Mobile Computing – Global System for Mobile Communications Prof. Rajesh M. More*, Lecturer, GSMCOE, MCA, Pune-45 (Abstract – Global System for Mobile Communications (GSM) is a digital mobile telephone system which is widely used in Europe and other part of the world. In 1982 it was recognized as a standard for digital wireless communications adopted firstly by Europe and then by Asia, Africa etc. INTRODUCTION To develop a standard for mobile telephone the European Conference of Postal and Telecommunication Administrations (CEPT) created the Groupe Special Mobile (GSM) which was used across Europe.
The GSM standard has been an advantage to both consumers and network operator. Consumer can get benefit from the ability to roam and switch carriers without switching phones whereas Network Operators can choose equipment from any of the many vendors implementing GSM. GSM also pioneered a low-cost (for network carrier) alternative to voice calls. The Short Message Service (SMS), also called “text messaging”, which is now also supported on other mobile standards were backward-compatible with the original GSM phones. For example, Release ‘97 of the standard added packet data capabilities, by means of General Packet Radio Service (GPRS).
Release ’99 introduced higher speed data transmission using Enhance Data Rates for GSM Evolution (EDGE). GSM ARCHITECTURE GSM is a cellular network where each cell is connected or served by Base Transceiver Station (BTS). BTS is equipment which facilitates wireless connection between user equipment (mobile) and network. Several BSTs are connected to the Base Station Controller (BSC). BSC controls all the calls in all the connected base stations. Base Stations can be directly connected to BSC forms star configuration, in the form of chain i. e. one to another or in the form of loop configuration.
The GSM network can be divided into three broad parts – The Mobile Station (which is carried by the subscriber), The Base Station Subsystem (which controls the radio link with the Mobile Station) and The Network Subsystem (which performs the switching of calls between the mobile and other fixed or mobile network users as well as management of mobile services). *more. [email protected] com Mobile Station The mobile station (MS) consists of the physical equipment (i. e. the radio transceiver, display and digital signal processors and a smart card called the Subscriber Identity Module (SIM).
The SIM provides personal mobility, so that the user can have access to all subscribed services irrespective of both the location of the terminal. SIM card needs to be inserted into GSM cellular phone, so that the user is able to receive and make calls from that phone or receive other subscribed services. The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI), identifying the subscriber, a secret key for authentication, and other user information.
The IMEI and the IMSI are independent. The SIM card may be protected against unauthorized use by a password or personal identity number. Base Station Subsystem The Base Station Subsystem is consisting of two parts – The Base Transceiver Station (BTS) and The Base Station Controller (BSC). The Base Transceiver Station houses the radio transceivers that define a cell and handles the radio-link protocols with the Mobile Station. Depending upon the area, there will potentially be a large number of BTSs deployed. The Base Station Controller manages the radio resources for one or more BTSs.
It handles radio channel setup, frequency hopping, and handovers. The BSC is the connection between the mobile and the Mobile service Switching Center (MSC). The BSC can translate the 13 kbps voice channel used over the radio link to the standard 64 kbps channel used by the Public Switched Telephone Network or ISDN. Network Subsystem The central component of the Network Subsystem is the Mobile services Switching Center (MSC). MSC acts like a normal switching node of the PSTN or ISDN, also provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, andovers, and call routing to a roaming subscriber. These services are provided in by several functional entities, which together form the Network Subsystem. Together with the MSC, The Home Location Register (HLR) and Visitor Location Register (VLR), provide the call routing and roaming capabilities of GSM. The HLR contains all the administrative information of each subscriber (registered in the corresponding GSM network), along with the current location of the mobile.
The current location of the mobile is in the form of a Mobile Station Roaming Number (MSRN) which is a regular ISDN number used to route a call to the MSC (where the mobile is currently located). Although it may be implemented as a distributed database, logically there is one HLR per GSM network. The Visitor Location Register (VLR) contains selected administrative information from the HLR, which is used for call control and provision of the subscribed services, VLR controls each mobile those currently located in the geographical area. EIR and AC registers are used for authentication and security purposes.
The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where each mobile station is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is not type approved. The Authentication Center is a protected database that stores a copy of the secret key stored in each subscriber’s SIM card, which is used for authentication and ciphering of the radio channel. GSM SECURITY GSM was designed with a moderate level of security.
The system was designed to authenticate the subscriber using a pre-shared key (a pre-shared key or PSK is a shared secrete key which was previously shared between the two parties using some secure channel before it needs to be used. Such systems almost always use Symmetric Key cryptographic algorithms) and challenge –response (challenge-response authentication) is a family of protocols in which one party presents a question (“challenge”) and another party must provide a valid answer (“response”) to authenticate). Communications between the subscriber and the base station can be encrypted.
The development of UMTS (Universal Mobile Telecommunications System) introduces an optional USIM (Universal Subscriber Identity Module), that uses a longer authentication key to give greater security, as well as mutually authenticating the network and the user – whereas GSM only authenticates the user to the network (and not vice versa). The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation (Non-repudiation is the concept of ensuring that a party in a dispute cannot repudiate, or refute the validity of a statement or contract).
GSM FREQUENCIES GSM networks operate in a different frequency ranges – GSM Frequency Ranges for 2G and UMTS Frequency Bands for 3G. 2G GSM networks usually operate in the 900 MHz or 1800 MHz bands. Few countries in Americas (including Canada and the United States) use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated. Most 3G GSM networks in Europe operate in the 2100 MHz frequency band. The rarer 400 and 450 MHz frequency bands are assigned in some countries where these frequencies were previously used for 1G system.
GSM-900 uses 890–915 MHz to send information from the Mobile Station to the Base Station (unlink) and 935–960 MHz for the Base Station to Mobile Station direction (downlink). In some countries the GSM-900 band has been extended to cover a larger frequency range known as ‘extended GSM’ (E-GSM) uses 880–915 MHz (uplink) and 925–960 MHz (downlink), adding 50 channels (channel numbers 975 to 1023 and 0) to the original GSM-900 band. CONCLUSION GSM is a true personal communication system. Together with international roaming and support for a variety of services like telephony, data transfer, short message services and other supplementary services.
GSM come close to fulfilling the requirements for a personal communication system of 21st century. . BIBLIOGRAPHY 1] [SCH03] Schiller Jochen, “Mobile Communication”, Pearson Education 2003. 2] [GAR01] Garg V. , Joseph E. Wilkes, “Wireless and personal Communications Systems”, Prentice Hall, 2001. 3] [RAP06] Theodore S. Rappaport, “Wireless Communications”, Prentice Hall, 2006. 4] [FLO05] J. E. Flood, “Telecommunication Switching, Traffic and Networks”, Pearson Education, 2005. 5] [PRA] Prabhu, “Mobile Computing – A Book of Reading”, Universities Press, 2002.