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, 3G and 4G networks.
In GSM networks, ASU maps to RSSI (received signal strength indicator, see TS 27.007[2] sub clause 8.5).
dBm = 2 × ASU - 113, ASU in the range of 0..31 and 99 (for not known or not detectable).
In UMTS networks, ASU maps to RSCP level (received signal code power, see TS 27.007[2] sub clause 8.69 and TS 25.133 sub clause 9.1.1.3).
dBm = ASU - 116, ASU in the range of -5..91 and 255 (for not known or not detectable).
In LTE_(telecommunication) networks, ASU maps to RSRP (reference signal received power, see TS 36.133, sub-clause 9.1.4). The valid range of ASU is from 0 to 97. For the range 1 to 96, ASU maps to
(ASU - 141) ≤ dBm < (ASU - 140).
The value of 0 maps to RSRP below -140 dBm and the value of 97 maps to RSRP above -44 dBm.
On Android devices however, the original GSM formula may prevail for UMTS.[3] Tools like Network Signal Info[4] can directly show the signal strength (in dBm), as well as the underlying ASU.
ASU shouldn't be confused with "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.[5]
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