RSSI units

I read a report which show that a cellphone receieves a RSSI of -549?
What could be the units of such value?

Isn't it considered very low?

Moreoever, how is the RSSI process going?

Does the handset perform periodic measurements to search for cells, and as part of it, it receive RSSIs from neighbor cells?
And eventually it select the cell which returns the highest RSSI?

Thank you.

My understanding is that RSSI is an arbitrary and unit-less measure of signal strength. Each manufacturer uses his own values and thus you can not compare one cellphone RSSI value with another.

I would assume that the handset selects the strongest signal from neighbor cells, but there may be other factors involved (such as how busy a particular tower is).