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Echo occurs within transmission channels. There are two types of echo. Talker echo, which occurs when the speaker’s electrical speech signal is reflected back along the transmission line to the speaker, and listener echo, which is caused by a current circulating in a transmission path. Echo Return Loss (ERL) is a type of measurement used to determine the amount of echo. The decibel level is taken along two or more points in the transmission line. Signal Return Loss occurs when there is an imbalance on both hybrid circuits and echo cycles, over and over until the amplifiers are over-driven.

To correct echo for a short-haul carrier facility, a compensation circuit is built into the hybrid device. This circuit is adjusted to balance the echo. Echo encountered on long-haul carrier facilities is countered by two types of devices namely, Echo suppressers and Echo cancellers. Echo suppressers are voice-activated devices that do not affect the speaker’s transmission. However, they reduce the level of transmission moving towards the speaker by about 35 dB. This prevents people from hearing their own voice as echo. Echo cancellers are used at the receiving end of a long-haul carrier facility (usually satellite). The echo canceller constantly monitors the incoming signal and produces a negative image signal of amplitude equal to the echo, thus canceling the echo.

There are two impairments associated with lightwave signals namely Attenuation and Distortion. Attenuation, or signal loss, in a fibre occurs because of absorption of the signal. Absorption may occur in three ways - Direct absorption occurs when some impurity in the core of the fibre stops a part of the signal. Secondly, Scattering occurs when part of the signal strikes an impurity in t.he core. It is then reflected into the cladding and absorbed. Finally, Refractive loss occurs when part of the signal strikes the boundary between the core and the cladding. It is then refracted, or bent, into the cladding and absorbed.

Distortion occurs when a part of the signal strikes the core/cladding boundary and is reflected back into the core. The reflected signal has farther to travel, and so arrives at the end of the fibre slightly later than the direct signal. One has to understand that Impairments are always there. They limit the capabilities of virtually every transmission system. The received signal always differs to some degree from the transmitted signal. When the differences are not acceptable, impairments need to be reduced.