May 4, 2017 at 11:41The past and future of cellular communications in areas where there is no GSM
Despite the fact that telecom operators are constantly increasing the number of installed base stations, even in large cities there are still areas with unstable reception.
A subscriber who, by chance, finds himself in such a zone, either has to reconcile and wait until the operator thinks about the coverage, or correct the situation on their own. We will not consider ways of “circumventing” our operator through third-party services, but talk about how services can be organized by the same provider in such “dead” zones.
It is clear that communication cannot be organized from scratch. When there is no coverage by base stations, you have to use a different transmission medium. And today the most affordable is the Internet.
The idea of making calls over the Internet without switching to another service provider has been in the industry for a long time. At first, it was implemented as an additional device - femtocells, which allows creating a kind of “bridge” between user devices and the network of a mobile operator via the public Internet of any provider. Such a “bridge” helps locally improve reception quality. The only limitation is that the user device must support the communication standard used by the femtocell (in general, it may not coincide with the standard used in the carrier network).
Femtocells never entered the mass market when they were replaced by Wi-Fi calling - a technology that allows you to simply call via Wi-Fi using your smartphone, but without using additional intermediary devices. But let's talk about everything in order.
The concept of femtocells has been developing since 2001 as part of the idea of small cells. Among the “brethren” femtocells are distinguished by the fact that they are subscriber, not operator devices, and work through a channel that is not controlled by the operator - a regular Internet channel.
The idea of creating a “miniature cell” arose even earlier: in parallel with the development of mobile communications, interesting solutions appeared, but in the 1990s they were far from modern femtocells. The first product, reminiscent of today's femtocell (a self-configuring home GSM base station), was announced by Alcatel in 1999, but did not enter the mass market because it turned out to be quite voluminous and expensive. It took five years of miniaturization of the components and further technological growth for several companies, including picoChip and Ubiquisys, which are still arguing who is the author of the term popular today, to work closely on femtocells.
Since 2007, the concept was developed by the international organization Femto Forum, which included several equipment manufacturers (now it is the Small Sell Forum, which is responsible for all small cells, both user and operator). In 2008, the world saw several solutions that made it possible to build commercial services based on femtocells. The cost and consumer characteristics of these devices no longer impeded the introduction of the concept to the masses. In the same 2008, with the direct participation of the Femto Forum, femtocells were standardized. The document became part of the 3GPP standard, and dozens of equipment manufacturers and operators around the world became interested in the topic.
The first operator services involving the rental or sale of femtocells were launched in the United States and Japan in 2009. By 2011, the concept came to Russia, despite certain restrictions on the law, more on that later.
Now femtocell is a device the size of a familiar Wi-Fi router distributed by mobile operators. Femtocells may vary in performance - for home, office use or for outdoor use. They may include a set of functions of a router or other network devices. True, unlike the same routers, at the moment in Russia femtocells cannot be purchased from an independent supplier without reference to the operator.
The power of femtocells is not more than 100 mW. Thus, they are designed to service small rooms - for example, an office or apartment. The coverage area depends on the type of room (configuration and wall material), the electromagnetic environment, in particular, the presence of other signal sources operating in the same range. On average, it has a radius of 10 - 30 m.
The most common devices are UMTS (MegaFon and other Russian operators, for example, use the UMTS2100), but there are femtocells that support other communication standards. But talking about exotic is beyond the scope of this article.
In Russia, a femtocell operates in a licensed frequency range, but for its installation, the user does not need to obtain permission (following the example of radio amateurs who used to transport even radios operating at 27 MHz to the Ministry of Communications). These issues are solved by the operator, and the subscriber only needs to not change the “registration address” of the device.
The device’s movement is monitored by the operator by IP address. Some manufacturers even integrate GPS into femtocells (whether it is used or not depends on the operator). But sometimes small movements are still possible - in particular, Megafon, using the lock on the surrounding cells, allows femtocells to be moved within a radius of 250 meters. And with the notification of the service provider, you can transfer the femtocell within the licensed territory of the operator’s branch.
The described difficulties with relocation are associated with the operator’s obligations in certain situations to provide information about calls to the Ministry of Emergencies and the Ministry of Internal Affairs, as well as to service emergency calls, as required by law.
