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LTE Protocol and Procedures

Training Manual 1 EPS Architecture

Issue 01 (2010-05-01) Huawei Proprietary and Confidential

Copyright Huawei Technologies Co., Ltd

1-1

1 EPS ArchitectureObjectives

On completion of this section the participants will be able to:

1.1 State the main functions of the network elements.

1.2 List the EPS interfaces.


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1.1 EPS Network ElementsThe term EPS (Evolved Packet System) relates to the Evolved 3GPP Packet SwitchedDomain. In contrast to the 2G and 3G networks defined by the 3GPP, LTE can be simplydivided into a flat IP based bearer network and a service enabling network. The former can be

further subdivided into the E-UTRAN (Evolved - Universal Terrestrial Radio Access Network)and the EPC (Evolved Packet Core) where as support for service delivery lies in the IMS (IPMultimedia Subsystem). This reference architecture can be seen in Figure 1-1.

Figure 1-1 LTE Reference Architecture

Whilst UMTS is based upon WCDMA technology, the 3GPP developed new specifications

for the LTE air interface based upon OFDMA (Orthogonal Frequency Division MultipleAccess) in the downlink and SC-FDMA (Single Carrier - Frequency Division Multiple Access)in the uplink. This new air interface is termed the E-UTRA (Evolved - Universal TerrestrialRadio Access).

1.1.1 User Equipment

Like that of UMTS, the mobile device in LTE is termed the UE (User Equipment) and iscomprised of two distinct elements; the USIM (Universal Subscriber Identity Module) and the

ME (Mobile Equipment).

The ME supports a number of functional entities including:

l RR (Radio Resource) - this supports both the Control Plane and User Plane and in sodoing, is responsible for all low level protocols including RRC (Radio Resource Control),

PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control), MAC (MediumAccess Control) and the PHY (Physical) Layer.

l EMM (EPS Mobility Management) - is a Control Plane entity which manages the

mobility management states the UE can exist in; LTE Idle, LTE Active and LTE

Detached. Transactions within these states include procedures such as TAU (TrackingArea Update) and handovers.


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l ESM (EPS Session Management) - is a Control Plane activity which manages the

activation, modification and deactivation of EPS bearer contexts. These can either bedefault EPS bearer contexts or dedicated EPS bearer contexts.

Figure 1-2

User Equipment Functional Elements

UE

EPS Mobility & EPSSession Management

IP AdaptationFunction

Radio Resource

ControlPlane

UserPlane

EPS Session ManagementBearer ActivationBearer ModificationBearer Deactivation

Radio ResourceRRC, PDCP, RLC, MAC &

PHY Layer Protocols

EPS Mobility ManagementRegistration

Tracking Area UpdateHandover

In terms of the Physical Layer, the capabilities of the UE may be defined in terms of the

frequencies and data rates supported. Devices may also be capable of supporting adaptivemodulation including QPSK (Quadrature Phase Shift Keying), 16QAM (16 QuadratureAmplitude Modulation) and 64QAM (Quadrature Amplitude Modulation).

In terms of the radio spectrum, the UE is able to support several scalable channels including;1.4MHz, 3MHz, 5MHz, 10MHz, 15MHz and 20MHz whilst operating in FDD (Frequency

Division Duplex) and/or TDD (Time Division Duplex). Furthermore, the UE may alsosupport advanced antenna features such as MIMO (Multiple Input Multiple Output).

Table 1-1 UE Categories

UE Category MaximumDownlinkData Rate

Number ofDownlinkData Streams

MaximumUplinkData Rate

Support forUplink64QAM

1 10.3Mbit/s 1 5.2Mbit/s No




5 302.8Mbit/s 4 75.4Mbit/s Yes


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UE Identities

An LTE capable UE will be allocated / utilize a number of identities during operation within

the network. These include:

l

IMSI (International Mobile Subscriber Identity) - this complies with the standard 3GPPformat and is comprised of the MCC (Mobile Country Code), MNC (Mobile Network

Code) and the MSIN (Mobile Subscriber Identity Number). This uniquely identifies asubscriber from within the family of 3GPP technologies - GSM, GPRS, UMTS etc.

l IMEI (International Mobile Equipment Identity) - is used to uniquely identify the ME. Itcan be further subdivided into a TAC (Type Approval Code), FAC (Final Assembly Code)and SNR (Serial Number).

l GUTI (Globally Unique Temporary Identity) - is allocated to the UE by the MME

(Mobility Management Entity) and identifies a device to a specific MME. The identity iscomprised of a GUMMEI (Globally Unique MME Identity) and an M-TMSI (MME -Temporary Mobile Subscriber Identity).

l

S-TMSI (Serving - Temporary Mobile Subscriber Identity) - is used to protect asubscriber !s IMSI during NAS (Non Access Stratum) signaling between the UE andMME as well as identifying the MME from within a MME pool. The S-TMSI iscomprised of the MMEC (MME Code) and the M-TMSI.

l IP Address - the UE requires a routable IP address from the PDN (Packet Data Network)

from which it is receiving higher layer services. This may either be an IPv4 or IPv6address.

1.1.2 Evolved Node B

In addition to the new air interface, a new base station has also been specified by the 3GPPand is referred to as an eNB (Evolved Node B). These, along with their associated interfaces

form the E-UTRAN and in so doing, are responsible for:l RRM (Radio Resource Management) - this involves the allocation to the UE of the

physical resources on the uplink and downlink, access control and mobility control.

l Date Compression - is performed in both the eNB and the UE in order to maximize theamount of user data that can be transferred on the allocated resource. This process isundertaken by PDCP.

l Data Protection - is performed at the eNB and the UE in order to encrypt and integrity

protect RRC signaling and encrypt user data on the air interface.

l Routing - this involves the forwarding of Control Plane signaling to the MME and User

Plane traffic to the S-GW (Serving - Gateway).

l Packet Classification and QoS Policy Enforcement - this involves the "marking# of

uplink packets based upon subscription information or local service provider policy. QoS(Quality of Service) policy enforcement is then responsible for ensuring such policy isenforced at the network edge.


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Figure 1-3 Evolved Node B Functional Elements

Security in LTE is not solely limited to encryption and integrity protection of information passing acrossthe air interface but instead, NAS encryption and integrity protection between the UE and MME also

takes place. In addition, IPSec may also be used to protect user data within both the E-UTRAN andEPC.

eNB Identities

In addition to the UE identities already discussed, there are a number of specific identities

associated with the eNB. These include:

l TAI (Tracking Area Identity) - is a logical group of neighboring cells defined by theservice provider in which UEs in LTE Idle mode are able to move within without

needing to update the network. As such, it is similar to a RAI (Routing Area Identity)

used in 2G and 3G packet switched networks.

l ECGI (E-UTRAN Cell Global Identifier) - is comprised of the MCC, MNC and ECI

(Evolved Cell Identity), the later being coded by each service provider.

Femto Cells

In order to improve both network coverage and capacity, the 3GPP have developed a new type

of base station to operate within the home or small business environment. Termed the HeNB(Home Evolved Node B), this network element forms part of the E-UTRAN and in so doingsupports the standard E-UTRAN interfaces. However, it must be stated that HeNBs do not

support the X2 interface.

The architecture may include an HeNB-GW (Home Evolved Node B - Gateway) whichresides between the HeNB in the E-UTRAN and the MME / S-GW in the EPC in order toscale and support large numbers of base station deployments.

1.1.3 Mobility Management Entity

The MME is the Control Plane entity within the EPC and as such is responsible for thefollowing functions:

l NAS Signaling and Security - this incorporates both EMM (EPS Mobility Management)and ESM (EPS Session Management) and thus includes procedures such as TrackingArea Updates and EPS Bearer Management. The MME is also responsible for NASsecurity.


