Global system for mobile communications

The global system for 2G mobile communications (from the English Global System for Mobile communications, abbreviated as GSM, and originally from the French groupe spécial mobile) is a standard system, developed by the European Telecommunications Standards Institute (ETSI) royalty-free, for digital mobile telephony.

A GSM client can connect through your phone to your computer and send and receive email messages, faxes, surf the Internet, securely access a company's computer network (local network/Intranet), as well as use other digital data transmission features, including short message service (SMS) or text messaging.

Logo to identify compatible terminals and systems.

GSM is considered, due to its transmission speed and other characteristics, a second generation (2G) standard. Its extension to 3G is called UMTS and differs in its higher transmission speed, the use of a slightly different network architecture and above all in the use of different radio protocols (W-CDMA).

Global reach and percentage of use

The GSM Association (GSMA or GSM Association), says that GSM is the most widespread mobile telecommunications standard in the world, with 82% of the world's terminals in use. GSM has more than 3 billion users in 159 different countries, being the predominant standard in Europe, South America, Asia and Oceania, and to a great extent in North America.

The ubiquity of the GSM standard has been an advantage both for consumers (who benefit from the roaming capacity and the ease of changing operators without changing terminals, simply by changing the SIM card) and for network operators (who can choose among multiple providers of GSM systems, as it is an open standard that does not require license fees).

GSM first implemented the short text message service (SMS), which was later extended to other standards. In addition, GSM defines a single emergency number worldwide, 112, which makes it easier for travelers from anywhere in the world to communicate emergency situations without having to know a local number.

Frequencies

The GSM radio interface has been implemented in different frequency bands.

History and development

The first GSM teams of 1991.

The GSM standard was developed starting in 1982. At the CEPT telecommunications conference of that year, the working group Groupe Spécial Mobile or GSM, was created, whose task was to develop a European standard for digital mobile telephony. It sought to avoid the problems of analog mobile telephone networks, which had been introduced in Europe at the end of the 1950s, and were not fully compatible with each other despite using, in part, the same standards. The GSM group involved 26 European telecommunications companies.

In 1990 the specifications for the first GSM-900 standard were finalized, followed by DCS-1800 a year later. In 1991 the first GSM telephone equipment was presented as prototypes. In parallel, the name of the group was changed to Standard Mobile Group (SMG) and the initials GSM from this moment on were used for the standard itself.

In 1992 the first European GSM-900 networks began their activity, and the same year the first GSM mobile phones were introduced to the market, the first being the Nokia 1011 in November of this year. In the following years, the GSM competed with other digital standards, but also ended up prevailing in Latin America and Asia.

In 2000, the GSM standardization working group was transferred to the TSG GERAN (Technical Specification Group GSM EDGE Radio Access Network) group of the 3GPP cooperation program, created to develop the third generation mobile telephony (3G). The successor to GSM, UMTS, was introduced in 2001, however its acceptance was slow, so a large part of mobile phone users in 2010 were still using GSM.

Network architecture

Division of available spectrum

When designing the network structure for a mobile phone system, the problem to face is the limitation in the range of available frequencies. Every "conversation" (or each client of data traffic) requires a minimum of bandwidth so that it can be transmitted correctly. Each operator in the market is assigned a certain bandwidth, in certain delimited frequencies, which must be distributed for sending and receiving traffic to the different users (who, on the one hand, receive the signal from the other end, and on the other, another send their part of the “conversation”). Therefore, a single antenna cannot be used to receive the signal from all the users at the same time, since the bandwidth would not be enough; and in addition, the ranges in which some users emit must be separated to avoid interference between their shipments. This problem, or rather its solution, is often referred to as spectrum sharing or media access control. The GSM system bases its channel access division on combining the following available spectrum distribution models. The first is decisive when specifying the network architecture, while the rest is resolved with circuitry in the operator's terminals and antennas:

• Use of adjacent cells at different frequencies to better distribute frequencies (SDMA, Space Division Multiple Access or multiple access by division of space); reuse of frequencies in non-contiguous cells;
• Division of time in emission and reception by TDMA (Time Division Multiple Access, or multiple access by division of time;
• Separation of bands for broadcasting and reception and radio channel subdivision (FDMA protocol, Frequency Division Multiple Access or multiple access by frequency division;
• Pseudo random variation of the transmission frequency from terminal to network (FHMA, Frequency Hops Multiple Access or multiple access by frequency jumps).

The BSS, the lower layer of the architecture (user terminal – BS – BSC), solves the problem of terminal access to the channel. The next layer (NSS) will be in charge, on the one hand, of routing (MSC) and on the other of subscriber identification, billing and access control (HLR, VLR and other operator databases). This paragraph with so many acronyms is explained more calmly below, but it serves as a general summary of the network architecture used.

