Describe four type of noiseWhich is likely to pose the greatest problem to network managers?

Answer 1

The following list summarizes the major sources of error. The first six are the most important; the last three are more common in analog rather that digital circuits.

Line outages are a catastrophic cause of errors and incomplete transmission. Occasionally, a communication circuit fails for a brief period. This type of failure may be caused by faulty telephone end office equipment, storms, loss of the carrier signal, and any other failure that causes a short circuit. When constructing and designing redundant networks that are fault survivable, this is usually called designing for the "farmer with a back hoe" problem.

White noise or gaussian noise (the familiar background hiss or static on radios and telephones) is caused by the thermal agitation of electrons and therefore is inescapable. Even if the equipment was perfect and the wires were perfectly insulated from any and all external interference, there still would be some white noise. White noise usually is not a problem unless it becomes so strong that it obliterates the transmission. In this case, the strength of the electrical signal is increased so it overpowers the white noise; in technical terms, we increase the signal to noise ratio.

Impulse noise (sometimes called spikes) is the primary source of errors in data communications. Some of the sources of impulse noise are voltage changes in adjacent lines, lightning flashes during thunderstorms, fluorescent lights, and poor connections in circuits.

Cross-talk occurs when one circuit picks up signals in another. It occurs between pairs of wires that are carrying separate signals, in multiplexed links carrying many discrete signals, or in microwave links in which one antenna picks up a minute reflection from another antenna. Cross-talk between lines increases with increased communication distance, increased proximity of the two wires, increased signal strength, and higher frequency signals. Wet or damp weather can also increase cross-talk. Like white noise, cross-talk has such a low signal strength that it normally is not bothersome.

Echoes can cause errors. Echoes are caused by poor connections that cause the signal to reflect back to the transmitting equipment. If the strength of the echo is strong enough to be detected, it causes errors. Echoes, like cross-talk and white noise, have such a low signal strength that they normally are not bothersome. In networks, echo suppressors are devices that reduce the potential for this type of error. Echoes can also occur in fiber optic cables when connections between cables are not properly aligned.

Attenuation is the loss of power a signal suffers as it travels from the transmitting computer to the receiving computer. Some power is absorbed by the medium or is lost before it reaches the receiver. This power loss is a function of the transmission method and circuit medium. High frequencies lose power more rapidly than low frequencies during transmission, so the received signal can thus be distorted by unequal loss of its component frequencies. Attenuation increases as frequency increases or as the diameter of the wire decreases, or as the distance of the transmission increases. Repeaters can be used in a digital environment to correct for attenuation due to distance, where amplifiers can be used to boost diminishing or attenuating analog signals over longer distances. A repeater will perfectly replicate the incoming, distorted digital signal and send it on deeper into the network as if new. An amplifier will boost an attenuating analog signal, but also boost the error noise in the signal as it does so. Fewer repeaters are necessary as compared to amplifiers to correct for attenuation, thus helping to make digital more cost effective when compared to analog transmission in controlling for noise.

Intermodulation noise is a special type of cross-talk. The signals from two circuits combine to form a new signal that falls into a frequency band reserved for another signal. On a multiplexed line, many different signals are amplified together, and slight variations in the adjustment of the equipment can cause intermodulation noise. A maladjusted modem may transmit a strong frequency tone when not transmitting data, thus producing this type of noise.

Jitter may affect the accuracy of the data being transmitted because minute variations in amplitude, phase, and frequency always occur. The generation of a pure carrier signal in an analog circuit is impossible. The signal may be impaired by continuous and rapid gain and/or phase changes. This jitter may be random or periodic.

Harmonic distortion usually is caused by an amplifier on a circuit that does not correctly represent its output with what was delivered to it on the input side.

Phase hits are short-term shifts "out of phase," with the possibility of a shift back into phase.