Figure 2.3: E-UTRAN, UTRAN and GERAN architecture. GPRS one tunnel approach.
will make it possible to support intra SGSN/MME and inter P/S-GW/GGSN node mobility between the different accesses.
2.6
Interfaces overview
This section contains a brief overview of the LTE/SAE interfaces.
Gi
Gi is the interface to external packet data networks (e.g. Internet) and contains the end-user’s IP Point of Presence (PoP). All user-plane and control-plane functions that use the Gi interface are handled above the end-user’s IP layer, whereas all terminal mobility within 3GPP is handled below the Gi interface.
S1
S1 is the interface between eNB and MME and between eNB and S-GW. In the user plane this interface will be based on GTP User data tunnelling (GTP-U) (similar to Iu and Gn interface in UMTS). In the control plane the interface is more similar
Figure 2.4: Typical implementation of LTE/SAE. Combined SGSN/MME one tunnel approach.
to RAN Application Part (RANAP), with some simplifications and changes due to the different functional split and mobility within EPS.
It has been agreed to split the S1 interface into a S1-CP (control plane) and S1- UP (user plane) part. The signalling transport on S1-CP will be based on Stream Control Transmission Protocol (SCTP). The signalling protocol for S1 is called S1 Application Protocol (S1AP). S1AP protocol has the following functions:
• EPS Bearer management function.
This overall functionality is responsible for setting up, modifying and releasing EPS bearers, which are triggered by the MME The release of EPS bearers may be triggered by the eNB as well.
• Initial context transfer function.
This functionality is used to establish an S1 UE context in the eNB, to setup the default IP connectivity, to setup one or more SAE bearer(s) if requested by the MME, and to transfer NAS signalling related information to the eNB if needed.
• Mobility functions for UEs in LTE ACTIVE in order to enable:
2.6 Interfaces overview
S1 interface (with EPC involvement),
◦ a change of RAN nodes between different RAT (inter-3GPP-RAT han-
dovers) via the S1 interface (with EPC involvement).
• Paging.
This functionality provides the EPC the capability to page the UE.
• S1 interface management functions:
◦ Reset functionality to ensure a well defined initialisation on the S1 inter-
face.
◦ Error Indication functionality to allow a proper error reporting/handling
in cases where no failure messages are defined.
◦ Overload function to indicate the load situation in the control plane of
the S1 interface.
• NAS signaling transport function between the UE and the MME is used
to:
◦ transfer NAS signalling related information and to establish the S1 UE
context in the eNB,
◦ transfer NAS signalling related information when the S1 UE context in
the eNB is already established.
• S1 UE context release function.
This functionality is responsible to manage the release of UE specific context in the eNB and the MME.
S1 is a many-to-many interface.
X2
X2 is the interface between eNBs. The interface is mainly used to support active mode UE mobility (Packet Forwarding). This interface may also be used for multi- cell Radio Resource Management (RRM) functions. The X2-CP interface consists of a signalling protocol called X2 Application Protocol (X2AP) on top of SCTP. The X2-UP interface is based on GTP-U. The X2-UP interface is used to support loss-less mobility (packet forwarding).
The X2-AP protocol provides the following functions:
• Mobility Management (MM).
This function allows the eNB to move the responsibility of a certain UE to another eNB. Forwarding of user plane data is a part of the mobility manage- ment.
• Load management.
This function allows eNBs to indicate overload and traffic load to each other.
This function allows reporting of general error situations, for which function specific error messages have not been defined.
The X2 interface is a many-to-many interface.
S3
S3 is a control interface between the MME and 2G/3G SGSNs. The interface is based on Gn/GTP Control plane (GTP-C) (SGSN-SGSN), possibly with some new functionality to support signalling free idle mode mobility between E-UTRAN and UTRAN/GERAN. S3 will not support packet forwarding; instead this will be sup- ported on the S4 interface.
S3 is a many-to-many interface.
The S3 interface is similar to the S10 interface between MMEs which will be used for intra-LTE mobility between two MME pool areas.
S4
S4 is the interface between the P-GW and 2G/3G SGSNs. The interface is based on Gn/GPRS Tunnelling Protocol (GTP) (SGSN-GGSN). The user plane interface is based on GTP-U (same as S1-UP and Iu-UP) and the control plane is based on GTP-C (similar to S11).
S4 is a many-to-many interface.
The S4 interface is backwards compatible with the Gn interface.
S6
S6a enables transfer of subscription and authentication data for authenticating/au- thorizing user access to the evolved system (Authentication, authorisation and ac- counting (AAA) interface) between MME and Home Subscriber Server (HSS). S6d is between the SGSN and the HSS. S6 is based on Diameter.
S5/S8
S5/S8 is the interface between the S-GW and P-GW. In principle S5 and S8 is the same interface, the difference being that S8 is used when roaming between different operators while S5 is network internal. The S5/S8 interface will exist in two variants one based on Gn/GTP (SGSN-GGSN) and the other will use the Internet Engineer- ing Task Force (IETF) specified Proxy Mobile IP (PMIP) for mobility control with additional mechanism to handle QoS.
The usage of PMIP or GTP on S5/S8 will not be visible over the S1 interface or in the terminal. In the non roaming case the S-GW and P-GW functions can be performed in one physical node.
It has been agreed in 3GPP that the usage of PMIP or GTP on S5 and S8 should not impact RAN behaviour or impact the terminals.
2.6 Interfaces overview
In the roaming case S8 is providing user and control plane between the S-GW in the Visited PLMN (VPLMN) and the P-GW in the Home PLMN (HPLMN). S8 is the inter Public Land Mobile Network (PLMN) variant of S5.
S5/S8 is a many-to-many interface.
S9
S9 provides transfer of QoS policy and charging control information between the Home Policy and Charging Rules Function (PCRF) and the Visited PCRF in order to support local breakout function.
S10
S10 is a control interface between the MMEs which will be very similar to the S3 interface between the SGSN and MME. The interface is based on Gn/GTP-C (SGSN-SGSN) with additional functionality.
S10 is a many-to-many interface.
S11
S11 is the interface between the MME and S-GW. The interface is based on Gn/GTP-C (interface between SGSN and GGSN) with some additional functions for paging coordination, mobility compared to the legacy Gn/GTP-C (SGSN-GGSN) interface.
S11 is a many-to-many interface.
S12
S12 is the interface between UTRAN and S-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
S13 enables UE identity check procedure between MME and Equipment Identify Register (EIR).
SGi
SGi is the interface between the P-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 IP Multimedia Subsystem (IMS) services. This interface corresponds to Gi for 3GPP accesses.
Rx
Gx
Gx provides transfer of QoS policy and charging rules from PCRF to PCEF in the P-GW.