On the other hand, the communications that are established will travel through different systems. To simplify, a communication channel established between one system and another is called a communication channel, regardless of the method actually used to establish the connection. In GSM there are defined a series of logical channels for the traffic of calls, data, signaling and other purposes.

Radio and Radio Control Layer: Base Station Subsystem or BSS

This network layer is in charge of providing and controlling the access of the terminals to the available spectrum, as well as the sending and receiving of the data.

Cell division: base stations or BS

General outline of a GSM network.

The system must be able to support a large user load, with many of them using the network at the same time. If there was only one antenna for all users, the available radio space would quickly become saturated due to lack of bandwidth. One solution is to reuse available frequencies. Instead of putting a single antenna for an entire city, several are placed, and the system is programmed so that each antenna uses different frequencies from its neighbors, but the same as other antennas outside its range. A certain frequency range is reserved for each antenna, which corresponds to a certain number of radio channels (each of the frequency ranges in which an antenna sends data). Thus, the channels assigned to each antenna in the operator's network are different from those of neighboring antennas, but can be repeated between non-adjacent antennas.

In addition, the antennas are provided with the necessary network electronics to communicate with a central control system (and the next logical layer of the network) and so that they can take charge of managing the radio interface: the set of the antenna with its electronics and its link to the rest of the network is called a Base Transceiver Station (BTS). The geographic area that a station provides coverage base is called cell or cell (from English cell, which is why the GSM system belongs to the family of cellular systems).

The use of cells requires an additional network layer that is new in the GSM standard compared to previous systems: it is the base station controller, or BSC, (Base Station Controller) that acts as an intermediary between the backbone and the base stations. Among other functions, it is in charge of assigning radio resources (radio channel and time slot) to users, controlling their power or managing the handover procedure. The set of base stations coordinated by a BSC provide the link between the user's terminal and the next network layer, and the main one, which we will see later. As a network layer, the set of BSs + BSC is called base station subsystem, or BSS (Base Station subsystem).

A GSM base station can reach a coverage radius around it from several hundred meters (in urban stations) to a practical maximum of 35 km (in rural areas), depending on its power and the geography of the environment. However, the number of users that each BS can serve is limited by the bandwidth (subdivided into channels) that the BSC assigns to each station, and although it might be thought that base stations should have great power to cover a larger area, they have a nominal power of 320 W maximum (compared to FM or television antennas, which have emission powers of thousands of Watts, an almost negligible value) and in fact always emit at the lowest possible power level to avoid interfering with cells that could use the same range of frequencies, which is why it is rare for models of more than 40 W to be installed. Moreover, in highly populated urban areas or tunnels, a greater number of BSs with very limited power (less than 2.5 W) to allow the creation of pico and microcell calls, which allow better frequency reuse (the more stations, the more frequency reuse and the more users admissible at the same time) or They do not provide coverage in places that a normal BS does not reach or require a large capacity (underground or highway tunnels, very crowded spaces, highly populated cities).

Therefore, in areas where there is a high concentration of users, such as cities, a large number of BSs with very limited power must be installed, and in areas with lower density of use, such as rural areas, the number of stations can be reduced. and expand its power. This also ensures longer battery life for the terminals and lower power usage for the base stations.

In addition, the terminal is not transmitting during the entire call. To save battery and allow more efficient use of the spectrum, the TDMA (Time Division Multiple Access,) transmission scheme is used. Time is divided into basic units of 4.615 ms, and these in turn into 8 time slots or time slots of 576.9 μs. During a call, the first time slot is reserved for synchronization, sent by the BS; A few slots later, the terminal uses one slot to send from the terminal to the BS and another to receive, and the rest are free for use by other users on the same BS and channel. This allows a good use of the available spectrum and a superior battery life, by not using the terminal emitter constantly but only a fraction of the time.

Handover: The Base Station Controller or BSC

At the same time, communication should not be interrupted because a user moves (roaming) and leaves the coverage area of a BS, deliberately limited so that the cell system works well. Both the user terminal and the BS calibrate the power levels with which they send and receive the signals and report this to the base station controller or BSC (Base Station Controller). In addition, usually several base stations at the same time can receive the signal from one terminal and measure its power. In this way, the base station controller or BSC can detect if the user is going to leave one cell and enter another, and notifies both MSCs (Mobile Switching Center) and to the terminal for the process of jumping from one BS to another: it is the process known as handover or transfer between cells, one of the three tasks of the BSC, which allows talking even when the user is moving.

