LTE Interfaces

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)

CableFree LTE Evolved Packet Core
CableFree LTE Evolved Packet Core

LTE Interfaces

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

Interfaces allow the LTE MME, SGW and PGW to cooperate 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 the 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.Let us start with a high level situation in which theuser is not roaming:
CableFree-LTE-Non-roaming-arch
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.
The 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,
CableFree-LTE-Roaming-scenario-routed-traffic
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.
CableFree LTE Roaming scenario-with-application by home operators only
Fig. 3. Roaming architecture for local breakout, with home operator’s application functions only
CableFree LTE Roaming scenario with application by visitor operator only
Fig. 4. Roaming architecture for local breakout, with home visitor’s application functions only
We should note that 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.
CableFree-eNB-MME--S1-MME
Fig. 5. Protocol stack of S1-MME interface
Where:
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.

CableFree-SGSN-MME--S3
Fig. 6. Protocol stack of S3 interface
Where:
GTP-C (GPRS Tunnelling Protocol for the Control Plane): This protocol tunnels signalling messages between SGSN and MME
UDP (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.
CableFree-SGSN-SGW--S4
Fig. 7. Protocol stack of S4 interface
Where:
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.
CableFree-SGW-PGW--S5orS8
Fig. 8. Protocol stack of interface S5 or S8
Where:
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.

CableFree-MME-MME--S10
Fig. 9. Protocol stack of S10 interface
Where:
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.
CableFree-MME-HSS--S6a
Fig. 10. Protocol stack of S11 interface
Where:
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 interface
Where:
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.
CableFree-MME-EIR--S13
Fig. 12. Protocol stack of S13 interface
Where:
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.
CableFree-CBC-MME--S1-MME--SBc
Fig. 13. Protocol stack of SBc interface
Where:
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.
CableFree-UserPlane
Fig. 14. User Plane
S1-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.
CableFree-eNB-SGW--S1-U
Fig. 15. Protocol stack of S1-U interface
Where:
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.
CableFree-UE-PGW-2g-via-S4-interface
Fig. 16. Protocol stacks of S4 interfaces used to connect UE from 2G network to PDN
Where:
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.
CableFree-UE-PGW-3g-via-S4-interface LTE interfaces
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.
CableFree-UE-PGW-3g-via-S12-interface LTE Interfaces
Fig. 18. Protocol stack  of S12 interface used to connect UE from 3G network to PDN
Where:
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 packets
UDP/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.

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