telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...

, signal reflection occurs when a

signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...

is transmitted along a

transmission medium A transmission medium is a system or substance that can mediate the propagation of signals for the purposes of telecommunication. Signals are typically imposed on a wave of some kind suitable for the chosen medium. For example, data can modulat ...

, such as a copper cable or an

optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to ...

. Some of the signal power may be reflected back to its origin rather than being carried all the way along the cable to the far end. This happens because imperfections in the cable cause impedance mismatches and non-linear changes in the cable characteristics. These abrupt changes in characteristics cause some of the transmitted signal to be reflected. In

radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...

(RF) practice this is often measured in a dimensionless ratio known as

voltage standing wave ratio In radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the trans ...

(VSWR) with a VSWR bridge. The ratio of energy bounced back depends on the

impedance mismatch In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize sign ...

. Mathematically, it is defined using the

reflection coefficient In physics and electrical engineering the reflection coefficient is a parameter that describes how much of a wave is reflected by an impedance discontinuity in the transmission medium. It is equal to the ratio of the amplitude of the reflected w ...

. Because the principles are the same, this concept is perhaps easiest to understand when considering an optical fiber. Imperfections in the glass create mirrors that reflect the light back along the fiber. Impedance discontinuities cause

attenuation In physics, attenuation (in some contexts, extinction) 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 att ...

attenuation distortion Attenuation distortion is the distortion of an analog signal that occurs during transmission when the transmission medium does not have a flat frequency response across the bandwidth of the medium or the frequency spectrum of the signal.Rowe, St ...

standing waves In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect ...

ringing Ringing may mean: Vibrations * Ringing (signal), unwanted oscillation of a signal, leading to ringing artifacts * Vibration of a harmonic oscillator ** Bell ringing * Ringing (telephony), the sound of a telephone bell * Ringing (medicine), a ri ...

and other effects because a portion of a transmitted signal will be reflected back to the

transmitting In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to th ...

device rather than continuing to the receiver, much like an

echo In audio signal processing and acoustics, an echo is a reflection of sound that arrives at the listener with a delay after the direct sound. The delay is directly proportional to the distance of the reflecting surface from the source and the lis ...

. This effect is compounded if multiple discontinuities cause additional portions of the remaining signal to be reflected back to the transmitter. This is a fundamental problem with the

daisy chain Daisy chain may refer to: * Daisy chain, a garland created from daisy flowers * Daisy chain (climbing), a type of strap * Daisy chain (electrical engineering), a wiring scheme * Daisy chain (fishing), a type of fishing lure * Daisy chain (knot), ...

method of connecting electronic components. When a returning reflection strikes another discontinuity, some of the signal rebounds in the original signal direction, creating multiple echo effects. These

forward echo In telecommunications, a forward echo is the propagation of a signal reflection in the same direction as the original signal and consisting of energy reflected back by one discontinuity and then forward again by another discontinuity. Forward ech ...

es strike the receiver at different intervals making it difficult for the receiver to accurately detect data values on the signal. The effects can resemble those of

jitter In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a significa ...

Because damage to the cable can cause reflections, an instrument called an electrical

time-domain reflectometer A time-domain reflectometer (TDR) is an electronic instrument used to determine the characteristics of electrical lines by observing reflected waveforms. It can be used to characterize and locate faults in metallic cables (for example, twisted pa ...

(ETDR; for electrical cables) or an

optical time-domain reflectometer An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber. It is the optical equivalent of an electronic time domain reflectometer which measures the impedance of the cable or transmission ...

(OTDR; for optical cables) can be used to locate the damaged part of a cable. These instruments work by sending a short pulsed signal into the cable and measuring how long the reflection takes to return. If only reflection magnitudes are desired, however, and exact fault locations are not required, VSWR bridges perform a similar but lesser function for RF cables. The combination of the effects of signal attenuation and impedance discontinuities on a

communications link A data link is the means of connecting one location to another for the purpose of transmitting and receiving digital information (data communication). It can also refer to a set of electronics assemblies, consisting of a transmitter and a recei ...

is called

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 inserti ...

. Proper network operation depends on constant

characteristic impedance The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in ...

in all cables and connectors, with no impedance discontinuities in the entire cable system. When a sufficient degree of

impedance matching In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize signal ...

is not practical,

echo suppressor 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 is already present. In addition to improving subjective audio quality, echo suppression i ...

s or

echo canceller 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 is already present. In addition to improving subjective audio quality, echo suppression ...

s, or both, can sometimes reduce the problems. The

Bergeron diagram The Bergeron diagram method is a method to evaluate the effect of a reflection on an electrical signal. This graphic method—based on the real characteristic of the line—is valid for both linear and non-linear models and helps to calculate the d ...

method, valid for both linear and non-linear models, evaluates the reflection's effects in an electric line.

See also