? Problem 5.8 Explain the structure of a GSM network. What are the main network ele- ments and what are their roles in the operation of GSM? Mobile Communications Problem 5.9 Explain the multiple-access method of GSM. Problem 5.10 Explain how location update is performed in GSM. What triggers it and what happens after that? Problem 5.11 Explain how a call is routed from a GSM MS to another MS of the same home network. Assume that (a) both are located in home network and (b) both have roamed to another country. Problem 5.12 Explain how handover is performed in the GSM network. Problem 5.13 Explain the security functions implemented in the GSM network. Problem 5.14 What are the main new network elements that are needed in GPRS network? What are their roles in GPRS operation? Problem 5.15 What are the basic operating differences between GPRS and circuit-switched References [1] Redl, M. S., M. K. Weber, and M. W. Oliphant, An Introduction to GSM, Norwood, MA: Artech House, 1995. [2] Mouly, M., and M. B. Pautet, The GSM System for Mobile Communications, Paris, France: Michel Mouly and Marie-Bernadette Pautet, 1992. Heine, G.A., and Inacon GmbH, GPRS from A-Z, Norwood, MA: Artech House, 2000. Data Communications In the beginning of this chapter we clarify some key terms that we need to describe a certain data communications principle or a system. Then we intro- duce the concept of data communication protocols, trying to convey a con- crete feel for the layered data communication protocol stacks and the reason why we define data communication architectures with the help of the proto- col layers. Then we describe various data communications systems used in access networks and for local- and wide-area data communications. In the latter half of this chapter, we concentrate on the Internet and describe its structure, operation, and services. 6.1 Principles of Data Communications The first data communications system was the telegraph. It was invented more than 100 years ago. The letters to be transmitted were converted into 