ing technology called X.25 [17] was in wide use. At the time, the data Basics of the 3GPP Technologies Core Network Figure 2.3 GSM CS network architecture. service provided over the GSM system was purely CS based, as was the whole network architecture. However, there was an increasing interest in packet data. The GSM architecture consists of network elements and the interfaces between those elements. (One has to be careful with the word 'interface'. In the 3GPP context, it means the whole vertical protocol stack between two network elements - sometimes also called a reference point, but in the IP language it means the network connection of an IP node.) You can see these described in the GSM network architecture in Figure 2.3. The network elements are the labeled boxes. In the following, we will briefly describe the main elements and the interfaces between them: Mobile Station (MS) is the phone, handset, or other device that is used to connect to the network. Base Transceiver Station (BTS) is also known as the base station. It is the radio network element that is directly connected to the MS over the air interface. Base Station Controller (BSC) controls a group of base stations. It performs the mobility management between the BTSs under its control. Home Location Register (HLR) is the database that contains the subscriber data. This subscriber data includes the authentication data used to authenticate the user, user profiles including what services the user is subscribed to, and the location where the user currently is. Mobile-services Switching Centre (MSC) is the local exchange responsible for a set of BSCs, and the MSs under those base stations in a geographical area. It performs authentication, mobility management, and exchanges the subscribers' calls. In Figure 2.3, the interfaces are shown as lines between the labeled network elements, looking from left to right, Um for the MS to BTS interface, Abis between BTS and the Deploying IPv6 in 3GPP Networks Gn/Gp Packet Core Network Figure 2.4 2G-GPRS network archi