iOS Field Test Mode Access
Call *3001#12345#* to access Field Test Mode on iOS.
- Works on iPhone 16 Pro Max, iOS 18.4 (developer beta)
- Alternative
3001#12345#(no asterisks) may not work on newer iOS versions
Network Identifiers
| Field | Purpose | Scope |
|---|---|---|
| Cell ID | Network routing identifier | Globally unique per carrier |
| PCI (Physical Cell ID) | RF interference management | Locally unique (0-503 range) |
| TAC (Tracking Area Code) | Groups towers in region | Regional grouping |
| PLMN | Network identifier | Carrier identification (e.g., 311 480 = Verizon) |
IMPORTANT
Key Insight: Cell ID vs PCI serve different purposes - Cell ID for network routing, PCI for RF management. PCI can change frequently as you move or network rebalances.
Signal Quality Metrics
Signal Quality Reference Values
| Metric | Good | Fair | Poor |
|---|---|---|---|
| RSRP | > -85 dBm | -85 to -100 dBm | < -100 dBm |
| RSRQ | > -10 dB | -10 to -15 dB | < -15 dB |
| SINR | > 10 dB | 5 to 10 dB | < 5 dB |
RSRP (Reference Signal Received Power)
Measures the power of the LTE/5G reference signals received by your device.
- Units: dBm (decibel-milliwatts)
- Range: Typically -40 dBm to -140 dBm
- What it means: Raw signal strength from the tower
- Impact: Lower (more negative) values mean weaker signal
RSRQ (Reference Signal Received Quality)
Measures signal quality by comparing desired signal to interference and noise.
- Units: dB (decibels)
- Range: -3 dB to -20 dB
- What it means: How “clean” the signal is
- Impact: Lower values indicate more interference/noise
SINR (Signal to Interference plus Noise Ratio)
Compares useful signal power to combined interference and noise.
- Units: dB (decibels)
- Range: -20 dB to +30 dB
- What it means: Signal clarity for data transmission
- Impact: Higher values = better data performance
Tools
Cell Tower Location Methods
- CellMapper.net: Crowdsourced real coverage data with actual user measurements
- AntennaSearch.com: FCC database search by address
- OpenSignal: Useful for signal maps but doesn’t provide specific tower information
- CellMapper app (iOS): Unusable in unpaid version
Coverage Shape Reality
CellMapper shapes based on actual user measurements, not theoretical models. Reflects real terrain/interference effects.
Troubleshooting Approach
Signal vs Network Issues
- Poor RSRP: Distance/obstruction problem - move closer to tower or switch carriers
- Poor RSRQ: Interference problem - network congestion or competing signals
- Poor SINR: Data performance problem - affects throughput more than connectivity
Signal vs Congestion Diagnosis
Speed test at different times:
- 3 AM vs 7 PM vs noon
- Consistently poor = RF/antenna problem
- Time-variable = deprioritization/congestion
Implications: RF problems require infrastructure changes or carrier switch. Congestion problems may improve with higher priority plans.
Sector Connection Problems
Problem Pattern: Connecting to wrong directional antenna sectors on cell towers, causing poor signal despite tower proximity.
Limited User Control:
- Network reset (temporary)
- Physical location changes
- Band locking (Android, requires root)
- Reality: Network algorithm controls sector assignment
Identification: Use CellMapper to see if you’re connecting to sectors pointing away from your location.
Coverage Gap Analysis
Signs of Coverage Gaps:
- Poor signal despite proximity to towers
- Field test shows connection to suboptimal sectors
- CellMapper coverage shapes don’t overlap your location
- Signal strength varies dramatically by small location changes
Solution Strategy: Different carriers use different tower locations and antenna patterns - coverage gaps for one carrier may not exist for others.
Common Problem Patterns
- All metrics poor: Coverage gap or tower distance issue
- RSRP good, RSRQ/SINR poor: Network congestion or interference
- Metrics vary by time: Likely congestion/prioritization issue
- Wrong sector connection: Check CellMapper for directional antenna coverage
Links: QCI Priority System, US Mobile Network Options, Network Reset Limitations
Resetting Network Settings
In iOS it’s reported that resetting network settings is helpful under certain circumstances. To do this go under Settings -> Reset -> Reset Network Settings
WARNING
This additionally removes all saved WiFi passwords and network information