f the control and user-plane parts. Gn/Gp-interface is the interface between the SGSN and the GGSN. The Gn-interface is used when the MS is in the home network, whereas the Gp-interface is used when the subscriber is roaming in another network. Thus, the Gp-interface is the interface between two network operators. The Gn-interface is also used between SGSN during a handover to exchange MS related information, and for forwarding user packets. Gr-interface is the interface between the SGSN and the HLR. Gi-interface connects the GPRS network to the external IP-networks. There are elements in the GPRS architecture that pass both control and user data. These include the whole BSS, and the SGSN and the GGSN. The interfaces connecting those elements have two modes - the user-plane, and the control-plane modes. Figure 2.5 shows the 2G-GPRS user-plane protocols. The protocols shown in the interface figure are described in more detail in Section 2.3. Here we will just give an overview. Deploying IPv6 in 3GPP Networks SNDCP SNDCP GTP-U GTP-U BSSGP BSSGP Network Network Service Service GSM RF GSM RF L1Bis L1Bis Figure 2.5 2G-GPRS user-plane protocols. GSM radio protocols (GSM Radio Frequency (RF), Media Access Control (MAC) [22], Radio Link Control (RLC) [22], and Logical Link Control (LLC) [23]) provide the GSM radio functionality including ciphering, reliability, and other radio procedures. Base Station System GPRS Protocol (BSSGP) provides routing-related and Quality of Service (QoS)-related information between the BSC and the SGSN [24]. The network service in the Gb-interface originally was defined as Frame Relay, but now also IP based configurations are possible [25]. SubNetwork Dependent Convergence Protocol (SNDCP) is a protocol that enables the transport of the multiple networking protocols over the GSM radio service [18]. GTP User Plane (GTP-U) is a tunneling protocol that allows the transportation of different networking protocols over the 3GPP core network. Figure 2.6 shows the 2G-GPRS control-plane protocols. Man