What PLMN a UE select when in Roaming?

Hi Experts,
If we have national roaming, the UE which operator will select?
Is there any specific criteria or priority?

When roaming, a UE obtains services from a VPLMN other than its HPLMN.
For example, the UE may obtain services from a PLMN in another country (an international roaming area).
When the UE with the roaming service moves out of the coverage area of its HPLMN, the UE can obtain services from another PLMN according to the PLMN information stored on the USIM.
The operator determines whether roaming is allowed for a UE.
After the UE registers with a VPLMN by roaming, it periodically searches for its HPLMN and attempts to return to it.
The time interval between consecutive searches for the HPLMN is defined by the operator and stored on the USIM.

SIM has priority of operators built in.
It follows that order.

As i know, the UE will select the best coverage operator.

From my own experience there is strict priority and UEs follows that.
Only checks next operator if goes out of coverage in one.

PLMN selection is a complex topic.

At the very onset, we assume that the user has been receiving normal service (when the user was already receiving limited service, the procedure hardly makes a material difference as far as user experience is concerned).

MS carries out the PLMN selection process either
a) at switch on or
b) faces lack of coverage from serving PLMN).
c) MS user requests for a PLMN re-selection

There are two modes for selection
a) automatic mode where no user interaction is needed
b) manual mode where the process is driven by user input

Sequence goes as follows:
On switch on (or any of the triggers mentioned above), MS tries to look for cells in serving PLMN (where the MS was last registered). If successful, MS goes ahead with cell selection followed by location registration in the PLMN and if all these are fine, MS is successfully camped on the selected cell of PLMN. However, if the serving PLMN is not in coverage, MS either follows automatic or manual mode of PLMN selection.

In automatic mode, it follows below sequence

  • look for HPLMN (home network)
  • consult PLMN priority list in SIM/USIM in the same order
  • Other available PLMNs with signal level of over -85 dBm i.e., dB below 1 mW) in random order
  • All other PLMNs in decreasing order of signal strength

If any of the above steps go through, selected PLMN is the new registered PLMN. Otherwise, MS declares that service is currently not available.

It is to be noted that, in International Roaming scenario, the steps above may involve several repetitions. This is almost always the case, because in typical commercial settings HPLMN operator signs several roaming agreements with VPLMN operators mostly in order to reduce wholesale outbound roaming cost. These costs are driven by Inter Operator tariff between the VPLMN and HPLMN. Overall framework for the same is guided by GSMA IR.73 PRD. There are a number of Steering of Roaming products in market that help HPLMN operator to select right partner network.

In manual mode, same procedure as automatic mode is followed except the step for PLMN selection where User is presented by the MS with a list of PLMNs which are available.