Hi  @JSINCO 

In a TP - Link EAP215 (or similar access point) setup, typically the RSSI (Received Signal Strength Indicator) value shown usually represents the signal strength that a client device or a secondary unit (sub) is receiving from the main access point (main bridge).

However, in many enterprise - grade access point setups like the EAP215, there may not be a direct and obvious indicator that shows the signal strength that the main is receiving from the sub in the same way as the client - to - main display. But you can often infer it based on the link quality metrics and communication status.

If the communication between the main and the sub is stable and working well, it's a good indication that the signal strength in both directions is sufficient.

It's not always safe to assume that the signal strength in both directions is the same. There are many factors that can affect the signal propagation in one direction differently from the other, such as antenna orientation, interference in one particular path, and differences in transmitter power settings between the main and sub units.