This process can also occur if the station closest to the user is saturated –that is, if all the channels assigned to the BS are in use–. In this case, the BSC sends the terminal to another adjacent station, less saturated, even though the terminal has to transmit with more power. That is why it is common to perceive communication cuts in areas where there are many users at the same time. This indicates the second and third tasks of the BSC, which are to control the power and frequency at which both the terminals and the BTSs emit to avoid outages with the least possible battery drain.

Signage

In addition to using the spectrum for calls, reserving the necessary channels for it while in use, the standard provides for the terminal to send and receive data for a number of signaling uses, such as initial registration on the network when turning on the terminal, the output of the network when turning it off, the channel in which the communication will be established if a call comes in or goes out, information on the number of the incoming call... And it also provides that from time to time the terminal notifies the network that is turned on to optimize the use of the spectrum and not reserve capacity for terminals that are turned off or out of coverage.

This use of the transmitter, known as signalling bursts, occupies very little network capacity and is also used to send and receive SMS short messages without the need to assign a radio channel. It is easy to hear a burst of signaling if the phone is near a device that is likely to pick up interference, such as a radio or television set.

In GSM, a series of channels are defined to establish communication, which group the information to be transmitted between the base station and the telephone. The following channel types are defined:

• Traffic channels (Traffic Channels, TCH): they host the ongoing calls supporting the base station.
• Control or signaling channels:
  • Dissemination channels (Broadcast ChannelsBCH.
    • Channel of control broadcast (Broadcast Control Channel, BCCH): communicates basic information and system parameters from the base station to the mobile.
    • Frequency control channel (Frequency Control Channel, FCCH): communicates to the mobile (from BS) the BS carrier frequency.
    • Synchronism Control Channel (Synchronization Control Channel, SCCH): informs the mobile about the training sequence (training) in force in the BS, for the mobile to incorporate it into its gusts.
  • Dedicated control channels (Dedicated Control ChannelsDCCH.
    • Slow associated control channel (Slow Associated Control ChannelSACCH).
    • Quick associated control channel (Fast Associated Control ChannelFACCH).
    • dedicated control channel between BS and mobile (Stand-Alone Dedicated Control ChannelSDCCH).
  • Common control channels (Common Control ChannelsCCCH).
    • Calling Notice Channel (Paging Channel, PCH): allows the BS to notify the mobile that there is an incoming call to the terminal.
    • Random access channel (Random Access Channel, RACH): hosts requests for access to the mobile network to the BS.
    • Access recognition channel (Access-Grant Channel, AGCH): Processes the acceptance, or not, of the BS of the request for access of the mobile.
• Cell Dissemination Channels (Cellular Dissemination Channels)Cell Broadcast ChannelsCBC.

Network and Switching Subsystem or NSS

The network and switching system (NSS), also called the core network ), is the logical layer for call routing and data storage. Let's note that, up to now, we only had a connection between the terminal, the base stations BS and its controller BSC, and no way was indicated to establish a connection between terminals or between users of other networks. Each BSC connects to the NSS, and it is this that is in charge of three matters:

• Routing the transmissions to the BSC in which the user is called (mobile switching center or MSC);
• Interconnect with the networks of other operators;
• Connect with the subscriber identification subsystem and the operator's databases, which give permission to the user to use the network's services according to their type of subscription and payment status (base and visitor locations, HLR and VLR).

Mobile Switching Center or MSC

The mobile switching central or MSC (mobile switching central) is responsible for initiating, terminating and channeling calls through the BSC and BS corresponding to the called subscriber. It is similar to a fixed network telephone switchboard, although since users can move within the network it performs more updates in its internal database.

Each MSC is connected to the BSCs in its area of influence, but also to its VLR, and must have access to the HLRs of the different operators and interconnection with the telephone networks of other operators.

Home and Visitor Location Records (HLR and VLR)

The HLR (home location register, or base location register) is a database that stores the user's position within the network, if you are connected or not and the characteristics of your subscription (services you can and cannot use, type of terminal, etc.). It is of a rather permanent character; each mobile phone number is assigned to a certain and unique HLR, which is managed by your mobile operator.

On receiving a call, the MSC asks the HLR corresponding to the called number if and where it is available (ie which BSC to ask to notify it) and routes the call or gives an error message.

The VLR (visitor location register or visitor location register) is a more volatile database that stores, for the area covered by an MSC, the identifiers, permissions, types of subscription and locations in the network of all active users at that moment and in that section of the network. When a user registers in the network, the VLR of the leg to which the user is connected contacts the home HLR of the user and checks whether or not he can make calls according to his subscription type. This information remains stored in the VLR while the user terminal is turned on and is periodically refreshed to prevent fraud (for example, if a prepaid user runs out of balance and his VLR does not know it, it could allow him to make calls).

