At switch on or recovery from loss of coverage UE should select the last registered-PLMN (RPLMN) if exists and start with the PLMN selection operating mode that was used before switch off/loss of coverage
Two different modes of PLMN selection
i)Automatic PLMN Selection Mode
This mode utilizes a list of PLMN's in priority order. The highest priority PLMN which is available and allowable is selected.
Ms selects and attempts registration on other PLMN/access technology combinations, if available and allowable in the following order
1. Either HPLMN (if EHPLMN list not present/empty on SIM) or the highest EHPLMN that is available (if EHPLMN list present)
2. PLMN/access technology combination in "User Controlled PLMN selector with access technology" data file in SIM.
3. PLMN/access technology combination in "Operator Controlled PLMN selector with access technology" data file in SIM.
4. Other PLMN/access technology combinations with received high quality signal in random order
5. Other PLMN/access technology combinations in order of decreasing signal quality
ii) Manual PLMN Selection Mode
Here the MS indicates to the user which PLMN's are available, Only when the user makes a manual selection does the MS try to obtain service on the PLMN.
MS indicates the list of PLMN's with all supported access technologies, this list includes PLMN's present in the "Forbidden PLMNs" list, "Forbidden PLMNs for GPRS Service" list
and PLMNs which offers services not supported by the MS.
Plmns meeting above criteria are presented in following order.
1. Either HPLMN (if EHPLMN list not present/empty on SIM) or the highest EHPLMN that is available, if more EHPLMNs are available display either the highest priority one or all based on the flag on the SIM.
2. PLMN/access technology combination in "User Controlled PLMN selector with access technology" data file in SIM.
3. PLMN/access technology combination in "Operator Controlled PLMN selector with access technology" data file in SIM.
4. Other PLMN/access technology combinations with received high quality signal in random order
5. Other PLMN/access technology combinations in order of decreasing signal quality
Once the UE has registered on a PLMN selected by the user, the UE shall not automatically register on a different PLMN unless
i) The new PLMN is indicated as an equivqlent PLMN by the registered PLMN or
ii) The user selects automatic mode
If no such PLMN selected or that PLMN is no longer available, the MS shall attempt to camp on any acceptable cell and enter the limited service state.
Roaming:
If the MS is roaming and in registered on a VPLMN, the MS shall periodically attempt to select HPLMN (if EHPLMN list is empty) or one of its EHPLMNs (if list is available on SIM) or a higher priority PLMN/access technology combinations listed
in "User and Operator controlled PLMN's list.
A value T minutes stored in the SIM which can range from 6 minutes to 8 hours in steps of 6 minutes to perform periodic attempts.
If no value is present in SIM a default value of 60 minutes is used for T.
The attempts to access the HPLMN or an EHPLMN 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, and not while the MS is attached for emergency bearer services or has a PDN connection for emergency bearer services;
b) The MS shall make the first attempt after a period of at least 2 minutes and at most T minutes:
c) The MS shall make the following attempts 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
PLMN - Public Land Mobile Network
HPLMN - Home PLMN
EHPLMN - Equivalent HPLMN
RPLMN - Registered PLMN
Reference : 3GPP TS 23.122
Two different modes of PLMN selection
i)Automatic PLMN Selection Mode
This mode utilizes a list of PLMN's in priority order. The highest priority PLMN which is available and allowable is selected.
Ms selects and attempts registration on other PLMN/access technology combinations, if available and allowable in the following order
1. Either HPLMN (if EHPLMN list not present/empty on SIM) or the highest EHPLMN that is available (if EHPLMN list present)
2. PLMN/access technology combination in "User Controlled PLMN selector with access technology" data file in SIM.
3. PLMN/access technology combination in "Operator Controlled PLMN selector with access technology" data file in SIM.
4. Other PLMN/access technology combinations with received high quality signal in random order
5. Other PLMN/access technology combinations in order of decreasing signal quality
ii) Manual PLMN Selection Mode
Here the MS indicates to the user which PLMN's are available, Only when the user makes a manual selection does the MS try to obtain service on the PLMN.
MS indicates the list of PLMN's with all supported access technologies, this list includes PLMN's present in the "Forbidden PLMNs" list, "Forbidden PLMNs for GPRS Service" list
and PLMNs which offers services not supported by the MS.
Plmns meeting above criteria are presented in following order.
1. Either HPLMN (if EHPLMN list not present/empty on SIM) or the highest EHPLMN that is available, if more EHPLMNs are available display either the highest priority one or all based on the flag on the SIM.
2. PLMN/access technology combination in "User Controlled PLMN selector with access technology" data file in SIM.
3. PLMN/access technology combination in "Operator Controlled PLMN selector with access technology" data file in SIM.
4. Other PLMN/access technology combinations with received high quality signal in random order
5. Other PLMN/access technology combinations in order of decreasing signal quality
Once the UE has registered on a PLMN selected by the user, the UE shall not automatically register on a different PLMN unless
i) The new PLMN is indicated as an equivqlent PLMN by the registered PLMN or
ii) The user selects automatic mode
If no such PLMN selected or that PLMN is no longer available, the MS shall attempt to camp on any acceptable cell and enter the limited service state.
Roaming:
If the MS is roaming and in registered on a VPLMN, the MS shall periodically attempt to select HPLMN (if EHPLMN list is empty) or one of its EHPLMNs (if list is available on SIM) or a higher priority PLMN/access technology combinations listed
in "User and Operator controlled PLMN's list.
A value T minutes stored in the SIM which can range from 6 minutes to 8 hours in steps of 6 minutes to perform periodic attempts.
If no value is present in SIM a default value of 60 minutes is used for T.
The attempts to access the HPLMN or an EHPLMN 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, and not while the MS is attached for emergency bearer services or has a PDN connection for emergency bearer services;
b) The MS shall make the first attempt after a period of at least 2 minutes and at most T minutes:
c) The MS shall make the following attempts 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
PLMN - Public Land Mobile Network
HPLMN - Home PLMN
EHPLMN - Equivalent HPLMN
RPLMN - Registered PLMN
Reference : 3GPP TS 23.122