23-09-2013, 04:56 PM
FROM GSM TO LTE - AN INTRODUCTION TO MOBILE NETWORKS AND MOBILE BROADBAND
INTRODUCTION TO MOBILE .pdf (Size: 3.62 MB / Downloads: 93)
Global System for Mobile
Communications (GSM)
At the beginning of the 1990s, GSM, the Global System for Mobile Communications triggered an
unprecedented change in the way people communicate with each other. While earlier analog wireless
systems were used by only a few people, GSM is used by over 3 billion subscribers worldwide in
2010. This has mostly been achieved by the steady improvements in all areas of telecommunication
technology and the resulting steady price reductions for both infrastructure equipment and mobile
devices. This chapter discusses the architecture of this system, which also forms the basis for the
packet-switched extension called General Packet Radio Service (GPRS), discussed in Chapter 2, for
the Universal Mobile Telecommunications System (UMTS), which is described in Chapter 3 and Long-
Term Evolution (LTE), which is discussed in Chapter 4. While the first designs of GSM date back to
the middle of the 1980s, GSM is still the most widely used wireless technology worldwide and it is
not expected to change any time soon. Despite its age and the evolution toward UMTS and LTE, GSM
itself continues to be developed. As shown in this chapter, GSM has been enhanced with many new
features in recent years. Therefore, many operators continue to invest in their GSM networks in addition
to their UMTS and LTE activities to introduce new functionality and to lower their operational cost.
Circuit-Switched Data Transmission
Initially, GSM was designed as a circuit-switched system that establishes a direct and exclusive con-
nection between two users on every interface between all network nodes of the system. Section 1.1.1
gives a first overview of this traditional architecture. Over time, this physical circuit switching has
been virtualized and many network nodes are connected over IP-based broadband connections today.
The reasons for this and further details on virtual circuit switching can be found in Section 1.1.2.
Virtual Circuit Switching over IP
While in the 1990s voice calls were the dominating form of communication, this has significantly
changed today with the rise of the Internet. While voice calls still remain important, other forms of
communication such as e-mail, instant messaging (IM), social networks (e.g. Facebook), blogs, wikis
and many more play an even bigger role. All these services share the Internet Protocol (IP) as a
transport protocol and globally connect people via the Internet.
While circuit switching establishes an exclusive channel between two parties, the Internet is based
on transferring individual data packets. A link with a high bandwidth is used to transfer the packets
of many users. By using the destination address contained in each packet, each network node that
the packet traversed decides over which outgoing link to forward the packet. Further details can be
found in Chapter 2.
Owing to the rise of the Internet and IP-based applications, network operators now have to maintain
two separate networks: a circuit-switched network for voice calls and a packet-switched network for
Internet-based services.
Standards
As many telecom companies compete globally for orders of telecommunication network operators,
standardization of interfaces and procedures is necessary. Without standards, which are defined by the
International Telecommunication Union (ITU), it would not be possible to make phone calls interna-
tionally and network operators would be bound to the supplier they initially select for the delivery of
their network components. One of the most important ITU standards discussed in Section 1.4 is the
Signaling System Number 7 (SS-7), which is used for call routing. Many ITU standards, however,
only represent the smallest common denominator as most countries have specified their own national
extensions. In practice, this incurs a high cost for software development for each country as a differ-
ent set of extensions needs to be implemented in order for a vendor to be able to sell its equipment.
Furthermore, the interconnection of networks of different countries is complicated by this.
GSM, for the first time, set a common standard for Europe for wireless networks, which has also
been adopted by many countries outside Europe. This is the main reason why subscribers can roam
in GSM networks across the world that have roaming agreements with each other. The common
standard also substantially reduces research and development costs as hardware and software can now
be sold worldwide with only minor adaptations for the local market. The European Telecommunication
Standards Institute (ETSI), which is also responsible for a number of other standards, was the main
body responsible for the creation of the GSM standard. The ETSI GSM standards are composed of
a substantial number of standards documents, each of which is called a technical specification (TS),
which describe a particular part of the system. In the following chapters, many of these specifications
are referenced and can thus be used for further information about a specific topic. All standards are
freely available on the Internet at http://www.etsi.org [1] or at http://www.3gpp.org [2]. 3GPP is the
organization that took over the standards maintenance and enhancement at the beginning of the UMTS
standardization, as described in Chapter 3.