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 ...

, signal reflection occurs when a signal 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 Copper has been used in electrical wiring since the invention of the electromagnet and the telegraph in the 1820s. The invention of the telephone in 1876 created further demand for copper wire as an electrical conductor. Copper is the electr ...

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 Hair is a protein filament that grows ...

. 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 up ...

(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 tran ...

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

. 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 var ...

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,

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 ring ...

and other effects because a portion of a transmitted signal will be reflected back to the transmitting 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 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 echoes 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 signific ...

Because damage to the cable can cause reflections, an instrument called an electrical time-domain reflectometer (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 transmissi ...

(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 i ...

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 si ...

is not practical, echo suppressors 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 method, valid for both linear and non-linear models, evaluates the reflection's effects in an electric line.

See also