People keep quoting dB and SNR numbers at me – what do they mean?
dB Definition
dB stands for decibels and is a relative measure of two different power levels. However, whilst the hospitality industry often uses dB in its discussion, it is actually usually referring to dBm which is the measured power referenced to one milliwatt.
As an aside, the decibel scale is a logarithmic scale with the result that a 3 dB difference is actually twice the power and a 10 dB difference is ten times the power.
The distance that a signal can be transmitted depends on several factors which include among other things:
- Transmitter power
- Location of the two antenna
- How far apart the antenna are
- Whether there is line of sight or obstacles in the way
- Receiver sensitivity
When we undertake a site survey we are looking to see what receiver sensitivity is required (i.e. the minimum signal power level that is necessary for the receiver to accurately decode a given signal). Because dBm is compared with milliwatts, 0 dBm is a relative point (a bit like 0 Centigrade) and negative numbers mean that significantly less power is required for the signal to be decoded. The relative levels are as follows:
|
dBm |
mW |
|
0 |
1 |
|
-10 |
0.1 |
|
-20 |
0.01 |
|
-30 |
0.001 |
|
-40 |
0.0001 |
|
-50 |
0.00001 |
|
-60 |
0.000001 |
|
-70 |
0.0000001 |
How do dBm numbers translate into the real world?
The information below is suggested as a guideline and is not meant to be definitive:
|
Laptops |
Tablets & smartphones |
VoWiFi devices |
|
| – 72 dBm |
YES |
NO |
NO |
| – 65 dBm / – 67 dBm |
YES |
YES |
MAYBE |
| – 62 dBm |
YES |
YES |
YES |
What dBm should I require?
The hospitality industry industry standard is often -65 dBm and will work in the vast majority of guest facing situations. Some hoteliers prefer to standardise on -67 dBm.
SNR Definition
SNR stands for Signal to Noise Ratio and is used to compare the level of a desired signal to the level of background noise.
It is defined as the ratio of signal power to the noise power and can be expressed in decibels. A high number is therefore good. As you can imagine, if there is a lot of background noise, it is more difficult for a device to “hear” an access point.
How do SNR numbers translate into the real world?
The information below is suggested as a guide and is not meant to be definitive:
- > 40 dB SNR => excellent signal – device always associates to AP and data transfer is very fast
- 25 dB to 40 dB => good signal – device always associates to AP and data transfer is fast
- 15 dB to 25 dB => low signal – device always associates to AP and data transfer is usually OK
- 10 dB to 15 dB => very low signal – device generally associates to AP but data transfer is slow
- 5 dB to 10 dB => no signal – device won’t associate to AP and hence no data is transferred
What SNR should I require?
The hospitality industry standard for SNR is 20 dB and works in most guest facing situations. However, as an alternative example Cisco recommends 25 dB for their wireless voice telephony systems.
What next?
If you or your team would like either technical training or non technical explanations of what the implications of WiFi for your location are then please contact Veridicum.