These same requirements previously hindered the development of femtocells in the direction of private users: legally “operator” equipment could not be leased to home users, only to business. Since 2013, some femtocells have finally ceased to be considered "operator" equipment (Rules for the use of subscriber terminals of mobile radio telephone systems of the UMTS standard with frequency duplex and frequency-code division of radio channels operating in the frequency range 2000 MHz, approved by the Order of the Ministry of Information Technology and Communications of the Russian Federation Federation of August 27, 2007 No. 100 (Order of the Ministry of Information Technologies and Communications of the Russian Federation No. 147 of June 25, 2013). For this, its power must be limited, and the manufacturer must go through the certification procedure of the device as a home one. True,no . The attempts of some operators to introduce such a service have not yet been crowned with success. Radio licensing also does not allow the creation and sale of operator-independent femtocells, i.e. to bring from abroad (order from China) more interesting or cheaper analogues, as users are used to doing with other “network” electronics.
For the femtocell to work, you must be connected to the mains and the Internet. The minimum Internet connection speed, providing the transfer of technical information and servicing one client device in 3G, is at least 500 Kbps (better than 2-5 Mbps), but with an increase in the number of connected users and the services they use, the required speed, of course, grows proportionally.
High-quality voice transmission additionally requires stability of the connection
(without short breaks, jitter fluctuations of more than 50 ms and loss of more than 1.5% of packets) and the absence of blocking of ports and protocols used by the femtocell.
Unlike a large-scale base station, the femtocell is installed and launched not by the operator, but by the end user, respectively, here is a slightly different approach to configuration: everything works out of the box and is controlled by the operator. All that is needed from the user is to set the network parameters for connecting to the Internet and a list of “allowed” numbers for maintenance (that is, specify SIM cards that will be allowed to use the device). All this can be done through the web interface, similar to setting up the same network router.
Authorization in the operator network is carried out using a technological SIM-card. Connection to it is carried out through a security gateway (SeGW) - the femtocell is not important, it uses a static or dynamic IP address. To transmit voice call packets, a secure communication channel is established with the operator using the IPsec protocol.
Although information on the vulnerability detection of certain femtocells periodically appeared on the network, allowing attackers to make calls at the expense of subscribers served by the device and even “clone” their phones, all these incidents related to certain firmware versions of specific devices (tied to certain operators) and allowed fraud only within the range of femtocells. The femtocell itself is comparable in vulnerability to a user’s smartphone. Known problems with femtocells in most situations were solved by updating the firmware; Moreover, Russian operators have not yet appeared in such messages.
After installation and the simplest setup, the femtocell behaves almost like a “large” base station: when entering the reception zone (when its signal is stronger than the signal of the base station), the user’s mobile device switches to it, regardless of the standby / talk mode. The network receives a notification about this event, which can be used to track visitors through third-party tools or SMS messages (the availability of this service depends on the operator’s policy).
The device can support 4 - 32 simultaneous voice calls and 4 - 16 channels of packet data transmission (depending on the configuration and operator settings). In the case of UMTS femtocells, end devices must support 3G to connect to the network.
For an operator, a femtocell is a way to expand network coverage without installing “full-fledged” base stations, as well as partially reduce the load from existing segments by connecting part of the subscribers to the femtocells. Well, do not forget about additional monetization of the services provided.
Yes, and from the point of view of users, femtocells can come up with quite a few areas of application.
Firstly, it is the organization of mobile communications where it does not exist or the quality leaves much to be desired (but where there is a broadband access channel). This is relevant primarily for offices on the territory of industrial zones and in industrial premises where walling shields the signal of “ordinary” base stations or there is no cellular coverage, as well as for those who are expanding their activities in the literal sense of “in the field”.
Femtocell can also work via satellite Internet, which provides maintenance of cell phones even in a dense forest. True, satellite Internet is characterized by large delays, respectively, delays / disconnections will be in the conversation. Femtocell via satellite is rather exotic.
When using a more stable wired Internet, femtocells can not only improve the quality of voice services, but also reduce the overall electromagnetic background, since the phone, in the immediate vicinity of the cell, automatically reduces the radiation power (setting it at a level sufficient for stable signal transmission).
Secondly, a femtocell can become a tracking tool, both in a good and a bad sense.
The simplest phone tracking tool is notifications about the appearance of a specific subscriber in the femtocell coverage area. With it, restless parents can set up an alert to register their children’s phones in the “home network” to know that they have returned home from school; supervisors can control the time employees come to work in the same way. Automation can also monitor alerts, starting, for example, synchronizing data on a smartphone with an office server.
