SAE Architecture Components

Question 1

MME

Answer 1

MME (Mobility Management Entity)
The key control-node for the LTE access-network.

Involved in the bearer activation/deactivation process.
The MME also terminates the S6a interface towards the HSS for roaming UEs.
Provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN.
Responsible for authenticating the user (by interacting with the HSS).
Responsible for choosing the SGW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation.
The Non Access Stratum (NAS) signaling terminates at the MME
Termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management.
Responsible for generation and allocation of temporary identities to UEs.
Checks the authorization of the UE to camp on the service provider’s Public Land Mobile Network (PLMN) and enforces UE roaming restrictions.
Supports lawful interception of signaling.
Responsible for idle mode UE (User Equipment) paging and tagging procedure including retransmissions.

Question 2

SGW

Answer 2

SGW (Serving Gateway)

Routes and forwards user data packets
Acts as the mobility anchor for the user plane during inter-eNodeB 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 PGW).
For idle state UEs, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the UE.
Manages and stores UE contexts, e.g. parameters of the IP bearer service, network internal routing information.
Performs replication of the user traffic in case of lawful interception.

Question 3

PGW

Answer 3

PGW (Packet Data Network Gateway)

Provides connectivity from the UE to external packet data networks (PDN) by being the point of exit and entry of traffic for the UE.
Performs policy enforcement (PCEF for QoS), packet filtering for each user, charging support, lawful interception and packet screening.
Another key role of the PGW is to act as the anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX and 3GPP2 (CDMA 1X and EvDO).
A UE may have simultaneous connectivity with more than one PGW for accessing multiple PDNs.

Question 4

HSS

Answer 4

HSS (Home Subscriber Server)

Central database that contains user-related and subscription-related information.
The functions of the HSS include functionalities such as mobility management, call and session establishment support, user authentication and access authorization.
The HSS is based on pre-Rel-4 Home Location Register (HLR) and Authentication Center (AuC).

Question 5

ePDG

Answer 5

ePDG (Evolved Packet Data Gateway)

The main function of the ePDG is to secure the data transmission with a UE connected to the EPC over untrusted non-3GPP access, e.g. VoWi-Fi. For this purpose, the ePDG acts as a termination node of IPsec tunnels established with the UE.

Question 6

ANDSF

Answer 6

Access network discovery and selection function

An entity within an evolved packet core (EPC) of the system architecture evolution (SAE) for 3GPP compliant mobile networks.
The purpose of the ANDSF is to assist user equipment (UE) to discover non-3GPP access networks – such as Wi-Fi or WIMAX – that can be used for data communications in addition to 3GPP access networks (such as HSPA or LTE) and to provide the UE with rules policing the connection to these networks.