PLMN: what it is, what it consists of, where it is used

Surely many of you have already heard the term PLMN, aka Public land mobile network or public land mobile network. But not everyone knows what it is. But there is nothing complicated or incomprehensible here. It’s enough just to understand all the nuances, in fact, that’s what we’ll do now. Before we go into theory and terms, we note that any wireless communication network that exists today and that is intended for use by clients who travel on foot or by car — this is the public land mobile network. This is PLMN. A person who uses cell phone — this is the most typical example of the end use of this network. Recently, the functionality of this network has expanded somewhat due to the fact that more and more people use their smartphones, as well as other portable gadgets, not only for calls, but also for accessing the Internet.

Now let's move on to a more detailed acquaintance with the PLMN network and its features.

What is a PLMN network?

In more technical terms, Public land mobile network — this is the mobile network service area. In the Russian version we can also find a similar term, namely public cellular communication network (SSOP). At their core, both technologies are quite similar. Some represent the totality of all hundreds of one cellular network operator.  ;That is, it turns out that in one country there can be several PLMN networks at the same time. It all depends on how many cellular operators are represented in it.

We would also like to draw your attention to the fact that if the same operator is represented in different countries, then these will still be different public land mobile networks. In this case, one subscriber will be able to belong to only one home PLMN network (meaning within the country). It is also called Home PLMN (HPLMN). If this subscription is currently outside the intranet space, in particular in roaming, then it is said to be connected to the Visited PLMN (VPLMN) — that is, the guest PLMN network.

To identify the public land mobile network, a globally unique PLMN code is used, which consists of 5 or 6 digits. Here we are already identifying both the country and the cellular operator represented in this country. That is, the PLMN code will simultaneously consist of two separate codes:

1. Mobile country code: MCC.
2. Mobile network code: MNC.

Today the PLMN code may look like 001-01 or 001-001. It turns out that it is he who globally identifies the public land mobile network. You can see lists of PLMN codes on SIM cards that operators provide to their users.

Different cellular technologies can be combined within a public land mobile network, as long as they are offered by the same operator within the same country. Often, modern mobile service providers offer formats such as GSM/2G, UMTS/3G and LTE/4G. It can be assumed that 5G will soon be added here.

Now we will take a closer look at the structure of the PLMN, namely those elements of which the public land mobile network is a part.

Technologies that use PLMN

Public land mobile networks have become widespread in practice today. In particular, they can be called components of the following solutions:

1. Location area identifier — LAI.
2. Global cell identity — CGI
3. International Mobile Subscriber Identity — IMSI.

Let's look at these 3 identifiers in more detail.

Location area identifier

Location Area Identity (LAI) is a parameter that uniquely describes a specific location within an international carrier network. It is intended to determine the geographic positioning of cellular subscribers. This identifier includes:

• 3 characters mobile country code (MCC);
• 2-3 characters of the mobile network code, which will identify GSM PLMN in a specific country;
• 16-character location code (LAC): one GSM network operator can use up to 65536 LAC parameters.

As a result, we receive a unique code specific to each cellular network operator operating in a particular country. As an example, let's take LAI for OJSC MegaFon. It will roughly look like this: 2500253640. In this case, the number 250 indicates the mobile code of the Russian Federation, 02 – This is directly the Megafon operator code, 53640 – this is the location code, that is, LAC.

The location zone identifier is broadcast regularly over the broadcast control channels. This code is then recognized by one or another mobile station and stored in its Subscriber Identification Module (SIM).

If we talk about the location area itself, then this term means a region where a mobile station (which is your smartphone or any other gadget equipped with a SIM card) can move freely, but without updating the location. If it happens that one user leaves this area, then the location area is automatically changed and a request is sent to the Visitors Location Register (VLR). In turn, the location update request starts your roaming connection.

We would like to draw your attention to the fact that when connecting to roaming, only the location area of the person is determined. And this is only a few hundred. If a person is abroad and someone calls him, then the VLR network will send requests to absolutely all cells that are within the coverage area of the identified location area. After receiving the request, the mobile station sends a response that will already contain detailed information about the cell in which it is located. This is how the connection is configured.

Why does the system need to determine the location of the subscriber? Everything here is logical and explainable. First of all, this is the need to correctly charge its activities on the network. 2nd nuance — This is to ensure that incoming calls are correctly connected to the correct subscriber.

If the location area is small, then the system does not make sense to send requests to absolutely all cells. But in this case, the mobile station will need to indicate the VLR zone of its new location much more often when moving. As a result — The battery of a smartphone or tablet runs out much faster, and the consumption of radio resources increases significantly.

If, on the contrary, the location area is very large, then during the connection with your device, a so-called survey of a large number of base stations is automatically launched in order to identify the cell in which your mobile station is currently located. This is also not good, since quite impressive resources are consumed.

Grouping in a Location Area in a particular location area allows you to eradicate both of these problems or at least minimize their negative impact. Thanks to this, a balance is maintained between the accuracy of information about the geographical location of subscribers at the current time, which is stored in roaming networks, as well as the power consumption of the radio channel and the loss of charge from the smartphone’s power supply during the process of updating the location.