A different level of surveillance can be achieved through the use of “modified” femtocells, such as spy equipmentInfoWatch The device announced by the company intercepts and records conversations of subscribers connected to the femtocell, protecting the interests of the state or an individual business. Technologies can come up with a lot of applications, both “honest” and not very, from a banal analysis of the effectiveness of the call center to total control over employees, even using personal smartphones.
In general, the functionality of a femtocell depends on the tariff policy of the operator, the options supported by it and the accepted principles of network expansion.
For example, femtocells can be used to combat network congestion, namely, a relatively quick reaction to increasing the number of connected devices. This is relevant, for example, for events held in non-standard venues (outside exhibition complexes, geared to a large flow of visitors). It may be important for the organizers that the participants of the event at any time can communicate with each other or with the outside world. The load can be distributed both for all devices located in the area of the femtocell, and only for “their own” (SIM-cards from the “white list”). Although in Megafon such decisions are not applied.
Femtocells did not have time to enter the Russian consumer market en masse, and they were replaced by another way to organize communications in areas not covered by base stations - Wi-Fi calling. This is a service that allows you to make calls from smartphones through an available Wi-Fi network in the absence of GSM connection.
Wi-Fi calling is also part of a broader concept - the so-called Unlicensed Mobile Access (UMA) - a way to connect external IP-subscribers to the carrier network. This access is provided by the Generic Access Network (GAN), and the transmission of IP packets from the unmanaged Internet to the operator network is carried out by the corresponding controller - GAN Controller (in this sense, the femtocell is standard GAN equipment).
The methodology for providing such access was described in 3GPP Release 6. And the first commercial implementations of UMA appeared in 2005-2006.
From the subscriber’s side, the function should be implemented in the smartphone software (in the form of an integrated or additionally installed software module). It is activated through the settings of the device and works if it is supported by a mobile operator.
Wi-Fi calling technology supports seamless transfer of a call from a cellular network to Wi-Fi and vice versa, so it doesn’t look like usual Skype or Viber (though there may be a disconnection when switching from the service area of one Wi-Fi router to another - this determined by Wi-Fi technology itself). When you enable the smartphone software option, you can choose the priority - where to send the call if both networks are available; Further, the device and the operator network will control how the call goes, independently.
From the point of view of any other subscriber, a Wi-Fi calling looks like a normal call. You can call the numbers of any operators. When such a call arrives, the caller’s phone number will be displayed on the screen of the interlocutor’s device, and the call will be paid in accordance with the selected tariff.
It should be noted that Wi-Fi calling does not allow to bypass expensive international roaming. Part of the standard is the search for existing GSM towers of any operators when the device is turned on, which automatically gives information about the location of the subscriber. Thus, the possibility of saving on calls via Wi-Fi calling is completely determined by the operator’s policy.
Wi-Fi calling is already certified in Russia. Its introduction a couple of years ago was hindered by a legislative conflict prohibiting, in accordance with the license, providing local telephone subscribers with access to the voice network. But the issue was resolved last year.
Wi-Fi calling services are already available to subscribers of Russian operators (at least in some regions). Testing this opportunity and Megaphone.
From a user point of view, the technology has a number of advantages against the background of femtocells. The main one is the lack of additional equipment, i.e. additional costs and bureaucratic difficulties associated with licensing frequencies. Intermediate devices are not used at all, which means there is no talk of getting any permissions, binding something to the “place of registration”, etc. (as we discussed above in relation to femtocells).
The only “but” is that Wi-Fi calling is not available on all devices. To support Wi-Fi calling (and UMA in principle) it is necessary that the phone is equipped not only with GSM, but also with a Wi-Fi module, which is initially more expensive - i.e. the cheapest “dialers” will never receive support for this technology.
The lack of intermediate devices leads to the fact that Wi-Fi calling, unlike femtocells, does not allow to deploy any additional services. Thus, for femtocells, there is still a place in the market. Therefore, these devices continue to develop rapidly. The need for them is primarily in the corporate sector, where it is important to provide communication to all visitors to a certain area. This problem has not yet been solved via Wi-Fi calling.
Thus, the relatively old and new technologies complement each other, jointly solving the issue of providing voice services where GSM coverage leaves much to be desired.