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l S-GW and PDN-GW Selection - upon receipt of a request from the UE to allocate a

bearer resource, the MME will select the most appropriate S-GW and PDN-GW. Thisselection criterion is based on the location of the UE in addition to current loadconditions within the network.

l

Tracking Area List Management and Paging - whilst in the LTE Idle state, the UE istracked by the MME to the granularity of a Tracking Area. Whilst UEs remain within theTracking Areas provided to them in the form of a Tracking Area List, there is norequirement for them to notify the MME. The MME is also responsible for initiating the paging procedure.

l Inter MME Mobility - if a handover involves changing the point of attachment within the

EPC, it may be necessary to involve an inter MME handover. In this situation, theserving MME will select a target MME with which to conduct this process.

l Authentication - this involves interworking with the subscriber !s HSS (Home SubscriberServer) in order to obtain AAA (Access Authorization and Accounting) information with

which to authenticate the subscriber. Like that of other 3GPP system, authentication is based on AKA (Authentication and Key Agreement).

Figure 1-4 MME Functional Elements

1.1.4 Serving Gateway

The S-GW terminates the S1-U Interface from the E-UTRAN and in so doing, provides thefollowing functions:

l Mobility Anchor - for inter eNB handovers, the S-GW acts as an anchor point for the

User Plane. Furthermore, it also acts as an anchor for inter 3GPP handovers to legacynetworks - GPRS and UMTS.

l Downlink Packet Buffering - when traffic arrives for a UE at the S-GW, it may need to

be buffered in order to allow time for the MME to page the UE and for it to enter theLTE Active state.

l Packet Routing and Forwarding - traffic must be routed to the correct eNB on thedownlink and the specified PDN-GW on the uplink.

l Lawful Interception - this incorporates the monitoring of VoIP (Voice over IP) and other packet services.

l GTP/PMIP Support - if PMIP (Proxy Mobile IP) is used on the S5/S8 Interfaces, the

S-GW must support MAG (Mobile Access Gateway) functionality. Furthermore, support

for GTP/PMIP chaining may also be required.


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Figure 1-5 S-GW Functional Elements

1.1.5 Packet Data Network - GatewayThe PDN-GW is the network element which terminates the SGi Interface towards the PDN(Packet Data Network). If a UE is accessing multiple PDNs, there may be a requirement for

multiple PDN-GWs to be involved. Functions associated with the PDN-GW include:

l Packet Filtering - this incorporates the deep packet inspection of IP datagrams arriving

from the PDN in order to determine which TFT (Traffic Flow Template) they are to beassociated with.

l Lawful Interception - as with the S-GW, the PDN-GW may also monitor traffic as it

passes across it.

l IP Address Allocation - IP addresses may be allocated to the UE by the PDN-GW. This is

included as part of the initial bearer establishment phase or when UEs roam betweendifferent access technologies.

l Transport Level Packet Marking - this involves the marking of uplink and downlink packets with the appropriate tag e.g. DSCP (Differentiated Services Code Point) basedon the QCI (QoS Class Identifier) of the associated EPS bearer.

l Accounting - through interaction with a PCRF (Policy Rules and Charging Function), the

PDN-GW will monitor traffic volumes and types.

Figure 1-6 PDN-GW Functional Elements


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1.2 EPS Interfaces

1.2.1 E-UTRAN Interfaces

As with all 3GPP technologies, it is the actual interfaces which are defined in terms of the protocols they support and the associated signaling messages and user traffic that traverse

them. Figure 1-7 illustrates the main interfaces in the E-UTRAN.

Figure 1-7 E-UTRAN Interfaces

Uu Interface

The Uu Interface supports both a Control Plane and a User plane and spans the link betweenthe UE and the eNB / HeNB. The principle Control Plane protocol is RRC (Radio Resource

Control) while the User Plane is designed to carry IP datagrams.

X2 Interface

The X2 interface interconnects two eNBs and in so doing supports both a Control Plane and

User Plane. The principle Control Plane protocol is X2AP (X2 Application Protocol).

S1 Interface

The S1 interface can be subdivided into the S1-MME interface supporting Control Plane

signaling between the eNB and the MME and the S1-U Interface supporting User Plane traffic between the eNB and the S-GW. The principle Control Plane protocol is S1AP (S1

Application Protocol).

1.2.2 EPC Interfaces

Figure 1-8 illustrates the fundamental architecture of the EPC and in so doing identifies the

key interfaces which exist between the network elements. It should be stated however thatthere exists additional interfaces which link the EPC with the IMS and legacy 3GPP / Non3GPP architectures.


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Figure 1-8 EPC Architecture and Interfaces

1.2.3 Additional Network Elements and Interfaces

In addition to the network elements, interfaces and associated protocols discussed so far, theEPC connects with numerous other nodes and networks. These are illustrated in Figure 1-9.


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Figure 1-9 Additional Network Elements and Interfaces

These include, but are not limited to the:

l HSS (Home Subscriber Server) - this can be considered a "master # database within thePLMN. Although logically it is considered as one entity, the HSS in practice is made upof several physical databases depending upon subscriber numbers and redundancy

requirements. The HSS holds variables and identities for the support, establishment andmaintenance of calls and sessions made by subscribers. It is connected to the MME viathe S6a Interface which uses the protocol Diameter.

l PCRF (Policy and Charging Rules Function) - this supports functionality for policy

control through the PDF (Policy Decision Function) and charging control through the

CRF (Charging Rules Function). As such, it provides bearer network control in terms ofQoS and the allocation of the associated charging vectors. The PCRF downloads thisinformation over the Gx Interface using the Diameter protocol.

l ePDG (evolved Packet Data Gateway) - which is used when connecting to Untrusted

Non 3GPP IP Access networks. It provides functionality to allocate IP addresses inaddition to encapsulating / de-encapsulating IPSec (IP Security) and PMIP tunnels. Itconnects to the PDN-GW via the S2b Interface.

l RNC (Radio Network Controller) - which forms part of the 3GPPs UTRAN (Universal

Terrestrial Radio Access Network), the RNC connects to the S-GW to support thetunneling of User Plane traffic using GTP-U. The interface linking these networkelements is the S12 Interface.

l

SGSN (Serving GPRS Support Node) - this forms part of the 3GPPs 2G and 3G packetswitched core domain. It connects to both the MME and S-GW in order to support


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packet switched mobility and uses the GTPv2-C and GTP-U protocols respectively. TheSGSN connects to the MME via the S3 Interface and the S-GW via the S4 Interface.

l EIR (Equipment Identity Register) - this database enables service providers to validate a particular IMEI (International Mobile Equipment Identity) against stored lists. It

connects to the MME via the S13 Interface and uses the Diameter protocol for messagetransfer.


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Training Manual 2 EPS Protocols



2-1

2 EPS ProtocolsObjectives


2.1 Explain how signaling takes place between the UE and the EPC.

2.2 State the main functions of Radio Resource Control, Packet Data Convergence Protocol,

Radio Link Control, Medium Access Control, the Physical Layer and their relations.

2.3 Explain the interaction of the E-UTRAN protocols and the mapping of logical, transport

and physical channels.


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2.1 EPS SignalingThe connectivity between the UE and the EPS can be split into a Control Plane and a UserPlane. Both of these can further split into the NAS (Non Access Stratum) and AS (AccessStratum). The Access Stratum consist of the protocols and signaling involved with the

E-UTRAN, i.e. maintain both the air interface and S1 interfaces. In contrast, the Non AccessStratum, as its name suggests, is not part of the Access Stratum and is defined as higher layersignaling and traffic (IP datagrams).

Control Plane

Figure 2-1 illustrates the concept of NAS and AS signaling, i.e. the Control Plane. It is worth

noting that the NAS signaling is effectively transparent to the E-UTRAN. Access Stratumsignaling provides a mechanism to deliver NAS signaling, as well as the lower layer signaling

required to setup, maintain and manage the connections. The X2 interfaces are also part ofthis methodology and as such it also is part of Access Stratum signaling.

Figure 2-1

NAS and AS Control Plane

User Plane

The User Plane focuses on the delivery of IP datagrams to and from the EPC, namely the

S-GW and PDN-GW. Figure 2-2 illustrates this concept.

Figure 2-2 NAS and AS User Plane


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In the case of the User Plane the higher layer NAS is an IP datagram. This effectively is

delivered between the UE and the PDN-GW, with the eNB and S-GW acting as lower layerrelaying devices.

