ell size. The BSs and MSs (telephone) are controlled to keep their transmission power as low as possible. This low-power transmission does not interfere with other users of the same frequency (reuse of frequencies) some cells away from this cell. This is how each frequency channel can be used again and again and, in principle, a network operator can increase capacity without Introduction to Telecommunications Network Engineering Rural Urban Same channels (carrier frequencies) in use Figure 5.2 Cellular structure of a mobile radio network. limit by reducing cell size. Naturally, this requires investment in additional BS sites. How often each carrier frequency is used is termed the frequency reuse factor and it depends on the system. Note that in the CDMA cellular system, which is introduced in Section 5.4.5, neighbor cells may use the same carrier frequency and there channeling is based on the spreading code instead of frequency (and time slot). The consequences of reduced cell size are handier and less expensive telephones as well as longer operational life for the battery. Low transmission power also provides a safety improvement from the users' point of view. Because of public concern about handheld terminals and their adverse effects on health, low transmission power has become increasingly important. In a conventional fixed network, telephone calls are always routed to one fixed telephone socket, as we saw in Chapter 2. In a cellular network a subscriber is located in one cell at a time. Now the network has to include additional intelligence to be able to connect a call to the cell where the called subscriber is available at that time. To succeed at this, the cellular networks have two databases or registers, a home location register (HLR) and a visitors location register (VLR), and with them the network is able to manage the mobility of its subscribers. 5.2.2 HLR and VLR When subscribers purchase a mobile telephone, they are registered in the HLR of their own mobile telephone operator. The HLR stores their u