## Sources of link degradation

### Distortion #### The common types of link distortion are: #### frequency-dependent phase shifts, giving rise to differential group delay #### gain variations with frequency caused by the channel filtering effect #### gain variations with time as experienced in a radio/infra red channel #### frequency offsets between transmitter and receiver due to Doppler Shift or local oscillator errors. |

#### Distortion can be introduced within the transmitter, the receiver and the channel. In some cases it can be corrected using channel equalizers, and gain and frequency control systems. Unlike noise and interference, distortion disappears when the signal is turned off.