The GSM has a number of features that differentiate it within the universe of mobile communications. Born in the 80 result of unprecedented cooperation in Europe, the systems share common elements with other technologies used in mobile telephony, as the digital transmission of voice and data and the use of cells. This article presents the basic technical characteristics of the system and its capabilities.
GSM Network Architecture
A GSM network is composed of three elements: the terminal-base station (BSS) and the network subsystem or node. Additionally, there are operation centers established by the operators, to monitor network status.
- Substation Base System (System of substation Base)
- Network Subsystem (Network Subsystem)
- TRX: Transceiver (Transrecibidor)
- EIR: Equipment Identity Register (Equipment Identification Register)
- MS: Mobile Station (Mobile Station)
- AC: Authentication Center (Central Authentication)
- SIM: Subscriber Identity Module (Subscriber Identification Module)
- HLR: Home Location Register (Register of caller location)
- BTS: Base Transceiver Station (Base Station Transrecibidora)
- BSC: Base Station Controller (Base Station Control)
- MSC: Mobile services Switching Center (Central Exchanging Mobile Services)
- VLR: Visitor Location Register (Visitor Location Register)
- ISDN: Integrated Services Digital Network (Integrated Services Digital Network)
- PSTN: Public Switched Telephone Network (Analog Public Telephone Network)
- SMSC: Short Message System Center (Central Short Messaging System)
The mobile station or terminal, contains the SIM card, which is used to identify the user within the network. The SIM gives personal mobility to the user of the card, allowing you to access network services regardless of mobile phone use or location. The SIM can be protected against misuse through a code (PIN) that you have to dial each time you connect the phone with the SIM inserted. There is also a number that identifies each terminal individually SUBSCRIBE the International Mobile Identity (IMEI), but is independent of the SIM.
The base-station controls the radio connection between mobile phone and network and is also known per cell, as it covers a certain geographic area. A BSS is composed of two elements: the BTS (Base Transceiver Station) and BSC (Base Station Controller). Each BSS may have or more BTS. The team are home BTS transmitter / receiver (the TRX or transceivers) and manage the radio protocols with the mobile terminal. In urban areas there are more BTS than in rural areas and in some cases with particular physical or geographical characteristics (eg, tunnels) are placed to ensure service broadcasters. Each station uses digital techniques to allow multiple users to the network is liguen and to allow to make and receive calls simultaneously. This step is called Multiplexing.
The BSC manages the radio resources of one or more BTS. Its functions include handoff (which occurs when the user moves from one cell to another, allowing the ligation is maintained), the establishment of the radio channels used and frequency changes. Finally, set the ligation between the mobile and the Mobile Service Switching Center (MSC), the heart of the GSM system.
The MSC, as already mentioned, is the center of the network, through which ligation is made between a call made from a mobile was the other fixed network (the PSTN analog or digital ISDN) or mobile. The node which is also has a series of equipment for controlling various functions such as service recovery, security and sending SMS messages.
The Home Location Register (HLR) contains all administrative information on customer service and the current location of the terminal. It is through the network HLR checks whether a mobile you are trying to link service has a valid contract. If the answer is yes the MSC sends a message back to the terminal informing that he is authorized to use the network. The name of the operator then appears on screen, reporting that can make and receive calls. When the MSC receives a call destined for a mobile it will check the location to the HLR. In parallel, the terminal from time to time sends a message to the network to inform the site where it is (this process is called polling).
The Visitor Location Register (VLR) is used to control the type of connections that a terminal can do. For example, if a user has restrictions on long distance calls, VLR prevents these are made, blocking and sending a message back to the mobile phone informing the user.
The Equipment Identity Register (EIR) and the Authentication Center (AC) are used both to ensure system security. The EIR has a list of IMEI of terminals that have been reported as stolen or that are not compatible with the GSM network. If the mobile phone is on that blacklist, the EIR does not let you connect to the network. Within the AC is a copy of the security code of the SIM. Authorization occurs when the AC generates a random number that is sent to your mobile. The two appliances, then, use that number, along with the code of the SIM and an encryption algorithm called A3, to create another number which is sent back to the AC. If the number sent by the terminal is the same as calculated by the AC, the user is authorized to use the network.
Finally, the Short Message System Center (SMSC) is responsible for generating the short text messages. Other equipment used in GSM can attach the collection of calls, Internet access, the voicemail box, etc..
How the GSM transmission
The GSM 900 system uses two sets of frequencies in the 900 MHz band, the first in the 890-915MHz, used for transmission of the terminal and the second in the 935-960MHZ, for transmissions from the network.
The method used by GSM frequencies to administer a combination of two technologies: TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access). The FDMA divides the 25 MHz frequency available in 124 channels with a width of 200 kHz and a data transmission capacity of around 270 Kbps One or more of these frequencies is assigned to each base station and split again in a matter of time using the TDMA, in eight timeslots (timeslots). The terminal uses a timeslot for reception and one for broadcast. They are separated temporarily so that the mobile does not receive and transmit simultaneously. This division of time is also called full rate. Networks can also divide the frequencies in 16 spaces, a process designed half-rate, but the transmission quality is lower.
The voice is encoded in a complex way, so that transmission errors can be detected and corrected. Then it is sent in timeslots, each with a duration of 577 milliseconds and a capacity of 116-bit encrypted. Each terminal has a frequency agile and can move between the timeslots used for sending, receiving and control, in a complete frame. It verifies a mobile phone other channels to determine if the signal is stronger and change the transmission for the same, if the answer is yes.
GSM Features
The GSM system has a number of functionalities that can be implemented by operators in their networks. Various features include:
- Possibility of using the terminal and the SIM card in GSM networks in other countries (roaming).
- "Short Message Service (SMS) through which can be sent and received messages with up to 126 characters.
- Call forwarding to another number.
- Transmission and reception of data and fax speeds up to 9.6 Kbps
- Cell Broadcast - messages with up to 93 characters can be sent to all mobile phones in a geographic area. The messages are received when the terminal is not being used and can be received every two minutes.
- CLIP (Calling Line Identification Presentation) - View on screen the number that is calling us. In contrast, the CLIR (Calling Line? Identification Restriction) prevents the calling number is seen by someone (anonymous) thanks to CLIP.
- Ability to display credit / costs.
- Restricted group of users - allow registered phones in the groups are used for different phone extensions or account.
- Ligations without static.
- Notice of call waiting, when we're on the phone.
- Possibility to place a call on hold while you take another.
- "The calls are encrypted, which prevents them from being heard by others.
- Ability to prevent the reception / transmission of certain calls.
- Emergency Calls - 112 can always be marked on any network, even without SIM.
- Possibility of several users shall speak to one another at the same tempo - Conferencing service
