the capability to transmit the same multimedia streams (for example, radio and video) to many users simultaneously. Infrastructure in Second- and Third- Generation Mobile Networks Third-generation (3G) networks use packet-switched technology for data. Voice is car- ried separately by using circuit-switched technology, which saves a path for the entire voice call. By contrast, fourth-generation (4G) protocols specify packet switching for voice as well as data. Figure 7-2 illustrates the 3rd Generation Partnership Project Second-, Third-, and Fourth-Generation Digital Networks Internet Billing, roaming, Short Message Service (databases and services) Mobile switching center NodeB NodeB = Voice and data traffic = Signaling traffic Radio access GGSN = GPRS Gatway Support Node network SGSN = Signaling Gateway Service Node RNC = Radio Network Controller Figure 7-2 The architecture in third-generation (3G) Wideband Code-Division Mul- tiple Access (WCDMA) networks. (3GPP)-specified architecture for WCDMA mobile networks. Figure 7-3 depicts devices in a GSM network. Although the figures represent them as separate devices, in practice the controller functions for both second-generation (2G) and 3G networks are often located within the same piece of equipment. 3G networks are backward compatible with earlier GSM and 2.5G (EDGE and GPRS) networks. In addition, traffic from these earlier-generation air interfaces can be transported from cell sites to the core on the same (backhaul) facilities. (See the sec- tion "Connecting Cell Sites and Core Networks," later in this chapter, for information about backhaul networks.) The 3GPP2 and CDMA2000 specifications have similar architectures to those for WCDMA. 3GPP2 specifies backward compatibility to earlier CDMA technology SO that carriers can mix older and newer cellular technologies using the same spec- trum and existing base stations and mobile switching centers. Currently, when carriers upgrade to 3G services, they select WCDMA mostly because of its predominance worldwide. See Ta