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Interfaces and their protocol stacks After familiarising with main network elements, time has come to better know interfaces between those elements.Interfaces are allowing MME, SGW and PGW cooperating with other network elements( e.g. HSS or PCRF).Each one of them is built in standardised way described by 3GPP.org. Each interface described here is taken from 23.401 3GPP.org documentation.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.

Lets start with big picture of situation in which user is not roaming.

Fig.1. Non-roaming architecture by 3GPP

Figure above is showing only 4G interfaces, additional interfaces for 2G and 3G are described in TS 23.060.As I probably wrote earlier Serving Gateway (SGW) and PDN Gateway (PGW) could be put in one chassis.As it comes to scenario with roaming architecture standards are describing two ways of dealing with it.Two roaming scenarios:

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

Keeping that in mind, now we can go straight through interfaces their functions and protocol stack.

Information flow could be divided into two groups, one is Control Plane, and the other is User Plane.The Control Plane consists of protocols for control and support of the user plane functions:

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.

Control Plane interfaces:S1-MME interface between eNodeB and MME.Reference point for the control plane protocol between E-UTRAN and MME.

Fig. 5. Protocol stack of S1-MME interfaceWhere:S1-AP (S1 Application Protocol): Application Layer Protocol between the eNodeB and MME.SCTP (Stream Control Transmission Protocol): This protocol guarantees delivery of signalling messages between MME and eNodeB (S1). SCTP is defined in RFC 4960

S3 interface between SGSN and MME.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 interfaceWhere:GTP-C (GPRS Tunnelling Protocol for the Control Plane): This protocol tunnels signalling messages between SGSN and MMEUDP (User Datagram Protocol): This protocol signaling messages. UDP is defined in RFC 768.

S4 interface between SGSN and SGW.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.

Fig. 7. Protocol stack of S4 interfaceWhere:GTP-C (mentioned above): This protocol tunnels signalling messages between SGSN and SGW.UDP: This protocol transfers signalling messages. UDP is defined in RFC 768.

S5 or S8 interface between SGW and PGW.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.S8: Inter-PLMN reference point providing user and control plane between the Serving GW in the VPLMN  (Visited PLMN) and the PDN GW in the HPLMN (Home PLMN). S8 is the inter PLMN variant of S5.

Difference between those two interfaces is S5 is used in one network entity(no roaming scenario), and S8 is being used to connect Visiting PLMN where user is with his Home PLMN.

Fig. 8. Protocol stack of interface S5 or S8Where:GTP-C: This protocol tunnels signalling messages between SGW and PGW.UDP: This protocol transfers signalling messages between SGW and PGW. UDP is defined in RFC 768.

S10 interface between MME and other MME. Reference point between MMEs for MME relocation (e.g handover) and MME to MME information transfer.

Fig. 9. Protocol stack of S10 interfaceWhere:GTP-C: This protocol tunnels signalling messages between MMEs.UDP: This protocol transfers signalling messages between MMEs. UDP is defined in RFC 768.

S11 interface between MME and SGW.Reference point between MME and Serving GW.

Fig. 10. Protocol stack of S11 interfaceWhere:GTP-C: This protocol tunnels signalling messages between MME and SGW.UDP: This protocol transfers signalling messages between MME and SGW. UDP is defined in RFC 768.

S6a interface between MME and HSS.It enables transfer of subscription and authentication data for authenticating/authorizing user access to the evolved system (AAA interface) between MME and HSS.

Fig. 11. Protocol stack of S6a interfaceWhere:Diameter: This protocol supports transferring of subscription and authentication data for authenticating/authorizing user access to the evolved system between MME and HSS (S6a). Diameter is defined in RFC 3588.SCTP: This protocol transfers signalling messages. SCTP is defined in RFC 4960.

S13 interface between MME and EIR.It enables UEEIR.

Fig. 12. Protocol stack of S13 interfaceWhere:Diameter: This protocol supports UE identity check procedure between MME and EIR (S13). Diameter is defined in RFC 3588.SCTP: This protocol transfers signalling messages. SCTP is defined in RFC 4960.

SBc interface between CBC and eNodeB.Reference point between CBC and MME for warning message delivery and control functions.Cell Broadcast Center (CBC) was a solution for the special requirement of an Earthquake and Tsunami warning system (ETWS) created for Japan, introduced in Rel. 8. It utilizes the existing interfaces between UE and MME in control plane. Additionally the MME is connected to the CBC via the SBc interface. In LTE/4G SBc interface is fully standardized and based on SCTP.

Fig. 13. Protocol stack of SBc interfaceWhere:SBc-AP (SBc Application Protocol): Application Layer Protocol between CBC and MME. This protocol supports transfer of warning messages.S1-AP (S1 Application Protocol): Application Layer Protocol between the eNodeB and the MME.SCTP: This protocol guarantees delivery of signalling messages between MME and eNodeB (S1). SCTP is defined in RFC 4960.

User Plane interfaces:

User Plane in a big picture.

Fig. 14. User PlaneS1-U interface between eNodeB and SGW.Reference point between E-UTRAN and Serving GW for the per bearer user plane tunnelling and inter eNodeB path switching during handover.

Fig. 15. Protocol stack of S1-U interfaceWhere: GTP-U (GPRS Tunnelling Protocol for the user plane): This protocol tunnels user data between eNodeB and SGW.UDP: This protocol transfers user data. UDP is defined in RFC 768.

S4 interface between UE with 2G access and PGW.S4 interface is also being used to connect UE with 3G access and PGW.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.

Fig. 16. Protocol stacks of S4 interfaces used to connect UE from 2G network to PDNWhere:

GTP U: This protocol tunnels user data between SGSN and the S GW as well as between the S GW and the P GW in the backbone network. GTP shall encapsulate all end user IP packets.UDP/IP: These are the backbone network protocols used for routing user data and control signalling.Protocols on the Um and the Gb interfaces are described in TS 23.060.

Fig. 17. Protocol stacks of S4 interfaces used to connect UE from 3G network to PDN

S12 interface between UE from 3G network and PGW.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.

Fig. 18. Protocol stack  of S12 interface used to connect UE from 3G network to PDNWhere:

GTP U: This protocol tunnels user data between UTRAN and the S GW as well as between the S GW and the P GW in the backbone network. GTP shall encapsulate all end user IP packetsUDP/IP: These are the backbone network protocols used for routing user data and control signalling.Protocols on the Uu interface are described in TS 23.060.SGSN controls the user plane tunnel establishment and establish a Direct Tunnel between UTRAN and S GW as shown in Fig. 18.