Roaming

Roaming (engl. roaming from engl. roam — to wander, wander) — the procedure of provision of services (cellular communication, Wi-Fi) to the subscriber out of service area of "home" network of the subscriber using resources of other (guest) network. At the same time the subscriber does not need to sign the agreement with the accepting operator, and the service fee is charged off his account. At telephone roaming at the subscriber his phone number remains.

From the technical point of view, service of the subscriber of a cellular network by the base station attributed to other switch already is roaming. But most often mean service in network of other operator by roaming. Such service requires the preliminary mutual arrangement between operators.

Roaming in Russia

Main article: Roaming in Russia

Roaming types

Intra network (regional) roaming

An opportunity to move from one region to another in a covering of one operator (when operators offer rates for a certain region). Due to the development of mobile technologies and falling of the prices intra network roaming is seldom offered to clients, except for the countries with the large territory (the USA, Russia, India, Brazil, etc.).

National internetwork roaming

An opportunity to use network of other mobile operator in the same country. For the commercial reasons, this type of roaming is resolved only in certain conditions. Usually it occurs when the new company obtains the license for rendering mobile services and tries to enter the market, signing with other operators the agreement on roaming in their networks, own network will not be built yet. In such countries as India where the number of regional operators is high and the country is separated into areas, this type of roaming meets rather often. Such type of roaming is most often paid by clients as though they were serviced by the network.

International roaming

An opportunity to use services of a mobile network of foreign provider. The international roaming the easiest works in the GSM standard, and more than 80% of the international mobile operators are used. However, even in this case there can be problems since in the different countries different frequencies under GSM communication are selected (there are 2 groups of the countries: the majority uses 900/1800 MHz, but the USA and some other countries in America use 850/1900 MHz). That phone worked in the country with other frequencies, it should support one or both frequencies set in this country and thus to be three - or four-band.

Interstandard roaming

Opportunity it is transparent to move between networks of different standards.

As technologies of mobile communication developed independently on different continents, there are considerable difficulties in achievement of such roaming. A large number of standards forced the largest producers to begin collaboration in achievement of interaction between technologies for the purpose of establishing interstandard roaming. Now these works are still conducted.

Even if the subscriber is in the territory where there are no cellular networks of the same standard, as at it, it can use roaming in networks of other standard. For example, the subscriber of GSM can use CDMA network or satellite (for example, Thuraya). For this purpose it needs phone working in different standards. Or he can use phone of the local standard with the SIM card.

Roaming operation principle in GSM network

Details of process of roaming differ depending on type of cellular transmission networks, but generally, process proceeds as follows:

• at first attempts to detect a home network, then, in case of failure are made, networks of the list which is stored on the SIM card are looked for further all other networks in process of decrease of the signal level get over. If between networks roaming is not prohibited, then the device tries to be registered in network. If the guest network is in the same country, as house, then phone will be times to check whether house appeared. In the manual mode all detected networks including prohibited are brought to the display;
• the guest network connects to house and requests the service information about the device by number IMSI. First of all it becomes clear whether the service is resolved;
• in case of success the guest network begins to service the time user account for the device, and house configures redirection on it

If the call on phone which is in the roaming mode was made, the telephone network sends a call to provider of services at whom number which then should send it to guest network is originally registered. This network should provide the internal temporary phone number for this mobile phone. As soon as temporary number is provided, the home network directs an incoming call to the temporary phone number.

In order that the user could "enter" into guest network, there has to be an agreement on roaming between guest and home networks. This agreement is established after series of the processes of testing called by IREG and TADIG. IREG testing checks functioning of the established connection while TADIG is intended for testing of billing on calls.

The transactions made by the user in guest network register in the file under the name TAP (Transferred Account Procedure) for GSM/CIBER (Cellular Intercarrier Billing Exchange Roamer) and are transferred to a home network. The TAP/CIBER file contains data on all calls made by the user. Payment amounts which users should pay to home operator are calculated for these files. Billing term to the Subscriber for the services rendered in roaming can reach 36 days from the moment of rendering services. This term is regulated by GSMA|the International GSM Association.

Signaling during a call

Process of signaling is considered on the example of a call of the subscriber who is in roaming in guest network.

• The calling party dials the mobile number MSISDN of the called party which is in roaming.
• Based on information which contains in MSISDN (the national code of the direction and the country code) the call is routed in the switching center gateway of GMSC. It is implemented using IAM (Initail Address Message) of an ISUP subsystem.
• To define location of the called party, GMSC sends HLRU SRI (Send Routing Information) of a subsystem of MAP (Mobile Application Part). The SRI message of a subsystem of MAP contains MSISDN number which HLR will complement with the identifier IMSI of the called party.
• As the called party was already registered in guest network, home HLR already knows what VLR services the subscriber. HLR sends to VLR of guest network the message of MAP PRN (Provide Roaming Number) to learn the roaming subscriber number of MSRN. After that home HLR will be capable to route a call to the correct address MSC.
• Together with IMSI which is contained in the message of MAP PRN, VLR appoints to the subscriber in guest network the temporary number known as the roaming subscriber number of MSRN. This number MSRN goes back home MSC using RIA (Routing Information Acknowledgement).
• Now, together with number MSRN, GMSC knows where exactly to route a call. The call using ISUP-(or TUP-) signallings between home GMSC and guest MSC is made. ISUP generates the IAM message supporting MSISDN and IMSI of the called party.
• Having received the IAM message, MSC of guest network distinguishes MSRN and IMSI for which this MSRN was selected. After this MSC sends the message of MAP SI (Send Information) for information request about the subscriber (available services  , etc.). If the called party is successfully authorized and ready to receive a call, VLR sends the message of the CC (Complete Call) subsystem of MAP for MSC.

Chronicle

2020: Telefónica, Deutsche Telekom and Vodafone started a blockchain system for calculations in roaming

On July 13, 2020 it became known of that, three cellular operators - Telefónica (Spain), Deutsche Telekom (Germany) and Vodafone (Great Britain) - completed testing and began full to use a solution blockchain which will allow in the automatic mode of a message calculations for payments for roaming. It is supposed that this project will reduce capital costs and will provide more operational accounting of profit and mutual calculations. Read more here.

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