Intelecommunications,signal reflection happens when asignal istransmitted along atransmission medium (such as acopper cable or anoptical fiber) and part of it is reflected back toward the source instead of reaching the end. This reflection is caused by imperfections or physical variations in the cable (such as abrupt changes in its geometry) that lead toimpedance mismatches.[1] These mismatches disrupt the signal and cause some of it to bounce back. Inradio frequency (RF) systems, this is typically measured using thevoltage standing wave ratio (VSWR), with a device called a VSWR bridge. The amount of reflected energy depends on the degree ofimpedance mismatch and is mathematically described by thereflection coefficient.[2]
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.[3]
Impedance discontinuities causeattenuation,attenuation distortion,standing waves,ringing and other effects because a portion of a transmitted signal will be reflected back to thetransmitting device rather than continuing to thereceiver, much like anecho. 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 thedaisy chain method of connecting electronic components.[4]
When a returning reflection strikes another discontinuity, some of the signal rebounds in the original signal direction, creating multiple echo effects. Theseforward 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 ofjitter.
Because damage to the cable can cause reflections, an instrument called an electricaltime-domain reflectometer (ETDR; for electrical cables) or anoptical time-domain reflectometer (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 forRF cables.
The combination of the effects of signal attenuation and impedance discontinuities on acommunications link is calledinsertion loss. Proper network operation depends on constantcharacteristic impedance in all cables and connectors, with no impedance discontinuities in the entire cable system. When a sufficient degree ofimpedance matching is not practical,echo suppressors orecho cancellers, or both, can sometimes reduce the problems.
TheBergeron diagram method, valid for both linear and non-linear models, evaluates the reflection's effects in an electric line.