RSSI (Received Signal Strength Indicator) is a measure of the total radio power a device's antenna is receiving across the channel. It is reported in dBm (decibels relative to one milliwatt) as a negative number, with values closer to zero indicating stronger signal.

In short: Typical cellular RSSI values range from around -50 dBm (very strong, usually within metres of a cell tower) to -110 dBm or worse (very weak, often unusable). Values between -65 and -85 dBm indicate good signal for most cellular applications. RSSI was the headline signal metric for 2G and 3G, but for LTE and 5G it has been partly superseded by RSRP and RSRQ, which measure the strength and quality of the cell's reference signal specifically rather than total channel power.

RSSI is still useful as a first check when troubleshooting a deployment. A router reporting RSSI of -100 dBm or worse is unlikely to deliver acceptable throughput, regardless of which cellular generation it is using. RSSI is also useful for comparing relative signal strength between different antenna positions during site survey: if moving the antenna improves RSSI by 10 dBm or more, that position is materially better.

For LTE and 5G troubleshooting, RSRP (which measures only the reference signal strength) and RSRQ (which measures signal quality relative to interference and noise) are more diagnostic than RSSI alone. A router with strong RSSI but poor RSRQ is in a noisy or congested cell, which can produce worse throughput than a quieter cell with weaker raw signal.