Networks, SIM/IMEI and Mobile Communication Basics
The Networks, SIM/IMEI and Mobile Communication Basics domain links the phone to the operator's network. Questions cover bands used in India, SIM identity, IMEI structure, APN settings and simple signal troubleshooting.
Cellular basics
- 2G GSM in India uses 900 and 1800 MHz; 4G LTE uses bands 1, 3, 5, 8, 40 and 41; 5G uses n78 among others. A phone must support the band the operator uses.
- The cell tower handles registration, paging and handover as you move. Signal strength is reported in dBm, where about -60 dBm is strong and -110 dBm is weak.
- VoLTE carries voice over LTE; without it a 4G-only phone may not make calls.
SIM and identity
- The SIM holds the ICCID (card serial) and the IMSI (subscriber identity). The network authenticates the subscriber with keys stored on the SIM.
- The IMEI is 15 digits: 8-digit TAC, 6-digit serial, 1 check digit. The IMSI identifies the subscriber; the IMEI identifies the handset. Changing or cloning an IMEI is illegal in India.
Data settings
- APN, username and password come from the operator; a wrong APN means no data even with full signal.
Worked example
Check the IMEI 490154203237518. Take the first 14 digits, double every second digit from the right, and add the digits of each result: doubled digits give 2+5+4+0+8+2+9 = 30; the other digits give 5+3+3+2+5+0+4 = 22. Total 52, so the check digit is 10 - 2 = 8, matching the IMEI. Reasoning: a mismatch means a mistyped or altered IMEI.
Traps that cost marks
- Confusing IMSI with IMEI.
- Blaming the phone's antenna for weak indoor coverage.
- Setting an APN without matching the operator's fields.
- Offering to change an IMEI.