2.2 EPS Protocols

2.2.1 Uu Interface

The Uu Interface supports both a Control Plane and a User plane and spans the link betweenthe UE and the eNB / HeNB. The principle Control Plane protocol is RRC in the Access

Stratum and EMM (EPS Mobility Management)/ ESM (EPS Session Management) in the Non Access Stratum. In contrast, the User Plane is designed to carry IP datagrams. However, both Control and User Planes utilize the services of the lower layers, namely PDCP (Packet

Data Convergence Protocol), RLC (Radio Link Control) and MAC (Medium Access Control),as well as the PHY (Physical Layer).

Figure 2-3 Uu Interface Protocols

2.2.2 Uu Interface - EMM and ESM

The NAS signaling between the UE and the EPC is identified as EMM or ESM. Table 2-1

illustrates the main EMM and ESM signaling procedures.

Table 2-1 NAS EMM and ESM Procedures

EMM Procedures ESM Procedures

Attach Default EPS Bearer Context Activation

Detach Dedicated EPS Bearer Context Activation

Tracking Area Update EPS Bearer Context Modification

Service Request EPS Bearer Context Deactivation


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Extended Service Request UE Requested PDN Connectivity

GUTI Reallocation UE Requested PDN Disconnect

Authentication UE Requested Bearer Resource Allocation

Identification UE Requested Bearer Resource Modification

Security Mode Control ESM Information Request

EMM Status ESM Status

EMM Information

NAS Transport

Paging

EMM ProceduresThe key EMM procedures include:

l Attach - this is used by the UE to attach to an EPC (Evolved Packet Core) for packet

services in the EPS (Evolved Packet System). Note that it can be also used to attach tonon-EPS services.

l Detach - this is used by the UE to detach from EPS services. In addition, it can also beused for other procedures such as disconnecting from non-EPS services.

l Tracking Area Updating - this procedure is always initiated by the UE and is used for thevarious purposes. The most common include normal and periodic tracking area updating.

l Service Request - this is used by the UE to get connected and establish the radio and S1

bearers when uplink user data or signaling is to be sent.

l Extended Service Request - this is used by the UE to initiate a Circuit Switched fallback

call or respond to a mobile terminated Circuit Switched fallback request from thenetwork.

l GUTI Reallocation - this is used to allocate a GUTI (Globally Unique Temporary

Identifier) and optionally to provide a new TAI (Tracking Area Identity) list to a particular UE.

l Authentication - this is used for AKA (Authentication and Key Agreement) between the

user and the network.

l Identification - this is used by the network to request a particular UE to provide specific

identification parameters, e.g. the IMSI (International Mobile Subscriber Identity) or the

IMEI (International Mobile Equipment Identity).

l Security Mode Control - this is used to take an EPS security context into use, andinitialize and start NAS signaling security between the UE and the MME with thecorresponding NAS keys and security algorithms.

l EMM Status - this is sent by the UE or by the network at any time to report certain errorconditions.

l EMM Information - this allows the network to provide information to the UE.

l Transport of NAS messages - this is to carry SMS (Short Message Service) messages in

an encapsulated form between the MME and the UE.

l Paging - this is used by the network to request the establishment of a NAS signaling

connection to the UE. Is also includes the Circuit Switched Service Notification


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EMM Procedures

The key ESM procedures include:

l Default EPS Bearer Context Activation - this is used to establish a default EPS bearer

context between the UE and the EPC.l Dedicated EPS Bearer Context Activation - this is to establish an EPS bearer context

with specific QoS (Quality of Service) and TFT (Traffic Flow Template) between the UE

and the EPC. The dedicated EPS bearer context activation procedure is initiated by thenetwork, but may be requested by the UE by means of the UE requested bearer resourceallocation procedure.

l EPS Bearer Context Modification - this is used to modify an EPS bearer context with a

specific QoS and TFT.

l EPS Bearer Context Deactivation - this is used to deactivate an EPS bearer context or

disconnect from a PDN by deactivating all EPS bearer contexts to the PDN.

l UE Requested PDN Connectivity - this is used by the UE to request the setup of a

default EPS bearer to a PDN.l UE Requested PDN Disconnect - this is used by the UE to request disconnection from

one PDN. The UE can initiate this procedure to disconnect from any PDN as long as it isconnected to at least one other PDN.

l UE Requested Bearer Resource Allocation - this is used by the UE to request an

allocation of bearer resources for a traffic flow aggregate.

l UE Requested Bearer Resource Modification - this is used by the UE to request a

modification or release of bearer resources for a traffic flow aggregate or modification ofa traffic flow aggregate by replacing a packet filter.

l ESM Information Request - this is used by the network to retrieve ESM information, i.e.

protocol configuration options, APN (Access Point Name), or both from the UE during

the attach procedure.l ESM Status - this is used to report at any time certain error conditions detected upon

receipt of ESM protocol data.

2.2.3 Uu Interface - RRC

The main air interface control protocol is RRC (Radio Resource Control). For RRC messagesto be transferred between the UE and the eNB it uses the services of PDCP, RLC, MAC and

PHY. Figure 2-4 identifies the main RRC functions. In summary, RRC handles all thesignaling between the UE and the E-UTRAN, with signaling between the UE and Core Network, i.e. NAS (Non Access Stratum) signaling, being carried by dedicated RRC

messages. When carrying NAS signaling, RRC does not alter the information but instead,

provides the delivery mechanism.


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Figure 2-4 Main RRC Functions

2.2.4 Uu Interface - PDCP

LTE implements PDCP in both the User Plane and Control Plane. This is unlike UMTS,where PDCP was only found in the User Plane. The main reason for the difference is that

PDCP in LTE takes on the role of security, i.e. encryption and integrity. In addition, Figure2-5 illustrates some of the other functions performed by PDCP.

Figure 2-5 PDCP Functions

In the Control Plane, PDCP facilitates encryption and integrity checking of signalingmessages, i.e. RRC and NAS. The User Plane is slightly different since only encryption is

performed. In addition, the User Plane IP datagrams can also be subjected to IP headercompression techniques in order to improve the system!s performance and efficiency. Finally,PDCP also facilitates sequencing and duplication detection.

2.2.5 Uu Interface - RLC

The RLC (Radio Link Control) protocol exists in the UE and the eNB. As its name suggests it provides "radio link # control, if required. In essence, RLC supports three delivery services tothe higher layers:


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l TM (Transparent Mode) - this is utilized for some of the air interface channels, e.g.

broadcast and paging. It provides a connectionless service for signaling.

l UM (Unacknowledged Mode) - this is like Transparent Mode, in that it is aconnectionless service; however it has the additional features of sequencing,

segmentation and concatenation.l AM (Acknowledged Mode) - this offers an ARQ (Automatic Repeat Request) service.

As such, retransmissions can be used.

These modes, as well as the other RLC features are illustrated in Figure 2-6. In addition to

ARQ, RLC offers segmentation, re-assembly and concatenation of information.

Figure 2-6 RLC Modes and Functions

2.2.6 Uu Interface - MAC

MAC (Medium Access Control) provides the interface between the E-UTRA protocols and

the E-UTRA Physical Layer. In doing this it provides the following services:

l Mapping - MAC maps the information received on the LTE Logical Channels into the

LTE transport channels.

l Multiplexing - the information provided to MAC will come from a RB (Radio Bearer) ormultiple Radio Bearers. The MAC layer is able to multiplex different bearers into thesame TB (Transport Block), thus increasing efficiency.

l HARQ (Hybrid Automatic Repeat Request) - MAC utilizes HARQ to provide error

correction services across the air. HARQ is a feature which requires the MAC andPhysical Layers to work closely together.

l Radio Resource Allocation - QoS (Quality of Service) based scheduling of traffic and

signaling to users is provided by MAC.

In order to support these features the MAC and Physical Layers need to pass various

indications on the radio link quality, as well as the feedback from HARQ operation.


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Figure 2-7 Medium Access Control Functions

2.2.7 Uu Interface - Physical

The PHY (Physical Layer) in LTE provides a new and flexible channel. It does however

utilize features and mechanisms defined in earlier systems, i.e. UMTS. Figure 2-8 illustratesthe main functions provided by the Physical Layer.

