Overview
What do signal bars indicate? Are signal bars equivalent to signal strength?
Signal bars are the phone reporting detected radio conditions to the network. The network uses this report to select technologies, bands, and channels for subsequent communication. People often use the number of bars to infer signal strength. For example, when a phone shows two bars, users tend to assume the signal is weak, but the reality can be more complex than that.
Signal bars are not determined solely by raw signal strength. The following case study lists common observations and analyzes factors related to signal bar behavior.
Observed symptom
An iPhone 13 showed signal bars repeatedly toggling between filled and empty, as shown below.
Analysis
Terminal log analysis at the corresponding timestamps is as follows.
After establishing a PDU session bearer with 5QI=5, the core network sent a PDU Session Release Command to delete the terminal's 5QI=5 SIP PDU session bearer. This delete-and-recreate process repeated. The core network indicated cause 39, which means the network requested the PDU session be reactivated. Therefore, repeated deletion and reestablishment of the 5QI=5 SIP bearer is suspected to cause the UE signal bars to flicker. The root cause for the core network issuing the PDU Session Release Command needs further analysis.
The core network replied that after sending an SMS-related message over the SIP bearer, the terminal responded with SIP 404 Internal Error. This caused the core network to request PDU session reactivation and rebuild the 5QI=5 SIP bearer. The repeated loop of delete and rebuild for the 5QI=5 SIP bearer explains the observed flickering. The terminal must analyze why it returned SIP 404.
The terminal analysis showed the destination user number in the SMS-related message was +86139****3934, while the number printed by the terminal lower layers was +86139****3949. This number mismatch caused SMS validation to fail, so the UE replied with SIP 404 Internal Error. Lower-layer logs indicate the mismatch resulted from a manually modified number. It is suspected the user entered an incorrect number in Settings > Phone > My Number.
Retest: when the number was set incorrectly, the signal bar flickering could be reproduced. After correcting the number, the device returned to normal behavior.
Resolution
The iPhone 13 user had manually set an incorrect number in Settings > Phone > My Number, causing the phone to read different numbers from two sources (SIM and the UI). The number mismatch caused SMS validation errors, so the terminal replied with SIP 404 when the core network sent SMS-related messages over the SIP bearer. The core network then issued a PDU session reactivation request, which triggered repeated deletion and reestablishment of the 5QI=5 SIP bearer, producing the observed signal bar flicker.
When manually setting the device phone number in the UI, ensure the entered number is correct.
