Definition of GSM
Global System for Mobile Communications abbreviated as GSM is a digital mobile communication standard formulated by the European Telecommunications Standards Organization ETSI.
Its air interface uses time division multiple access technology. Since it was put into commercial use in the mid-1990s, it has been adopted by more than 100 countries around the world.
The ubiquity of the GSM standard makes international roaming common for users after signing roaming agreements between mobile phone operators.
The biggest difference between GSM and its previous standards is that its signaling and voice channels are digital, so GSM is regarded as a second-generation (2G) mobile phone system.
The Global System for Mobile Communications (GSM) is by far the most successful global mobile communications system.
Its development began in 1982. The European Telecommunications Management Conference (CEPT), the predecessor of the European Telecommunications Standards Institute (ETSI), established the Groupe Speciale Mobile, which has been authorized to improve many of the recommendations related to the pan-European digital mobile communication system.
The two goals tried to accomplish were:
First, better and more effective technical solutions for wireless communications-at that time, digital systems were superior to analog, which was still popular at the time, in terms of user capacity, ease of use, and the number of possible additional services. The system is already obvious.
Second, to achieve a unified standard across Europe to support roaming across national borders. This was impossible in the past because all countries use incompatible analog systems.
The success of GSM has exceeded everyone’s expectations. Although it was originally developed as a European system, at the same time it was promoted and applied in Europe, GSM has been widely used throughout the world. Australia is the first non-European country to sign a basic agreement.
Since then, GSM has gradually become a global mobile communication standard, with more than one billion users in 2004. Of course, there are a few exceptions: Japan and South Korea have never adopted GSM.
There are three versions of GSM, each of which uses a different carrier frequency. The original GSM system used a carrier frequency of around 900 MHz. Later, GSM-1800, also known as DCS-1800, was added to support the ever-increasing number of users.
The carrier frequency used by GSM is around 1800MHz, and the total available bandwidth is about three times the available bandwidth of around 900MHz, which reduces the maximum transmit power of the mobile station.
GSM-1800 is exactly the same as the original GSM. Signal processing, switching technology, etc. can be used in the same way without any changes.
A higher carrier frequency means greater path loss, and at the same time, a reduction in transmit power will result in a significant reduction in cell size. This practical effect, together with the wider available bandwidth, allows the network capacity to be considerably expanded.
The third system is called GSM1900MHz or PCS-1900MHz (Personal Communication System), which works on the 1900MHz carrier frequency and is mainly used in the United States.
GSM is an open standard. This means that only the interface is specified, and the specific implementation form is not restricted. As an example, let’s consider the modulation method adopted by GSM, namely GMSK.
The GSM standard specifies the upper limit of out-of-band emission, phase jitter, and intermodulation products. How to achieve the required linearity depends on the equipment manufacturer.
When using a proprietary system, the service provider can only select the equipment supplier once during the initial network construction stage. For GSM (and other open standards), service providers can first purchase base stations from a certain manufacturer, and then in order to achieve network expansion, they can purchase base stations from another manufacturer with a more reasonable price.
Service providers can also buy some parts from one company and other parts from another company.
System composition
The GSM system is mainly composed of four parts: mobile station (MS), mobile network subsystem (NSS), base station subsystem (BSS), and operation and maintenance center (OMC).
A mobile station is a device used by users in a public GSM mobile communication network and is also a device in the entire GSM system that users can directly contact. The types of mobile stations include not only handheld stations, but also vehicle-mounted stations and portable stations.
With the development trend of the GSM standard digital handheld station further being smaller, lighter, and more functional, the users of the handheld station will account for a large part of the entire user.
Base Station Subsystem (BSS)
The base station subsystem (BSS) is the most direct basic component of the GSM system with the wireless cellular aspect. It is directly connected with the mobile station through a wireless interface and is responsible for wireless transmission and reception and wireless resource management.
The base station subsystem is connected to the mobile service switching center (MSC) in the network subsystem (NSS) to realize the communication connection between mobile users or between mobile users and fixed network users and transmit system signals and user information.