Figure 2-8 Physical Layer Functions

2.2.8 X2 Interface

As previously mentioned, the X2 interface interconnects two eNBs and in so doing supports both a Control Plane and User Plane. The principle Control Plane protocol is X2AP (X2Application Protocol). This resides on SCTP (Stream Control Transmission Protocol) whereas the User Plane IP is transferred using the services of GTP-U (GPRS Tunneling Protocol -

User) and UDP (User Datagram Protocol).

Figure 2-9 illustrates the X2 User Plane and Control Plane protocols.


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Figure 2-9 X2 Interface Protocols

2.2.9 X2 Interface - X2 Application Protocol

The X2AP is responsible for the following functions:

l Mobility Management - this enables the serving eNB to move the responsibility of a

specified UE to a target eNB. This includes Forwarding the User Plane, Status Transferand UE Context Release functions.

l Load Management - this function enables eNBs to communicate with each other in order

to report resource status, overload indications and current traffic loading.

l

Error Reporting - this allows for the reporting of general error situations for whichspecific error reporting mechanism have not been defined.

l Setting / Resetting X2 - this provides a means by which the X2 interface can be setup /

reset by exchanging the necessary information between the eNBs.

l Configuration Update - this allows the updating of application level data which is neededfor two eNBs to interoperate over the X2 interface.

2.2.10 X2 Interface - Stream Control Transmission Protocol

Defined by the IETF (Internet Engineering Task Force) rather than the 3GPP, SCTP wasdeveloped to overcome the shortfalls in TCP (Transmission Control Protocol) and UDP whentransferring signaling information over an IP bearer. Functions provided by SCTP include:

l Reliable Delivery of Higher Layer Payloads.

l Sequential Delivery of Higher Layer Payloads.

l Improved resilience through Multihoming.

l Flow Control.

l Improved Security.

SCTP is also found on the S1-MME Interface which links the eNB to the MME.


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2.2.11 X2 Interface - GPRS Tunneling Protocol - User

GTP-U tunnels are used to carry encapsulated PDU (Protocol Data Unit) and signalingmessages between endpoints or in the case of the X2 interface. Numerous GTP-U tunnels may

exist in order to differentiate between EPS bearer contexts and these are identified through aTEID (Tunnel Endpoint Identifier).

GTP-U is also found on the S1-U Interface which links the eNB to the S-GW and may also be used onthe S5 Interface linking the S-GW to the PDN-GW.

2.2.12 S1 Interface

The S1 interface can be subdivided into the S1-MME interface supporting Control Planesignaling between the eNB and the MME and the S1-U Interface supporting User Plane traffic between the eNB and the S-GW.

Figure 2-10

S1 Interface Protocols

2.2.13 S1 Interface - S1 Application Protocol

The S1AP spans the S1-MME Interface and in so doing, supports the following functions:

l E-RAB (E-UTRAN - Radio Access Bearer) Management - this incorporates the setting

up, modifying and releasing of the E-RABs by the MME.

l Initial Context Transfer - this is used to establish an S1UE context in the eNB, setup the

default IP connectivity and transfer NAS related signaling.

l UE Capability Information Indication - this is used to inform the MME of the UECapability Information.

l Mobility - this incorporates mobility features to support a change in eNB or change inRAT.

l Paging.

l S1 Interface Management - this incorporates a number of sub functions dealing with

resets, load balancing and system setup etc.


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l NAS Signaling Transport - this is used for the transport of NAS related signaling over

the S1-MME Interface.

l UE Context Modification and Release - this allows for the modification and release ofthe established UE Context in the eNB and MME respectively.

l

Location Reporting - this enables the MME to be made aware of the UEs currentlocation within the network.

2.2.14 S1 Interface - SCTP and GTP-U

The S1-MME and S1-U lower layer protocols are similar to the X2 interface. As such, theyalso utilize the services of SCTP (discussed in Section 2.2.10 ) and GTP-U (discussed in

Section 2.2.11 ).

2.2.15 S11 Interface

The S11 Interface links the MME with the S-GW in order to support Control Plane signaling.

In so doing, it utilizes GTPv2-C (GPRS Tunneling Protocol version 2 - Control) which, likeall other interfaces which use variants of GTP, uses the services of UDP and IP.

Figure 2-11

S11 Interface Protocols

GTPv2-C is also found on the S5/S8 Interface between the S-GW and PDN-GW and the S10 Interface between MMEs. Furthermore, it can also be found on the S3 and S4 interfaces when interconnectingwith an SGSN (Serving GPRS Support Node).

2.2.16 GPRS Tunneling Protocol version 2 - Control

GTPv2-C supports the transfer of signaling messages between the MME and the S-GW and as

such is responsible for the exchange of the following message types:

l Path Management - this incorporates Echo Request and Echo Response messages to

ensure ongoing connectivity across the link.

l Tunnel Management - these messages are used to activate, modify and delete the EPS bearers and sessions spanning the network.

l Mobility Management - these messages ensure mobility is supported through acombination of relocation and notification procedures.


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l CS (Circuit Switched) Fallback - this incorporates suspend and resume procedures

during fallback to circuit switched operation.

l Non 3GPP Access - these messages support the establishment of tunnels to forward packet data between the 3GPP and Non 3GPP networks.

2.2.17 S5/S8 Interface

The S5/S8 Interface links the S-GW with the PDN-GW and supports both a Control Plane andUser Plane. The term S5 is used when these elements reside within the same PLMN (PublicLand Mobile Network) and S8 when the interface spans a HPLMN (Home Public Land

Mobile Network) / VPLMN (Visited Public Land Mobile Network).

The GTPv2-C protocol operates on the Control Plane for both of these interfaces whereas

GTP-U or PMIP is used on the User Plane.

2.2.18 Proxy Mobile IP

Defined by the IETF, PMIP supports mobility when a UE moves from one S-GW to anotherduring a handover procedure. Data is tunneled between the PDN-GW, which supports HA(Home Agent) functionality and the S-GW, which acts as the FA (Foreign Agent).

It is anticipated that PMIP will be used by 3GPP2 based networks migrating to LTE as they

already utilize PMIP within their 3G architectures. 3GPP based networks however areexpected to use GTP-U instead.

Figure 2-12 S5/S8 Interface Protocols

2.2.19 S10 Interface

The S10 Interface links two MMEs in order to pass Control Plane signaling. In so doing, ituses the services of GTPv2-C.


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Figure 2-13 S10 Interface Protocols

2.2.20 SGi Interface

The SGi Interface connects the PDN-GW to an external PDN. This could be the publicInternet, Corporate Intranets or a service provider !s network supporting services such as the

IMS. Although defined by the 3GPP, the protocols which operate over the SGi Interface aredefined by the IETF and include TCP, UDP in addition to a host of application specific protocols.

Figure 2-14 SGi Interface Protocols

2.3 E-UTRAN Channel MappingThe concept of "channels# is not new. Both GSM and UMTS defined various channelcategories, however LTE terminology is closer to UMTS. Broadly there are four categories ofchannel.


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Figure 2-15 LTE Channels

2.3.1 Logical Channels

In order to describe Logical Channels it is best to identify where Logical Channels are locatedin relation to the LTE protocols and the other channel types. Figure 2-16 shows Logical

Channels located between the RLC and the MAC layers.

Figure 2-16 Location of Channels

Logical channels are classified as either Control Logical Channels, which carry control datasuch as RRC signaling, or traffic Logical Channels which carry User Plane data.

Control Logical Channels

The various forms of these Control Logical Channels include:

l BCCH (Broadcast Control Channel) - this is a downlink channel used to send SI (SystemInformation) messages from the eNB. These are defined by RRC.

l PCCH (Paging Control Channel) - this downlink channel is used by the eNB to send

paging information.

Figure 2-17

BCCH and PCCH Logical Channels


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l CCCH (Common Control Channel) - this is used to establish a RRC (Radio Resource

Control) connection, also known as a SRB (Signaling Radio Bearer). The SRB is

discussed further in Section"#$%&'()

. The SRB is also used for

re-establishment procedures. SRB 0 maps to the CCCH.l

DCCH (Dedicated Control Channel) - this provides a bidirectional channel for signaling.Logically there are two DCCH activated:

%

SRB 1 - this is used for RRC messages, as well as RRC messages carrying high priority NAS signaling.

