LTE Architecture and Network Elements

The 3GPP evolution for the 3G mobile system defined the UTRAN Long Term Evolution (LTE) and System Architecture Evolution (SAE) network. These standards define an all-IP network as a base for the LTE/SAE. The LTE/SAE does not have separate packet switched data traffic and circuit switched voice network. Both data and user plane communicates over the same network, which is called Evolved Packet System (EPS) network. 

LTE/SAE network includes many new network elements like MME and SAE GW. Only remaining element in radio access network is (enhanced) eNode B.

The figure shows the evolved system architecture of LTE

eNB

eNode B (eNB) is the base station in the LTE/SAE network that interfaces with the UE and hosts the PHYsical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers. It also hosts Radio Resource Control (RRC) functionality corresponding to the control plane. Its main functions are

•  Radio resource management
•  IP header compression and encrypting of user data stream
• Selection of an MME at UE attachment
•  Routing of user  plane data towards SAE gateway
• Measurement and measurement reporting configuration for mobility and scheduling

Mobility Management Entity (MME)

The main task of MME is to take care signaling of control plane, and especially for mobility management and idle-mode handling.  It is also manages and stores UE context for the idle state, for example UE/user identities, UE mobility state and user security parameters.

Serving Gateway (SGW)

The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW).

Packet Data Network Gateway (PDN GW)

The PDN GW provides connectivity to the UE to external packet data networks by being the point of exit and entry of traffic for the UE. A UE may have simultaneous connectivity with more than one PDN GW for accessing multiple PDNs. The PDN GW performs policy enforcement, packet filtering for each user, charging support, lawful Interception and packet screening. 

Both SGW and PDN GW gateways are used to process the user-plane data and handle the tasks related to the mobility management inside LTE and between other 3GPP radio technologies.

Home Subscriber Server (HSS)

Home Subscriber Server (HSS) can be compared to Home Location Register (HLR) in UMTS network. It contains the subscription-related information, performs authentication and authorization of the user, and can provide information about the subscriber's location and IP information.

Policy and Charging Rules Function (PCRF)

Policy and Charging Rules Function (PCRF) is the generic name for the entity within the LTE SAE EPC which detects the service flow, enforces charging policy. For applications that require dynamic policy or charging control, a network element entitled the Applications Function, AF is used.

LTE network combined with GSM and UMTS networks will be complex because the network will have so many symbiotic relationships between the various elements within the system. For example voice handover between LTE and UMTS networks is not possible to do. Facing these challenges will generate work for years to come.