Mobile Communication Systems and Networks
This domain tests how handsets register on public mobile networks and how radio, identity and packet-data settings produce or block service. It spans 2G GSM, 3G UMTS, 4G LTE and 5G NR at a service level, not deep radio engineering.
Identity and registration
A SIM or USIM stores the IMSI and authentication keys; the network assigns a temporary identity. The IMEI identifies the handset equipment, not the subscriber. On attach, the phone searches supported bands, synchronises to a cell, reads system information, authenticates and establishes a data bearer. LTE uses EPS bearers; 5G NR can use non-standalone or standalone architectures. VoLTE carries voice as IP traffic over LTE, so a correct IMSI does not guarantee calls if VoLTE or IMS is misconfigured.
Signal and data settings
Coverage metrics include RSRP (reference signal received power), RSRQ and SINR; higher SINR generally means better quality. APN settings tell the handset which packet gateway to use. Airplane mode disables radios, and dual-SIM phones may assign data to one SIM only. Network selection can be automatic or manual, and a locked handset may reject a SIM from another operator.
Worked example
A phone shows “No service” but Wi-Fi works. The technician confirms the SIM is seated and active, checks airplane mode and network selection, compares the IMEI in settings with the label, then inspects the antenna flex and connector. If another known-good SIM also fails and signal is strong nearby, the fault is likely handset-side radio or antenna hardware; if only one SIM fails, suspect SIM provisioning or account status.
Traps
Wi-Fi working does not prove the cellular radio works. IMSI and IMEI are not interchangeable. A phone can show bars yet fail data if the APN or data allowance is wrong. Do not promise coverage fixes without checking the operator’s band support.