Transmission of voice, data and image information into an electrical signal, modulated, and moved to the spectrum suitable for a medium transport band, and some facilitate transmission of electromagnetic waves to each other, and then restored by demodulation signal, which includes the modulation-demodulation process communication-transmission equipment combined. Transport systems as a channel connecting the two terminals constitute a telecommunication system, as the link you can connect network nodes switching systems telecommunications networks.
Transmission system in the process of transmitting signals, inevitably introduced some of the factors contributing to the deterioration of signal quality, such as attenuation, crosstalk, interference, noise, distortion, fading, etc. In order to continuously improve the transmission quality, expand capacity, and get technical and economic performance, transmission technologies must constantly develop and improve. Transmission system development mainly in the transmission medium for development and progress as the symbol of modulation, transmission quality and system capacity, affordability, adaptability, reliability, maintainability, as measured in terms of. Work to extend the absolute frequency bandwidth and compression modulated signal bandwidth to improve spectrum utilization, are effective means to expand transmission capacity.