What is dBm or mW?
Short answer: dBm is decibels relative to 1 milliwatt and mW is a milliwatt (one thousandth of a watt). The next obvious question is: what are watts and decibels? A watt is a unit of power named after the Scottish engineer James Watt that is used to express the rate of energy conversion or transfer with respect to time. It can be calculated relative to an object’s velocity as Joules per Second (J/S) or in terms of electromagnetism as the rate at which work is done when one Ampere of current flows through an electrical potential difference (Volt), aka Watt = Volts x Amperes. A decibel is a logarithmic unit used to express a ratio between two values. The decibel is used to conveniently represent very large or small values and allow for simple mathematical calculations that would otherwise be difficult for most humans. For the sake of saving time and not derailing the purpose of this post, I recommend checking out WLAN Pros’ Easy dB Math in 5 Minutes to understand how to work with dBms.
How is signal strength displayed in Ekahau Site Survey (ESS)?
In ESS the signal strength visualization displays the signal strength of the selected set of Access Points (APs) in dBm. By default, the signal strength of the strongest AP per location will be displayed. To display the signal strength for just one or few access points, select “Show Signal Strength for Selected Access Points” and click to select APs from the map or from the AP list as shown in the screenshot below. More about the AP selections on our next blog post.
Signal strength has the following options accessible via the Visualization Options:
- Viewed Signal allows you to disregard the signals from the strongest access points, visualizing the second or third strongest signals instead of the strongest one. This is useful for visualizing AP failover scenarios and redundancy.
- Signals at Channel limits the signal strength visualization to only show the signals at a selected channel.
- Adapter selection allows you to visualize the signal strength as the selected adapter would “see” it. For example, the signal strength may be sufficient using a high-quality network adapter, but may not be sufficient using a wireless VoIP phone with different antenna characteristics.
Below is an example of a signal strength visualization:
- Signal strength does not meet the requirement. The signal strength is below -65 dBm and the visualization is greyed-out.
- High signal strength.
- Low signal strength. The signal strength is below the cutoff value (-75 dBm) and the visualization is not shown.
- Click or slide to adjust the signal strength visualization requirements, range, spacing, and coloring. Right-click for more options.
Why is signal strength important for a Wi-Fi network?
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.
What signal strength should I design my network to?
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, where can you determine what your signal strength requirements should be? You first need to 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. Once you know what data rate you would like to design towards, you then need to find the corresponding 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. Often, 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. For instance, 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.
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. Many vendors have design guides that can help define requirements based on the network needs. This is often an iterative process of determining network needs and defining signal strength, SNR, and other parameter requirements.
To be continued… next time we’ll look at how to select APs used for visualizations, and then we’ll jump into Signal to Noise Ratio (SNR).
Stay warm and best regards!