The PHY Data Rate is based upon the pings that the AirCheck G2 performs during the connection test. This is to report the rate of data traffic which can be at 802.11n or 802.11ac rates, and not the rate of management and control traffic which will be at legacy rates. If, upon completion of the connection test, the AirCheck G2 roams to another AP or otherwise disconnects from the initial AP, it will not send additional pings and therefore, the PHY Data Rate is no longer reported. However, if you perform a roaming test, the AirCheck G2 will send continuous pings even after roaming, and therefore will continue to report a PHY Data Rate unless it disconnects altogether.