ble 7-1 for infrastructure in 2G and 3G networks. Mobile and Wi-Fi Networks Voicemail and text messaging servers BTS = Base Transceiver Station BSC = Base Station Controller C.R.S E MSC = Mobile Switching Center PSTN = Public Switched Telephone Network Figure 7-3 The architecture in second-generation (2G) Global System for Mobile Communications (GSM) networks. Base stations are the most expensive portions of a mobile carrier's network. In rural areas, cell sites might cover up to 30 miles. In densely populated cities with high concentrations of traffic, a cell site might cover less than a mile. Therefore, network upgrades to services such as next-generation mobile service or acquisition of different spectrum that requires additional base stations or upgrades to existing cell sites are hugely expensive. Mobile central office switches that route traffic within mobile networks and to landline networks are also part of all mobile networks. Newer mobile switches are built on softswitch technology, which supports Voice over Internet Protocol (VoIP) and multimedia applications. For information about softswitches, see Chapter 4, "Carrier Networks." All of the aforementioned network types as well as 4G networks depend on trans- port networks. This is a generic term for protocols (T1, Gigabit Ethernet) and media (fiber optics, microwave) that are used on links between equipment such as base sta- tions and equipment in the core network, including controllers, signaling devices, and switches in mobile networks. The transport network includes links to other networks. Second-, Third-, and Fourth-Generation Digital Networks Table 7-1 2G and 3G Infrastructure Generation Cell Site Core Functions Base Transceiver Station Base station Controls handover of calls (BTS) contains antennas controller to other cell sites and to and radios, and performs the mobile switching cen- similar functions to NodeBs ter (mobile central office). in 3G networks of manag- Connects to other network ing traffic between mobile controllers and radio base 