GSMSIM: an educational simulation tool for teaching GSM-based mobile communications in laboratory lectures

International Journal of Electrical Engineering Education, Jul 2009 by García-Díaz, Pilar, Salcedo-Sanz, Sancho, Portilla-Figueras, Antonio, Núñez-Clemente, David

Abstract This paper describes an educational simulation tool, GSMSIM, which has been developed to teach GSM mobile communications in laboratory lectures. The tool has been programmed in OMNET , a flexible environment which allows its extension to different aspects of GSM technology, such as the simulation of calls, traffic, handoffs, etc. This tool has been used in the courses 'Communication Systems' and 'Telecommunication Networks I', at the University of Alcalá, Madrid, Spain. We report some of the results obtained in three different scenarios simulating a small, medium and large city, with different characteristics of network coverage.

Keywords discrete-even simulation; GSM system; laboratory lectures; mobile communications

Mobile telecommunications are currently an important part of our daily life. Mobile technology is one of the easiest and fastest ways to communicate, so it has taken over other telecommunication technologies as the most used in the world (see Fig. 1; note that in both cases the penetration rate of mobile communications is much higher than other technologies1). Its economic importance is large: the contribution of the mobile market to the UK economy was 2.2% in 2003, more than three times the average; and it is forecast to increase to up to 7.5% in 2013.2 This growth and the evolution to 3rd Generation (3G) and systems beyond 3G, means that this industry requires many specialists. Responding to this trend, almost all technical schools and universities around the world offer specialised courses in mobile telecommunications. Specifically, most universities where electrical engineering is taught offer basic courses about the GSM system (2nd Generation Mobile, or 2G), before offering other courses on 3G.

Teaching of the GSM system is usually done in two stages. First, the students receive theoretical lectures about how the system works, with details of all its components. Then, the students have the chance to do some laboratory work on the system. This laboratory work can be done either with real equipment or with simulators. 3-5 The former are obviously very expensive, and not available in the majority of technical schools. On the other hand, simulators are usually commercial software, usually expensive, and also quite complicated to use, because they are oriented to the real operation and design of GSM networks used in communications companies. Therefore these simulators are usually not suitable for teaching GSM mobile communications. Having considered the above points, we have developed a GSM Radio Access Simulator for its application in laboratory lectures about mobile communications using the GSM system. This paper describes the developed simulator (GSMSIM), showing its main properties and advantages over commercial software. Specifically, we have used a popular simulation platform called OMNeT , which accepts modules in C , so the simulator can be extended or modified by the students in an easy way. We report the results of using this software in laboratory lectures for the courses 'Communication Systems' and 'Telecommunication Networks I', in the BS degree in Telecommunications offered at Alcalá University, Madrid, Spain.

GSM: history and architecture

GSM history

Global System for Mobile (GSM) communications was born due to several reasons, most importantly the increase in demand for mobile services in the mid-1980s which could not be met by the very resource-limited 1st Generation of Mobile Telecommunications. In 1982 the European Conference of Postal and Telecommunications created the Group Special Mobile for the development of 2nd Generation of Mobile Telecommunications, named GSM. The main milestones of the development of this system are the following:

* Commission of the Group Special Mobile, (1982);

* GSM agreed to use digital communications (1985);

* Selection of Time Division Multiple Access (TDMA) as physical medium access. (1986);

* Memorandum of Undestanding (MoU) signed by 13 European countries to adopt the GSM standard (1987);

* GSM becomes a ETSI group (1989);

* End point of the release 1 of the GSM specification. System is pre-operational (1990);

* First commercial implementations (1991).

The rest of the history is well known. About 1995 the GSM mobile service became a social communication driver, and about 2000 the penetration in Europe was about 90%.

GSM system architecture

Figure 2 shows a simplified scheme of the GSM architecture. Three blocks can be distinguished:

Network Switching Subsystem (NSS), also referred to as the core network of GSM. It provides the switching, intelligence and signalling functions of the network. It also performs inter-working functions with external networks.

Base Station Subsystem (BSS), also known as the access network It enables the user to communicate with the network.

Operating and Support System (OSS). It performs the management and administration functions.

In the NSS the most important element is the Mobile services Switching Center (MSC). It performs the telephony switching functions of the system. It also controls calls to and from other telephony and data systems, such as the Public Switched Telephone Network (PSTN) and other Public Land Mobile Networks by means of the Gateway Inter-working Unit (GIWU). In this case, the MSC is also named GMSC. In summary, the MSC is responsible among others for the following functions: