La microempresa

The LTE system is designed for the packet switched services providing IP con- nectivity between the Packet Data Network (PDN) and the User Equipment (UE) without service interruption even during mobility. The LTE system can be di-

vided into two main branches: Evolved Universal Terrestrial Radio Access Net- work (E-UTRAN) and System Architecture Evolution (SAE). The E-UTRAN evolved from the UMTS radio access network; it is sometimes also referred to as LTE. The SAE supports the evolution of the packet core network, also known as Evolved Packet Core (EPC). The combination of both the E-UTRAN and the SAE compose the Evolved Packet System (EPS). Figure 3.1 shows the general LTE network architecture.

According to [SBT09], an EPS bearer is defined to be an IP packet flow between the PDN-GW and the UE with predefined Quality of Service (QoS) characteristics. Both the EPC and the E-UTRAN are responsible for setting and releasing such a bearer depending on the application QoS requirements. In LTE multiple bearers can be established for users with multiple services, e.g., a user can have a voice call using the Voice over Internet Protocol (VoIP) and at the same time be downloading a file using File Transfer Protocol (FTP), or browse the web using the Hypertext Transfer Protocol (HTTP). Each of these services can be mapped to a different bearer. More detailed explanations on the quality of service and the bearers in LTE are given in section 3.5. In the next subsection a brief description of the important LTE nodes will be presented.

3.3.1 User Equipment (UE)

As the name suggests, a UE is the actual device that the LTE customers use to connect to the LTE network and establish their connectivity. The UE may take several forms; it can be a mobile phone, a tablet, or a data card used by a computer/notebook. Similar to all other 3GPP systems, the UE consists of two main entities: a SIM-card or what is also known as User Service Identity Mod- ule (USIM), and the actual equipment known as Terminal Equipment (TE). The SIM-card carries the necessary information provided by the operator for user iden- tification and authentication procedures. The terminal equipment on the other hand provides the users with the necessary hardware (e.g., processing, storage, operat- ing system) to run their applications and utilize the LTE system services.

3.3.2 Evolved UTRAN (E-UTRAN)

The E-UTRAN in LTE consists of directly interconnected eNodeBs which are connected to each other through the X2 interface and to the core network through the S1 interface. This eliminates one of the biggest drawbacks of the former 3GPP systems (UMTS/HSPA): the need to connect and control the NodeBs through the

Radio Network Controller (RNC), which make the system vulnerable against RNC failures. The LTE E-UTRAN architecture can be seen in Figure 3.5.

MME/ aGW MME/ aGW eNodeB eNodeB eNodeB X2 X2 X2 S1 S1 S1 S1 EPC E-UTRAN

Figure 3.5: LTE E-UTRAN architecture

The enhanced NodeB (eNodeB) entity works as a bridge between the UE and the EPC. It provides the necessary radio protocols to the user equipment, so as to be able to send and receive data and it tunnels the users data securely over the LTE transport to the PDN-GW and vice versa. The GTP tunneling protocol is used, which works on top of the UDP/IP protocols. The eNodeB is also respon- sible for the scheduling which is one of the most important radio functions. The eNodeB schedules the frequency spectrum resources among the different users by exploiting both the time and frequency, while guaranteeing different quality of service for the end users. In addition, the eNodeB also has some mobility man- agement functionalities, e.g., radio link measurements and handover signaling for other eNodeBs.

3.3.3 Evolved Packet Core (EPC)

As shown in Figure 3.1, the EPC (also known as the LTE core network) consists of three main entities: Mobility Management Entity (MME), Serving Gateway (S-GW) and the Packet Data Network Gateway (PDN-GW). In addition, there are some other logical entities like the Home Subscriber Server (HSS) and Policy and Charging Rules Function (PCRF). The main purpose of the EPC is to provide the

necessary functionalities to support the users and establish their bearers [SBT09]. Each of the EPC main entities and their functionalities is described briefly in the next paragraphs. A more detailed description can be found in [36.11a].

The MME entity provides control functions as well as signaling for the EPC. The MME is only involved in the control plane. Some of the MME supported functions include: authentication, security, roaming, default/dedicated bearer es- tablishment, tracking user mobility and handover. The S-GW is the main gateway for the user traffic, where all the users IP traffic goes through. It is the local mobil- ity anchor point for inter-eNodeB handover, as well as the mobility anchoring for inter-3GPP mobility [36.11a]. In addition the S-GW provides several other func- tions like: routing, forwarding, charging/accounting information gathering. The packet data network gateway PDN-GW acts as the user connectivity point for the user traffic, it is responsible for assigning the users IP addresses as well as clas- sifying the user traffic into different QoS classes. In addition, the PDN-GW acts as the mobility anchor point for inter-working with non 3GPP technologies, like Wireless LAN and WiMax.