indicated). The GGSN also provides additional information to configure the IP stack in the UE. This information includes for instance the DNS server information. The UE configures and brings up a network interface in its operating system. (Interface here refers to an IP-interface as described in Chapter 1, not a 3GPP interface, as the connections between 3GPP network elements are called in this chapter. The Internet technical community and the 3GPP community use the same word for two different definitions. It is confusing, we know.) After the PDP Context is up and the IP stack is configured, the UE is ready and connected to the Internet. Packet Delivery After PDP Context activation, the UE can start communicating to the Internet. The IP stack in the UE creates a packet with the above allocated IP address as the source address, and sends it to the GPRS interface. There the GPRS modem sends it first over the radio interface to the BTS or the NodeB. Before sending, the packet might be header compressed. In 2G-GPRS, the packet is decompressed back into its original form in the Basics of the 3GPP Technologies SGSN, whereas in 3G-GPRS the packet is put back together in the RNC. The RNC encapsulates the user's packet into a GTP tunnel (the packet is put inside a GTP packet). The tunnel is between this RNC, and the SGSN serving the UE. The SGSN receives the GTP encapsulated packet, and decapsulated the IP packet sent by the UE. The SGSN's context table and the GTP tunnel identifier allows the SGSN to find the upstream GTP- tunnel. The SGSN re-encapsulates the IP packet to the GTP tunnel going to the GGSN where the PDP Context is anchored. The GGSN then liberates the packet and sends it off further into the Internet. The first TTL-field decrease is done at the GGSN. Even though the packet has hopped over multiple network elements, and the connections between them, from the point of view of the IP packet that the UE sent, the packet has just done one hop. The network has been completely transparent to it. In the downlink d