How does a device choose its carrier?

Some of the most common inquiries people have in relation to multi-carrier functionality are

  • “Is it possible to switch carriers when communication with the currently connected carrier fails?”.

  • “Can I choose the carrier with the strongest signal from among multiple carriers?”

  • “I want to disconnect to a specific carrier due to a weak signal or other reasons.”

  • “Why does it take a few minutes to connect to the network when I turn on my device for the first time or after a long time?”

In this blog, we will review “3GPP TS 23.122”, a technical document published by 3GPP, a standardization project that studies and creates communication specifications such as LTE and 5G, to understand how a device connects to a network. In doing so we hope to explain the answers to the above questions.

What is 3GPP TS 23.122?

The questions we have just introduced have one thing in common: they all want to obtain good communication conditions. However, radio waves are constantly changing due to environmental influences, for example, the strength of the signal can be affected by humidity (e.g., rain) or by shielding (materials that do not allow radio waves to pass through) between the signal and the base station.

The final choice of what carrier to use is actually made within the device, with only some influence from the SIM card. In fact, there is a standardized document on this choice as a specification to be followed: 3GPP TS 23.122 “Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode”.

This document specifies the "protocol stack functions that should be retained by devices that are not yet connected to any carrier.”

What is a PLMN?

In reading 3GPP TS 23.122, one acronym to remember is “PLMN (Public Land Mobile Network)”. The definition of PLMN is found in 3GPP TS 22.011 “Service accessibility”:

“A Public Land Mobile Network (PLMN) is a network established and operated by an Administration or RPOA for the specific purpose of providing land mobile communication services to the public. It provides communication possibilities for mobile users. For communications between mobile and fixed users, interworking with a fixed network is necessary.

A PLMN may provide service in one or a combination of frequency bands.

As a rule, a PLMN is limited by the borders of a country. Depending on national regulations, there may be more than one PLMN per country.

A relationship exists between each subscriber and his home PLMN (HPLMN). If communications are handled over another PLMN, this PLMN is referred to as the visited PLMN (VPLMN).”

It is undeniable that the definition of PLMN is a bit difficult to understand, as it requires preexisting knowledge of terms and technologies involved with cellular connections. For simplicity, it can be easiest to remember that a PLMN is a globally unique, 5 or 6-digit, mobile network identification number provided by a telecommunications carrier.

PLMN numbers differ by carrier company, the frequency band used, and country/region. As such, there are often multiple PLMNs in a country.

The organization responsible for PLMN assignment also differs by country but is most often a part of that country’s government. For example, in the United States, PLMN codes are assigned to carriers by the Federal Communications Commission. While in Japan, they are assigned by the Ministry of Internal Affairs and Communications.

What is PLMN Selection?

PLMN Selection is the process in which a device with a SIM selects a PLMN to connect to from among multiple PLMNs.

PLMN Selection generally has two modes: Automatic mode and Manual mode. This article describes the more commonly used Automatic mode.

As you may have guessed by now, a SIM with multi-carrier support means that the SIM supports multiple carriers, i.e., multiple PLMNs.

In fact, SIMs and devices maintain a list of PLMNs of various types for different applications. The PLMNs listed in 3GPP TS 23.122, “4.4.3 PLMN selection”, are introduced at a glance.

Acornym Full Name Definition
RPLMN Registered PLMN Holds only one registered PLMN, i.e., a PLMN that has had a successful connection at least once.
EPLMN Equivalent PLMN A PLMN that can be treated as an RPLMN; includes PLMNs that receive the same services as an RPLMN — distributed by the carrier at the time of connection.
HPLMN Home PLMN PLMN of Home Operator, which can be identified from IMSI by the operator that issued the SIM card.
EHPLMN Equivalent HPLMN A PLMN that can be treated equivalently to an HPLMN.
UPLMN User-Controlled PLMN User-configurable preferred PLMN list. Some modules can edit this via AT command. More than one PLMN may be set in order of priority.
OPLMN Operater-Controlled PLMN A carrier-configurable list of preferred PLMNs, configurable by the Over the Air updates and maintained on the SIM…
Other PLMN Other PLMN Other PLMN determined by radio wave quality and other factors.
FPLMN Forbidden PLMN A PLMN that the carrier has denied connection to (or is not eligible for connection); it is held in the SIM and, depending on the module, can be edited with AT commands.
VPLMN Visited PLMN A PLMN that is not part of either HPLMN or EHPLMN, e.g. for roaming.

