Which of the following types is not an example of noise in the communication channel?

Electronic noise

IT staff mainly deals with electronic noise, created in the radio or network systems that transmit data, or in the medium -- such as wire and air -- through which signals are transmitted.

Thermal noise

Thermal noise occurs in all transmission media and communication equipment, including passive devices. It arises from random electron motion and is characterized by a uniform distribution of energy over the frequency spectrum with a Gaussian distribution of levels; the higher the temperature of the components or the medium, the greater the level of thermal noise.

Intermodulation noise

Intermodulation (IM) effects result when two or more signals pass through a nonlinear device or medium and interact with each other in ways that produce additional signals, such as harmonics and subharmonics of input signal frequencies. These resulting IM components may be inside or outside the frequency band of interest for a particular device; it is only when they are inside the band of interest that IM effects become IM noise.

IM noise is a significant concern in radio communications, including cellular telephony and data networks. Component manufacturers test for it -- it can result from either design characteristics of a system, or from manufacturing defects. But it can become an issue post-manufacture due to damage to a component, so field engineers need to watch for it, too.

Cross-talk

Cross-talk refers to signals interfering with each other electromagnetically. There are essentially three causes of cross-talk:

• Electrical coupling between transmission media, like adjacent wires in a multilane serial interface connection -- for example, Ethernet or Fibre Channel;
• Poor control of frequency response -- i.e., defective filters or poor filter design; and
• Nonlinear performance in analog multiplex systems. High levels of cross-talk increase bit-error rates and degrade a digital path's performance.

Impulse noise

Impulse noise is a noncontinuous series of irregular pulses or noise spikes of short duration, broad spectral density and of relatively high amplitude. Impulse noise can be caused by positioning a communications cable near a source of intermittent but strong electromagnetic pulses, such as an elevator motor. It degrades telephony only marginally, if at all, but can seriously corrupt data transmissions.

Shot noise 

Shot noise, also called quantum noise, is the variation in a signal that is caused by the quantized nature of the light and electricity making up the signal. We tend to think of a signal, whether a beam of light or a stream of electrons, as being uniform: a steady stream of particles traversing a path. The physical reality, though, is not one of uniform and constant movement, but of clumpy movement that only looks smooth on average across long, large flows of light or electricity -- as measured by intensity of light or by electrical current density.

Shot noise has become a major concern, as circuits get smaller and faster, reducing the time over which flows can be averaged down to -- and past -- the nanosecond level, and current flows down to a nanoampere or less. Chip and system designs increasingly need to account for shot noise, as the drive to shrink components and increase component speeds continues.

Transit-time noise

Transit-time noise is a similar phenomenon to shot noise in that it affects systems more as they get smaller due to the quantized nature of electricity. Transit-time noise results when a signal frequency's period is the same as the time an electron takes to travel from sender to receiver. The noise results from the statistical variations in actual electron flow.

Acoustic noise

All these types of electronic noise above are distinct from acoustic noise, which encompasses sounds in an environment, including:

• Continuous noise that's steady in tone and volume, such as noise created by some machinery in industrial environments, like conveyor belts or worm gears moving materials along a production line; and retail environments, such as vent fans in a restaurant kitchen; as well as by things like fluorescent lighting -- the 60-cycle hum -- and air-conditioning in all kinds of environments.
• Low-frequency noise, also called infrasound, which is below the range of sounds normally audible to humans -- i.e., at or below about 20 hertz -- but which can be very disturbing to many people. Infrasound can be generated by machinery, and even by the vibration of buildings in response to wind or other forces.
• Workplace noise, variable in volume and tone, such as what's typically heard in the background in call centers and open-plan offices, or is experienced by workers in factories, kitchens and other environments -- often in addition to continuous noise.

As far as networks are concerned, these types of noise are part of the signal -- the sound being transmitted -- since they are not artifacts of the technology used to transmit the sound.

Acoustic noise is mitigated through a combination of workplace design principles, like  breaking up large open offices; and furnishings, such as using sound-damping coverings on walls and incorporating plantings in a space; work practices, like staggering shift breaks in call centers, for instance; and human interface technologies, such as noise-cancelling headphones, which help people lower their voices, and directional microphones, which pick up less background noise.