When we talk about LTE architecture, the first picture that comes in our mind UE,eNodeB and Core connection in simplified form.

So yes, when we talk about LTE architecture it includes various components, nodes and interfaces.

We will discuss each node with their functionalities and interfaces between nodes.

In fig.1 you can see the overall LTE architecture consisting of E-UTRAN and EPC (Evolved Packet Core).

The Various LTE Nodes are

• eNodeB
• MME
• SGW
• PGW
• HSS
• PCRF
• OCS
• OFC
• SPR

Note: A PDN is an internal or external IP domain of any operator that a UE wants to latch or communicate with. PDN provides UE the services like the Internet or IMS.

Function of LTE Nodes:

eNodeB Functions:

• Terminates the complete radio interface
• Performs Radio Bearer Control
• Performs Admission Control
• Perform Mobility Control
• Uplink & Downlink Scheduling
• Does Measurement Configuration
• Selection of an MME at UE attachment
• Scheduling and transmission of paging messages, Broadcast Info.

MME Functions:

MME handles the NAS function and it is signaling entity inside EPC.

• NAS signaling & security – The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs
• MME handles attaches and detaches.
• MME handles Tracking management to track the position of Idle mode UE
• TA list management, Paging retransmission also handled by MME
• MME is responsible for P-GW & S-GW selection
• MME selection when HO with MME change is taken care by MME.
• Interface towards the HSS which stores the subscription relevant information and the currently assigned MME in its permanent data base.
• User authentication and roaming with HSS is tackled by MME.
• Bearer Control Function

SGW Functions:

SGW provides the data place functionalities such as packet routing and forwarding functionalities. It connects via the S1-U interface towards eNB and receives uplink packet data from here and transmits downlink packet data on it. SGW is controlled by one or more MMEs via s11 interface.

• Local Mobility anchor for inter-eNB handover
• Only one SGW per UE allocation.
• It works as Mobility anchor between Inter-3GPP (2G,3G,LTE)
• Packet routing & forwarding -relays the packet data within EPC via the S5/S8 interface to or from the PDN gateway.
• Charging function is done by SGW (UE, PDN, QCI level)
• Act as Non-3GPP anchor (for roaming).

PGW Functions:

PGW provides the connection between EPC and PDN. It works as anchor point for handover mobility between 3gpp and non-3gpp.

• Mobility anchor for mobility between 3GPP access systems and non-3GPP access systems.
• It performs Policy Enforcement Function (PCEF)

Service level charging, gate control, rate enforcement

• There may be Several PGW could exist for a UE (Multiple PDN)
• Per-user based packet filtering is done by PGW.
• User IP address allocation is done by PGW.
• UL/DL Bearer binding & check UL Bearer binding for Terminal

HSS Function:

• HSS works as main data base or Centre. All the user profiles are being stored there.
• HSS provides the information related to authentication and the user profile.

PCRF Function:

A PCRF is the policy and charging control entity.

• PCRF provided the service data flow gating.
• It Creates charging and policy for each service data flow.
• PCRF sets QOS for each services data flow.
• It enables bearer QOS control.

OCS :

Online charging system (OCS) is a system allowing a communications service provider to charge their customers, in real time, based on service usage.

OFC :

Offline Charging System. In OFCS, charging events are received by Charging Data Function (CDF). CDF generates CDRs (charging data records) and transmit the same to CGF (Charging Gateway Function). CGF functions as Gateway between the 3GPP LTE network and Billing related systems.

SPR :

SPR is the repository to store all business assets, technical assets and configuration items used by the PCRF Server, Central Management Server. This is a mandatory component to run PCRF Server.

Interfaces between LTE Nodes:

• S1-MME :- Reference point for the control plane protocol between E-UTRAN and MME.
• S1-U:- Reference point between E-UTRAN and Serving GW for the per bearer user plane tunnelling and inter eNodeB path switching during handover.
• S5:- It provides user plane tunnelling and tunnel management between Serving GW and PDN GW.
• S6a:- It enables transfer of subscription and authentication data for authenticating/authorizing user access to the evolved system (AAA interface) between MME and HSS.
• S8:- S8 is the inter PLMN variant of S5.
• S10:- Reference point between MMEs for MME relocation and MME to MME information transfer.
• S11:- Reference point between MME and Serving GW.
• Gx:- It provides transfer of (QoS) policy and charging rules from PCRF to Policy and Charging Enforcement Function (PCEF) in the PDN GW.
• SGi:- It is the reference point between the PDN GW and the packet data network.