rk, and stays in the same topological location within it. Hence, the 3GPP link emulates a wired access link. 3GPP mobility management does not only hide mobility within one 3GPP access technology, but also during handovers between the different 3GPP radio technologies. In addition the mobility being invisible for the IP stack, the radio network and the whole network infrastructure is invisible for the UE's IP stack. (This is true for GPRS and EPS in GTP mode. However, the PMIP variant has a small deviation from this, which is described in more detail when describing bearers in Section 4.1.1.) A PDP Context, or an EPS Bearer seems like a single-hop link. In addition, the UE is alone on that link as the PDP Contexts and bearers of one UE are separated from other UE's PDP Contexts and bearers. The mobile gateway, GGSN or PGW, is the first-hop router for the UE. Practically this means that the IP packet does not see all the infrastructure and network elements during transit over the network, and therefore cannot address the network elements in the infrastructure either. Hence, also communication between the UEs in the 3GPP network has to always go through the network, and the GGSN or PGW. Note that within the scope Basics of the 3GPP Technologies of this book we do not consider or cover Local IP Access (LIPA) [29] or Selective IP Traffic Offload (SIPTO) [29] 3GPP system architectural extensions that may change the earlier stated communication assertion. Special Link Characteristics Although, as we explained above, the 3GPP access technologies emulates wireline IP- connectivity, it is still really a wireless system with different characteristics. Whereas fixed network links provide stable conditions with relatively stable bandwidth and latency char- acteristics, the mobile networks characteristics can significantly change due to mobility, load, and changing radio conditions. In mobile networks, the radio conditions can change drastically very quickly. This can even cause considerable packet loss for extended periods, 