Background information regarding “automatic network selection mode procedure”, following some extracts from 3GPP TS 23.122
The MS normally operates on its home PLMN (HPLMN). However a visited PLMN (VPLMN) may be selected, e.g., if the MS loses coverage. There are two modes for PLMN selection:
i) Automatic mode This mode utilizes a list of PLMNs in priority order. The highest priority PLMN which is available and allowable is selected.
ii) Manual mode Here the MS indicates to the user which PLMNs are available. Only when the user makes a manual selection does the MS try to obtain normal service on the VPLMN.
There are two cases:
- International Roaming This is where the MS receives service on a PLMN of a different country than that of the HPLMN.
- National Roaming This is where the MS receives service from a PLMN of the same country as that of the HPLMN, either anywhere or on a regional basis. The MS makes a periodic search for the HPLMN while national roaming.
The MS selects and attempts registration on other PLMNs, if available and allowable, in the following order:
i) HPLMN (if not previously selected);
ii) each PLMN in the "User Controlled PLMN Selector with Access Technology" data field in the SIM (in priority order);
iii) each PLMN in the "Operator Controlled PLMN Selector with Access Technology" data field in the SIM (in priority order);
iv) other PLMN/access technology combinations with received high quality signal in random order;
v) other PLMN/access technology combinations in order of decreasing signal quality.
A high quality signal is defined by:
- 3GPP TS 43.022 for the GSM radio access technology as a signal level is above ‑85 dBm,
- 3GPP TS 25.304 for the UMTS radio access technology as a signal level is above -96 dBm.
If the MS is in a VPLMN, the MS shall periodically attempt to obtain service on its HPLMN or higher priority PLMN listed in "user controlled PLMN selector" or "operator controlled PLMN selector" by scanning in accordance with the requirements that are applicable to i), ii) and iii) as defined in the Automatic Network Selection Mode in clause 22.214.171.124.1. In the case that the mobile has a stored "Equivalent PLMNs" list the mobile shall only select a PLMN if it is of a higher priority than those of the same country as the current serving PLMN which are stored in the "Equivalent PLMNs" list. For this purpose, a value T minutes may be stored in the SIM, T is either in the range 6 minutes to 8 hours in 6 minute steps or it indicates that no periodic attempts shall be made. If no value is stored in the SIM, a default value of 60 minutes is used.
The attempts to access the HPLMN or higher priority PLMN shall be as specified below:
a) The periodic attempts shall only be performed in automatic mode when the MS is roaming;
b) After switch on, a period of at least 2 minutes and at most T minutes shall elapse before the first attempt is made;
c) The MS shall make an attempt if the MS is on the VPLMN at time T after the last attempt;
d) Periodic attempts shall only be performed by the MS while in idle mode;
e) If the HPLMN or higher priority PLMN is not found, the MS shall remain on the VPLMN;
f) In steps i), ii) and iii) the MS shall limit its attempts to access higher priority PLMNs to PLMNs of the same country as the current serving VPLMN;
g) Only the priority levels of Equivalent PLMNs of the same country as the current serving VPLMN shall be taken into account to compare with the priority level of a selected PLMN.
So in “Automatic mode” selection, the main difference between “International roaming” and “national roaming” is that in national roaming the MS will search the HPLMN periodically.