LTE Interfaces and Network Elements

The following are LTE Interfaces : (Ref: TS 23.401 v 841)

• 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 [6].
• SBc:- Reference point between CBC and MME for warning message delivery and control functions.

Protocol assumption:
- The S1-U is based on GTP-U protocol;
- The S3 is based on GTP protocol;
- The S4 is based on GTP protocol;
- The S5 is based on GTP protocol. PMIP variant of S5 is described in TS 23.402 [2];
- The S8 is based on GTP protocol. PMIP variant of S8 is described in TS 23.402 [2].
- S3, S4, S5, S8, S10 and S11 interfaces are designed to manage EPS bearers LTE Network elements
E-UTRAN
E-UTRAN is described in more detail in TS 36.300 [5].
In addition to the E-UTRAN functions described in TS 36.300 [5], E-UTRAN functions include:
- Header compression and user plane ciphering;
- MME selection when no routeing to an MME can be determined from the information provided by the UE;
- UL bearer level rate enforcement based on UE-AMBR and MBR via means of uplink scheduling(e.g. by limiting the amount of UL resources granted per UE over time);
- DL bearer level rate enforcement based on UE-AMBR;
- UL and DL bearer level admission control;
- Transport level packet marking in the uplink, e.g. setting the DiffServ Code Point, based on the QCI of the associated EPS bearer.
MME
MME functions include:
- NAS signalling;
- NAS signalling security;
- Inter CN node signalling for mobility between 3GPP access networks (terminating S3);
- UE Reachability in ECM-IDLE state (including control and execution of paging retransmission); - Tracking Area list management;
- PDN GW and Serving GW selection;
- MME selection for handovers with MME change;
- SGSN selection for handovers to 2G or 3G 3GPP access networks;
- Roaming (S6a towards home HSS);
- Authentication;
- Bearer management functions including dedicated bearer establishment.
- Lawful Interception of signalling traffic.
- Warning message transfer function (including selection of appropriate eNB).
- UE Reachability procedures.
NOTE: The Serving GW and the MME may be implemented in one physical node or separated physical nodes.
Gateway General
Two logical Gateways exist:
- Serving GW (S‑GW);
- PDN GW (P‑GW).
NOTE: The PDN GW and the Serving GW may be implemented in one physical node or separated physical nodes.
Serving GW
The Serving GW is the gateway which terminates the interface towards E-UTRAN.
For each UE associated with the EPS, at a given point of time, there is a single Serving GW.
The functions of the Serving GW, for both the GTP-based and the PMIP-based S5/S8, include:
- the local Mobility Anchor point for inter-eNodeB handover;
- sending of one or more “end marker” to the source eNodeB, source SGSN or source RNC immediately after switching the path during inter-eNodeB and inter-RAT handover, especially to assist the reordering function in eNodeB.
- Mobility anchoring for inter-3GPP mobility (terminating S4 and relaying the traffic between 2G/3G system and PDN GW);
- ECM-IDLE mode downlink packet buffering and initiation of network triggered service request procedure;
- Lawful Interception;
- Packet routeing and forwarding;
- Transport level packet marking in the uplink and the downlink, e.g. setting the DiffServ Code Point, based on the QCI of the associated EPS bearer;
- Accounting on user and QCI granularity for inter-operator charging;
- UL and DL charging per UE, PDN, and QCI(e.g. for roaming with home routed traffic).
- Interfacing OFCS according to charging principles and through reference points specified in TS 32.240 [51].
Additional Serving GW functions for the PMIP-based S5/S8 are captured in TS 23.402 [2].
Connectivity to a GGSN is not supported.
PDN GW
The PDN GW is the gateway which terminates the SGi interface towards the PDN.
If a UE is accessing multiple PDNs, there may be more than one PDN GW for that UE, however a mix of S5/S8 connectivity and Gn/Gp connectivity is not supported for that UE simultaneously.
PDN GW functions include for both the GTP-based and the PMIP-based S5/S8:
- Per-user based packet filtering (by e.g. deep packet inspection);
- Lawful Interception;
- UE IP address allocation;
- Transport level packet marking in the uplink and downlink, e.g. setting the DiffServ Code Point, based on the QCI of the associated EPS bearer;
- UL and DL service level charging as defined in TS 23.203 6;
- Interfacing OFCS through according to charging principles and through reference points specified in TS 32.240 [51].
- UL and DL service level gating control as defined in TS 23.203 [6];
- UL and DL service level rate enforcement as defined in TS 23.203 6;
- UL and DL rate enforcement based on APN-AMBR(e.g. by rate policing/shaping per aggregate of traffic of all SDFs of the same APN that are associated with Non-GBR QCIs);
- DL rate enforcement based on the accumulated MBRs of the aggregate of SDFs with the same GBR QCI(e.g. by rate policing/shaping);
- DHCPv4 (server and client) and DHCPv6 (client and server) functions;
- The network does not support PPP bearer type in this version of the specification. Pre-Release 8 PPP functionality of a GGSN may be implemented in the PDN GW;
- packet screening.
Additionally the PDN GW includes the following functions for the GTP-based S5/S8:
- UL and DL bearer binding as defined in TS 23.203 [6];
- UL bearer binding verification as defined in TS 23.203 [6];
- Functionality as defined in RFC 4861 [32].
The P‑GW provides PDN connectivity to both GERAN/UTRAN only UEs and E‑UTRAN capable UEs using any of E‑UTRAN, GERAN or UTRAN. The P‑GW provides PDN connectivity to E‑UTRAN capable UEs using E‑UTRAN only over the S5/S8 interface.
SGSN
In addition to the functions described in TS 23.060 [7], SGSN functions include:
- Inter EPC node signalling for mobility between 2G/3G and E-UTRAN 3GPP access networks;
- PDN and Serving GW selection: the selection of S‑GW/P‑GW by the SGSN is as specified for the MME;
- MME selection for handovers to E-UTRAN 3GPP access network.
GERAN
GERAN is described in more detail in TS 43.051 [15].
UTRAN
UTRAN is described in more detail in TS 25.401 [16].
PCRF
General
PCRF is the policy and charging control element. PCRF functions are described in more detail in TS 23.203 [6].
In non-roaming scenario, there is only a single PCRF in the HPLMN associated with one UE’s IP-CAN session. The PCRF terminates the Rx interface and the Gx interface.
In a roaming scenario with local breakout of traffic there may be two PCRFs associated with one UE’s IP-CAN session:
- H-PCRF that resides within the H-PLMN;
- V-PCRF that resides within the V-PLMN.
Home PCRF (H-PCRF)
The functions of the H-PCRF include:
- terminates the Rx reference point for home network services;
- terminates the S9 reference point for roaming with local breakout;
- associates the sessions established over the multiple reference points (S9, Rx), for the same UE’s IP-CAN session (PCC session binding).
The functionality of H-PCRF is described in TS 23.203 [6].
Visited PCRF (V-PCRF)
The functions of the V-PCRF include:
- terminates the Gx and S9 reference points for roaming with local breakout;
- terminates Rx for roaming with local breakout and visited operator’s Application Function.
The functionality of V-PCRF is described in TS 23.203 [6].
PDN GW’s associated AAA Server
The PDN Gateway may interact with a AAA server over the SGi interface. This AAA Server may maintain information associated with UE access to the EPC and provide authorization and other network services. This AAA Server could be a RADIUS or Diameter Server in an external PDN network, as defined in TS 29.061 [38]. This AAA Server is logically separate from the HSS and the 3GPP AAA Server.