How roaming works - international, national, emergency / VimpelCom (Beeline) Blog

The behavior of the phone in roaming sometimes seems not quite clear and predictable, for example, in terms of network selection.

Let's try to understand from a technical point of view in some issues:

• What is the phone guided by when choosing a network in roaming?
• How do blacklists of networks work?
• Is it possible to influence the choice of network in roaming?
• What is roaming management?
• When and why you can be in roaming without even leaving Russia?
• How Emergency Firewall Roaming (AMR) Works

First of all, the following applies not only to phones, but to all GSM-UMTS-LTE mobile terminals - tablets, modems, routers, and so on. To understand the process, it is important that each base station has a transceiver that constantly transmits radio signals containing “system information”. System information is grouped into system messages (in the GSM network) or system blocks (in UMTS and LTE networks) that carry the information necessary for mobile stations in order to navigate in the environment and make decisions to perform certain actions when gaining access to services and work in a mobile network.

In particular, system information contains the following parameters:

• Country and network code (MCC and MNC - Mobile Country Code and Mobile Network Code), which determine the ownership of a particular base station transceiver in a particular operator’s network.
• The levels of the received signal at which access to a particular BS is allowed.
• The output power levels that mobile phones can use at the beginning of a communication session.
• Location Area Code (LAC) - identifier of the zone (group of base stations) within the network, which are allocated for registering the location of mobile devices and distributing signal traffic.
• Cell ID - identifier of a specific radio transmitter serving the sector (“cell”).
• In relation to packet information transfer, identifiers are transmitted:
• Routing Area ID - service area identifier in a packet information network.
• Tracking Area ID (TAC) is a unique analogue of LAC and RAC for LTE network.

and other information needed by the phone.

Further, for simplicity, we assume that this all works both at base stations and on the phone, providing the higher levels of software in the phone with the necessary information.

Turn on the phone and ...

So, having flown to another country, we turn on the phone to get access to mobile services.

After you turn on, you need to complete a number of tasks to access your mobile phone services. To speed up access to services, some tasks are performed in parallel, but there are several key procedures that must be performed.

And which networks work in the district?

Obviously, to access the telephone services, it is necessary to establish a communication channel with one of the base stations. To do this, the phone must search for base station signals for all supported radio access technologies and in all supported frequency ranges. It is clear that this task is quite lengthy, which may not cause the most pleasant emotions for the user, especially for models with the greatest capabilities for supported radio access technologies and frequency ranges. Therefore, standards allow manufacturers to optimize this process.

As a result of the search, the phone for each supported radio access technology should make a list of detected BS signals, from the signals of which it selects the information necessary for deciding on further actions when choosing a network:

• Network Code (MCC + MNC).
• LAC, CellID.
• Received signal level in order to select the communication channel with the best conditions
• Information about restrictions on access to the cell imposed by the operator, etc.

Golden Key to network services

In order to provide access to mobile communication services, the operator needs to make sure that these services can be provided to the subscriber - he has a subscription to these services. The key to accessing services is a (U) SIM card. Therefore, immediately after turning on the power of the mobile terminal and establishing the operating voltages for its nodes, the initialization procedure of the (U) SIM card begins, during which the phone searches for files and tries to read from them the information necessary for deciding on the choice of network and subsequent registration in the selected network.

6F7B - EF fplmn (Forbidden PLMN) - a list of "forbidden" networks

This is a small file for recording MCC + MNC codes for four networks organized in a "queue" (FIFO). When making a (U) SIM card, this file has no entries (FF value is written in all fields). When the phone receives a refusal with the reason code # 11 PLMN not allowed when trying to register on a “foreign” network, the MCC + MNC code of this network is written to this file. Then, when choosing a network for automatic registration, the terminal no longer attempts to automatically register in networks whose codes are currently in this file.

If the user manually selects the network whose code is contained in this file, the phone will try to register on this network, despite the fact that the network code is in the “forbidden” list.

If the registration is successful, the phone will remove the code for this network from the list of "forbidden" (fill the space with FF value). If manual registration is denied, then everything will remain unchanged.

6F30 - EF plmnsel (PLMN selector)

This is one of the oldest files that affect the choice of network in roaming. It contains network codes (at least 8 MCC + MNC codes) in the priority order of their choice in roaming. This list can be created both by the user through the telephone interface and by the operator, moreover, not only in the manufacture of (U) SIM-cards, but also subsequently, downloading information “by air”.

In the course of further development of standards and technologies, several more files appeared in which the influence of the user and operator were already separated, but information about the radio access technologies available in each network was added to the files.

