heir payload. The addressing, IP version, and the traffic are completely separate from the transport layer. These two layers are completely separated, and thus independent of each other. This separation allows different IP versions to be used by the user from those used by the network. Hence, the operator can provide an IPv6 service to the user even if the operator's internal network is not yet fully IPv6 capable. The opposite is also possible: in the case of fully IPv6 capable network, the operator can provide IPv4 services for legacy devices. 2.3 3GPP Protocols Above, we became familiar with the 3GPP architecture variants, and saw that they are a bit different, although there are similarities in all of them. We also learned the difference between the control and user planes. In this section, we will drill a bit deeper on what are the different functions that the control and user planes actually perform. In addition, we will look a bit more closely at the main protocols used in the 3GPP networks. The different interfaces throughout the network have very different characteristics. The radio interface is a wireless interface, where it is important to make sure that the messages - network control signals or user packets - can get over that hop with adequate robustness. Some other interfaces within or between the radio access and packet core networks can actually be transported over a complete network with both wireline and wireless segments. The network topology of those networks does not necessarily reflect the clean architecture pictures shown in this book and in other literature. The different interfaces are not necessarily in their different networks, or segments, but in some cases are in the very same network segment. Depending on the network, the network elements can be either co-located in the very same data centers, or on very different locations. However, obviously, the base stations and (e)NodeBs have to be located relatively close to the subscriber to actually create a wide area wireless network. The dif