Global cell identity

Cell Global Identity (CGI) is a unique worldwide identifier that operates in mobile communication networks with a receiving base of transmitting stations. It includes 4 separate groups of components:

• country code used by mobile operators: MCC;
• direct mobile network code: MNC;
• location code: LAC;
• Cell ID: DI.

All these parameters — mandatory for all mobile networks. They are part of the 3GPP specification. With their help, individual base stations are identified, including for Handover — handover, namely the transfer of the current data session or call from one channel to another, connected to the same core network. That is, based on this technology, current telephone calls are transferred between various mobile technologies or separately controlled base stations.

Country code for mobile communications and the mobile network code itself — these are the key components of the PLMN identifier, but the PLMN identifier itself and the location area code — this is the LAI location area identifier, which we discussed in the previous paragraph. That is, thanks to the connection of these technologies, the location zone of a specific cellular operator network is accurately identified. That is, in its essence, such a parameter as the global identity of a cell — this is the location zone identifier with additional cell identification. This provides the additional ability to identify a specific base station in a given location area.

In simpler terms, the main area of application of this technology — This is an approximate setting of the real geographical location of the mobile phone in the current forward time. If your smartphone or any other gadget is connected to a GSM network, then the system can easily identify its geolocation. For this, global cell identity cells are used, which provide coverage for your gadget.

Another point that we would like to draw your attention to is that the mobile network code can have either a two-digit or three-digit form. And the leading role here is played by zeros. That is why indicators 01 and 001 — These are all the same mobile network codes, they are just different. Accordingly, if the code of a public land mobile network looks like 001-03 or 001-003, then they will be geographically located in the same country, but belong to different operators, in particular with code 03 and 003. But still, When assigning a PLMN code, such similarities between two- and three-digit mobile network codes are still best avoided.

Today there are quite a lot of technologies that were developed on the basis of CGI. But they all work in approximately the same direction: they allow you to determine location with a fairly high level of accuracy. Here are a few solutions that are actively used in practice today:

1. CGI + TA. This is a global cell identification with a time advance function.
2. CGI (E-CGI). Improved global cell identity
3. WCDMA. We are talking about a cell identifier, which today is used by most cellular operators in order to provide broadband radio access as part of ensuring stable operation of the 3G connection.
4. U-TDOA and ATI (Any Time Interrogation). These are parameters indicating the difference in uplink arrival time.
5. A-GPS. A high-precision terminal-based method for global positioning system.

International Mobile Subscriber Identity

International Mobile Subscriber Identity (IMSI) is an individual number for each cellular network subscriber. That is, this is a parameter that is associated with any person who uses mobile communication standards such as GSM, UMTS or CDMA. As soon as a user registers on the network, his smartphone or any other gadget transmits this code, which is subsequently identified by the system. One of the most important tasks today in the field of mobile communications security — prevent interception of this number. Because of this, these numbers are sent extremely rarely, mainly solely to ensure that the user is authenticated. In all other cases, a one-time, randomly generated TMSI code is used.

In the systems of cellular network operators, the international mobile subscriber identifier is hidden on user SIM cards. This is an EF elementary file with ID 6F07. The format in which the international mobile subscriber identifier should be stored is specified in the GSM 11.11 specification. In addition, the identifier is used by absolutely every cellular network that is connected to CDMA or EVDO according to the same principle as is implemented in GSM networks. Here the number is linked directly to the phone. If we are talking about the CDMA system, then the binding is to an analogue of SIM cards, namely to the R-UIM card.

IMSI identifier is quite long. Most of it consists of 15 digits. In practice, the PLMN code — this is the first part of the IMSI code. That is, it identifies the SIM card or USIM of the subscriber. Based on this parameter, the mobile device determines roaming. That's why, when you change your location outside of your network operator's range within a specific country, you may see an "R" in the area of the icon that usually indicates connection strength. This will indicate that you are connected to a public land mobile network whose code is different from the beginning of the IMSI

So, if the code of your PLMN network is 352-22, then the IMSI code belonging to it will be written as 352220000000001.

Public land mobile network services

Today, all mobile subscribers can receive a fairly impressive range of services through the services of the public land mobile network. Among the main points we highlight

1. Making calls to emergency services: ambulance, fire service, police stations. This is possible even if there is no mobile connection at the moment.
2. Providing voice calls to other users of the PLMN cellular network, as well as PSTN/VoIP landline lines from them.
3. The ability to use short message services (SMS) to and from any other PLMN or SIP service. By the way, SIP — it is the original form of text messages sent to cell phones and has largely replaced additional messaging applications.
4. Use of additional unstructured USSD data services that provide interaction with a specific operator. As an example, we are talking about dialing numbers that allow you to determine the current balance on your mobile phone account. This also includes connecting to the Internet via IuPS in UMTS or LTE, GPRS in GSM for the purpose of transferring data for arbitrary services.
5. Forwarding MMS multimedia messages to and from any other PLMN or SIP service. That is, here we are talking about both cellular and landline communications.
6. Providing access to the Internet.

You should also understand that the quality, availability and capacity of these services directly depends on what technology the operator uses in practice to implement the public land mobile network. But in any case, the functionality here is quite wide, as is the scope of application of this technology.

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