%

SRB 2 - this is used for RRC carrying low priority NAS signaling. Prior to itsestablishment low priority signaling is sent on SRB1.

Figure 2-18 CCCH and DCCH Signaling

Traffic Logical Channels

Release 8 LTE has one type of Logical Channel carrying traffic, namely the DTCH(Dedicated Traffic Channel). This is used to carry DRB (Data Radio Bearer) information, i.e.IP datagrams.

Figure 2-19 Dedicated Traffic Channel

The DTCH is a bidirectional channel that can operate in either RLC AM or UM mode. This is

configured by RRC and is based on the QoS (Quality of Service) of the E-RAB (E-UTRAN -Radio Access Bearer).

2.3.2 Transport Channels

Historically, Transport Channels were split between common and dedicated channels.However, LTE has moved away from dedicated channels in favor of the common/sharedchannels and the associated efficiencies provided. The main Release 8 Transport Channels

include:


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l BCH (Broadcast Channel) - this is a fixed format channel which occurs once per frame

and carries the MIB (Master Information Block). Note that the majority of SystemInformation messages are carries on the DL-SCH (Downlink - Shared Channel).

l PCH (Paging Channel) - this channel is used to carry the PCCH, i.e. paging messages. It

also utilizes DRX (Discontinuous Reception) to improve UE battery life.l DL-SCH (Downlink - Shared Channel) - this is the main downlink channel for data and

signaling. It supports dynamic scheduling, as well as dynamic link adaptation. Inaddition, it supports HARQ (Hybrid Automatic Repeat Request) operation to improve performance. As previously mentioned it also facilitates the sending of SystemInformation messages.

l RACH (Random Access Channel) - this channel carries limited information and is used

in conjunction with Physical Channels and preambles to provide contention resolution procedures.

l UL-SCH (Uplink Shared Channel) - this is similar to the DL-SCH, this channel supportsdynamic scheduling (eNB controlled) and dynamic link adaptation by varying the

modulation and coding. In addition, it also supports HARQ (Hybrid Automatic Repeat

Request) operation to improve performance.

Figure 2-20 LTE Release 8 Transport Channels

2.3.3 Physical Channels

The Physical Layer facilitates transportation of MAC Transport Channels, as well as providing scheduling, formatting and control indicators.

Downlink Physical Channels

There are a number of downlink Physical Channels in LTE. These include:

l PBCH (Physical Broadcast Channel) - this channel carries the BCH.

l

PCFICH (Physical Control Format Indicator Channel) - this is used to indicate thenumber of OFDM symbols used for the PDCCH.

l PDCCH (Physical Downlink Control Channel) - this channel is used for resource

allocation.

l PHICH (Physical Hybrid ARQ Indicator Channel) - this channel is part of the HARQ

process.

l PDSCH (Physical Downlink Shared Channel) - this channel carries the DL-SCH.

Uplink Physical Channels

There are a number of Uplink Physical Channels in LTE. These include:


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l PRACH (Physical Random Access Channel) - this channel carries the Random Access

Preamble. The location of the PRACH is defined by higher layer signaling, i.e. RRCsignaling.

l PUCCH (Physical Uplink Control Channel) - this channel carries uplink control and

feedback. It can also carry scheduling requests to the eNB.l PUSCH (Physical Uplink Shared Channel) - this is the main uplink channel and is used

to carry the UL-SCH (Uplink Shared Channel) Transport Channel. It carries bothsignaling and user data, in addition to uplink control. It is worth noting that the UE is notallowed to transmit the PUCCH and PUSCH at the same time.

2.3.4 Radio Channels

The term "Radio Channel# is typically used to describe the overall channel, i.e. the downlinkand uplink carrier for FDD or the single carrier for TDD.

Figure 2-21

Radio Channel

2.3.5 Channel Mapping

There are various options for multiplexing multiple bearers together, such that LogicalChannels may be mapped to one or more Transport Channels. These in turn are mapped into

Physical Channels. Figure 2-22 and Figure 2-23 illustrate the mapping options.


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Figure 2-22 Downlink Channel Mapping


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2-19

Figure 2-23 Uplink Channel Mapping

In order to facilitate the multiplexing from Logical Channels to Transport Channels, the MAC

Layer typically adds a LCID (Logical Channel Identifier).


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Training Manual 3 LTE/SAE Quality of Service



3-1

3 LTE/SAE Quality of ServiceObjectives


3.1 Explain the purpose of EPS Bearer Services and E-UTRA Radio Bearers.

3.2 List the different attributes of the E-UTRA Radio Bearer and explain how they are used.


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3.1 EPS Bearer Services and E-UTRA Radio Bearers

3.1.1 QoS in Packet Switched NetworksIn order to support a mixture on non-real time and real time applications such as voice andmultimedia, the issues associated with radio access based contention means that delay and jitter may become excessive if the flows of traffic are not coordinated. Modern packet

switches are now termed "QoS aware#, in that they are able to classify, schedule and forwardtraffic based on the destination address, as well as the type of media being transported. Figure3-1 illustrates how the concept of packet classification and scheduling is part of the eNB,

S-GW and PDN-GW responsibilities.

Figure 3-1 QoS Packet Scheduling

The main functions associated with QoS in a packet switch (router) are the:

l Packet Classifier - this function analyses packets and based on a set of filters classifiesthe packet. As such, it receives the correct packet forwarding treatment and scheduling.

l Packet Scheduler - this schedules packets based on priority. In so doing various methodsare used to ensure low latency data, e.g. voice, is optimally scheduled.

3.1.2 LTE Bearers

The LTE system utilizes the concept of bearers. In so doing, a bearer has been defined to bethe aggregate of one or more IP flows related to one or more services.

Figure 3-2 illustrates the main bearer terminology in LTE. Note that if the system employsPMIP (Proxy MIP) on the S5/S8 interfaces then the EPS Bearers effectively terminate on theS-GW.


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Figure 3-2 LTE Bearers

End to End Bearer Service

The end to end service runs between the UE and the peer entity, such as a call server, web

server etc. This is supported by an EPS Bearer plus external bearers that may support theequivalent QoS across the external networks, i.e. beyond the SGi Interface.

EPS Bearer Service

The EPS Bearer extends between the UE and the PDN-GW. It is defined as a logical

aggregate of one or more SDF (Service Data Flow). The EPS Bearer QoS is managed andcontrolled in the EPC / E-UTRAN. Figure 3-3 illustrates the concept of Service Data Flowsmapping into the same EPS bearer. Note that the S-GW and eNB are both unaware of the

mapping.

Figure 3-3 Service Data Flows

EPS Radio and Access Bearer

The EPS Bearer consist of two parts the EPS Radio Bearer and the EPS Access Bearer. The

EPS Radio Bearer facilitates the transport of the EPS Bearer traffic between the UE and theeNB. Note that the eNB manages the QoS. The EPS Access Bearer service provides the


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transport between the S-GW / PDN-GW and eNB according to the EPS QoS profile

associated with each EPS Bearer.

3.1.3 The Default EPS Bearer

LTE enables the UE to operate as "always on#. This is achieved by establishing a default EPS

bearer during the LTE Attach process. The default EPS bearer is configured as non-GBR (non- Guaranteed Bit Rate) and carries all traffic which is not associated with a dedicated bearer.

Figure 3-4 Default and Dedicated EPS Bearers

It is possible for the UE to establish more than one default EPS bearer, however this is via a differentAPN (Access Point Name).

3.1.4 Dedicated EPS BearersDedicated EPS bearers carry traffic for IP flows that have been identified to require a specific

QoS. This classification is achieved using a TFT (Traffic Flow Template) at the PDN-GW andUE. The TFT, i.e. filters, for the UE to utilize for each dedicated EPS bearer are passed to theUE in NAS ESM signaling.

Dedicated EPS bearers may be established during the Attach. For example, in the case ofservices that require "always-on# connectivity and higher QoS than that provided by thedefault bearer. Dedicated bearers can be either GBR (Guaranteed Bit Rate) or non-GBR.

