Signal-to-Noise Ratio

By Lee Copeland

January 15, 2001 12:00 PM ET

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Computerworld - Noise is the real enemy of data communications. Whenever electrons travel over a wire or radiate through space, they generate some electromagnetic noise. Therefore, whenever a data signal is sent across a transmission channel, such as a copper wire or a radio-frequency broadcast, background electromagnetic interference (EMI), or noise, accompanies that signal.

			S/N: It's Not Just for Electronics Although the term S/N originated in the field of electrical engineering as a specific, quantitative measure, the concept itself properly applies to any method of communication, electronic or otherwise. For example, smoke signals can be an effective means of long-distance communication for people who don't have electricity - as long as there isn't much surrounding environmental "noise," such as fog or rain. Or consider a large cocktail party, where dozens of people may be talking at once. If you want to communicate with someone else, you have to move close enough that your voice (the signal) can be heard above the din of everyone else's chatter (the noise). In other words, you have to achieve a sufficiently high S/N. Finally, look at the thousands of Internet newsgroups known collectively as the Usenet. One frequent comment I've heard from longtime Internet users (and indeed made myself on numerous occasions) - especially about many of the REC (recreational) or ALT (alternative, meaning totally unregulated) groups - is that "there isn't a high enough signal-to-noise ratio." In other words, too many people are posting messages with too little content; this is often the case when "flame wars" break out on a particular topic.

Signal-to-noise ratio (S/N) measures the amount of unwanted electromagnetic noise relative to a signal's strength. If the background noise on a data channel is higher than the signal, it can cause a reduction in data speed or a disruption in system circuit functioning.

That's why airlines require passengers to shut off all electronic devices, such as cell phones and laptops, before takeoff. This is a precaution to ensure that no electrical noise from these devices will disrupt the airplane's navigational, radio or fly-by-wire systems - admittedly a slight possibility.

Getting Wired

In the wired world, S/N issues are relatively easy to manage because of the closed nature of the environment. Take a typical corporate network, for example, where low-level noise from radios and other devices can cause interference. An Ethernet system running at 10 MHz would include a noise filter at the receiving end to eliminate unwanted signals above 10 MHz. Another method for increasing the S/N ratio is to amplify the transmission signal voltage.

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