Arbitrary Strength Unit (ASU) is an integer value proportional to the received signal strength measured by the mobile phone.
It is possible to calculate the real signal strength measured in dBm (and thereby power in Watts) by a formula. However, there are different formulas for 2G and 3G networks.
In GSM networks, ASU is equal to the RSSI (received signal strength indicator, see TS 27.007 sub clause 8.5).
dBm = 2 × ASU - 113, ASU in the range of 0..31 and 99 (for not known or not detectable)
It is widely misbelieved[by whom?] that ASU = "Active Set update". The Active Set Update is a signalling message used in handover procedures of UMTS and CDMA mobile telephony standards. On Android phones, the acronym ASU has nothing to do with Active Set Update. It has not been declared precisely by Google developers.