LTE / SAE Architecture

UTRAN Long Term Evolution (LTE) refers to the long term evolution of the 3GPP
radio access technology and is considered the successor of the current UMTS
system with the rollout anticipated to begin with trials in 2009.

The LTE work in 3GPP is closely aligned to the 3GPP system architecture evolution
(SAE) framework which is concerned with the evolved core network architecture. The
LTE/SAE framework defines the flat, scalable, IP-based architecture of the Evolved
Packet System (EPS) consisting of a radio access network part (Evolved UTRAN)
and the Evolved Packet Core (EPC).

Note that the Evolved Packet System is purely packet based. Voice transport is thus
based on Voice over IP (VoIP) technology. Circuit-switched (CS) voice traffic is
supported by either using the CS fallback (CSFB) or the single radio voice call
continuity (SR-VCC) interworking solution.

Move your mouse pointer over the items in the architecture figure for a short
introduction to each item.

·  LTE is the evolution towards a flat network architecture.

· The LTE radio interface (air interface, LTE-Uu) is between the user equipment
(UE) and the eNB.

· The evolved Node B (eNodeB, eNB) supports the LTE radio interface and
provides the packet-switched functionality of a traditional radio network
controller (RNC). As a result, the Evolved UTRAN does not require a
separate RNC network element, in other words the architecture is “flat”
(architecture contains fewer types of network entities and interfaces)

· The X2 interface between two eNB network elements is used during an intereNB
handover.

· The S1-MME interface carries control plane signalling information between the
eNodeB and Mobility Management Entity.

· The S1-U interface between the eNodeB and Serving Gateway carries the user
plane data over a so-called GTP tunnel.

· The S4 interface between the S-GW and SGSN provides a GTP tunnel for the
user plane during an inter-system handover.

· The S3 interface carries signalling between the MME and Serving GPRS
Support Node (SGSN) located in a 2G/3G packet-switched core network.

· The S11 interface carries signalling messages between the Serving Gateway
and the Mobility Management Entity.

· The S6a interface is used for transferring subscription and authentication data
between the Home Subscriber Server (HSS) and MME.

The Gx interface is between PGW and PCRF,The PCRF provides policy control decisions and flow based charging controls. The PCRF
determines how a service data flow shall be treated in the enforcement function (PGW in
this case) and ensure that the user plane traffic mapping and treatment is in accordance
with the user’s profile

· The SGi interface is between the PDN Gateway and the packet data network
(PDN). The packet data network may be an operator-external public or private
IP network, or an IP network belonging to the operator, for instance providing
IP Multimedia Subsystem (IMS) services. Legacy Gn/Gp interface
connectivity to the EPS is also supported.

· The Serving Gateway (S-GW) and PDN Gateway (P-GW) provide the user
plane connectivity between the access network and the external packet data
network (PDN). In the Nokia Siemens Networks LTE solution, it is possible to
implement these functional entities within a single node.

· The Mobility Management Entity (MME) provides the basic control plane
functionality in the Evolved Packet Core network. Note that user plane traffic
does not go through the MME.