o be transmitted in order. Although 3GPP removed X.25 support after there was clear consensus that it would not be needed anymore, sometimes reaching consensus on a feature being outdated can be difficult. PPP has been used in very few networks, and practically it was never supported by mainstream mobile phones. Still, the support for PPP is included in the specifications. 3G-GPRS The 3G-GPRS user-plane protocol architecture is shown in Figure 2.8. Even at first glance it is clear that there are similarities with the 2G-GPRS architecture. However, there are also some obvious differences: the radio interface protocols are only between the UE and Basics of the 3GPP Technologies the NodeB, and GTP extends to the radio access network. In the following, we take a look at the two protocols that transport the users' packets over the 3G-GPRS network - the Packet Data Convergence Protocol (PDCP) and the GTP. PDCP is responsible for transferring the packets over the radio interface, and providing header compression. The segmentation and reassembly of the user packets, and mapping the PDP Contexts to the radio bearers is provided by the underlying RLC [35]. In 3G-GPRS, the functionality that was in one protocol, in the SNDCP in 2G, has been distributed into PDCP and to the RLC protocol layers. In addition, PDCP and RLC are only used between the UE and the RNC, and they are not extended to the SGSN as SNDCP did in 2G. The header compression algorithms supported by PDCP are Degermark [20] and RoHC [21]. GTP in 3G-GPRS is the same protocol as in 2G-GPRS. Therefore the description of the protocol itself in the previous section applies also for 3G. However, in addition to the core network, GTP is also used inside the radio access network. In the radio access network, GTP is used in the user plane in the Iu-interface, which is an interface specific to the 3G-GPRS architecture. GTP is used in the core network in the Gn- and Gp-interfaces, which also exist in 2G-GPRS. If the 2G and 3G-GPRS architectures are very similar with few di