LTE ARCHITECTURE DESCRIPTION

Evolved NodeB:

• It is the only network element defined as part of EUTRAN. 
• It replaces the old Node B / RNC combination from 3G.
• It provides all radio management functions
• To enable efficient inter-cell radio management for cells not attached to the same eNB.
• The interface between eNodeB's is X2.

Serving Gateway (SGW):

• Manages the user data path  within EPC.
• It connects via the S1-U interface towards eNB and receives uplink packet data from here and transmits downlink packet data on it.
• The serving gateway has packet data anchoring function within EPC.
• It relays the packet data within EPC via the S5/S8 interface to or from the PDN gateway.
• A serving gateway is controlled by one or more MMEs via S11 interface. 

Packet Data Network Gateway (PGW): 

• The PDN gateway provides the connection between EPC and a number of external data networks.
• PDN Gateway is comparable to GGSN in 2G/3G networks.
• Charging & Lawful Interception support.
• IP Address Allocation for UE.
• Packet screening (firewall functionality).

Mobility Management Entity (MME): 

• It is a pure signaling entity inside the EPC.
• SAE uses tracking areas to track the position of idle UEs. The basic principle is identical to location or routing areas from 2G/3G.
• MME handles attaches and detaches to the SAE system, as well as tracking area updates.
• Interface towards the HSS which stores the subscription relevant information and the currently assigned MME in its permanent data base.

Home Subcriber Server (HSS): 

• Permanent and central subscriber database.
• Stores mobility and service data for every subscriber.
• Contains the Authentication Center (AuC) functionality.

Policy and Charging Rule Function (PCRF):

• The PCRF major functionality is the Quality of Service (QoS) coordination.