In this chapter we shall see what are the different interfaces available in LTE.
First, what is an interface?
Interface is a channel that is used to communicate between two elements in an LTE network.
Why do we need an interface?
We need interfaces to deliver signaling or user data information to/from the network element.
Who defines these interfaces?
3PGG is the organization that will define these interfaces.
Below are the Interfaces available in LTE.
LTE Uu S1-MME X2 S11 S5 S1-U S6a Sp Gx Gy Gz SGi
Interface: LTE Uu
Protocol Used: E-UTRA (control plane and user plane)
Description:
It is the air interface between UE and EUTRAN (eNB).
The signaling connection is the RRC connection represented by Signaling Radio Bearer.
The user plane connection is the logical channels represented by Data Radio Bearers.
Interface: S1-MME
Protocol Used: S1-AP
Description:
It is the interface for the control plane between an E-UTRAN(eNB) and an MME.
Interface: X2
Protocol Used: X2-AP for control plane
GTP-U for user plane
Description:
X2 interface is used between 2 eNB. It is used during X2 handover and for Self Organizing Network related functions.
X2-C to transfer control plane information between 2 eNB.
X2-U to transfer user plane information between 2 eNB.
Interface: S11
Protocol Used: GTP-C
Description:
It is used as an control plane interface between an MME and an S-GW. It is used to provide GTP tunnel Per user.
Interface: S5
Protocol Used: GTP-C for control plane
GTP-U for user plane
Description:
It is an interface between S-GW and a P-GW for control and user plane.
S5 interface provides a GTP tunnel per bearer for the user plane and GTP tunnel management per user for the control plane.
Interface: S1-U
Protocol Used:GTP-U
Description:
It is an user plane interface wetween en eNB and an S-GW. It provides a GTP tunnel per bearer.
Interface: S6a
Protocol Used:Diameter
Description:
An interface for control plane between HSS and an MME.
Interface: S7
Protocol Used:Gx
Description:
It provides transfer of QoS service and charging rules from Policy and PCRF to Policy and Charging Enforcement Function (PCEF) in PDN-GW.
Interface: S10
Description:
It is a reference point between MMEs for MME relocation and MME to MME information transfer.
Interface: S11
Description:
It is a reference point between MMEs for MME relocation and MME to SGW.
Interface: Sp
Protocol Used: Diameter
Description:
An interface for control plane between SPR and a PCRF.
Interface: Gx
Protocol Used: Diameter
Description:
An interface for control plane between a PRCF and a P-GW. It is used to transfer policy control and charging rules form the PCRF to the P-GW to support QoS.
Interface: Gy
Protocol Used: Diameter
Description:
An interface between OCS and a P-GW
Interface: Gz
Protocol Used: GTP
Description:
An interface between OFCS and a P-GW
Interface: SGi
Protocol Used:IP
Description:
An interface for control and user plane between a P-GW and a PDN.
S2a, S2b, S2c Interfaces:
All these 3 interfaces are between P-GW to PDN. Based on the functionality they are divided into 3 parts. Below we shall see the functionality of each interface:
Interface: S2a
It provides user plane with related controls and mobility support between trusted non-3GPP IP access and PDN-GW.
It is based on PMIP.
Interface: S2b
It provides the user plane with the related control and mobility support between evolved Packet Data Gateway (ePDG) and PDN-GW.
It is based on proxy mobile IP.
Interface: S2c
It provides user plane with related controls and mobility support between UE and PDN-GW. It is implemented over trusted and/or untrusted non-3GPP access and/or 3GPP access.
Note:
UE to eNB is called as LTE
eNB to Core Network (EPC) is called as SAE : System Architecture Evolution