3.1.5 EPS QoS Parameters

EPS Bearers may support Guaranteed or Non Guaranteed Bit Rate services. As such various parameters are used to control and identify the QoS.

GBR QoS Information

The GBR QoS Information parameter provides the eNB with information on the uplink and

downlink rates. It can include:

l E-RAB Maximum Downlink Bit Rate.

l E-RAB Maximum Uplink Bit Rate.

l E-RAB Guaranteed Downlink Bit Rate.

l

E-RAB Guaranteed Uplink Bit Rate.


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AMBR (Aggregate Maximum Bit Rate)

Non Guaranteed EPS Bearers are subject to control through an AMBR (Aggregate Maximum

Bit Rate). The AMBR applies to both the subscriber and APN (Access Point Name) associated

with the subscriber.

l UE AMBR (User Equipment Aggregate Maximum Bit Rate) - this value applies to the

total bit rate that can be allocated to a subscriber for all its non-GBR services.

l APN AMBR (Access Point Name Aggregate Maximum Bit Rate) - this value applies tothe total bit rate that can be allocated to the subset of a subscriber !s services associatedwith a particular APN.

QoS Class Indicator

QCI (QoS Class Indicator) provides a simple mapping from an integer value to specific QoS

parameters that controls bearer level packet forwarding treatment. Currently eight label typeshave been defined, these are illustrated in Table 3-1.

Table 3-1 QCI Attributes

QCI Type Priority PacketDelayBudget (ms)

PacketError Rate

Example Service

1 GBR 2 100 10-2

Conversational Voice

2 GBR 4 150 10-3

Conversational Video

3 GBR 3 50 10-3

Real Time Gaming

4 GBR 5 300 10-6

Non-Conversational Voice

5 Non-GBR 1 100 10-6

IMS Signaling

6 Non-GBR 6 300 10-6

Video, TCP Based

7 Non-GBR 7 100 10-3

Voice, Video, InteractiveGaming

8 Non-GBR 8 300 10-6

Video, TCP Based

9 Non-GBR 9 300 10-6

Video, TCP Based

ARP (Allocation and Retention Priority)

The ARP (Allocation and Retention Priority) indicates if a bearer establishment or

modification request can be accepted. In addition, it may be used to indicate which bearers aredropped when there is congestion in the network. The main parameters include:

l Priority Level (0 to 15) - Value 15 means "no priority", whereas values between 1 and 14

are ordered in decreasing order of priority, i.e. 1 is the highest and 14 the lowest, withvalue 0 being reserved.


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l Pre-emption Capability - this indicates the pre-emption capability on other E-RABs. In

so doing, it indicates whether the E-RAB will not pre-empt other E-RABs or, the E-RABmay pre-empt other E-RABs.

l Pre-emption Vulnerability - this indicates the vulnerability of the E-RAB to preemption

of other E-RABs.

3.2 E-UTRA Radio BearersThe LTE air interface has two types of radio bearers, namely Signaling Radio Bearers and

Data Radio Bearers.

3.2.1 Signaling Radio Bearers

A SRB (Signaling Radio Bearer) is a RB (Radio Bearer) that is only used for the transmission

of RRC and NAS messages. More specifically, the following three SRBs are defined:

l SRB0 - this is for RRC messages using a CCCH logical channel, e.g. RRC Connection

Request, Setup and Re-establishment.

l SRB1 - this is mainly for RRC messages using a DCCH logical channel. It can also beused for NAS messages prior to the establishment of SRB2.

l SRB2 - this is for NAS messages using a DCCH logical channel. Note that SRB2 has alower-priority than SRB1 and is always configured by the E-UTRAN after securityactivation.

Figure 3-5 Signaling Radio Bearers

Uu S1-U S5/S8 SGi

UEeNB S-GW PDN-GW

SRB 2

SRB 1RRC (High Priority)

NAS (Lower Priority)

3.2.2 Data Radio Bearers

In addition to Signaling Radio Bearers, at least one DRB (Data Radio Bearer) needs to beestablished for the Default EPS bearer. There are various identities used in LTE at differentlayers to identify the EPS bearers. The main higher layer identifier is the EPS Bearer Identity,

this has a value between 0 to 15. In a UMTS network this is referred to as a NSAPI (Networklayer Service Access Point Identifier). When the EPS bearer is established an associated DRBIdentity is assigned. These have values between 1 and 32. Finally, the lower layers, i.e. MAC,

allocate the LCID (Logical Channel Identity). There are only 10 available for Radio Bearers,with the values 1 and 2 mapping to SRB1 and SRB2 respectively. In so doing, the remainingeight LCID are available for Data Radio Bearers (1 Default EPS Bearer and 7 Dedicated EPS

Bearers).

Figure 3-6 illustrates how the Data Radio Bearer relates to an EPS bearer. In this case the

Default EPS Bearer.


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Figure 3-6 Data Radio Bearers

3.2.3 Radio Bearer QoS

The QoS for Data Radio Bearers is provided to the eNB by the MME using the standard QoSattributes such as QCI and ARP, as well as maximum and guaranteed bit rates in the uplinkand downlink direction. Based on these the eNB configures the UE E-UTRA layers and

manages the ongoing scheduling of uplink and downlink traffic.

Figure 3-7 E-RAB QoS Parameters to the eNB


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E-UTRA Configuration

In order to achieve the QoS for the E-RAB the eNB configures the lower layer protocols,

namely PDCP, RLC, MAC and the Physical Layers.

Figure 3-8

E-UTRA E-RAB QoS

There are various parameters that could be configured/modified to influence the performanceof the E-UTRA and thus aid the eNB QoS scheduling requirements. These include:

l PDCP Compression.

l RLC AM or UM.

l RLC AM Polling Configuration.

l Uplink MAC Priority.

l Uplink MAC Prioritized Bit Rate.

l Uplink MAC Bucket Size Duration.

l HARQ Configuration and re-transmissions.

l

BSR (Buffer Status Report) Configuration.

l SPS (Semi Persistent Scheduling) Configuration.

l Physical Channel and Power Configuration.


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Training Manual 4 X2/S1 Interface and Protocols



4-1

4 X2/S1 Interface and ProtocolsObjectives


4.1 Explain the main functions and procedures of X2AP signaling protocol.

4.2 Explain the main functions and procedures of S1AP signaling protocol.

4.3 Explain the main functions and procedures of the User Plane protocol GTP.


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4.1 X2AP Functions and ProceduresThe X2 interface links an eNB to its neighbors. It is sub-divided into a Control Plane and UserPlane, these are illustrated in Figure 4-1. The messages required to invoke the X2 interfaceservices are carried by X2AP (X2 Application Part) in the Control Plane. The User Plane

utilizes the services of GTPv1-U (GPRS Tunneling Protocol Version 1 - User Plane). TheE-UTRAN interfaces use similar terminology to that of UMTS, in that the interfaces aredivided into a RNL (Radio Network Layer) and a TNL (Transport Network Layer). The Radio

Network Layer supports the higher layer functions, incorporating X2AP and the user !s IPstreams.

Figure 4-1 X2 Control and User Plane

The Transport Network Layer Control Plane and User Plane both use the service of IP;however a reliable robust delivery protocol in the form of SCTP (Stream ControlTransmission Protocol) exists within the Control Plane. In contrast, the User Plane utilizes

GTP-U and the services of the UDP (User Datagram Protocol). Note that an eNB may haveone or multiple IP addresses at the Transport Network Layer for both the Control and UserPlanes.

4.1.1 Functions of the X2 Application Protocol

The X2AP has the following functions:

l Mobility Management - this function allows the eNB to move the responsibility of a

certain UE to another eNB. Forwarding of User Plane data, Status Transfer and UEContext Release function are parts of the mobility management.

l Load Management - this function is used by eNBs to indicate resource status, overloadand traffic load to each other.

l Reporting of General Error Situations - this function allows reporting of general error

situations, for which function specific error messages have not been defined.

l Resetting the X2 - this function is used to reset the X2 interface.

l Setting up the X2 - this function is used to exchange necessary data for the eNB for setup

the X2 interface and implicitly perform an X2 Reset.

l eNB Configuration Update - this function allows updating of application level dataneeded for two eNBs to interoperate correctly over the X2 interface.


