onality in their roaming agreements. It may contain fire- wall functions to ensure secure connections over the inter-PLMN backbone network. The BG may be integrated into the GGSN. The GPRS network contains some network elements that are not shown in Figure 5.16. Two important examples of these are the charging gateway (CG), which collects charging information from SGSN and GGSN Introduction to Telecommunications Network Engineering and sends it to the billing system, and the domain name server (DNS), which maps logical domain names to IP address numbers the same way as in any IP network (see Section 6.6.10). 5.7.2.5 GPRS Network Interfaces Several interfaces are defined in GPRS standards, the most important of which are as follows: Gb, between BSS and SGSN; Gn, between SGSN and GGSN; Gi, between GGSN and external network; Gs, between SGSN and VLR; Gr, between SGSN and HLR. Interface specifications define protocols needed for packet-switched operation. Typically IP packets are transmitted from the MS to the external PDN and additional IP tunneling is used in the GPRS core network. 5.7.2.6 MS GPRS requires completely new terminals, which can be regular mobile tele- phones, PC cards, or specific modules built in to a machine. These terminals are divided into three classes: 1. Class A terminals can handle both circuit-switched and packet- switched services simultaneously and independently. 2. Class B terminals can handle either circuit- or packet-switched serv- ice at one time. It can automatically switch between these two modes. For example, in the case of an incoming circuit-switched call, it may suspend packet transfer and resume it afterward. 3. Class C terminals must be manually set into one of the modes. In the circuit-switched mode, it is cannot be accessed for packet- switched traffic and vice versa. A special case of class C mobile is a packet-only terminal integrated into laptop. 5.7.3 Operation of GPRS GPRS provides genuine packet-switched radio access and packet service users share the radio channels allo