cellular standards were developed in Nordic countries, the United States, and Japan at the end of 1970s. In this chapter we introduce first the idea and operation of cellular radio systems in general. The common principles of cellular systems are valid for any public land mobile network. Then we will review other mobile sys- tems such as paging systems, cordless telephones, and WLANs. In the last section of this chapter, we review the structure and operation of the GSM network. Our goal in this chapter is to provide the reader with an under- standing of what is required of the network to enable someone to receive or initiate a call anywhere in the world. The natural requirement for this is that compatible service be available. We use GSM as an example of a digital cellu- lar system because it is currently the dominant global digital technology. Introduction to Telecommunications Network Engineering 5.1 Cellular Radio Principles The main problem of conventional radio telephone networks was low capac- ity because of the limited frequency band available for this service. Cellular networks provide a solution for this by using the same frequencies in multi- ple areas inside the network. This principle of frequency reuse with the help of a cellular network structure was invented at Bell Laboratories during the 1960s. The technical development of radio-frequency control, the micro- processor, and software technologies made cellular networks feasible by the end of 1970s. Here is a list of the most important common characteristics of cellular systems: Frequency reuse provides a much larger number of communication channels than the number of channels allocated to the system. Automatic intercellular transfer, or a handover, ensures continuity of communication when there is a need to change BSs. Continuous monitoring of communication between the mobile and BS verifies the quality and detects the need for a cell transfer. Automatic location of mobile stations within the network ensures that calls can be routed to mobiles. Mob