PLMN selection — Choosing a network before choosing a cell
The device — identity, state, mobility
Where it sits
What it is
What it is. Choosing a network — before, and separately from, choosing a cell. TS 23.122 §4. A device first decides whose network it wants to be on, then looks for a suitable cell belonging to it.
The order is the point. Cell selection finds the best radio; PLMN selection decides which radios are even eligible. A device does not camp on the strongest cell in the area — it camps on the strongest suitable cell of a permitted network, and permission is decided first.
Two modes:
Automatic. The device works down a priority order: the HPLMN (or the equivalent home networks), then the User Controlled PLMN Selector list on the USIM — which the subscriber may edit — then the Operator Controlled list, then any other PLMN found, by signal strength.
Manual. The device presents what it found and the user picks. This is the one place in the whole system where a person's choice enters directly, and the ordering above is precisely why it has to exist: the automatic order encodes the operator's preferences, and they are not always the user's.
"Higher priority PLMN search" is where this reaches back into the radio. A device camped in a VPLMN — roaming — periodically looks for a higher-priority network, because the automatic order says it should be on one if it can.
That search is expensive: scanning other frequencies costs power and interrupts nothing useful. So it
is done on a timer, and TS 38.304 gives it a lever in the
cell selection criterion itself: Qrxlevminoffset and Qqualminoffset
are "only applied when a cell is evaluated for cell selection as a result of a periodic search for a
higher priority PLMN while camped normally in a VPLMN".
The device holds a candidate to a stricter standard when it is considering leaving a working network — a hysteresis for networks rather than for cells, and the reason a roaming phone does not flicker between two operators at a border.
Forbidden PLMNs are remembered. A network that rejects a registration with certain causes goes on a forbidden list stored on the USIM, and the device stops trying it — otherwise a device in range of a network it may not use would retry indefinitely, which is expensive for both sides.
What the device sees to do any of this. The PLMN identity — MCC and MNC — is broadcast in SIB1, so a device can read a cell's network identity from the system information after acquiring the SSB and PBCH, before any connection exists.
Read on
This concept is read as part of one argument in ref-ue-mobility, alongside the rest of its group.
Before this concept, the hierarchy says to learn the following — the full chain, in order: