LTE INTERFACES
* Figure 3 illustrates the most important interfaces for the radio access network
* The air-interface connection between the User Equipment (UE) and the eN ode B is known as the Uu. The UE and eN ode B make use of the Uu interface whenever they transmit or receive across the LTE air-interface
* The X2 interface connects one eN ode B to another eN ode B. This allows both signalling and data to be transferred between neighbouring eNode B
o the control plane of the X2 (X2-CP) interface allows signalling between eN ode B
o the user plane of the X2 (X2-UP) interface allows the transfer of application data between eN ode B
* The Sl interface connects an eNode B to the Evolved Packet Core (EPC). This allows both signalling and data to be transferred between the Evolved Packet Core (EPC) and Evolved UMTS Radio Access Network (E-UTRAN)
o the control plane of the S 1 (S 1-MME) interface allows signalling with the MME
o the user plane of the S 1 (S 1-UP) interface allows application data transfer through the Serving Gateway
* Application Protocols have been specified to define the signalling procedures and message types which can be sent across the X2 and S1 interfaces, i.e. X2-AP and Sl-AP
* Both the X2 and S 1 interfaces are based upon IP
* Figure 3 illustrates a logical representation of the interfaces within E-UTRAN. In practice, the X2 and Sl are likely to use a single physical connection at the eN ode B, i.e. a single Ethernet cable can be used for both the X2 and Sl interfaces
* Figure 4 illustrates an example physical representation of the X2 and Sl interfaces. The eNode Bare connected to an IP backhaul transport network using a single Ethernet cable. This cable transfers information for both the X2 and S 1 interfaces
* In the case of the X2 interface, the IP routers within the transport network receive data from one eN ode Band direct it towards another eNode B. In the case of the Sl interface, the IP routers provide connectivity between the eN ode Band the Evolved Packet Core
* The Ethernet connection between the eN ode B and transport network could be based upon either an electrical or optical Gigabit Ethernet cable
* IP Quality of Service (QoS) can be used to differentiate and prioritise packets transferred across the IP backhaul
* Timing over Packet (ToP) can be used to provide the eNode B with synchronisation information. ToP is specified within IEEE 1588.Alternatively, Global Positioning System (GPS) satellites can be used, or a synchronisation signal can be provided by a co-sited BTS
* A more complete set of interfaces associated with L TE and the Evolved Packet Core is shown in Figure 5. Only a single eNode B is shown in this figure so the X2 interface does not appear. The control plane of the S1 interface is shown as the S1-MME, while the user plane is shown as the S1-UP
* The S 11 interface connects the MME to the Serving Gateway. This allows signalling information for mobility and bearer management to be transferred. Application data does not use the S11 interface
* The S5 interface connects the Serving Gateway to the Packet Data Network (PDN) Gateway. Both control plane signalling and user plane data use the S5 interface. The PDN Gateway provides connectivity to the set ofiP services so the S5 represents the main connection for application data across the Evolved Packet Core
* The S8 interface is similar to the S5 interface but it terminates at a PDN Gateway belonging to a different PLMN. This interface is used by end-users who are roaming away from their home PLMN
* The S6a interface connects the MME to the Home Subscriber Server (HSS). The HSS hosts a database containing subscription related information for the population of end-users. The HSS represents an evolution of the Home Location Register (HLR) used by earlier network architectures
* The S 13 interface connects the MME to the Equipment Identity Register (EIR). The EIR stores the International Mobile Equipment Identities (IMEI) of the end-user devices used within the network. These IMEI can be 'white listed', 'grey listed' or 'black listed' to control access to the network
* The Gx interface connects the Policy and Charging Enforcement Function (PCEF) within the PDN Gateway to the Policy and Charging Rules Function (PCRF). The PCRF provides QoS and charging information to the PDN Gateway. The Gx interface is also known as the S7 interface in some references
* The SGi interface provides connectivity between the PDN Gateway and a packet data network. The packet data network could be an external network (either public or private), or could belong to the operator. The SGi interface corresponds to the Gi interface in earlier network architectures
* The S3 interface allows the transfer of control plane signalling between the MME and an SGSN. The SGSN could belong to either a UMTS or GPRS network. The main purpose of the signalling is to allow mobility between the various access technologies
* The S4 interface allows the transfer of application data between the Serving Gateway and SGSN when a 'Direct Tunnel' is not established between the RNC and Serving Gateway. This interface may be used when a UE roams from the L TE network across to a UMTS network
* The S2a interface provides connectivity between the PDN Gateway and a non-3GPP access technology. Figure 5 illustrates the non- 3GPP technology as a wireless LAN. WiMax is a non-3GPP access technology which could be connected using the S2a interface
* The S 12 interface allows the transfer of application data between the Serving Gateway and RNC when a 'Direct Tunnel' is established.The S4 interface represents the alternative when a 'Direct Tunnel' is not established. Both the S12 and S4 interfaces are applicable when a UE roams from the L TE network across to a UMTS network
* 3GPP References: TS 36.410, TS 36.420, TS 23 .002, TS 23.402
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