When used together Received Signal Strength Indicator (RSSI) and Fast Roaming options help ensure fast, strong Wi-Fi connections. Use RSSI to ensure that users always experience a strong signal to their Access Point (AP). Use Fast Roaming to speed up security connections when you are using WPA2 Enterprise Security.
RSSI measures the strength of a radio signal. Any RSSI value lower than -80 dBm is considered poor signal strength. Based on the client implementation some clients consider -75 dBm as poor strength as well, and will start roaming to a better Access Point, so values in the range -70 to -80 dBm are client dependent.
For example, on IOS devices, clients maintain a connection until the RSSI hits a value lower than -70 dBm. Any signal lower than this will trigger the client to try and roam to a different SSID.
RSSI values between -50 to -70 dBm are examples of ranges in which the clients would see the AP or vice versa. But if the wireless client is in very close vicinity to the AP it can receive signals stronger than -50 dBm as well, which is still OK.
Fast roaming
Fast Roaming, also known as IEEE 802.11r or Fast BSS Transition (FT), speeds up connections in WPA2 Enterprise Security environments. Devices authenticate when they connect to the first AP, and carry their credentials with them to subsequent APs. Roaming is more efficient due to quicker authentication. Mist supports configuring different Fast Roaming options. 11r is Fast BSS Transition, which allows a mobile client to re-establish existing security parameters before re-associating to a new AP.