Explaining the Interfaces in LTE:
Within an LTE network, LTE Interfaces connect the various components to or within the core.
Interfaces allow the MME, SGW and PGW to cooperate with other network elements (e.g. HSS or PCRF).
Each one of them is built in a standard way described by 3GPP.org. Each interface described here is taken from the 23.401 3GPP.org documentation.
These LTE interfaces are used within either between eNodeBs and the core, or within the Evolved Packet Core (EPC)
The following are LTE Interfaces:
- 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.
- S3:- It enables user and bearer information exchange for inter 3GPP access network mobility in idle and/or active state.
- S4:- It provides related control and mobility support between GPRS Core and the 3GPP Anchor function of Serving GW. In addition, if Direct Tunnel is not established, it provides the user plane tunnelling.
- S5:- It provides user plane tunnelling and tunnel management between Serving GW and PDN GW. It is used for Serving GW relocation due to UE mobility and if the Serving GW needs to connect to a non-collocated PDN GW for the required PDN connectivity.
- S6a:- It enables transfer of subscription and authentication data for authenticating/authorizing user access to the evolved system (AAA interface) between MME and HSS.
- Gx:- It provides transfer of (QoS) policy and charging rules from PCRF to Policy and Charging Enforcement Function (PCEF) in the PDN GW.
- S8:- Inter-PLMN reference point providing user and control plane between the Serving GW in the VPLMN and the PDN GW in the HPLMN. S8 is the inter PLMN variant of S5.
- S9:- It provides transfer of (QoS) policy and charging control information between the Home PCRF and the Visited PCRF in order to support local breakout function.
- S10:- Reference point between MMEs for MME relocation and MME to MME information transfer.
- S11:- Reference point between MME and Serving GW.
- S12:- Reference point between UTRAN and Serving GW for user plane tunnelling when Direct Tunnel is established. It is based on the Iu-u/Gn-u reference point using the GTP-U protocol as defined between SGSN and UTRAN or respectively between SGSN and GGSN. Usage of S12 is an operator configuration option.
- S13:- It enables UE identity check procedure between MME and EIR.
- SGi:- It is the reference point between the PDN GW and the packet data network. Packet data network may be an operator external public or private packet data network or an intra operator packet data network, e.g. for provision of IMS services. This reference point corresponds to Gi for 3GPP accesses.
- Rx:- The Rx reference point resides between the AF and the PCRF in the TS 23.203.
- SBc:- Reference point between CBC and MME for warning message delivery and control functions.
More Information on LTE Interfaces
LTE Interfaces and their protocol stacks
Each one of them is built in standardised way described by 3GPP.org.
Please keep in mind, that the documentation is (sometimes) bigger than we need it to be, so not every aspect of interfaces is described here.Let us start with a high level situation in which theuser is not roaming:
|Fig.1. Non-roaming architecture by 3GPP
- where traffic is routed from home network to UE by S8 interface,
|Fig. 2. Roaming architecture scenario with home routed traffic
- where there is local breakout with home operator’s application functions only, and separate from that with visited operator’s application functions only.
|Fig. 3. Roaming architecture for local breakout, with home operator’s application functions only
|Fig. 4. Roaming architecture for local breakout, with home visitor’s application functions only
- controlling the E-UTRA network access connections, such as attaching to and detaching from E-UTRAN;
- controlling the attributes of an established network access connection, such as activation of an IP address;
- controlling the routing path of an established network connection in order to support user mobility; and
- controlling the assignment of network resources to meet changing user demands.
|Fig. 5. Protocol stack of S1-MME interface
It enables user and bearer information exchange for inter 3GPP access network mobility in idle and/or active state.
|Fig. 6. Protocol stack of S3 interface
|Fig. 7. Protocol stack of S4 interface
|Fig. 8. Protocol stack of interface S5 or S8
Reference point between MMEs for MME relocation (e.g handover) and MME to MME information transfer.
|Fig. 9. Protocol stack of S10 interface
|Fig. 10. Protocol stack of S11 interface
|Fig. 11. Protocol stack of S6a interface
|Fig. 12. Protocol stack of S13 interface
|Fig. 13. Protocol stack of SBc interface
|Fig. 14. User Plane
|Fig. 15. Protocol stack of S1-U interface
|Fig. 16. Protocol stacks of S4 interfaces used to connect UE from 2G network to PDN
|Fig. 17. Protocol stacks of S4 interfaces used to connect UE from 3G network to PDN
|Fig. 18. Protocol stack of S12 interface used to connect UE from 3G network to PDN
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