All About PLMN Selection in LTE

What Is PLMN?
- Public Land Mobile Network
- PLMN ID - Unique identification of PLMN
- PLMN ID (not more than 6 digits) = MCC(Mobile Country Code) + MNC(Mobile Network Code)
- Each operator providing mobile services has its own PLMN.

Types Of PLMN:

- RPLMN : Registered PLMN
               - Derived from PLMN ID in Tracking Area Identity (TAI = PLMN + TAC).
               - Stored in EFEPSLI in USIM/NVMEM

- EPLMN : Equivalent PLMN
               - Received in Attach accept / Tracking Area Accept.
               - Regarded by the UE as equivalent to each other for PLMN selection and cell selection/re-selection.
               - UE shall update or delete this list at the end of each attach or combined attach or tracking area updating or combined tracking area updating procedure.
                - When the UE is switched off, it shall keep the stored list so that it can be used for PLMN selection after switch on.

- HPLMN : Home PLMN
               - Derived from IMSI
               - IMSI (not more than 15 digits) = PLMN ID + MSIN = MCC + MNC + MSIN 

- EHPLMN : Equivalent Home PLMN
               - Stored in USIM in EFEHPLMN

- UPLMN : User Controlled PLMN
               - Stored in USIM in EFPLMNwACT

- OPLMN : Operator Controlled PLMN
               - Stored in USIM in EFOPLMNwACT

- VPLMN : Visitor PLMN
               - Given by Physical Layer Search.

- FPLMN : Forbidden PLMN
               - Stored in USIM in EFFPLMN

PLMN Selection Modes: 
- Automatic PLMN Selection 
- Manual PLMN Selection

Automatic PLMN Selection Mode:   
- RPLMN and EPLMNs
- Either the HPLMN(if the EHPLMN (in USIM) list is not present or is empty) or the highest priority EHPLMN that is available(if the EHPLMN list is present).
- Each PLMN/Access Technology combination in the "User Controlled PLMN Selector with Access Technology"(UPLMNwACT) data file in the USIM(in Priority Order).
- Each PLMN/Access Technology combination in the "Operator Controlled PLMN Selector with Access Technology" (OPLMNwACT) data file in the USIM(in Priority Order).
- Other PLMN/Access Technology combinations with received high quality signal received in the random order (Phy Detected).
- Other PLMN/Access Technology combinations in order of decreasing signal quality (Phy Detected).

EXCEPTION:
- If the UE is in Automatic Network selection mode and if finds coverage of an EHPLMN, the UE may register to the highest priority EHPLMN available and not return to the register PLMN or equivalent PLMN.
- If the EHPLMN list is not present or is empty, and the HPLMN is available, the UE may register on the HPLMN and not return to the registered PLMN or equivalent PLMN.
- The Operator shall be able to control by USIM configuration whether an UE that supports this option is permitted to perform this alternate behaviour.

- EF - LRPLMNSI (Last RPLMN Selection Indication) plays an important role to decide this.
- EF - LRPLMNSI = '00' The UE shall attempt registration on the last RPLMN at switch on or recovery from out of coverage.
- EF - LRPLMNSI = '01' The UE shall attempt registration either on the EHPLMN or the last RPLMN at switch on or recovery from out of coverage.


Manual PLMN Selection Mode:   
- UE displays PLMNs to the USER.
- Includes FPLMNs as well.