Wireless Signal Basics Defined
What are some of the wireless signal basics?
- dB - Relative measure, or ratio, between two different power levels. 3dB is twice (or half) as much, 6dB is four times, 10dB is ten times, etc.
- dBm - A measure of signal strength. 0dBm = 1 milliwatt
- dBi - A measure of antenna gain compared to a perfect, isotropic antenna. “Isotropic” means “uniform in all directions”. So if this isotropic antenna existed, it would project a perfect sphere of signal. See “Antenna Gain” below.
Signal strength is measured on a logarithmic scale:
30 = 1000 (the same as 1 Watt)
20 = 100 (for 802.11 this is more or less the maximum power at the antenna connector)
10 = 10
0 = 1
-10 = .1
-20 = .01
-30 = .001
-40 = .0001
In North America, the defined maximum power output of IEEE 802.11 at the antenna connector is about 100 mW, or 20dBm. This maximum power varies a little based on the frequency, and it produces signal roughly in the shape a globe or sphere. However, antennae are allowed to play tricks with that power by changing the shape of the signal and increasing the perceived power - up to a maximum of 4 Watts. This is called “gain”.
Gain does not increase signal power, it just focuses the signal and increases the perceived power and sensitivity within the new signal shape.
- Example: The standard 2 dBi gain omnidirectional antenna that ships with most Access Points slightly flattens the theoretical transmit globe into a torus, (or doughnut shape), and increases the effective signal from 20 dBm (100 mW) to 22 dBm (160 mW).
- Another example: A 15 dBi gain unidirectional Yagi antenna takes the 20 dBm (100 mW) and focuses it from a globe into a narrow cone. Inside that cone the strength of the signal has a 15 dBi gain, which would make it about 35 dBm, (about 4 Watts).
- In contrast, amplifiers can increase the power without narrowing the signal. However, amplifiers do not increase sensitivity, (the ability to hear signal). It is great for FM radio, but not so great for 802.11 connections.
RSSI - Received Signal Strength Indicator
The RSSI scale is not specified by the IEEE. In addition, each chipset manufacturer has its own method for measuring RSSI. Most HP MSM products use an Atheros chipset, which measures RSSI as the dB relative to the noise floor. For this calculation, the floor is assumed to be a constant: -95 dBm. So signal minus the noise constant = RSSI. Signal strength corresponds to the RSSI value. A higher value equals a stronger signal.
- For example, the signal strength of -60dBm yields an RSSI of 35dB. (-60dBm minus -95dBm = 35dB).
- This is roughly analogous to Signal to Noise Ratio (SNR), but can vary significantly if background noise is high.
SNR (Signal to Noise Ratio)
Signal to Noise Ratio is similar to Atheros' version of RSSI because it expresses the difference between signal strength and the intensity of noise in the area. However, it is calculated with the actual noise sampled within the environment.
- We want signal to be strong: -35 dBm would be great.
- We want noise to be weak: -95 dBm would be great.
- When calculated, these values yield an SNR of 60 dB.
> 40dB = Excellent signal (5 bars) always associated; really fast.
25dB to 40dB = Very good signal (3 - 4 bars) always associated; fast.
15dB to 25dB = Low signal (2 bars) always associated; moderate speed.
10dB - 15dB = very low signal (1 bar) mostly associated; pretty slow.
5dB to 10dB = no signal; (no bars) difficult to associate.