re now based on general purpose operating systems, they have a bit of an issue with the multiple PDN Connections, and customization work is needed to support this concept. We believe that improvements are possible, as we will later discuss in Section 6.1. 2.6 User Equipment In the traditional 3GPP architecture, everything that is on the end user side is generally referred as a UE (or as an MS in the case of GPRS) as shown in Figure 2.7. The UE then consists of two main domains called the Universal Integrated Circuit Card (UICC) and the Mobile Equipment (ME), of which the ME can be further split down into the Mobile Terminal (MT) and the Terminal Equipment (TE) [54, 55]. The actual shapes and sizes for implementations of the abstract concept of UE, including those of MT and TE, can however vary significantly. In this section we will look at how the traditional UE model works, and how that has evolved into various Split-UE implementations. The first thing to understand is how responsibilities between MT and TE are split. The MT is the part that is always 3GPP aware, and its responsibilities include the follow- ing [54]: 1. Radio transmission termination and channel management. 2. Speech encoding and decoding. 3. Error protection for all information sent over the radio. 4. Flow control of signaling and user data. 5. Rate adaptation of user data, and data formatting for the transmission. 6. Support for multiple terminals. 7. Mobility management (of the part covered by 3GPP, not the IP layer mobility). The responsibilities left for the TE are then everything else that is protocol-wise 'above' the MT's responsibilities: starting from the IP stack itself. So to be clear, and for the following sections, the first significant part of the protocol stack that is on the TE is the IP stack, including the IPv6 that this book is about. Basics of the 3GPP Technologies 2.6.1 Traditional 3GPP UE Model For a long time a UE, and in particular a 2G MS, was simply a single device consisting of an MT and an ME tightly integrated into e