Let's take into account that the GSM system allows agreements between operators to share the network, so that a user abroad –for example- can connect to a network (MSC, VLR and radio layer) of another operator. When turning on the phone and registering in the foreign network, the VLR of the foreign operator takes note of the user's information, contacts the HLR of the user's home mobile operator and asks for information about the subscription characteristics to allow or not to make calls. Thus, the different VLRs and HLRs of the different operators must be interconnected with each other for everything to work. For this purpose there are special network protocols, such as SS7 or IS-41; operators decide which standard to choose in their bilateral roaming and interconnection agreements.

Other systems

In addition, MSCs are connected to other systems that perform various functions.

For example, the AUC (authentication user center) is in charge of signal encryption and user identification within the system; the EIR (equipment identification register, equipment identification register) keeps lists of permission to access the terminal, which is uniquely identified by its serial number or IMEI, to prevent stolen and reported terminals from using the network; SMSCs or short message centers; and thus several more systems, among which are included those for management, maintenance, testing, pricing and the set of transcoders necessary to be able to transfer calls between the different types of networks (fixed and different mobile standards).

Standard codes in GSM networks

All of the noted codes must be executed by pressing CALL, CALL, SEND or their equivalent to call a number.

Caller Identification (CID)

Activation of sending or hiding the number when making or receiving a call. These codes depend on the authorization of the service by the service provider. In some countries, such as Argentina, the companies Personal, Claro, Peru (Movistar) and Venezuela (Digitel) ignore the codes and the activation/deactivation of the service must be done from the menu of each phone.

When making a call:

• Activate: *31# [SEND]
• Cancel: #31# [SEND]
• State: *#31# [SEND]

Upon receiving

• Activate: *30# [SEND]
• Cancel: #30# [SEND]
• State: *#30# [SEND]

Temporary (for one call only)

• Don't show: #31#Number [SEND]
• Show: *31#Number [SEND]

Display the phone's IMEI code

• Mark *#06#

Call Waiting

• Activate: *43# [SEND]
• Deactivate: #43# [SEND]
• State: *#43# [SEND]

Call forwarding

• All calls - Activate: **21*Number#
• All calls - Deactivate: #21#
• All calls - State: *#21#
• Deflect if you don't answer - Activate: **61*
• Deflect if not available - Activate: **62*
• Deflect if occupied - Activate: **67*Number#

To deactivate forwarding, the same main code can be used, such as #67# to deactivate forwarding if busy. To deactivate all diversions no matter which one is active, we can use ##002#.

Network Restriction

• - Activate: *33*PASS#
• All exits - Deactivate: #33*PASS#
• All outgoing - State: *#33#
• Just voice - Activate: *33*PASS*11#
• Only SMS - Activate: *33*PASS*16#
• All incoming - Activate: *35*PASS#
• All incoming - Deactivate: #35*PASS#
• All incoming - State: *#35#
• Just voice - Activate: *35*PASS*11#
• Only SMS - Activate: *35*PASS*16#

Specific network restriction codes such as voice and SMS can be queried using the same status code and can be disabled in the same way. The word PASS must be replaced by a 4-digit code (called a restriction password), which is held by the telephone operator, although it can be used as network code 0000.

SIM card

One of the main features of the GSM standard is the subscriber identity module, commonly known as a SIM card. The SIM card is a removable smart card that contains the user's subscription information, network settings, and phone book. This allows the user to keep their information after changing their phone. At the same time, the user can also change the telephone operator, keeping the same equipment simply by changing the SIM card. Some operators introduce a lock so that the phone uses only one type of SIM card, or only one SIM card issued by the company where the phone was purchased, this practice is known as sim lock, and is illegal in some countries.

In Australia, North America and Europe, many mobile operators block the handsets they sell. This is done because the price of mobile telephony is typically subsidized with revenue from subscriptions, and operators to try to avoid subsidizing competing mobile phones may resort to this practice. Subscribers can contact the operator to remove the lock or use private services to remove the lock, or use software and websites to unlock the phone by themselves. While most websites offer unlocking at a fixed cost, some do it for free. The blocking applies to the phone, identified by its International Mobile Equipment Identity (IMEI) number, and not to the account (which is identified by the SIM card).

