When data is transmitted as a signal, due to various factors, the signal gets distorted or corrupted. It leads to errors in the received data, which can cause problems in communication systems. The errors must be detected and corrected.
In OSI model, the error detection and correction is done at the transport and data link layers. In the Internet model, the error detection and correction is done at the network layer. In this module, error detection and correction in data link layer is focused more.
Noise
Always present in real life. Can be caused by various factors such as electromagnetic interference, thermal noise, and atmospheric noise.
White Noise
Aka. thermal noise, gaussian noise. Continuous, random noise spread evenly across all frequencies. A type of random noise that has equal intensity at different frequencies, giving it a constant power spectral density. Relatively constant. Can be reduced by using higher signal power or better shielding. If gets too strong, can disrupt the communication.
Impulse Noise
Sudden, short-duration spikes in the signal. One of the most disruptive forms of noise. Can destroy 1 or more bits of information. Difficult to remove from an analog signal because it may be hard to distinguish from the original signal. Can damage more bits if the bits are closer together or transmitted at a faster rate.
Jitter
When received signal arrives at irregular intervals. Does not cause data loss. But can cause problems in real-time communications (like video calls, online gaming).
Caused by:
- Network congestion
- EM interference
Can be reduced by either:
- Jitter buffers: temporarily storing incoming data packets to ensure a steady stream of data to the receiver.
- Shielding: signal carrying cables are protected from EM or radio frequency interference.
Crosstalk
Unwanted coupling between two different signal paths. Relatively constant and can be reduced.
Echo
A delayed repetition of the original signal. The reflective feedback of a transmitted signal as the signal moves through a medium. Most often occurs on coaxial cable. If bad enough, it could interfere with original signal. Relatively constant. Can be significantly reduced.
Delay distortion
Occurs because the velocity of propagation of a signal through a medium varies with the frequency of the signal. Can be reduced using equalizers.
Attenuation
The reduction in signal strength as it travels through a medium. If the signal gets too weak, the receiver can’t detect the bits. Can be reduced using less lossy mediums, or amplifiers.
Distortion
As signal travels, it become distorted (changes shape). Successive bits may merge, making reception difficult.
Interference
Unwanted signal from outside sources. Often intermittent, difficult to diagnose.