A subscriber who, by chance, finds himself in such a zone, either has to reconcile and wait until the operator thinks about the coverage, or correct the situation on their own. We will not consider ways of “circumventing” our operator through third-party services, but talk about how services can be organized by the same provider in such “dead” zones.
It is clear that communication cannot be organized from scratch. When there is no coverage by base stations, you have to use a different transmission medium. And today the most affordable is the Internet.
The idea of making calls over the Internet without switching to another service provider has been in the industry for a long time. At first, it was implemented as an additional device - femtocells, which allows creating a kind of “bridge” between user devices and the network of a mobile operator via the public Internet of any provider. Such a “bridge” helps locally improve reception quality. The only limitation is that the user device must support the communication standard used by the femtocell (in general, it may not coincide with the standard used in the carrier network).
Femtocells never entered the mass market when they were replaced by Wi-Fi calling - a technology that allows you to simply call via Wi-Fi using your smartphone, but without using additional intermediary devices. But let's talk about everything in order.
Femtocells - the birth of an idea
The concept of femtocells has been developing since 2001 as part of the idea of small cells. Among the “brethren” femtocells are distinguished by the fact that they are subscriber, not operator devices, and work through a channel that is not controlled by the operator - a regular Internet channel.
The idea of creating a “miniature cell” arose even earlier: in parallel with the development of mobile communications, interesting solutions appeared, but in the 1990s they were far from modern femtocells. The first product, reminiscent of today's femtocell (a self-configuring home GSM base station), was announced by Alcatel in 1999, but did not enter the mass market because it turned out to be quite voluminous and expensive. It took five years of miniaturization of the components and further technological growth for several companies, including picoChip and Ubiquisys, which are still arguing who is the author of the term popular today, to work closely on femtocells.
Since 2007, the concept was developed by the international organization Femto Forum, which included several equipment manufacturers (now it is the Small Sell Forum, which is responsible for all small cells, both user and operator). In 2008, the world saw several solutions that made it possible to build commercial services based on femtocells. The cost and consumer characteristics of these devices no longer impeded the introduction of the concept to the masses. In the same 2008, with the direct participation of the Femto Forum, femtocells were standardized. The document became part of the 3GPP standard, and dozens of equipment manufacturers and operators around the world became interested in the topic.
The first operator services involving the rental or sale of femtocells were launched in the United States and Japan in 2009. By 2011, the concept came to Russia, despite certain restrictions on the law, more on that later.
Modern femtocell
Now femtocell is a device the size of a familiar Wi-Fi router distributed by mobile operators. Femtocells may vary in performance - for home, office use or for outdoor use. They may include a set of functions of a router or other network devices. True, unlike the same routers, at the moment in Russia femtocells cannot be purchased from an independent supplier without reference to the operator.
Service zone
The power of femtocells is not more than 100 mW. Thus, they are designed to service small rooms - for example, an office or apartment. The coverage area depends on the type of room (configuration and wall material), the electromagnetic environment, in particular, the presence of other signal sources operating in the same range. On average, it has a radius of 10 - 30 m.
frequency range
The most common devices are UMTS (MegaFon and other Russian operators, for example, use the UMTS2100), but there are femtocells that support other communication standards. But talking about exotic is beyond the scope of this article.
In Russia, a femtocell operates in a licensed frequency range, but for its installation, the user does not need to obtain permission (following the example of radio amateurs who used to transport even radios operating at 27 MHz to the Ministry of Communications). These issues are solved by the operator, and the subscriber only needs to not change the “registration address” of the device.
The device’s movement is monitored by the operator by IP address. Some manufacturers even integrate GPS into femtocells (whether it is used or not depends on the operator). But sometimes small movements are still possible - in particular, Megafon, using the lock on the surrounding cells, allows femtocells to be moved within a radius of 250 meters. And with the notification of the service provider, you can transfer the femtocell within the licensed territory of the operator’s branch.
The described difficulties with relocation are associated with the operator’s obligations in certain situations to provide information about calls to the Ministry of Emergencies and the Ministry of Internal Affairs, as well as to service emergency calls, as required by law.