To carry out operation and maintenance management of the BSS part, it is also necessary to establish a communication connection between the BSS and the Operation Support Subsystem (OSS).
Mobile Network Subsystem (NSS)
NSS consists of Mobile Services Switching Center (MSC), Home Location Register (HLR), Visited Location Register (VLR), Authentication Center (AUC), Equipment Identification Register (EIR), Operation and Maintenance Center (OMC-S), short message services Center (SC) composition.
The MSC is a functional entity that controls and exchanges traffic for the MS located in its coverage area and is also an interface entity between the mobile communication network and other communication networks. It is responsible for call control, mobility management, and radio resource management in the entire MSC area. VLR is a dynamic database that stores information about users and calls processing in its coverage area.
The MSC needs to retrieve information from the VLR in order to process the incoming and outgoing calls of the MS located in the coverage area. Generally, the VLR and the MSC are co-located in the same physical entity.
HLR is a database used for mobile user management, and each mobile user should be registered in its home location register. HLR mainly stores two types of information, one is business information about users, and the other is user location information.
Operation and Maintenance Center (OMC)
The Operation and Maintenance Center (OMC), also known as OSS or M2000, needs to complete many tasks, including mobile user management, mobile device management, and network operation and maintenance.
Safety requirements
The goal of GSM system security is to make the system as secure as the public switched telephone network (PSTN). The wireless path system in the system is the most vulnerable part because wireless signals can be easily intercepted.
Mobile stations have a little-known security problem, and it is technically possible to use MS to eavesdrop (for example, as a wiretap). Even if it is turned off, it can be turned on through the air interface, so the best way to protect it is to remove the battery.
The GSM MoU group (Memorandum of Understanding Group) believes that security technical features are only a small part of the security requirements, and the biggest threat comes from simpler attacks such as the leakage of encryption keys, insecure billing systems, or corruption.
Therefore, comprehensive measures must be taken to ensure that these safety processes meet safety requirements. In addition, the cost-effectiveness of safety measures must also be considered.
The security requirements of the GSM system take into account some potential weaknesses of the cellular network. The security of the system should be appropriate for both the system operator and the user. The system operator wants to ensure that the right person is charged and the service is not affected; customers demand privacy protection.
Summarize the following requirements:
(1) Make wireless networks as secure as fixed networks, which means anonymity and encryption to prevent listening.
(2) Adopt strong authentication to prevent operators’ billing from being deceived.
(3) Prevent operators from endangering the safety of others, whether unintentionally or under competitive pressure.
(4) It cannot cause the initial call setup delay or the subsequent communication delay to increase significantly.
(5) Cannot occupy more channel bandwidth.
(6) Lead to an increase in error rate or error propagation.
For this reason, the design of the operator’s GSM system must consider the environment and security processes, such as the generation and distribution of keys, the exchange of information between operators, and the confidentiality of algorithms.
Technical characteristics
Spectral efficiency
Due to the adoption of a high-efficiency modulator, channel coding, interleaving, equalization, and speech coding technology, the system has high spectral efficiency.
Capacity
As the transmission bandwidth of each channel is increased, the requirement for the co-frequency multiplexing to interference ratio is reduced to 9dB, so the co-frequency multiplexing mode of the GSM system can be reduced to 4/12 or 3/9 or even smaller (7/21 for analog systems) );
Coupled with the introduction of half-rate voice coding and automatic traffic allocation to reduce the number of handovers, the capacity efficiency of the GSM system (the number of channels per megahertz per cell) is 3 to 5 times higher than that of the TACS system.
Open interface
The open interface provided by the GSM standard is not limited to the air interface but also includes between networks and between various device entities in the network, such as the Abis interface.
Safety
Through the use of authentication, encryption, and TMSI number, the purpose of security is achieved. Authentication is used to verify the user’s network access rights. Encryption is used in the air interface and is determined by the key of the SIM card and the network AUC.
TMSI is a temporary identification number assigned to users by the business network to prevent someone from tracking and revealing their geographic location.
