RF Basic Background: Signal Strength and SNR

03/26/2020 916 People found this article helpful 485,893 Views

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Signal strength is one of an important factor needs to be taken into account when deploying wireless network. Signal attenuation or signal loss occurs as the signal passes through air and also affected by other factors, including natural and artificial phenomena and structures (such as walls, wireless interference, device power output, and even ambient humidity and temperature).

Object	Plasterboard wall	Class wall with metal frame	Cinder block wall	Office window	Metal door	Metal door in brick wall	Human body
Signal Attenuation	3dB	6dB	4dB	3dB	6dB	12dB	3dB

Signal attenuation after passes through physical objects

Although sometimes it's elusive, fortunately, signal strength can be measured. So far, different wireless manufacturer may use different measurements to indicate signal strength, such as mW (milliwatts), dBm("dB"-milliwatts), RSSI(Receive Signal Strength Indicator), or percentage, among which percentage is more popular used. All of these measurements are related to each other.

The "dBm" values can be directly converted to mW values. (100mW = 10*log100 =20dBm, 1mW = 0dBm, 25mW = -6dBm?0.0000009mW=-60dBm)
The RSSI value is an integer with an allowable range of 0-255 defined by IEEE. However, most vendors do not really use the whole range, which is depended on the chip manufacturers (e.g. SonicWall uses scale of 1 to 100, 100 is the RSSI_MAX value).
The percentage value represents the RSSI value divided by the RSSI_Max value and multiplied by 100. For example, RSSI_MAX is 100, a 50% signal strengthwill convert to the RSSI value 50.

SNR (Signal to Noise Radio) is actrually not a ratio but a decibel (dB) value measures the difference between the signal strength and the background noise. For instance, the signal strength is -56 dBm, noise is -86 dBm, the SNR is 30dB. SNR is also an vital factor to be considered during deployment. The higher SNR client receives usually means the better perfomance of the communication.

RF signal strength and wireless clients

RF signal strength and APs

Matching client and AP power

RF signal strength and wireless clients

Generally wireless client needs to be perceived at least −60 dBm or better in order to ensure the data rate of 11 Mbps and above. In fact, required signal strength varies with the modulation and channel bandwith. For instance, to reach the top rate (72 Mbps) of 802.11n in a 20 MHz channel, wireless client needs at least -64 dBm signal strength, the requirement rises to -59 dBm for the top rate (86 Mbps) of 802.11ac, single-stream in a 20 MHz channel, and needs -49 dBm for the top rate (866 Mbps) in a 160 MHz channel.

RF signal strength and APs

When deploy APs, a minimum of two APs with the recieved signal strength greater than 35 dBm at all times are required on non-overlapping channels, but do not deploy more than one AP with the recieved signal strength greater than 35 dBm on per overlapping channel. In another words, for each overlapping channel, the signal strength of deployed each AP should be less than 35 dBm ( Although the signal strength less than 35 dBm on overlapping channels, this situation can still cause interference. Hence, minimize this situation as much as possible. This interference or noise degrades voice quality. ).

Note: Noise is additive. If you have three extra APs on the same channel, all with low RSSIs, the situation can be as harmful as a single extra AP with a higher RSSI.

When deploying APs on the same channel, it is recommended to do as following:

Maintain an AP coverage overlap of less than15 percent to 20 percent.
Maintain a minimum signal−to−noise ratio (SNR) of 25 dB.

This above illustration shows a typical deployment, with a 15% to 20% overlap of a given AP cell from each of the adjoining cells. The -60 dBm for coverage boundary requirement is to minimize packet loss, and the -79 dBm is to minimize co-channel interference from other AP cells on the same channel.

Matching client and AP power

Wireless communication is not one-way, but a solid round-trip connection with equal performance (both to and from each client device) is required. Most of us do not explicitly consider the signal strength from the client back to the AP. Howerver? AP-client power mismatches sometimes will be a major cause of device connectivity problems for many clients. The reason is that client devices typically operate at lower transmitter power levels than APs. A symptom of this problem is having “5 bars” on the client device, but still having a a poor connection. The client can hear the AP but the AP cannot hear the client.

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