288-446: The scattering center, which is typically in the form of some specific microstructural feature. For example, since visible light has a wavelength scale on the order of one micrometer, scattering centers will have dimensions on a similar spatial scale. Thus, attenuation results from the incoherent scattering of light at internal surfaces and interfaces. In (poly)crystalline materials such as metals and ceramics, in addition to pores, most of

2340-526: The scattering of light in optical quality glass fiber is caused by molecular-level irregularities (compositional fluctuations) in the glass structure. Indeed, one emerging school of thought is that a glass is simply the limiting case of a polycrystalline solid. Within this framework, "domains" exhibiting various degrees of short-range order become the building-blocks of both metals and alloys, as well as glasses and ceramics. Distributed both between and within these domains are microstructural defects that will provide

2392-437: The shortest wavelengths (blue and violet) of the visible spectrum. In coastal waters where high concentrations of phytoplankton occur, the green wavelength reaches the deepest in the water column and the color of water appears blue-green or green . The energy with which an earthquake affects a location depends on the running distance . The attenuation in the signal of ground motion intensity plays an important role in

2444-418: The signal is echo. Although the suppressor effectively deals with echo, this approach leads to several problems which may be frustrating for both parties to a call. In response to this, Bell Labs developed echo canceler theory in the early 1960s, which then resulted in laboratory echo cancelers in the late 1960s and commercial echo cancelers in the 1980s. An echo canceller works by generating an estimate of

2496-491: The signal which would inevitably re-enter the transmission path, and cancel it out. Rapid advances in digital signal processing allowed echo cancellers to be made smaller and more cost-effective. In the 1990s, echo cancellers were implemented within voice switches for the first time (in the Northern Telecom DMS-250 ) rather than as standalone devices. The integration of echo cancellation directly into

2548-538: The signal. Some telephone switches or converters (such as analog terminal adapters) disable echo suppression or echo cancellation when they detect the 2100 or 2225 Hz answer tones associated with such calls, in accordance with ITU-T recommendation G.164 or G.165 . ISDN and DSL modems operating at frequencies above the voice band over standard twisted-pair telephone wires also make use of automated echo cancellation to allow simultaneous bidirectional data communication. The computational complexity in implementing

2600-597: The switch meant that echo cancellers could be reliably turned on or off on a call-by-call basis, removing the need for separate trunk groups for voice and data calls. Today's telephony technology often employs echo cancellers in small or handheld communications devices via a software voice engine , which provides cancellation of either acoustic echo or the residual echo introduced by a far-end PSTN gateway system; such systems typically cancel echo reflections with up to 64 milliseconds delay. The echo cancellation process works as follows: The primary challenge for an echo canceller

2652-484: The transmitted ultrasound amplitude decreases as a function of frequency. The attenuation coefficient ( α {\displaystyle \alpha } ) can be used to determine total attenuation in dB in the medium using the following formula: Attenuation is linearly dependent on the medium length and attenuation coefficient, as well as – approximately – the frequency of the incident ultrasound beam for biological tissue (while for simpler media, such as air,

SECTION 50

#1732801583899

2704-424: The wave-induced flow of the pore fluid relative to the solid frame. Attenuation decreases the intensity of electromagnetic radiation due to absorption or scattering of photons . Attenuation does not include the decrease in intensity due to inverse-square law geometric spreading. Therefore, calculation of the total change in intensity involves both the inverse-square law and an estimation of attenuation over

#898101