These same requirements previously hindered the development of femtocells in the direction of private users: legally “operator” equipment could not be leased to home users, only to business. Since 2013, some femtocells have finally ceased to be considered "operator" equipment (Rules for the use of subscriber terminals of mobile radio telephone systems of the UMTS standard with frequency duplex and frequency-code division of radio channels operating in the frequency range 2000 MHz, approved by the Order of the Ministry of Information Technology and Communications of the Russian Federation Federation of August 27, 2007 No. 100 (Order of the Ministry of Information Technologies and Communications of the Russian Federation No. 147 of June 25, 2013). For this, its power must be limited, and the manufacturer must go through the certification procedure of the device as a home one. True,no . The attempts of some operators to introduce such a service have not yet been crowned with success. Radio licensing also does not allow the creation and sale of operator-independent femtocells, i.e. to bring from abroad (order from China) more interesting or cheaper analogues, as users are used to doing with other “network” electronics.
Internet requirements
For the femtocell to work, you must be connected to the mains and the Internet. The minimum Internet connection speed, providing the transfer of technical information and servicing one client device in 3G, is at least 500 Kbps (better than 2-5 Mbps), but with an increase in the number of connected users and the services they use, the required speed, of course, grows proportionally.
High-quality voice transmission additionally requires stability of the connection
(without short breaks, jitter fluctuations of more than 50 ms and loss of more than 1.5% of packets) and the absence of blocking of ports and protocols used by the femtocell.
Customization
Unlike a large-scale base station, the femtocell is installed and launched not by the operator, but by the end user, respectively, here is a slightly different approach to configuration: everything works out of the box and is controlled by the operator. All that is needed from the user is to set the network parameters for connecting to the Internet and a list of “allowed” numbers for maintenance (that is, specify SIM cards that will be allowed to use the device). All this can be done through the web interface, similar to setting up the same network router.
Security
Authorization in the operator network is carried out using a technological SIM-card. Connection to it is carried out through a security gateway (SeGW) - the femtocell is not important, it uses a static or dynamic IP address. To transmit voice call packets, a secure communication channel is established with the operator using the IPsec protocol.
Although information on the vulnerability detection of certain femtocells periodically appeared on the network, allowing attackers to make calls at the expense of subscribers served by the device and even “clone” their phones, all these incidents related to certain firmware versions of specific devices (tied to certain operators) and allowed fraud only within the range of femtocells. The femtocell itself is comparable in vulnerability to a user’s smartphone. Known problems with femtocells in most situations were solved by updating the firmware; Moreover, Russian operators have not yet appeared in such messages.
Service continuity
After installation and the simplest setup, the femtocell behaves almost like a “large” base station: when entering the reception zone (when its signal is stronger than the signal of the base station), the user’s mobile device switches to it, regardless of the standby / talk mode. The network receives a notification about this event, which can be used to track visitors through third-party tools or SMS messages (the availability of this service depends on the operator’s policy).
The device can support 4 - 32 simultaneous voice calls and 4 - 16 channels of packet data transmission (depending on the configuration and operator settings). In the case of UMTS femtocells, end devices must support 3G to connect to the network.
Practice
For an operator, a femtocell is a way to expand network coverage without installing “full-fledged” base stations, as well as partially reduce the load from existing segments by connecting part of the subscribers to the femtocells. Well, do not forget about additional monetization of the services provided.
Yes, and from the point of view of users, femtocells can come up with quite a few areas of application.
Firstly, it is the organization of mobile communications where it does not exist or the quality leaves much to be desired (but where there is a broadband access channel). This is relevant primarily for offices on the territory of industrial zones and in industrial premises where walling shields the signal of “ordinary” base stations or there is no cellular coverage, as well as for those who are expanding their activities in the literal sense of “in the field”.
Femtocell can also work via satellite Internet, which provides maintenance of cell phones even in a dense forest. True, satellite Internet is characterized by large delays, respectively, delays / disconnections will be in the conversation. Femtocell via satellite is rather exotic.
When using a more stable wired Internet, femtocells can not only improve the quality of voice services, but also reduce the overall electromagnetic background, since the phone, in the immediate vicinity of the cell, automatically reduces the radiation power (setting it at a level sufficient for stable signal transmission).
Secondly, a femtocell can become a tracking tool, both in a good and a bad sense.
The simplest phone tracking tool is notifications about the appearance of a specific subscriber in the femtocell coverage area. With it, restless parents can set up an alert to register their children’s phones in the “home network” to know that they have returned home from school; supervisors can control the time employees come to work in the same way. Automation can also monitor alerts, starting, for example, synchronizing data on a smartphone with an office server.
