Category Archives: RSSI

How to read RSSI/signal and SNR/noise ratings ?


Source: Speed Guide

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Signal (a.k.a. RSSI) is the usable strength of the radio waves, expressed in decibels between 0db (strongest) and -120db (weakest). Smaller negative numbers represent a cleaner/stronger signal.

For wireless data communications,

  • Normal range is -45db to -87db.
  • Anything below -85db is generally unusable, and
  • Over -50db can be considered perfect.

Notes: 

  • 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 -100db and -80db.

  • 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 (dB, 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) = -55db, and noise = -85db, then:
(-55db signal) – (-85db 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 dB 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.

Notes:

  • 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.

How does RSSI (dBm) relate to signal quality (percent) ?


Source: Speed Guide

WiFi-300x249

Depending on your OS and application, WiFi signal strength is represented either as quality in percentage, or an RSSI value in dBm, i.e. -70db. RSSI is usually expressed in decibels from 0 (zero) to -120db and the closer it is to zero, the stronger the signal is. RSSI level less than -80db may not be usable, depending on noise.

While there is no simple precise solution that is used universally, we will try to explain the approximate correlation between signal (RSSI) and quality (percentage).

Generally, 

  • db >= -50 db = 100% quality
  • db <= -100 db = 0% quality

For RSSI signal between -50db and -100db,

  • quality ~= 2* (db + 100)
  • RSSI ~= (percentage / 2) – 100

For example:

  • High quality: 90% ~= -55db
  • Medium quality: 50% ~= -75db
  • Low quality: 30% ~= -85db
  • Unusable quality: 8% ~= -96db

 

Received Signal Strength Indication | RSSI


From Wikipedia, the free encyclopedia

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Related Posts:

In telecommunications, Received Signal Strength Indicator (RSSI) is a measurement of the power present in a received radio signal

RSSI is usually invisible to a user of a receiving device. However, because signal strength can vary greatly and affect functionality in wireless networking, IEEE 802.11 devices often make the measurement available to users.

RSSI is often derived in the intermediate frequency (IF) stage before the IF amplifier. In zero-IF systems, it is derived in the baseband signal chain, before the baseband amplifier. RSSI output is often a DC analog level. It can also be sampled by an internal ADC and the resulting codes available directly or via peripheral or internal processor bus.

In 802.11 Implementations 


In an IEEE 802.11 system, RSSI is the relative received signal strength in a wireless environment, in arbitrary units. RSSI is an indication of the power level being received by the receive radio after the antenna and possible cable loss. Therefore, the higher the RSSI number, the stronger the signal. Thus, when an RSSI value is represented in a negative form (e.g. −100), the closer the value is to 0, the stronger the received signal has been.

RSSI can be used internally in a wireless networking card to determine when the amount of radio energy in the channel is below a certain threshold at which point the network card is clear to send (CTS). Once the card is clear to send, a packet of information can be sent. The end-user will likely observe a RSSI value when measuring the signal strength of a wireless network through the use of a wireless network monitoring tool like Wireshark, Kismet or Inssider. As an example, Cisco Systems cards have an RSSI maximum value of 100 and will report 101 different power levels, where the RSSI value is 0 to 100. Another popular Wi-Fi chipset is made by Atheros. An Atheros-based card will return an RSSI value of 0 to 127 (0x7f) with 128 (0x80) indicating an invalid value.

There is no standardized relationship of any particular physical parameter to the RSSI reading. The 802.11 standard does not define any relationship between RSSI value and power level in milliwatts or decibels referenced to one milliwatt. Vendors and chipset makers provide their own accuracy, granularity, and range for the actual power (measured as milliwatts or decibels) and their range of RSSI values (from 0 to RSSI maximum).  One subtlety of the 802.11 RSSI metric comes from how it is sampled—RSSI is acquired during only the preamble stage of receiving an 802.11 frame, not over the full frame. 

As early as 2000, researchers were able to use RSSI for coarse-grained location estimates.  More recent work was able to reproduce these results using more advanced techniques.  Nevertheless, RSSI does not always provide measurements that are sufficiently accurate to properly determine the location. 

Received channel power indicator 

For the most part, 802.11 RSSI has been replaced with received channel power indicator (RCPI). RCPI is an 802.11[3] measure of the received radio frequency power in a selected channel over the preamble and the entire received frame, and has defined absolute levels of accuracy and resolution. RCPI is exclusively associated with 802.11 and as such has some accuracy and resolution enforced on it through IEEE 802.11k-2008. Received signal power level assessment is a necessary step in establishing a link for communication between wireless nodes. However, a power level metric like RCPI generally cannot comment on the quality of the link like other metrics such as travel time measurement (time of arrival).