Global system for mobile communications (GSM)

This paper describes the early history of GSM, who founded it, and why it was founded. Some services of GSM will be described like SMS, and the emergency call service. The network layout of GSM, and the layers of the protocol will also be described. Introduction GSM was conceived in the early 1980’s by the study group: Groupe Spécial Mobile. GSM had some ambitious goals, some were met, and some, like ISDN compatibility had to be toned down. By 1994 GSM networks were operating in 110 countries and provided service to 1.3 million people worldwide. By 1997 that number grew to over 55 million. GSM provides many services like telephony, data and fax transmission / reception, SMS, emergency services, call waiting, caller ID; the list goes on. Background In the early 1980’s cellular phones were gaining popularity in Europe. Each country had their own proprietary cellular networks. These individual networks had a limited market, driving cellular costs up. If someone from France were to go to Germany with their cellular phone it would be completely useless [1]. In 1982 a study group Groupe Spécial Mobile (GSM) was founded to develop a pan-European mobile system that would fit the following specifications: [1] • Good subjective speech quality • Low terminal and service cost • Support for international roaming • Ability to support handheld terminals • Support for range of new services and facilities • Spectral efficiency • ISDN compatibility Unlike its analog counterpart (cellular) GSM is digital. At the time digital technologies were unproven, but advances were being made consistently in compression algorithms and digital signal processors. The extensive documentation (8000 pages) of GSM allowed for improvements to be made without interfering with how things work [1]. GSM Services One of the more interesting features of GSM is the SIM card. The SIM card stores all the necessary information so that you can use the network. Say you were to get a new mobile phone, the SIM cards can be swapped without any additional modification. GSM provides the basic telephony service and some nice additions. GSM allows things like call conferencing call encryption, call hold on outgoing and incoming requests, and caller id. There is even an emergency call feature. If you dial a 911 type number, usually 112, that call is forwarded to the nearest service provider free of charge [2]. As stated earlier, originally GSM hoped to produce speeds equal to ISDN, or 64 kbs. But due to radio transmission limitations and cost effectiveness, the speeds were lowered [1]. For data transmission and reception, GSM provides 9.6 kbs. And like data transmission, fax transmission and reception is also 9.6 kbs, but requires an adaptor [2]. GSM was also the first to provide the SMS service. SMS is an instant messaging service. Each message can be up to 160 bytes. One of the more interesting features of SMS is the broadcast feature. You can broadcast a short message to all users of your particular service inside the current cell (a cell the area covered by the transmitter providing the service) [2]. GSM Network Architecture The GSM network is made up of three major parts: the mobile station (handheld device), the base station subsystem (receiver and transmitter), and the network subsystem (authentication, registration, etc) [1]. One of the biggest problems with cellular networks before GSM was the proprietary equipment. GSM sought to do away with this limitation by having well documented interfaces so that equipment could be manufactured by anyone allowing more competition and driving prices down. The Mobile Station is basically your mobile phone, and the SIM card. The mobile phone is responsible for providing you an interface to the services you subscribe to. The SIM card holds all information necessary to be able to uniquely identify you no matter which phone it is inserted in. A SIM card can be inserted into any mobile phone, it allows calls to be made or received, and use any other service that you may be subscribed to [1]. The Base Station Subsystem is responsible for relaying data between the network and the mobile device. This subsystem has two components: the base transceiver station, and the base station controller. The base transceiver station is the actual transmitter / receiver, and uses the radio-link protocols for data transfer. The base station controller “handles radio-channel setup, frequency hopping, and handovers [1].” The concept of a “cell” also comes into play with the base station subsystem. A cell is basically a hexagonal estimation of coverage for an area. They use a hexagon so it ensures some overlap between transceivers, so all customers can get service [3]. The size of these cells varies, the more demand the service is in an area; the smaller the cells and the more there are (limited number of frequencies). The Network Subsystem handles all the authentication and routing of calls. It’s in The Network Subsystem that the personal information in the SIM card is used. The core of the Network Subsystem is the Mobile services Switching Center (MSC); it is what is actually connected to the network. The MSC has four smaller, helper “registers”: The Equipment Identity Register, The Authentication Center, The Home Location Register, and the Visitor Location Register; the first two have to do with authentication, the last two have to do with routing. The Equipment Identity Register is used to identify valid (not stolen) mobile equipment. The Authentication Center has a copy of the secret key inside of your SIM, it uses that key from authentication and encryption [1]. GSM Network Layering Like other network protocols, GSM uses a layered approach. GSM has three layers: Layer 1, the physical layer; Layer 2, the data link layer; and Layer 3, the GSM protocol which is again divided into three parts. The first layer of Layer 3 is the Radio Resources Management. This layer “controls the setup, maintenance, and termination of radio and fixed channels, including handovers.[1]” Handover is the process of moving a on-going call to a different cell. The second layer of Layer 3 is Mobility Management. Mobility Management “Manages the location updating and registration procedures, as well as security and authentication.[1]” The question is, how does a call get to you? This layer solves that problem. Each cell is a member of a group of cells, this group of cells knows who is inside of it. When ever a mobile station moves between a group of cells, it’s location is updated, this is done to avoid excessive updating of position and waste of the radio channel. The third layer of Layer 3 is Connection Management. Connection Management “handles general call control, similar to CCITT Recommendation Q.931, and manages Supplementary Services and the Short Message Service.[1]” Each of the three major parts discussed in the GSM Network Architecture have a standardized interface between them. They did this to try and encourage competition between component builders. Between the mobile station and the BTS is the Um interface. Between the BTS and the BTC is the Abis interface. Between the BTC and the MSC is the A interface. ConclusionThe GSM committee had a giant task ahead of them. Europe had no standardized cellularservice or equipment. GSM accomplished the task of making well documentedInterfaces between components which allowed some competition driving prices down.GSM grew in popularity greatly, by growing from operating in 110 countries and having1.3 million subscribers, to having over 53 million subscribers only 3 years later. In myopinion GSM was a huge success. References[1] Overview of the Global System for Mobile CommunicationsBy John Scourias http://www.shoshin.uwaterloo.ca/~jscouria/GSM/gsmreport.html [2] GSM Overviewhttp://www.cellular.co.za/gsm-overviewpage.htm [3] GSM Overview #2http://www.cse.iitb.ac.in/~anil/MTP/GSM-Overview.pdf