Function of Network Element

As per the LTE Architecture from my previous post, the function of each network element is defined below:

eNB

• Interfaces with UE. The interface is known as Uu interface.
• Connects to MME with interface S1-MME or S1-C
• Connects to S-GW with interface S1-U
• Admission Control
• Scheduling
• Radio Resource Management
• Enforcement of negotiated UL QoS
• Cell Information Broadcast
• Ciphering/deciphering of user and Control Plane data
• Compression and decompression of DL/UL user plane packet headers

MME

 It is the main control node in the LTE access network.

• Connects to SGSN with interface S3 (provides Control plane function for Mobility between LTE and 2G/3G)
• Connects to another MME with interface S10
• Connects to HSS with interface S6a
• Responsible for IDLE mode UE tracking and PAGING procedure including re-transmissions.
• Authentication of user. This is achieved by interacting with HSS.
• NAS signaling terminates at the MME
• Generation and allocation of temporary ID's to UE
• Enforces UE roaming restrictions and also checks the authorization of UE to camp on the service provider's PLMN 
• Handles the security key management and is the termination point in the access network for ciphering and integrity protection for NAS signaling.
• Involves in bearer Activation/Deactivation
• Responsible for choosing S-GW for a UE during the Initial Attach and also during the Intra LTE Handover which also involves the core network node relocation

Serving-GW

• Connects to P-GW with interface S5/S8
• Connects to SGSN with interface S3
• Acts as Mobility anchor for the User plane during Inter eNB handover
• Anchor for mobility between LTE and other 3GPP technologies
• For Idle state UE's, the S-GW terminates the DL data path and triggers paging when DL data arrives for the UE.
• Manages and stores UE context (parameters of the IP bearer service, network's internal routing information)
• Performs replication of the user traffic in case of lawful interception

PDN-GW

• Acts as the entry and exit point of Traffic for the UE
• Provides connectivity to the UE to external packet data networks
• UE can have more than one PDN-GW for accessing multiple PDN's
• Connects to PCRF with S7 interface
• Connects to IMS with Gx interface
• Performs policy enforcement
• Packet filtering with each user
• Charging support
• Packet screening and lawful interception
• Anchor for mobility between 3GPP and non 3GPP technologies(WiMax) and 3GPP2 (CDMA 1x and EVDO)

The next post will explain the RACH procedure.