In some countries like Bangladesh, Belgium, Chile, Costa Rica, Cuba, Indonesia, Malaysia, Hong Kong and Pakistan, unlocked phones are sold. However, in Belgium, it is illegal for operators to offer any form of subsidy on the price of the phone. This was also the case in Finland until April 1, 2006, when the sale of combinations of subsidized phones and numbers became legal, although obligatory operators have to unlock the phones for free after a certain period (may be a maximum of 24 months). The case of Spain is similar, in which the operators are also obliged to release upon request once the contract is finished, although said operator could pass on the cost of the operation to the customer.

GSM in Spain

Mobile technology in Spain began in 1976 with a service for vehicles limited to Madrid and Barcelona called Automatic Vehicle Phone. This service evolved to accommodate more users with technologies such as TMA-450 and later TMA-900, reaching 900,000 in 1996.

In 1995, given the technological inferiority of the analogue service compared to the digital one provided by GSM, the first mobile digital network called Movistar was created. Subsequently, licenses were granted for a second mobile operator called Airtel (now Vodafone). In 1999 a third operator called Amena (now Orange) was created.

The latter was assigned frequencies only in the 1800 MHz band, which meant having to deploy more cells than if the 900 MHz band were used to cover the same area. Already in 2005, the government assigned Amena new frequencies in the 900 MHz band, but Movistar and Vodafone continued to have a greater number of frequencies in this band.

At the beginning of the year 2000, the closure of analogue networks and the allocation of licenses for 3G technology began, which would be followed years later by 3.5G technology. That same year, a license was granted to the fourth operator called Xfera (currently Yoigo), although it would not start operating until 2006.

Currently we live with 2G/3G/3.5G technology and, although 3.5G is technologically superior, companies like Vodafone use a dual network to offer greater coverage (if there is no 2G or 3G coverage, the mobile terminal may have 3.5G coverage and vice versa) and maximize the battery life of your mobiles.

According to the data offered by the Spanish Telecommunications Market Commission for the year 2009, it can be seen that the number of GSM base stations is considerably higher than that of 3G/UMTS stations.

After much commotion during 2013, the 4 big Spanish companies (Vodafone, Orange, Movistar and Yoigo) have implemented the first 4G in large urban centers.

In May 2013 a "war" between Yoigo and Orange to see who launches the 4G line faster, Movistar was on the side promising 4G at the end of the year (no change) and between Vodafone and Yoigo by mid-summer. The big surprise was made by Vodafone, who without saying anything, in the first week of May said that it would implement the 4G service for 7 large cities in June of this year, thus being the first operator to offer 4G and a mobile speed of 150Mbps in Spain. The cities that enjoyed 4G from June were: Madrid, Barcelona, Valencia, Bilbao, Seville, Malaga and Palma de Mallorca. The adaptation to new 4G lines in these 7 cities cost an investment of 12 billion euros by Vodafone, Orange and Yoigo. Orange launched it on July 8, 2013 and Yoigo on July 18, 2013, while Vodafone was available from June 3, 2013. Finally, Movistar announced in October 2013 the availability of 4G through the Yoigo network. until the 800 MHz frequency is released by the Government.

GSM in Latin America

According to the figures provided by the organization “3G Americas”, in Colombia 89 percent of mobile phones operate under the GSM standard, while in Argentina this figure reaches 97 percent (in 2008 the Movistar operators, Personal, and Claro only operate with GSM), in Chile (the first country in Latin America to operate GSM networks since 1997) 100% of mobile phones operate under GSM and the first Chilean mobile phone company to implement and debut under the technology GSM was ENTEL PCS, 80 percent in Mexico, 65 percent in Brazil, 100 percent in Paraguay and Uruguay, and 100 percent in Venezuela Digitel since it was the operator that started with this technology, Movistar is in the to expand its GSM network to 100%, and Movilnet operates in CDMA/GSM duality, countries like Cuba that began with TDMA, as of January 2009 exclusively uses GSM technology through the state company Cubacel.

In Colombia, the Communications Regulatory Commission (CRC) pointed out that since October 1, 2011, mobile phone companies are obliged to deliver cell phones with open bands (unblocked) so that they can work with any operator. With this measure, the Government seeks to promote competition in the cell phone market, in which the winner will be the end user, and to prevent theft and illegal trafficking of mobile phones, not only in Colombia, but throughout Latin America, according to dialogues between the different governments.

In Chile, two ways of providing terminals are used; Sale (mainly for prepaid subscribers, although there are postpaid customers who prefer to buy the terminal) and Leasing with purchase option (a very widespread modality in the postpaid modality, since the terminal is at a cheaper price); By law, all equipment, both prepaid and postpaid, is unlocked for all operators. The old equipment, which was delivered blocked by the operators can be unlocked for free.