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The X2AP consists of various EP (Elementary Procedures). Table 4-1illustrates the mapping

between the functions provided by the X2 interface and the actual Elementary Procedure(s)that are used to support this functionality.

Table 4-1

Mapping between X2AP Functions and X2AP EPs

Function Elementary Procedure(s)

Mobility Management a) Handover Preparation.

b) SN Status Transfer.

c) UE Context Release.

d) Handover Cancel.

Load Management a) Load Indication.

b) Resource Status Reporting Initiation.

c) Resource Status Reporting.

Reporting of General Error Situations Error Indication.

Resetting the X2 Reset.

Setting up the X2 X2 Setup.

eNB Configuration Update eNB Configuration Update.

4.1.2 X2 Elementary Procedures

The X2AP consists of various Elementary Procedures. Class 1 procedures, i.e. EPs includinga request and response, are illustrated in Table 4-2.

Table 4-2

Class 1 Elementary Procedures

ElementaryProcedure

InitiatingMessage

Successful Outcome Unsuccessful Outcome

Response message Response message

HandoverPreparation

HANDOVERREQUEST

HANDOVER

REQUESTACKNOWLEDGE

HANDOVER

PREPARATIONFAILURE

Reset RESET REQUEST RESET RESPONSE

X2 Setup X2 SETUPREQUEST

X2 SETUPRESPONSE

X2 SETUP FAILURE

eNB

ConfigurationUpdate

ENB

CONFIGURATIONUPDATE

ENB

CONFIGURATIONUPDATEACKNOWLEDGE

ENB CONFIGURATIONUPDATE FAILURE

Resource

Status

Reporting

Initiation

RESOURCE

STATUSREQUEST

RESOURCE STATUSRESPONSE

RESOURCE STATUSFAILURE


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The X2AP also supports various Class 2 procedures, i.e. EPs without a response message.

Elementary Procedure Initiating Message

Load Indication LOAD INFORMATION

Handover Cancel HANDOVER CANCEL

SN Status Transfer SN STATUS TRANSFER

UE Context Release UE CONTEXT RELEASE

Resource Status Reporting RESOURCE STATUS UPDATE

Error Indication ERROR INDICATION

The role of the X2 interface may be divided into two main groups. These are:

l X2AP Basic Mobility Procedures - these relate to procedures used to handle the UE

mobility within E-UTRAN.

l X2AP Global Procedures - these relate to procedures that are not related to a specificUE.

4.1.3 Message Formatting

X2AP messages and S1AP messages consist of individual IE (Information Elements) andgroups of Information Elements that are nested together. Each message must start with theelement defining the "Message Type#. This will be followed by a series of Information

Elements.

Presence

The presence of Information Elements within a message depends on a number of factors

including the scenario in which the message has been invoked. Consequently, InformationElements may be:

l M (Mandatory) - these IE are always included in the message.

l O (Optional) - these IE may or may not be included in the message.

l C (Conditional) - these IE are included in the message only if the condition is satisfied.

RangeThe Range indicates the number of copies of repetitive Information Elements that are allowed

in the message. E.g. there may be three cells configured and each has its associated parameters.

Criticality

In each protocol message, there is criticality information set for individual and/or groups of IE

that comprise it. This criticality information instructs the receiver how to act when receiving

an IE that is in error or not comprehended. This criticality information may be applied asfollows:

l

Null - no criticality information is applied explicitly.


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l Yes - criticality information is applied only for non-repeatable IE.

l Global - the Information Element and all its repetitions have common criticality

information.

l Each - each repetition of the Information Element has its own criticality information.

Based on the criticality information, the receiver may take the following action if errors are

encountered in the Information Element:

l Reject.

l Ignore.

l Ignore and Notify.

4.1.4 X2 Basic Mobility Procedures - Handover Preparation

Based on radio resource requirements the source eNB will decide to initiate a handover procedure with the target eNB. The source eNB initiates the procedure by sending the

Handover Request message to the target eNB. Note that the following messages are alsoincluded in mobility scenarios in Section "#$%&'().

Handover Request

The Handover Request message includes the following information:

l Old eNB UE X2AP ID - this provides the X2 signaling association for future messages

between the source and target eNBs.

l Cause - this element indicates to the MME the reason for the handover including reasonssuch as the radio network layer, transport network layer etc.

l ECGI - this is the global id of the eNB and is expressed as a PLMN identity plus the

entire 28bit cell identity.l GUMMEI (Globally Unique MME Identifier) - this is the identity of the MME that is

currently serving the UE.

l UE Context Information - this contains the following information:

% MME UE S1AP ID - this provides the target eNB with the signaling associationreference with the MME across the S1-MME interface for specific UE.

%

UE Security Capabilities - this information element defines the UE capabilities interms of its RF, E-RAB formats etc. These are typically defined by referencing thecategory of the LTE device.

%

AS Security Information - the purpose of the Security Context IE is to provide

security related parameters to the eNB. These are used to derive security keys forUser Plane traffic and RRC signaling messages and for security parameter generationfor subsequent X2 or intra eNB handovers.

% UE Aggregate Maximum Bit Rate - this element is used to define the total bandwidthin Mbit/s that can be allocated to the UE for all E-RABs that are established.

%

E-RABs to be Setup List - this identifies the E-RAB ID, E-RAB QoS, GTP

information and RRC Context for each EPS Bearer. The latter provides details on thecurrent configuration and the implementation of the air interface protocols.

l UE History Information -this is information about cells that a UE has been served by inthe active state prior to the target cell.

l Trace Activation - this O (Optional) parameter is able to start trace procedures on the

Target eNB. In so doing, it indicates which interfaces to trace and where to send theinformation.


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l SRVCC Operation Possible - this indicates to the target eNB whether SRVCC (Single

Radio Voice Call Continuity) is available, i.e. the UE can be handed over from theE-UTRAN to CS (Circuit Switched) 2G/3G systems.

Figure 4-2

X2 Handover Request

Handover Request Acknowledge

The allocation of E-RAB resources will be based on those received in the Handover Request

procedure. Note that if conflicts occur the target eNB can utilize the ARP (Allocation andRetention Priority) parameter (part of the E-RAB QoS) to help resolve the issue. In so doing,the target eNB admits the E-RABs and sends the Handover Request Acknowledge message

back to the source eNB. The message contains the following information:

l Old eNB UE X2AP ID - this is the X2 signaling association of the source eNB.

l New eNB UE X2AP ID - this is the X2 signaling association of the target eNB.

l E-RABs Admitted List - this details the list of E-RAB(s) that have been admitted basedon the resources available in the target eNB.

l

E-RABs Not Admitted List - this identifies the E-RAB(s) which are not admitted.


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l Target eNB To Source eNB Transparent Container- this includes handover information

for the UE. This, in essence, is an RRC Connection Reconfiguration message definingthe lower layer configuration on the new cell.

l Criticality Diagnostics - this is sent by the eNB when parts of a received message have

not been comprehended or were missing, or if the message contained logical errors.When applicable, it contains information about which parameters were notcomprehended or were missing.

Handover Preparation Failure

There are a number of reasons why the Handover Preparation Failure message may be sent,

typical examples include:

l If the target eNB does not admit at least one non-GBR E-RAB.

l The target eNB receives a Handover Request message and the RRC Context parameter

does not include required information.

l A failure occurs during the Handover Preparation.

In these instances, the target eNB sends the Handover Preparation Failure message to thesource eNB with the appropriate cause parameter indicated.

Figure 4-3 X2 Handover Preparation Failure

SN Status Transfer

The SN Status Transfer procedure is used to transfer the uplink and downlink PDCP (Packet

Data Convergence Protocol) SN (Sequence Number) and HFN (Hyper Frame Number) statusfrom the source eNB to the target eNB during an X2 handover for each respective E-RAB forwhich PDCP SN and HFN status preservation applies. These E-RAB(s) are identified in thehandover preparation phase based on the RRC Context parameters in the Handover Request.


