RSSI (Received Signal Strength Indication) and Interference

Well, in GSM you have a 200KHz channel in one of the GSM bands, assigned to a user/mobile.
So if you are measuring at the mobile, then whatever enters the 200KHz channel contributes to RSSI - interference, jammer signals, the actual signal that you are supposed to receive, etc.
When you measure RSSI at the Base Station, then you are measuring the whole band-range
that a TRX (Transceiver) board in the BTS handles. This band-range can be Nx200KHz, N being the number of ARFCNs (Absolute RF Channel numbers) the TRX handles. Here again whatever enters the band of the TRX contributes to the RSSI. But a TRX also measures the RSSI in the individual 200KHz band as well (to power-control the mobile correctly).

Long story short -high GSM RSSI does not necessarily mean high interference. If you have
a misbehaving phone that is screaming at the BTS tower, you will still measure high RSSI. If you are a phone too close to the tower, you measure high RSSI!