A different level of surveillance can be achieved through the use of “modified” femtocells, such as spy equipmentInfoWatch The device announced by the company intercepts and records conversations of subscribers connected to the femtocell, protecting the interests of the state or an individual business. Technologies can come up with a lot of applications, both “honest” and not very, from a banal analysis of the effectiveness of the call center to total control over employees, even using personal smartphones.
In general, the functionality of a femtocell depends on the tariff policy of the operator, the options supported by it and the accepted principles of network expansion.
For example, femtocells can be used to combat network congestion, namely, a relatively quick reaction to increasing the number of connected devices. This is relevant, for example, for events held in non-standard venues (outside exhibition complexes, geared to a large flow of visitors). It may be important for the organizers that the participants of the event at any time can communicate with each other or with the outside world. The load can be distributed both for all devices located in the area of the femtocell, and only for “their own” (SIM-cards from the “white list”). Although in Megafon such decisions are not applied.
Wifi calling
Femtocells did not have time to enter the Russian consumer market en masse, and they were replaced by another way to organize communications in areas not covered by base stations - Wi-Fi calling. This is a service that allows you to make calls from smartphones through an available Wi-Fi network in the absence of GSM connection.
Technology
Wi-Fi calling is also part of a broader concept - the so-called Unlicensed Mobile Access (UMA) - a way to connect external IP-subscribers to the carrier network. This access is provided by the Generic Access Network (GAN), and the transmission of IP packets from the unmanaged Internet to the operator network is carried out by the corresponding controller - GAN Controller (in this sense, the femtocell is standard GAN equipment).
The methodology for providing such access was described in 3GPP Release 6. And the first commercial implementations of UMA appeared in 2005-2006.
From the subscriber’s side, the function should be implemented in the smartphone software (in the form of an integrated or additionally installed software module). It is activated through the settings of the device and works if it is supported by a mobile operator.
Wi-Fi calling technology supports seamless transfer of a call from a cellular network to Wi-Fi and vice versa, so it doesn’t look like usual Skype or Viber (though there may be a disconnection when switching from the service area of one Wi-Fi router to another - this determined by Wi-Fi technology itself). When you enable the smartphone software option, you can choose the priority - where to send the call if both networks are available; Further, the device and the operator network will control how the call goes, independently.
From the point of view of any other subscriber, a Wi-Fi calling looks like a normal call. You can call the numbers of any operators. When such a call arrives, the caller’s phone number will be displayed on the screen of the interlocutor’s device, and the call will be paid in accordance with the selected tariff.
It should be noted that Wi-Fi calling does not allow to bypass expensive international roaming. Part of the standard is the search for existing GSM towers of any operators when the device is turned on, which automatically gives information about the location of the subscriber. Thus, the possibility of saving on calls via Wi-Fi calling is completely determined by the operator’s policy.
Russian reality
Wi-Fi calling is already certified in Russia. Its introduction a couple of years ago was hindered by a legislative conflict prohibiting, in accordance with the license, providing local telephone subscribers with access to the voice network. But the issue was resolved last year.
Wi-Fi calling services are already available to subscribers of Russian operators (at least in some regions). Testing this opportunity and Megaphone.
Advantages over Femtocells
From a user point of view, the technology has a number of advantages against the background of femtocells. The main one is the lack of additional equipment, i.e. additional costs and bureaucratic difficulties associated with licensing frequencies. Intermediate devices are not used at all, which means there is no talk of getting any permissions, binding something to the “place of registration”, etc. (as we discussed above in relation to femtocells).
The only “but” is that Wi-Fi calling is not available on all devices. To support Wi-Fi calling (and UMA in principle) it is necessary that the phone is equipped not only with GSM, but also with a Wi-Fi module, which is initially more expensive - i.e. the cheapest “dialers” will never receive support for this technology.
The lack of intermediate devices leads to the fact that Wi-Fi calling, unlike femtocells, does not allow to deploy any additional services. Thus, for femtocells, there is still a place in the market. Therefore, these devices continue to develop rapidly. The need for them is primarily in the corporate sector, where it is important to provide communication to all visitors to a certain area. This problem has not yet been solved via Wi-Fi calling.
Thus, the relatively old and new technologies complement each other, jointly solving the issue of providing voice services where GSM coverage leaves much to be desired.