This chapter covers the high-level architecture of LTE. We begin by describing the hardware components in an LTE network and by reviewing the software protocols that those components use to communicate. We then look in more detail at the techniques used for data transport in LTE before discussing the state diagrams and the use of radio spectrum. We will leave some more specialized architectural issues until later chapters, notably those related to quality of service, charging and inter-system operation.
Several specifications are relevant to this chapter. TS 23.401  and TS 36.300  are stage 2 specifications that include descriptions of the system architecture, while the relevant stage 3 specifications [3, 4] contain the architectural details. We will also note some other important specifications as we go along.
Figure 2.1 reviews the high-level architecture of the evolved packet system (EPS). There are three main components, namely the user equipment (UE), the evolved UMTS terrestrial radio access network (E-UTRAN) and the evolved packet core (EPC). In turn, the evolved packet core communicates with packet data networks in the outside world such as the internet, private corporate networks or the IP multimedia subsystem. The interfaces between the different parts of the system are denoted Uu, S1 and SGi.
The UE, E-UTRAN and EPC each have their own internal ...