Sunday, February 6, 2011

Protocol Layers and their location

In this section, we describe the functions of the different protocol layers and their location in the LTE architecture. Figures 2 and 3 show the control plane and the user plane protocol stacks, respectively [4]. In the control-plane, the NAS protocol, which runs between the MME and the UE, is used for control-purposes such as network attach, authentication, setting up of bearers, and mobility management.

All NAS messages are ciphered and integrity protected by the MME and UE. The RRC layer in the eNB makes handover decisions
based on neighbor cell measurements sent by the UE, pages for
the UEs over the air, broadcasts system information, controls UE
measurement reporting such as the periodicity of Channel Quality
Information (CQI) reports and allocates cell-level temporary identifiers
to active UEs. It also executes transfer of UE context from the
source eNB to the target eNB during handover, and does integrity
protection of RRC messages. The RRC layer is responsible for the
setting up and maintenance of radio bearers.

Network Sharing

Network Sharing

The LTE architecture enables service providers to reduce the cost of owning and operating the network by allowing the service providers to have separate CN (MME, SGW, PDN GW) while the E-UTRAN (eNBs) is jointly shared by them. This is enabled by the S1-flex mechanism by enabling each eNB to be connected to multiple CN entities. When a UE attaches to the network, it is connected to the appropriate CN entities based on the identity of the service provider sent by the UE.



A key feature of the EPS is the separation of the network entity that performs control-plane functionality (MME) from the network entity that performs bearer-plane functionality (SGW) with a well defined open interface between them (S11). Since E-UTRAN will provide higher bandwidths to enable new services as well as to improve existing ones, separation of MME from SGW implies that SGW can be based on a platform optimized for high bandwidth packet processing, where as the MME is based on a platform optimized for signaling transactions.

This enables selection of more cost-effective platforms for, as well as independent scaling
of, each of these two elements. Service providers can also choose optimized topological locations of SGWs within the network independent of the locations of MMEs in order to optimize bandwidth reduce latencies and avoid concentrated points of failure.

Packet Data Network Gateway

Packet Data Network Gateway (PDN GW)

The PDN GW provides connectivity to the UE to external packet data networks by being the point of exit and entry of traffic for the UE. A UE may have simultaneous connectivity with more than one PDN GW for accessing multiple PDNs.

The PDN GW performs policy enforcement, packet filtering for each user, charging support, lawful Interception and packet screening. Another key role of the PDN GW is to act as the anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX and 3GPP2 (CDMA 1X and EvDO).

Mobility Management Entity

Mobility Management Entity (MME)

The MME is the key control-node for the LTE access- network. It is responsible for idle mode UE tracking and paging procedure including retransmissions.

It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS).

The Non- Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. It checks the authorization of the UE to camp on the service provider’s Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME is the termination point in the network for ciphering/integrity
protection for NAS signaling and handles the security key management.

Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming UEs.

Serving Gateway

Serving Gateway (SGW)

The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW).

For idle state UEs, the SGW terminates the DL data path and triggers paging when DL data arrives for the UE. It manages and stores UE contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception

Evolved Radio Access Network

Functional Elements

The architecture consists of the following functionalelements:

Evolved Radio Access Network (RAN)

The evolved RAN for LTE consists of a single node,i.e., the eNodeB (eNB) that interfaces with the UE.The eNB hosts the PHYsical (PHY), Medium AccessControl (MAC), Radio Link Control (RLC), and PacketData Control Protocol (PDCP) layers that includethe functionality of user-plane header-compressionand encryption.

It also offers Radio Resource Control(RRC) functionality corresponding to the controlplane. It performs many functions including radioresource management, admission control, scheduling,enforcement of negotiated UL QoS, cell informationbroadcast, ciphering/deciphering of userand control plane data, and compression/decompressionof DL/UL user plane packet headers.


E-UTRA is expected to support different types of services including web browsing, FTP, videostreaming, VoIP, online gaming, real time video, push-to-talk and push-to-view. Therefore, LTE isbeing designed to be a high data rate and low latency system as indicated by the key performancecriteria shown in Table 1. The bandwidth capability of a UE is expected to be 20MHz for both transmissionand reception. The service provider can however deploy cells with any of the bandwidthslisted in the table.

This gives flexibility to the service providers’ to tailor their offering dependenton the amount of available spectrum or the ability to start with limited spectrum for lower upfrontcost and grow the spectrum for extra capacity.Beyond the metrics LTE is also aimed at minimizing cost and power consumption while ensuringbackward-compatibility and a cost effective migration from UMTS systems. Enhanced multicastservices, enhanced support for end-to-end Quality of Service (QoS) and minimization of the numberof options and redundant features in the architecture are also being targeted.The spectral efficiency in the LTE DownLink (DL) will be 3 to 4 times of that of Release 6 HSDPAwhile in the UpLink (UL), it will be 2 to 3 times that of Release 6 HSUPA.

The handover procedurewithin LTE is intended to minimize interruption time to less than that of circuit-switched handoversin 2G networks. Moreover the handovers to 2G/3G systems from LTE are designed to beseamless.

LTE Introduction

The recent increase of mobile data usage and emergence of new applications such as MMOG (Multimedia Online Gaming), mobile TV, Web 2.0, streaming contents have motivated the 3rd Generation Partnership Project (3GPP) to work on the Long-Term Evolution (LTE). LTE is the latest standard inthe mobile network technology tree that previously realized the GSM/EDGE and UMTS/HSxPA networktechnologies that now account for over 85% of all mobile subscribers.

LTE will ensure 3GPP’scompetitive edge over other cellular technologies.LTE, whose radio access is called Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), isexpected to substantially improve end-user throughputs, sector capacity and reduce user planelatency, bringing significantly improved user experience with full mobility.

With the emergence ofInternet Protocol (IP) as the protocol of choice for carrying all types of traffic, LTE is scheduled toprovide support for IP-based traffic with end-to-end Quality of service (QoS). Voice traffic will besupported mainly as Voice over IP (VoIP) enabling better integration with other multimedia services.Initial deployments of LTE are expected by 2010 and commercial availability on a larger scale 1-2years later.

Unlike HSPA (High Speed Packet Access), which was accommodated within the Release 99 UMTSarchitecture, 3GPP is specifying a new Packet Core, the Evolved Packet Core (EPC) network architectureto support the E-UTRAN through a reduction in the number of network elements, simplerfunctionality, improved redundancy but most importantly allowing for connections and hand-over toother fixed line and wireless access technologies, giving the service providers the ability to delivera seamless mobility experience

LTE has been set aggressive performance requirements that rely on physical layer technologies,such as, Orthogonal Frequency Division Multiplexing (OFDM) and Multiple-Input Multiple-Output(MIMO) systems, Smart Antennas to achieve these targets. The main objectives of LTE are to minimizethe system and User Equipment (UE) complexities, allow flexible spectrum deployment inexisting or new frequency spectrum and to enable co-existence with other 3GPP Radio AccessTechnologies (RATs).

LTE is backed by most 3GPP and 3GPP2 service providers who along with the other interested partiesaim to complete and agree the EUTRAN Standards by Q4-2007 and the EPC by Q1-2008.