How to read RSSI/signal and SNR/noise ratings ?
Signal (a.k.a. RSSI) is the usable strength of the radio waves, expressed in decibels relative to a milliwatt between 0dBm (strongest) and -120dBm (weakest). Smaller negative numbers represent a cleaner/stronger signal. For wireless data communications, normal range is -45dBm to -87dBm. Anything below -85dBm is generally unusable, and over -50dBm can be considered perfect.
RSSI values can be different depending on the chip vendor. Cisco can use range between 0 to -100, Atheros may use 0 to -127.
EIRP (Effective Isotropic Radiated Power) is the actual amount of signal leaving the antenna, measured in dB. EIRP = Tx power (dBm) + antenna gain (dBi) - cable loss (dB).
Noise (dBm) in wireless communications is a combination of all unwanted interfering signal sources, such as crosstalk, radio frequency interference, distortion, etc. This value is measured in decibels from zero to -120. The closer this value is to -120, the better, because that means there is little to no interference. Typical environments range between -100dBm and -80dBm.
Signal-to-noise ratio (SNR) is defined as the power ratio between a signal (meaningful information) and the background noise (unwanted signal): SNR = signal / noise
The more commonly used SNR margin, as described below is sometimes abbreviated as simply SNR as well.
SNR margin (dBm, a.k.a. noise margin) is the ratio by which the signal exceeds the minimum acceptable amount (minimum SNR) to sustain a certain speed. It is normally measured in decibels. SNR margin is often confused and used interchangeably with SNR. Many DSL modems and wireless devices (notably dd-wrt open source router firmware) use SNR margin, only denoted as "SNR". SNR margin is simply calculating the difference between signal (RSSI) and noise to get the SNR margin as a positive number expressed in db.
SNR margin = signal(dBm) - noise(dBm)
For example, if singal (RSSI) = -55dBm, and noise = -85dBm, then:
(-55dBm signal) - (-85dBm noise) = 30 SNR margin
Higher SNR/SNR margin numbers represent cleaner signals, with less noise. Utilizing full 54 Mbps data rate, for example requires at least 25 dBm of SNR margin.
Signal Quality this is a percent value between 0% and 100%, with the higher numbers representing better link quality. It is the percentage of the best theoretical ideal quality in regards to your local noise. It can be calculated differently, depending on the OS/device used. It is based on signal strength and SNR margin. Generally, signal quality above 25-30% is usable.
See also: How does RSSI relate to signal quality ?
The TX(transmit) power of a device, as well as antennas attached to it both factor into the signal level.
Some devices can display both the actual SNR, and the SNR margin as a separate value.