tion of the GSM architecture. However, the significant alterations to the architecture in the context of this book are related to the PS domain. These changes to the architecture were made when the PS domain was first introduced, and after that, when a new radio access technology was added. These bigger architecture steps have been the introduction of the PS domain - the addition of GPRS to GSM architecture, the move to the 3G radio system (and to 3GPP from ETSI) and hence the introduction of Universal Mobile Telecommunications System, and finally the latest new radio technology, Long Term Evolution. All of these three architectures are closely related, and they share many of the same technologies. However, GPRS and UMTS are SO closely related that they are often now together described as 'GPRS', or sometimes 2G-GPRS and 3G-GPRS. Therefore in the section below, these two architectures are discussed together. But LTE is a more radical change to the 3GPP architecture and it is described separately. We will now follow this evolution in the 3GPP system, starting with a high-level overview of the GSM architecture. After that we will move to a GPRS architecture overview covering both the 2G and the 3G variants. At the end of the section, we will cover the newest arrival to the 3GPP architecture family, LTE. 2.2.1 GSM System As described earlier, the 3GPP system has evolved from the original European mobile telephony system GSM. The GSM system [16] was introduced in the early 1990s to provide digital mobile telephony service. It was the first digital, cellular mobile phone system to become widely used. The GSM used Time Division Multiple Access (TDMA) based radio technology. It was very successful, and the usage of mobile telephony started growing. Data was becoming more important for the users of telecommunication systems. These users included enterprise users, but also consumer interest was growing. At the time, IP and the Internet access had not become as dominant as it is today, and still another packet data network