6F60, 6F61, 6F62

6F60 - EF plmnwact (User controlled HPLMN Selector with Access Technology)
6F61 - EF oplmnwact (Operator controlled PLMN Selector with Access Technology)
6F62 - EF hplmnwact (HPLMN Selector with Access Technology)
These three files have the same structure. They can contain a list of codes (MCC + MNC) for at least 8 networks, but with additional information about the radio access technologies supported by each of the networks (GSM, GSM-Compact, UTRAN, E-UTRAN, cdma2000 HRPD, cdma20001xRTT).

The 6F60 file can be populated by the user, usually via the telephone interface, if the telephone manufacturer provides such an interface.

The contents of file 6F61 are controlled by the operator who issued the (U) SIM card, both when ordering a batch of (U) SIM cards and subsequently when it can change the list by transferring the new contents of the file “over the air”.

6F31 - EF hpplmn (Higher Priority PLMN Search Period)

This is a short file containing a number that determines the length of the interval after which the phone should try to search for a network with a high priority (home, etc.).

So what does the phone choose?

So, there comes a time when the switched-on phone after initializing the (U) SIM card, finished reading the files necessary for making a decision on choosing a network, and as a result of the search, it found signals from the base stations of mobile networks, and collected the system information transmitted by them. How is a decision made about automatic network selection?

3GPP-ETSI TS 23.122 automatically instructs the phone to select the network (PLMN) with the highest priority from those networks that are available and not prohibited. The phone checks the “forbidden” networks in the 6F7B file - EF fplmn (Forbidden PLMN).

But what is the highest priority? The mobile phone sequentially goes through and tries to register in networks using different technologies in the following order:

1. Home network (HPLMN - Home Public Mobile Network), or its equivalent, with the highest priority.
2. Each of the networks and its supported radio access technologies in order of priority from the 6F60 file is EF plmnwact (User controlled HPLMN Selector with Access Technology).
3. Each of the networks and its supported radio access technologies in order of priority from the 6F61 file is EF oplmnwact (Operator controlled PLMN Selector with Access Technology). If the SIM card installed in the phone does not contain files containing information about the radio access technologies supported in the listed networks, then the phone should use the list of networks from the 6F30 file - EF plmnsel (PLMN selector), and all supported radio access technologies.
4. A randomly selected network and radio access technology from those whose signals are received with high quality.
5. Other networks and radio access technologies in decreasing order of signal quality.

Which mode of network selection, automatic or manual, is better to use in international roaming?

First of all, it should be remembered that the phone will begin to work in the network selection mode (automatic or manual) in which it was at the time the power was turned off. This seems like a trifle, but if immediately after switching on the phone was automatically inserted into the pocket, and at home the network was selected manually for some reason (after all, other networks were still not allowed to go!), Then when roaming the phone will not automatically select the network, and the subscriber can remain unconnected for an indefinite time!

In addition, you need to understand that when using the phone while roaming in the manual network selection mode and subsequently there are chances to remain without communication if you are in places where the coverage of the selected operator disappears - the phone will wait for the owner’s decision to choose a network from others available in this place. Therefore, it is useful to either select one of the local networks in manual mode and wait for successful registration, or put the phone into automatic network selection mode and make sure that it is registered in one of the local networks.

How can an operator influence the automatic selection of roaming networks by phone

The recommendations of the operator who issued the (U) SIM card are written to a file 6F61 - EF oplmnwact (Operator controlled PLMN Selector with Access Technology) and / or 6F30 - EF plmnsel (PLMN selector).

Many operators are part of groups of companies providing mobile services in several countries. Naturally, in such cases, operators are interested in subscribers receiving roaming services in partner networks. And the point here is often not so much in the desire to receive additional income from higher prices, but in the desire to leave income within the group of companies. Therefore, many operators are implementing roaming management systems (Steering of Roaming). Many, probably, turning on the phone upon arrival in the country, found a welcome text message from the operator, which contained practical recommendations on the use of voicemail, etc.

It appears because at the moment of registration in roaming the “home” operator receives a request from the “business” network for information that is necessary to provide services to the subscriber, which means that the “home” operator finds out which country the subscriber is in. Having such information, it is possible to send a special format “SMS-ku” to the subscriber’s phone, which will not be displayed to the user, but will be “downloaded” by the phone to the (U) SIM-card, where it will update the contents of the desired file. Using this mechanism, the operator tells the phone priorities when automatically selecting networks in roaming.

