Join Brad Wheeler for an in-depth discussion in this video Determining signal strength with a site survey, part of Extending and Optimizing a Wi-Fi Network for Small Businesses.
- View Offline
- Measuring the signal strength of a wireless network is something you already do at a basic level. Your phone or mobile device already has an obvious, if simple measure of signal strength built in, whether it's in the form of bars or some pseudo-qualitative measure like excellent or poor. When I say signal strength, what I mean is the amplitude of the access point's wireless signal that is being received by the device. First, let's take a closer look at what signal amplitude means. Signal amplitude refers to the energy of a radio signal, usually expressed in decibel milliwatts or dBm.
dBm is is directly related to wattage. A zero dBm signal has a power level of one milliwatt. dBm refers to the power of any radio signal, whether FM tower, GPS satellite, or Wi-Fi device. FM radio stations transmit at 80 dBm, whereas GPS satellites transmit at an amazingly low 127 dBm. These power levels refer to the broadcast power. In a Wi-Fi site survey, we're concerned with the strength of the signal that your device is receiving, not the power of the access point is broadcasting.
Your device may refer to the signal its receiving as signal strength or RSSI, Received Signal Strength Indication. In terms of Wi-Fi, dBm is always expressed as a negative value, from approximately negative 10 dBm, at best, to negative 100 dBm, at worst. In a real life scenario, a device is unlikely to get better than negative 35 dBm signal strength, and will start to suffer poor performance around negative 70 dBm. If the signal strength gets as low as negative 85 to negative 90 dBm, the device is likely to disconnect.
A higher RSSI is correlated with higher throughput, but it's not a linear relationship. Second, it is important to consider the device that is receiving this signal. Different devices will report different signal strength in identical circumstances. This depends on the number and size of antennas, Wi-Fi chipset, device construction, etc. I recommend using a mobile device when performing a signal strength site survey, since it's something of a worst case scenario.
If you can connect usefully to a network with a smartphone, you're likely to be able to connect with anything else. There are a large number of free apps to determine a device's RSSI. If you have access to an Android device, I recommend the WiFi Overview 360 app. It provides an RSSI reading as well as a nice graph of RSSI over time, as well as some additional details of the access point you're connected to. To perform the site survey, simply stand in a fixed location, orient the device so that its back is facing the access point, and hold it still for a moment.
The RSSI reading should stabilize. Record this value on your floor plan or table and then move to the next location and repeat. Once you've mapped your entire environment, file the site survey for comparison next time.
Need a more substantial solution? Check out the last chapter on hardware and special security measures for enterprise networks.
- Measuring performance
- Diagnosing problems
- Selecting channel and bandwidth
- Upgrading firmware
- Setting up wireless isolation and multiple SSIDs
- Using MoCA and powerline network extenders
- Exploring enterprise solutions