GSM overview and technical characteristics analysis

GSM is the abbreviation of Global System For Mobile CommunicaTIons. A digital mobile communication standard developed by the European telecommunications standards organization ETSI. GSM is an abbreviation of Global System of Mobile communicaTIon. 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 GSM standard equipment occupies more than 80% of the current global cellular mobile communication equipment market.

GSM is currently the most widely used mobile phone standard. More than 1 billion people in more than 200 countries and regions around the world are using GSM phones. All users can roam freely between mobile phone operators who have signed a "roaming agreement". 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. This shows that digital communication has been built into the system from an early age. GSM is an open standard currently developed by 3GPP.

From the user's point of view, the main advantage of GSM is that users can choose between higher digital voice quality and low cost [SMS]. The advantage of network operators is that they can customize their equipment configuration to different customers, because GSM as an open standard provides easier interoperability. In this way, the standard allows network operators to provide roaming services, and users can use their mobile phones worldwide.

As a standard that continues to be developed, GSM maintains backward compatibility with the original GSM phone. For example, the message exchange capability was added in Release '97 version of the standard, which is GPRS. High-speed data exchange was also introduced in Release '99 edition standard, mainly EDGE and UMTS standards.

At this stage, GSM has a wide range of applications, with the following technical characteristics:

Spectral efficiency. Due to the use of high-efficiency modulators, channel coding, interleaving, equalization and speech coding technologies, the system has high spectral efficiency.

capacity. As the transmission bandwidth of each channel increases, the requirement for co-channel multiplexing plant-to-plant ratio is reduced to 9dB, so the co-channel multiplexing mode of GSM system can be reduced to 4/12 or 3/9 or even smaller (7/21 for analog system) ); Plus the introduction of half-rate voice coding and automatic traffic distribution to reduce the number of handovers, making the capacity efficiency of the GSM system (number of channels per megahertz per cell) 3 to 5 times higher than the TACS system.

Voice quality. In view of the characteristics of digital transmission technology and the definition of air interface and voice coding in the GSM specification, when the threshold is exceeded, the voice quality always reaches the same level regardless of the wireless transmission quality.

Open interface. The open interface provided by the GSM standard is not limited to the air interface, but also between the newspaper network and the equipment entities in the network, such as the A interface and the Abis interface.

safety. Through authentication, encryption and the use of TMSI numbers, the purpose of security is achieved. Authentication is used to verify the user's network access rights. The encryption is used for the air interface and is determined by the key of the SIM card and the network AUC. TMSI is a temporary identification number assigned by the service network to the user to prevent someone from tracing and revealing its geographic location.

Interconnection with ISDN, PSTN, etc. The interconnection with other networks usually uses existing interfaces, such as ISUP or TUP. Roaming based on SIM card. Roaming is an important feature of mobile communications, which marks that users can automatically enter another network from one network. The global mobile communication system can provide global roaming, and of course also requires certain agreements between network operators, such as billing.  

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