TIA's Glossary of Telecommunication Terms
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transfer function
1. A mathematical statement that describes the transfer characteristics of a system, subsystem, or equipment. 2. The relationship between the input and the output of a system, subsystem, or equipment in terms of the transfer characteristics. Note 1: When the transfer function operates on the input, the output is obtained. Given any two of these three entities, the third can be obtained. Note 2: Examples of simple transfer functions are voltage gains, reflection coefficients, transmission coefficients, and efficiency ratios. An example of a complex transfer function is envelope delay distortion. Note 3: For a negative feedback circuit, the transfer function, T , is given by 
where e o is the output, e i is the input, G is the forward gain, and H is the backward gain, i.e., the fraction of the output that is fed back and combined with the input in a subtracter. 3 . Of an optical fiber, the complex mathematical function that expresses the ratio of the variation, as a function of modulation frequency, of the instantaneous power of the optical signal at the output of the fiber, to the instantaneous power of the optical signal that is launched into the fiber. Note: The optical detectors used in communication applications are square-law devices. Their output current is proportional to the input optical power. Because electrical power is proportional to current, when the optical power input drops by one-half (3 dB), the electrical power at the output of the detector drops by three-quarters (6 dB). [FAA]