Signal Strength - What everyone is looking for
The Holy Grail of Wi-Fi parameters, signal strength is considered as one of the most important design goal to meet. With the proliferation of high density deployments and higher data rates, other design parameters have grown in importance. You still always need a signal to transmit though.
What does signal strength mean?
Often mistakenly referred to as the Received Signal Strength Indicator (RSSI) value, which is an indication of the power level at the receiving antenna, aka the Wi-Fi client device.
While the RSSI can give you an indication of how strong a signal is relativistically, it does not provide an absolute measurement. There is no standardized method of determining RSSI between Wi-Fi NIC (Network Interface Card) vendors, leading to each providing different RSSI values for the same environment.
When engineers refer to power levels in dBm or mW, it is the actual measured signal strength and not a relative calculation when referring to RSSI.
Why is signal strength important?
Generally speaking, a higher signal strength will result in higher data rates. For any given receiver, higher power levels are required to support higher data rates.
The reason for this is that different data rates use different modulation techniques and encoding; as the data rates get higher the encoding is more susceptible to corruption.
Of course, there are also other factors that determine the data rate, such as Signal to Noise Ratio (SNR), interference, client Wi-Fi adapter compatibility (i.e. 802.11a/b/g/n/ac), and more.
How do I determine what signal strength I need?
It depends. Seriously, there is no absolute correct power level that meets the needs of all networks. A VoIP deployment will differ from basic connectivity coverage, which in turn will differ from a dense high speed environment.
So, how can you determine what your signal strength requirements should be?
1. Determine what data rate is required given the client application needs.
For example, HD video streaming will require higher data rates than typical web browsing.
2. Find the signal strength required to operate at that data rate.
To find the signal strength value required for a given data rate, a good place to start is the vendor-specific data sheets/design guides. Vendors list the signal strength and SNR required for a given data rate or Modulation and Coding Scheme (MCS) index in such documents, which may be posted on their website.
If they are not listed, you can try to ask one of their representatives for recommendations. Each vendor has their own recommended signal strength for a given data rate or application.
One vendor may recommend designing their VoIP solution at -67 dBm (perhaps the most widely-used value for VoIP deployment if I had to choose one), while another may say -70 dBm. In most cases the values will differ only by a couple dB.
What signal strength should I design my network to?
A common practice is to design for signal strengths in the -65 to -70 dBm range with two or more APs overlapping throughout the coverage area.
However, please note that every design is different and you need to consider capacity (number of clients), applications (HD video, Audio streaming), use cases (VoIP vs. Data), link quality, and more when determining a proper signal strength threshold.
This is often an iterative process of determining network needs and defining signal strength, SNR, and other parameter requirements.