LTE Network Architecture

Figure 2 shows a very basic architecture of the EPS when the User Equipment (UE) is connected to the EPC over E-UTRAN (LTE access network). The Evolved NodeB (eNodeB) is the base station for LTE radio. In this figure, the EPC is composed of four network elements: the Serving Gateway (Serving GW), the PDN Gateway (PDN GW), the MME and the HSS. The EPC is connected to the external networks, which can include the IP Multimedia Core Network Subsystem (IMS).

HSS

Basically, the HSS (for Home Subscriber Server) is a database that contains user-related and subscriber-related information. It also provides support functions in mobility management, call and session setup, user authentication and access authorization.

It is based on the pre-3GPP Release 4 - Home Location Register (HLR) and Authentication Centre (AuC).

Serving GW

The gateways (Serving GW and PDN GW) deal with the user plane. They transport the IP data traffic between the User Equipment (UE) and the external networks.

The Serving GW is the point of interconnect between the radio-side and the EPC. As its name indicates, this gateway serves the UE by routing the incoming and outgoing IP packets.

It is the anchor point for the intra-LTE mobility (i.e. in case of handover between eNodeBs) and between LTE and other 3GPP accesses.

It is logically connected to the other gateway, the PDN GW.

PDN GW

The PDN GW is the point of interconnect between the EPC and the external IP networks. These networks are called PDN (Packet Data Network), hence the name. The PDN GW routes packets to and from the PDNs.

The PDN GW also performs various functions such as IP address / IP prefix allocation or policy control and charging.

3GPP specifies these gateways independently but in practice they may be combined in a single "box" by network vendors.

MME

The MME (for Mobility Management Entity) deals with the control plane. It handles the signalling related to mobility and security for E-UTRAN access.

The MME is responsible for the tracking and the paging of UE in idle-mode. It is the termination point of the Non-Access Stratum (NAS).

From <http://www.3gpp.org/technologies/keywords-acronyms/100-the-evolved-packet-core>

The X2 Interface

A new interface (X2) has been defined between eNodeB, working in a meshed way (meaning that all Node Bs may possibly be linked together). The main purpose of this interface is to minimize packet loss due to user mobility. As the terminal moves across the access network, unsent or unacknowledged packets stored in the old eNodeB queues can be forwarded or tunnelled to the new eNodeB thanks to the X2 interface. 

From <https://sites.google.com/site/lteencyclopedia/lte-network-infrastructure-and-elements>