Frequency band
The GSM frequency band (frequency range) is the frequency of the cellular wireless communication system designated by the International Telecommunication Union for the operation of GSM mobile phones.
System | Frequency band | Uplink (MHz) | Downlink(MHz) | Channel number | Corresponding to UMTS/LTE frequency band number |
T-GSM-380 | 380 | 380.2–389.8 | 390.2–399.8 | Dynamic |
|
T-GSM-410 | 410 | 410.2–419.8 | 420.2–429.8 | Dynamic |
|
GSM-450 | 450 | 450.6–457.6 | 460.6–467.6 | 259-293 | 31 |
GSM-480 | 480 | 479.0–486.0 | 489.0–496.0 | 306-340 |
|
GSM-710 | 710 | 698.2–716.2 | 728.2–746.2 | Dynamic | 12 |
GSM-750 | 750 | 747.2–762.2 | 777.2–792.2 | 438-511 |
|
T-GSM-810 | 810 | 806.2–821.2 | 851.2–866.2 | Dynamic | 27 |
GSM-850 | 850 | 824.2–849.2 | 869.2–894.2 | 128-251 | 5 |
P-GSM-900 | 900 | 890.0–915.0 | 925.0–960.0 | 1-124 |
|
E-GSM-900 | 900 | 880.0–915.0 | 925.0–960.0 | 975-1023， | 8 |
0-124 | |||||
R-GSM-900 | 900 | 876.0–915.0 | 921.0–960.0 | 955-1023， |
|
0-124 | |||||
T-GSM-900 | 900 | 870.4–876.0 | 915.4–921.0 | Dynamic |
|
DCS-1800 | 1800 | 1,710.2–1,784.8 | 1,805.2–1,879.8 | 512-885 | 3 |
PCS-19001800 | 1900 | 1,850.2–1,909.8 | 1,930.2–1,989.8 | 512-810 | 2 |
P-GSM, the reference GSM-900 MHz frequency band
E-GSM, extend the GSM-900 MHz frequency band (including the reference GSM-900 MHz frequency band)
R-GSM, railway GSM-900 MHz band (including reference and extended GSM-900 MHz band)
T-GSM, trunking wireless system-GSM
GSM-900 MHz, GSM-1800 MHz and EGSM/EGSM-900 MHz
China mainly uses GSM-800 MHz, GSM-900 MHz, and GSM-1800 MHz frequency bands.
GSM-900 and GSM-1800 MHz are used in most areas of the world: Europe, the Middle East, Africa, Oceania, and most of Asia. Used by the following countries in South and Central America:
Paraguay – GSM-1800 MHz and GSM-1900 MHz
Peru – GSM-1900 MHz
Costa Rica – GSM-1800 MHz
Brazil – GSM-850 MHz, 900 MHz, 1800 MHz and 1900 MHz
Guatemala – GSM-850 MHz, GSM-900 MHz and 1900 MHz
El Salvador – GSM-850 MHz, GSM-900 MHz and 1900 MHz
Venezuela – GSM-850 MHz, GSM-900 MHz and 1900 MHz
GSM-900 uses 890-915MHz to send information from the device to the base station (uplink) and uses 935-960MHz to receive information (downlink). It provides 124 wireless channels (channel number 1-124), and each channel occupies 200kHz. The duplex interval is 45MHz. The protection bandwidth of 100kHz is placed at both ends of the frequency band.
GSM-1800MHz uses 1710–1785MHz to send information from the mobile station to the base station (uplink) and uses 1805–1880MHz to receive information (downlink). It provides 374 channels (channel numbers 512-885). The duplex interval is 95MHz.
GSM-1800MHz is also called DCS (Digital Mobile Phone Service) in the UK and PCS in Hong Kong. To avoid confusion, GSM-1900 is called PCS in other parts of the world. MCA uses GSM1800 MHz.
GSM-850 and GSM-1900 MHz
GSM-850 and GSM-1900MHz are used in Argentina, Brazil, Canada, the United States, and many other countries in the Americas.