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Figure 4-4 X2 SN Status Transfer

The source eNB initiates the SN Status Transfer procedure. In so doing, it stops the

assignment of PDCP SNs to downlink SDUs and stops delivering uplink SDUs towards theEPC (Evolved Packet Core). Finally it sends the SN Status Transfer message to the targeteNB.

For E-RAB that have had forwarding preservation agreed the source eNB forwards the uplink packets to the target eNB and routes downlink packets to the target eNB that will assign itsown sequence numbers to the packets based on the value of the PDCP DL Count received

from the target eNB.

The information in the SN Status Transfer message includes:

l Old eNB UE X2AP ID - this is the X2 signaling association of the source eNB.

l New eNB UE X2AP ID - this is the X2 signaling association of the target eNB.

l

E-RABs Subject to Transfer - this lists the E-RAB that have been identified to haveforwarding applied based on their QoS. Each E-RAB will have the following parametersdetailed for them:

%

Receive Status of UL PDCP SDUs - this optional parameter provides a bit map ofmissing PDCP Sequence Numbers.

% UL Count Value - this is the PDCP-SN and HFN of the next uplink SDU (ServiceData Unit) to be forwarded to the EPC.

% DL Count Value - this is the PDCP-SN and HFN of the first downlink SDU to beformatted into a PDCP SU for delivery to the UE.

UE Context Release

The UE Context Release message is sent once a handover has been successfully completed

and enables the source eNB to release all resources associated with the UE.


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Figure 4-5 X2 UE Context Release

UE Context Release

eNB eNB

UE Context Release

Old eNB UE X2AP IDNew eNB UE X2AP ID

Source Target

Handover Cancel

The Handover Cancel message is sent from the source eNB to the target eNB to cancel a

handover that is currently in progress.

Figure 4-6 X2 Handover Cancel

Handover Cancel

eNB eNB

Handover CancelOld eNB UE X2AP IDNew eNB UE X2AP IDCause

Source Target

4.1.5 X2 Load Indication

The Load Indication message transfers load and interference coordination information between neighboring eNBs that are operating on the same carrier frequency. It enables aneNB to indicate the interference it is experiencing on particular PRB (Physical Resource

Block) and the sensitivity to interference for each PRB.

The message contains the following information:

l Cell ID - this indicates the cell to which the report relates.

l UL Interference Load Indication - this is used to report to a neighbor eNB that specificPRBs are experiencing interference. This may be defined as high, medium or low. PRBare listed with PRB 0 being the first in the list, PRB 1 is the second and so on.

l UL High Interference Indication - this message indicates the sensitivity of PRB to

interference. A bit map is used, with a 0 indicating low sensitivity and 1 indicating highsensitivity.

l RNTP (Relative Narrowband Tx Power) - this indicates, per PRB, whether downlink

transmission power is lower than the value indicated by the RNTP threshold. The

receiving eNB may take such information into account when setting its scheduling policyand can consider the received RNTP value valid until reception of a new LoadInformation message carrying an update.


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Figure 4-7 X2 Load Indication

Figure 4-8 illustrates how two of the Load Indication message parameters can be set to

indicate the uplink overload and interference requirements on an eNB.

Figure 4-8 X2 Uplink Interference

The Load Indication message also provides the Relative Narrowband Tx Power bitmap andassociated parameters. This effectively indicates to neighboring cells the power levels

transmitted per PRB.

Figure 4-9 Downlink RNTP


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4.1.6 X2 Resource Status Reporting

Closely associated with load reporting is resource status reporting, this is used by an eNB torequest updated information regarding load information etc from its neighbors.

Resource Status Request

The Resource Status Request message is sent from one eNB to its neighbor. It is used to

register (start) measurement reports or to deregister (stop) these reports. It is also used torequest the periodicy of reports and to specify the specific cells on which reports are required.

Figure 4-10 X2 Resource Status Request

The Reported Characteristics parameter is used to indicate: PRB Periodic, TNL loadIndication Periodic or HW Load Indication Periodic.

Resource Status Response

The Resource Status Response message indicates if the request can be performed. Subsequent

messages are then sent in Resource Status Update messages.

Resource Status Failure

The Resource Status Failure message is sent when requested measurements cannot be

initiated.

Resource Status Update

The Resource Status Update message is used to send the requested results. It includes the

requested report.


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Figure 4-11 X2 Resource Status Update

4.1.7 X2 Setup

The purpose of the X2 Setup procedure is to exchange application level configuration dataneeded for two eNBs to interoperate correctly over the X2 interface. This procedure erases

any existing application level configuration data in the two nodes and replaces it by the onereceived. This procedure also resets the X2 interface in a similar fashion to a Reset procedure.

X2 Setup Request

The X2 Setup Request message includes:

l Global eNB ID - this is the global id of the eNB and is expressed as the first 20bits of thecell ID in the case of a macro eNB and for a home eNB it is the entire 28bit cell identity.

l Served Cells - this contains a list of the cells supported by the eNB. For each cell thefollowing information is provided:

% ECGI (E-UTRAN Cell Global Identifier).

% PCI (Physical Cell Identifier).

% EARFCN (E-UTRA Absolute Radio Frequency Channel Number).

% TAC (Tracking Area Code).

%

Broadcast PLMNs - including FDD and TDD configuration.

%

Neighbor Cells - including ECGI, PCI and EARFCN.

l GU Group ID (Globally Unique Group Identifier) - this is all the pools to which the eNB belongs to.


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Figure 4-12 X2 Setup Request

X2 Setup Request

X2 Setup Response

eNB eNB

X2 Setup ResponseGlobal eNB IDServed Cells- Served Cell Information- Neighbor Information-- ECGI-- PCI-- EARFCNGU Group Id List (C)Criticality Diagnostics

X2 Setup RequestGlobal eNB ID

Served Cells- Served Cell Information- Neighbor Information-- ECGI-- PCI-- EARFCNGU Group Id List (C)

X2 Setup Response

The X2 Setup Response message simply reflects the information included in the request but

this time the values are associated with the neighbor that received the request message.

4.1.8 X2 eNB Configuration

The purpose of the eNB Configuration Update procedure is to update application levelconfiguration data needed for two eNBs to interoperate correctly over the X2 interface.

eNB Configuration Update

The eNB Configuration Update includes updates and modification to the eNB configuration.

eNB Configuration Update Acknowledge

The eNB Configuration Update Acknowledge message is returned to indicate to the sending

eNB that the necessary updates have been completed in the target eNB.


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Figure 4-13 eNB Configuration Update

eNB Configuration Update Failure

If the eNB cannot accept the update it responds with an eNB Configuration Update Failuremessage and the appropriate cause value. If the message includes the Time To Wait parameter

the eNB waits at least for the indicated time before reinitiating the eNB Configuration Update procedure towards the same eNB. Both eNBs continue to communicate on the X2 interfacewith their existing configuration data.

4.2 S1AP Functions and ProceduresThe S1AP, which resides on the S1-MME interface within the E-UTRAN, uses the concept of

an EP (Elementary Procedure). These interactions comprise of a series of protocol messageswhich in turn consist of one or more IE (Information Element). Like the X2 interface, the S1interface can be split into a RNL (Radio Network Layer) and TNL (Transport network Layer).

Figure 4-14 illustrates this split, as well as the associated protocols.

Figure 4-14 S1 Control and User Plane

IP

Layer 2

Layer 1

SCTP

S1AP

IP

Layer 2

Layer 1

UDP

GTP-U

Control Plane User Plane

IPRNL

S1-MME S1-U


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4.2.1 S1AP Functions

S1AP is defined as being able to perform the following functions:

l E-RAB Management - this overall functionality is responsible for setting up, modifying

and releasing E-RABs, which are triggered by the MME. Note that the release ofE-RABs may be triggered by the eNB as well.

l Initial Context Transfer - this is used to establish an S1 UE context in the eNB, to setup

the default IP connectivity, to setup one or more E-RAB(s) if requested by the MME, aswell as to transfer NAS (Non Access Stratum) signaling related information to the eNB ifneeded.

l UE Capability Information Indication - this functionality is used to provide the UECapability Information

for huke lte protocols and procedures

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