So devices and carriers have a list of these PLMNs prepared in advance, which they use to determine which carrier to connect to. The list is prioritized in the order listed, but if a PLMN is not supported by the device (e.g., the communication module does not support 4G), that PLMN is ignored.

PLMN Selection Overview

The following is an overview of PLMN Selection with the order of priority from left to right. It is also important to note that Equivalent PLMN and Equivalent HPLMN are placed in parallel to Registered PLMN and Home PLMN, which are considered as interchangeable with each other.

This is a simplified overview diagram for illustrative purposes only; please refer to 3GPP TS 23.122 Figure 2a for a more detailed description of the state transition diagram.

The device performs PLMN Selection via the following steps when it is powered on or loses carrier coverage. However, if the selected PLMN is included in the Forbidden PLMN or if a connection to the selected PLMN cannot be established, the device skips to the next step.

  • Step 1: Receive an announcement from the base station
  • Step 2: Match RPLMN or EPLMN
  • Step 3: Check against EHPLMN or HPLMN
  • Step 4: Match against UPLMN
  • Step 5: Match against OPLMN
  • Step 6: Match against Other PLMN

First, the device receives an “alert” from the base station in its area (Step 1). This alert contains the PLMN. Next, the PLMN to be connected to is finally determined by comparing the list in Steps 2 to 6 with the alert information. Steps 2 and 3, which are particularly important, are described in detail below.

The Mechanism for Quick Network Connection (Step 2)

PLMN Selection is a lengthy process due to the number of PLMN lists that must be checked to ensure that each possible connection is approved.

Considering the fact that many networks also have multiple PLMNs, it would be time-consuming to identify each PLMN among those available, check it against your own PLMN, and try to connect to it every time you lose coverage. So, to quickly connect to the network, a "cache-like” mechanism is used to create the Registered PLMN (RPLMN) and Equivalent RPLMN.

As mentioned earlier, RPLMN is a list that stores PLMNs that have been successfully connected at least once. Also, after a successful connection, the carrier distributes EPLMNs that can be treated as equivalent to RPLMNs. These lists are first subject to checking in step 2, and if they can be matched with the information reported in step 1, the subsequent steps can be skipped.

However, this mechanism may work differently across different SIM cards. For example, when using a SIM that can connect to multiple carriers, if the signal environment weakens due to changes in the environment, it is easy to think that the SIM should connect to the carrier with the strongest signal, but this is not the case. Suppose the signal environment deteriorates further, and the connection to the RPLMN or EPLMN also fails. In that case, the procedure from step 3 onward may then be performed, and a connection to another carrier may be established.


Home, Cooperation, and Roaming (Step 3)

If no PLMN is determined in Step 2, the device proceeds to Step 3.

Here, the Equivalent HPLMN (EHPLMN) is matched against the Home PLMN (HPLMN), which, as the name implies, lists only one PLMN from the PLMN operator that is the SIM’s home, i.e., the SIM issuing operator. EHPLMN, on the other hand, is a slightly special PLMN list that lists PLMNs that can be handled equally with HPLMN.

Some carriers may borrow facilities from other companies to ensure coverage in areas where their own base stations are not well developed. In such cases, the EHPLMN is also used, and the PLMN of the collaborating carrier may be registered as the EHPLMN.

One more thing: Visited PLMN (VPLMN) does not appear in the PLMN Selection Overview figure. If the HPLMN obtained from IMSI is not included in the EHPLMN distributed by the carrier, the device knows it is staying in the roaming area.

And if “Roaming is disabled” in the device’s settings, the device will not make a connection in the VPLMN.

Subsequent steps

Step 4, User Controlled PLMN (UPLMN), is used when a user wants to connect to a specific PLMN individually.

Step 5, Operator Controlled PLMN (OPLMN) is used when a user wants to be connected to a PLMN controlled by the carrier, such as when specifying a destination for international roaming.

Step 6, Other PLMN is finally used when no PLMN list is applicable.

PLMN Selection determines the network to connect to by repeatedly checking against the device’s own PLMN list, selecting the PLMN, and testing the connection to the selected PLMN.

In Conclusion

We hope that this article has clarified some of the behavior that you can expect from Soracom’s Air SIM cards. However, the definitions involved may seem complicated, if you can remember the order that various PLMN lists are checked in, you will be able to troubleshoot your connections more accurately. If you have any further questions about the behaviors addressed above, our Support Team is always available to assist you.

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