LTE PLMN Selection and Different type of PLMN, HPLMN, VPLMN, EPLMN, IPLMN, FPLMN

PLMN stands for Public Land Mobile Network. The LTE compliant UE is responsible for selecting a PLMN for subsequent cell selection. A PLMN is identified by its PLMN identity broadcast within SIB-1. A single cell can belong to multiple PLMNs, so SIB-1 may broadcast a list of PLMN identities.

Following are two steps for PLMN selection:

STEP-1: The LTE UE NAS layer requests AS layer to report available PLMN.
STEP-2: The LTE NAS layer is responsible for selecting a PLMN from these list of reported PLMN.

STEP-1: Let us see how NAS layer gets requested information from AS layer:

• UE scans all the RF channels within its supported RF bands
• UE searches for strongest cell carrier and reads SIs to indentify PLMN 
• PLMN are reported to NAS as high quality if RSRP>=-110dBm
• PLMN not meeting high quality criteria are reported along with their RSRP to NAS
• UE can optimize PLMN search procedure using stored information such as RF carriers and cell parameters.
• NAS layer can stop search at any instant for example after finding the home PLMN.

STEP-2: Let us see how NAS layer selects PLMN from reported list. It utilizes USIM information for PLMN selection:

• IMSI defines Home PLMN (i.e. HPLMN) 
• HPLMN selector with access technology: Defines priorities of each RAT associated with HPLMN.
• User controlled PLMN selector with access technology allows end user to prioritize the PLMN and RAT.
• Operator controlled PLMN selector with access technology allows the operator to prioritize the PLMN and access technology. 
• Forbidden PLMNs: Defines PLMN which UE does not automatically attempt to access. Forbidden PLMNs are added to the list when network will reject an ATTACH REQUEST using cause value as 'PLMN not allowed'. 
• EHPLMN (Equivalent HPLMN) defines set of PLMN which are equivalent to PLMN with which LTE UE is registering. This list can be treated or deleted during the ATTACH or TRACKING AREA UPDATE procedures. EHPLMN are ordered in terms of their priority.

PLMN automatic and manual selection types
PLMN can either be selected manually or automatically as described below:

In the case of automatic selection, the LTE UE selects the PLMN and RAT (Radio Access Technology) using following order of priority:

1.       HPLMN or the highest priority EHPLMN

2.       PLMN and RAT combinations defined within User controlled PLMN selector.

3.       PLMN and RAT combinations defined within operator controlled PLMN selector.

4.       Other PLMN reported as high quality PLMN, selected in random order.

5.       Other PLMN selected in order of decreasing signal quality.

The LTE UE searches all supported access technologies before PLMN selection using last two steps 4 and 5 mentioned above.
In the case of manual selection, UE presents end user with available PLMN, listing them in the order as mentioned above.

REFERENCES: 3GPP TS 36.304, TS 23.122, TS 31.102, TS 24.301

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 behavior.

- 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.

In Layman Language

HPLMN Vs EHPLMN Vs VPLMN

The difference between HPLMN is EHPLMN is very small. Say if you are using TATA Docomo. If you are in a place where Docomo directly provide service and you camp on that, you are registered in you HPLMN.

Say if you move to some other place where some other operator say Vodafone provide service for Docomo as Docomo doesn’t have direct network service available there. There will be mutual agreement between these two operators. You will get the same feature sets like same call rates etc. when you use HPLMN. So here Vodafone is an EHPLMN. So you won’t feel any difference in real time even if you are in HPLMN or EHPLMN.

A VPLMN is similar, but there won’t be any agreement as mentioned above between the two operators. The have roaming agreement and call/data charges will be based on roaming rates.

PLMN: Preferred List of Mobile Networks or Public Land Mobile Network

MCC + MNC

HPLMN: Home PLMN

• Subscribers profile is held in the HLR of this PLMN.

• SMSC (short message service Centre) and SCP (service control point) are service nodes contained here.

VPLMN:

• When subscriber is roaming outside the HPLMN, subscriber's profile is temporarily stored here.

EPLMN: Equivalent PLMN

• Stored in SIM and updates itself upon successful Location Registrations/ Attach

• Used for cell/ PLMN selection and reselection when MS is off

• Useful when there are ICR pacts between operators

IPLMN: Interrogating PLMN

• Contains the GMSC which handles MT calls

• If, let’s say, there is an MT call, HPLMN mostly acts as the IPLMN since that is where the subscriber's information shall be checked for

• And during SOR (Support of Optimal Routing), IPLMN may be used

F-PLMN: Forbidden PLMN

• When No suitable cells in LA message is received by MS, then that cell is added to FPLMN - when MS is switched off, or SIM is removed - this FPLMN cell list is erased

• When PLMN not allowed message is received by MS, then that PLMN is added to FPLMN list - when upon Manual selection the MS can camp to this PLMN then this is erased from FPLMN list.