How a user can influence the automatic selection of roaming networks by his phone

Since it has already been noted that in international roaming, in order not to be left without a connection, it is better to use automatic network selection, but you do not want to rely on the operator completely, the question arises as to how a subscriber can influence the network selection in automatic mode.

First of all, it is useful to familiarize yourself in advance with the list of operators providing services in the visited country (or countries) and the tariffs for their services. Based on this information, you can create your own list of priorities for choosing networks in the host country. In the phone’s menu in the settings section, you need to find the network settings and a list of priority networks. Since this interface is not regulated by standards, the name and location of this menu item may be different in different models, and in some models this item may be completely absent! If an item is found, then the list of networks that may be present there should be edited in accordance with the selected preferences.

Why in the list of networks to select preferred only 2G or 3G networks are indicated

Do not be embarrassed if only one radio access technology is indicated in the list for a particular network, although it is known that the operator uses several technologies - GSM, UMTS, LTE. After the phone detects the desired network and gets access to it, he will be able to learn about the network support and other technologies from the system information transmitted by the BS, then to select the one that is optimal from the point of view of radio conditions.

It is also worth noting that manual tuning of the list of priorities for choosing networks is best done while in the country, after the operator could correct the list of preferences as you see fit. Or, at a minimum, verify that a prioritization of priorities has not changed. This is especially true when both the operator and the user use the same 6F30 file, EF plmnsel (PLMN selector), to manage priorities.

Should I buy a SIM-card of a local operator during the holidays?

Everyone can solve this issue based on their interests, needs and preferences. Recently, Russian operators have significantly reduced prices for roaming services, or began to offer special options or packages that optimize the cost of communication in roaming. Of course, if you intend to communicate a lot with the owners of local numbers, then using a local operator’s SIM can be really more profitable. However, it may turn out to be inaccessible for incoming calls from the homeland - after all, not everyone knows the phone number assigned to the local SIM card, and forcing people to make international calls is not always convenient. In general, everyone solves this issue individually, based on specific conditions.

Emergency Firewall Roaming (AMR)

This concept appeared several years ago, and even now it is not very well known to a wide audience.

The fact is that the Russian regulatory framework does not allow the provision of roaming services to subscribers of another operator that has licenses in the same territory (republic, territory, region). That is why files with lists of “forbidden” networks (6F7B) in (U) SIM-cards of Russian users usually contain codes of all “alien” networks working where the subscribers live. After all, operators put BS at different points, and you can almost always find a place (elevator, basement, the middle of a large reinforced concrete building, etc.), where the phone will lose its home network for a while, and at the same time it will find another operator’s network. For the first time in such a situation, the phone will try to access the services in the network of this operator. Due to the prohibition of roaming, the phone usually receives a denial of services with reason code # 11 PLMN not allowed.

But in cases of natural disasters (for example, the recent flood in the Far East), serious accidents in the networks of operators (for example, when a meteorite falls in the Chelyabinsk region), a very large number of citizens are left without communication. They cannot call for help, for example, a doctor, inform relatives about their whereabouts or find out about their condition. In cases of such serious disasters, it is customary to help the victims "all over the world." It is for such cases, at the initiative of the regulator (the Ministry of Communications) or at the request of the injured operator, the inclusion of Emergency Network Roaming in the territory covered by an emergency may be allowed. In this case, subscribers get the opportunity to use the services of any of the operators whose network is available at a particular time. Unlike emergency calls (to number 112), when AMP is on, you can call not only the Ministry of Emergency Situations,

In such difficult situations, it is important for subscribers not to be confused and to know what needs to be done to use the networks of other operators in an emergency, because without special actions on the part of the user, their phones will not be registered in the networks of other operators in order to become available for incoming calls.

If the phone has lost contact with the network of the “home” operator due to a failure of the network equipment, the phone will automatically find the signal of one of the base stations of the other operator that has remained operational and will be in the Limited Service state, waiting for the owner’s command to make an emergency call ( 112). In this case, you can simply try to call the desired number. If AMR is enabled, then the “foreign” network must provide the connection.

If the phone continues to receive signals from the base stations of the “home” network, but the network does not allow you to make calls, you need to manually search for available networks and select one of the “foreign” networks detected by the phone. If the “alien” network provides access, then AMR is turned on, and such a subscriber will be able to make outgoing calls and receive incoming calls. If the network denies access, then you should try to repeat the manual selection of other networks detected by the phone until registration in one of the networks occurs. If none of the “foreign” networks has completed the registration, this means that the AMR is not included in this place, and in the “foreign” networks you can use only the “Emergency Call” service.