LTE: LTE Network Interfaces

Network Interfaces:

Evolved Packet System (EPS) contains following network elements:

Ø LTE-Uu: Allows data transfer between the eNodeB and the UE’s. All the functions and protocols needed for this transfer and the control operations of the E-UTRAN Uu interface are implemented in the eNodeB.

Ø S1-C or S1-MME: Reference point for the control plane protocol between E-UTRAN and MME.

Ø S1-U: Reference point between E-UTRAN and Serving GW for the per bearer user plane tunneling and inter eNodeB path switching during handover.

Ø S5: It provides user plane tunneling 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.

Ø S6a: It enables transfer of subscription and authentication data for authenticating/authorizing user access to the evolved system (AAA interface) between MME and HSS.

Ø Gx: It provides transfer of (QoS) policy and charging rules from PCRF to Policy and Charging Enforcement Function (PCEF) in the PDN GW. The interface is based on the Gx interface.

Ø Gxa: It provides transfer of (QoS) policy information from PCRF to the Trusted Non-3GPP accesses.

Ø Gxc: It provides transfer of (QoS) policy information from PCRF to the Serving Gateway

Ø 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.

Ø S10: Reference point between MMEs for MME relocation and MME to MME information transfer.

Ø S11: Reference point between MME and Serving GW

Ø 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.

Ø X2: The X2 reference point resides between the source and target eNodeB.

LTE-Uu Interface or Radio Interface:

Ø Air Interface of E-UTRAN.

Ø Based on OFDMA in downlink and SC-FDMA in uplink

Ø Supports FDD and TDD mode

Ø Supports all 3GPP defined frequency bands

Ø Scalable bandwidth 1.4MHz, 3MHz, 5MHz, 10MHz, 15MHz and 20MHz.

Ø Theoretically peak data rate is 173 Mbps in downlink and 53 Mbps in uplink

Ø MIMO is a major component but optional.

Ø No user dedicated channel.

Radio Interface:

Ø The physical layer provides the basic bit transmission functionality over air.

Ø MAC main functions:

ü Mapping of logical channels onto transport channels

ü Error correction through HARQ (Hybrid Automatic Repeat Request).

Ø RLC main Functions:

ü Concatenation, segmentation and reassembly of higher layer data.

ü Error correction through ARQ (Automatic Repeat Request)

Ø PDCP main Functions:

ü Header error compression and de-compression

ü Ciphering and deciphering

Ø RRC main functions:

ü Broadcast of system information

ü RRC connection control

ü I-RAT mobility

ü Measurement configuration and reporting

ü Other functions (eg. Transfer NAS messages)

ü Generic protocol error handling

ü Support of self-configuration and self-optimization.

X2 Interface:

Ø Inter eNB interface.

Ø Intra LTE Access system mobility support for UE

ü Context transfer

ü Control of user plane tunnels

ü Handover cancellations

Ø Load Management (used for interference management)

Ø X2 management and error indication

Ø The X2-U interface provides non-guaranteed delivery of user plane PDUs between eNBs.

S1 Interface:

Ø S1 Paging function

Ø UE context Management function.

Ø Mobility function for UEs

ü Intra LTE handover

ü Inter 3GPP RAT handover

Ø eRAB Service Management function

Ø NAS signaling transport function

Ø S1 interface management functions (eg error indication and reset)

Ø RAN information management function

Ø Configuration transfer function (eg SON (Self Optimizing Network) information)

Ø The S1-U interface provides non-guaranteed delivery of user plane PDUs between the eNB and the SGW

S11 Interface:

Ø Interface between MME and a SGW

Ø Control plane only interface

Ø A single MME can handle multiple SGW each one with its own S11 interface

Ø Used to coordinate the establishment of SAE bearers within EPC

Ø SAE bearer setup can be started by the MME (default SAE bearer) or by SGW (dedicated SAE bearer).

S5/S8 Interface:

Ø Interface between SGW and PGW

Ø S5: If SGW and PGW belong to the same PLMN (non-roaming case)

Ø S8: If SGW and PGW belong to different PLMN (roaming case)

Ø S8 = S5 + inter-operator security functions

Ø Mainly used to transfer user packet data between PGW and SGW.

Ø Signaling on S5/S8 is used to setup the associated bearers resources

Ø S5/S8 can be implemented either on 3GPP solution (GTP) or IETF solution (MIPv6).

S6a Interface:

Ø Interface between the MME and the HSS

Ø The MME uses it to retrieve subscription information from HSS (handover/tracking area restrictions, external PDN allowed, QoS, etc) during attaches and updates.

Ø The HSS can during these procedures also store the user’s current MME address in its database.

Gx, SGi and Rx+ Interfaces:

Gx:

Ø Interface between PDN GW and PCRF

Ø It Allows:

ü The PCRF to request the setup of a SAE bearer with appropriate QoS

ü The PDN GW to ask the QoS of an SAE bearer to setup.

ü To indicate EPC status changes to the PCRF to apply a new policy rule

SGi:

Ø Interface used by the PDN GW to send and receive data to and from the external data network.

Ø It is typically either IPv4 or IPv6 based

Ø Downlink data coming from the external PDN must be assigned to the right SAE bearer of the right user by analysis of the incoming packet’s IP addresses, port numbers,etc.

Ø The interface corresponds to the Gi interface in 2G/3G networks

Rx+:

Ø Interface between PCRF and the external PDN network/operator IMS

S9 Interface: