e mobility management, makes very interesting read. However, 3GPP technology is built to hide the mobility from the upper layers. Therefore, it has little effect on the IP layer. Hence in this book, we will concentrate on the network functions that are needed for the user to connect to the Internet - and eventually disconnect from the network. The 2G and 3G GPRS variants work very much in a similar manner. Therefore, we are going to explore the control plane of the two GPRS variants together. However, EPS uses a slightly different approach, and we will study EPS separately in its own section. The aim of this book is to understand the functionality of the protocols. There is not enough space to dive into the details of the different protocols. There is better literature and much better authors for that purpose. An interested reader can take a look at the suggested reading section at the end of the chapter. We already described the protocol architecture in Section 2.2.2 for GPRS, and the EPS protocol architecture was defined in Section 2.2.3. In the following, we will show how the connections between the UE and the network are managed, by explaining the used signaling. The protocol messages described in the following sections are messages of the upper protocol layers of the control plane part of an interface. The layers below the uppermost layer make sure that the message is transported correctly over the interface. For 2G-GPRS, the control-plane protocols can be found in Figure 2.6, and for 3G-GPRS control plane protocols are in Figure 2.9. The EPS system control-plane protocols can be found in Figure 2.12. Before connecting to the Internet, a UE has to get access to the mobile network itself. This is done by performing a network attach function. The purpose of the attach function is for the UE and the network to get to know each other. The UE identifies itself to the network, and the subscriber database, the HLR, authenticates the user. During this authentication procedure the network and the UE also set up netwo