How YOU will your phone will choose among AT&T, Verizon, Sprint, T-Mobile, etc ?

Paradox of choice for UE until it finds PLMN of each operator

If all the phone+data plans in the country are available to you , with respect to all national operators and carriers. How will you decide which operator to go for ? Probably you will make this decision based on any of the following

1. Wallet
2. You are stuck with a family plan for 2 years
3. You like the girl in that famous unlimited data plan commercial
4. You wanted to buy a new phone and that operator enticed you with a steal deal
5. Etc.

However , your UE will decide about a particular carrier over the others keeping Public Land Mobile Network (PLMN) identity in mind.

Yes, from a UE’s perspective PLMN identifies one mobile operator from the other.

In order to make this decision UE is going to get help by reading SIB 1.

UE does not know about a particular operator/carrier, until it has read SIB 1 in order to fetch PLMN information .

Fetching PLMN value by reading SIB 1

Previously I discussed first four steps,what happens before PLMN selection.

So going on to Step 5. PLMN selection comes into play.

Step 5: PLMN Selection

SIB 1 contains Public Land Mobile Network (PLMN) information.  For example, UE will realize AT&T’s network  from Verizon , T-Mobile or Sprint etc. based on  PLMN .

PLMN arrives in contents of SIB1

UE is responsible for selecting PLMN for subsequent cell selection.

An operator is identified by its PLMN identity broadcast within System Information Block 1 (SIB 1).

How many Types of PLMN exist ?

Home PLMN (HPLMN): Universal Subscriber Identification Module (USIM) of the UE identifies and defines the Home PLMN. This is the primary PLMN which belongs to the operator or carrier you purchased connection  / subscription in the first place.

Operator Controlled PLMN with Access Technology: Operators can prioritize a specific PLMN among various PLMNs  based on the access technology. For example, they can prioritize 4G LTE PLMN over 3G etc.

User Controlled PLMN Selector with Access Technology: As an end user, you have the option available to select and prioritize a PLMN based on technology either 3G or 4G or 2G etc.

Forbidden PLMNs: These are the PLMNs which UE does not automatically attempt to access. Forbidden PLMNs are added to the list when the network rejects an Attach Request using the cause value as ‘PLMN not allowed’

Equivalent to Home PLMN (EHPLMN):  If you are traveling to an area, where your current carrier does not have any coverage but they do have a roaming agreement with local carriers to support you as a subscriber. For example , if you are going to Canada from USA on a business trip or vacation. In this case a set of PLMNs are treated as equivalent to Home PLMN with which your UE is going to register. This list can be updated or deleted by the network during attach or tracking area update procedures. EHPLMN are ordered in terms of their priority i.e., high priority EHPLMN appears first.

How PLMN Selection Takes Place ?

When a list of PLMNs are available to choose from. How does a UE go about to specify or prioritized a particular PLMN over the other ?

There are two modes of PLMN selection.

• Automatic PLMN Selection
• Manual PLMN Selection

Most likely your phone is set on Automatic PLMN selection too

Most of the times, as end users, you will not be selecting a particular PLMN, and the UE automatically selects a PLMN when you power on the phone. As you can see in the image above, my phone automatically selects a PLMN from the available list.

In case of manual PLMN selection mode, as an end user , you will choose which network you want the UE to get connected with.

In either case, UE will prioritize a list among all the available PLMNs.  Whichever PLMN comes out to be top on the list. That PLMN will be selected.

AUTOMATIC PLMN PRIORITIZATION PROCEDURE

UE will prioritize among all the PLMNs being reported in the following order

Priority 1:  First priority is given to Home PLMN. If HPLMN is not available then the highest priority Equivalent to Home PLMN (EHPLMN) is selected (if EHPLMN list is available)

Priority 2: Second priority is for end user controlled PLMN:  PLMN and access technology combinations defined within User controlled PLMN , will be selected

Priority 3: Third priority is for Operator Controlled: PLMN and access technology combinations defined within the Operator Controlled PLMN Selector

Priority 4: Fourth priority comes to other available PLMNs which DO have RSRP value >= -110 dBm . Those PLMN values are selected in random order

Priority 5: Last priority is given to PLMNs in order of decreasing RSRP values PLMNs: PLMNs which do not have RSRP value >= -110 dBm are reported and selected in order of decreasing signal strength .

Priority IV Priority V selection and Reporting

MANUAL PLMN SELECTION PRIORITIZED LIST

The difference between automatic selection and manual selection is subtle. The prioritized list in case of manual PLMN selection is pretty much the same as in case of automatic PLMN selection. In manual selection model, UE presents the end user with the available PLMN on the home screen and user can choose from the list.

The order is decided as follows

Priority 1:  Home PLMN or the highest priority Equivalent to Home PLMN (EHPLMN) (if EHPLMN list is available)

Priority 2: User Controlled :  PLMN and access technology combinations defined within User controlled PLMN

Priority 3: Operator Controlled: PLMN and access technology combinations defined within the Operator Controlled PLMN Selector

Priority 4: Other PLMNs which do have RSRP value >= -110 dBm . Those PLMN values are selected in random order

Priority 5: Other Decreasing RSRP values PLMNs: PLMNs which do not have RSRP value >= -110 dBm and reported selected in order of decreasing signal strength .

Prioritized Arrangement of Available PLMNs

The end user , finally then can select whichever PLMN comes on top of the prioritzed list. Hence the UE should attempt to access.

FAQs about PLMN Selection

Question: Can a cell belong to just one PLMN ?

Ans: A single cell can belong to multiple PLMN so SIB 1 may broadcast a list of PLMN identities.

Question: Will UE scan only the operator’s PLMN ?

Ans: UE scans all RF channels within supported frequency bands

Question: At what RSRP value a PLMN is selected by the UE ?

Ans: UE searches for the strongest cell on each carrier and reads the system information to identify the PLMN. A PLMN is selected if available RSRP is >= -110 dBm

For any questions, comments or criticism, please comment below.

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