Tài liệu Analog Transmission of Digital Data (Broadband Transmission)

There are many occasions when data need to be transmitted over a voice communication network.
Many people use a computer to connect to their Internet Service Provider via the telephone lines to transmit and receive data, check their email, surf the Web, etc.
Telephone networks were originally built for human speech, not for computer data and is unsuitable for digital communication in its raw form.



The transmission impairments and the bandwidth limitation makes digital signalling (have a wide spectrum) unsuitable for transmission over the local loop (designed for analog transmission).

Transmission Impairments:

Transmission lines suffer from three major problems:
Attenuation - is the loss of energy as the signal propagates outward. The amount of energy lost depends on the frequency. Each Fourier component is attenuated by a different amount.
Delay distortion - It is caused by the fact that different Fourier components travel at different speeds. For digital data, fast components from one bit may catch up and overtake slow components from the bit ahead, mixing the two bits and increasing the probability of incorrect reception.
Noise - unwanted energy from the sources other than the transmitter. Thermal noise, cross talk, impulsive noise.