4G INTERVIEW QUESTION .

LTE Frequently Asked Questions

What is LTE?

What is goal of LTE?

What speed LTE offers?

What is LTE Advanced?

What is LTE architecture?

What is EUTRAN?

What are LTE Interfaces?

What are LTE Network elements?

What are LTE protocols & specifications?

What is VoLGA?

What is CS Fallback in LTE?

How does LTE Security works?

How does measurements work in LTE?

What is Automatic Neighbour Relation?

How does Intra E-UTRAN Handover is performed?

What is SON & how does it work in LTE?

How does Timing Advance (TA) works in LTE?

How does LTE UE positioning works in E-UTRAN?

How many operators have committed for LTE?

How does Location Service (LCS) work in LTE network?

How does Lawful Interception works in LTE Evolved Packet System?

What is carrier aggregation in LTE-Advanced?

What is LTE?

LTEi (Long Term Evolution) is initiated by 3GPPi to improve the mobile phone standard to cope

with future technology evolutions and needs.

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What is goal of LTE?

The goals for LTE include improving spectral efficiency, lowering costs, improving services,

making use of new spectrum and reformed spectrum opportunities, and better integration with

other open standards.

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What speed LTE offers?

LTE provides downlink peak rates of at least 100Mbit/s, 50 Mbit/s in the uplink and RAN (Radio

Access Network) round-trip times of less than 10 ms.

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What is LTE Advanced?

LTE standards are in matured state now with release 8 frozen. While LTE Advanced is still under

works. Often the LTE standard is seen as 4G standard which is not true. 3.9G is more

acceptable for LTE. So why it is not 4G? Answer is quite simple - LTE does not fulfill all

requirements of ITU 4G definition.

Brief History of LTE Advanced: The ITU has introduced the term IMT Advanced to identify mobile

systems whose capabilities go beyond those of IMT 2000. The IMT Advanced systems shall

provide best-in-class performance attributes such as peak and sustained data rates and

corresponding spectral efficiencies, capacity, latency, overall network complexity and quality-

of-service management. The new capabilities of these IMT-Advanced systems are envisaged to

handle a wide range of supported data rates with target peak data rates of up to approximately

100 Mbit/s for high mobility and up to approximately 1 Gbit/s for low mobility.

See LTE Advanced: Evolution of LTE for more details.

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What is LTE architecture?

The evolved architecture comprises E-UTRAN (Evolved UTRAN) on the access side and EPC

(Evolved Packet Core) on the core side.

The figure below shows the evolved system architecture

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What is EUTRAN?

The E-UTRAN (Evolved UTRAN) consists of eNBs, providing the E-UTRA user plane

(PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs

are interconnected with each other by means of the X2 interface. The eNBs are also connected

by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME

(Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (S-GW) by

means of the S1-U.

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What are LTE Interfaces?

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.

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

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What are LTE Network elements?

eNB

eNB interfaces with the UE and hosts the PHYsical (PHY), Medium Access

Control (MAC), Radio Link Control (RLC), and Packet Data Control

Protocol (PDCP) layers. It also hosts Radio Resource Control (RRC)

functionality corresponding to the control plane. It performs many

functions including radio resource management, admission control,

scheduling, enforcement of negotiated UL QoS, cell information

broadcast, ciphering/deciphering of user and control plane data, and

compression/decompression of DL/UL user plane packet headers.

Mobility Management Entity

manages and stores UE context (for idle state: UE/user identities, UE mobility state, user

security parameters). It generates temporary identities and allocates them to UEs. It checks the

authorization whether the UE may camp on the TA or on the PLMN. It also authenticates the

user.

Serving Gateway

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).

Packet Data Network Gateway

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.

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What are LTE protocols & specifications?

In LTE architecture, core network includes Mobility Management Entity (MME), Serving Gateway

(SGW), Packet Data Network Gateway (PDN GW) where as E-UTRAN has E-UTRAN NodeB (eNB).

See LTE protocols & specifications for specification mappings.

Protocol links are as below

Air Interface Physical Layer

GPRS Tunnelling Protocol User Plane (GTP-U)

GTP-U Transport

Medium Access Control (MAC)

Non-Access-Stratum (NAS) Protocol

Packet Data Convergence Protocol (PDCP)

Radio Link Control (RLC)

Radio Resource Control (RRC)

S1 Application Protocol (S1AP)

S1 layer 1

S1 Signalling Transport

X2 Application Protocol (X2AP)

X2 layer 1

X2 Signalling Transport

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What is VoLGA?

VoLGA stands for "Voice over LTE via Generic Access". The VoLGA service resembles the 3GPP

Generic Access Network (GAN). GAN provides a controller node - the GAN controller (GANC) -

inserted between the IP access network (i.e., the EPS) and the 3GPP core network.

The GAN provides an overlay access between the terminal and the CS core without requiring

specific enhancements or support in the network it traverses. This provides a terminal with a

'virtual' connection to the core network already deployed by an operator. The terminal and

network thus reuse most of the existing mechanisms, deployment and operational aspects.

see VoLGA - Voice over LTE via Generic Access for more details.

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What is CS Fallback in LTE?

LTE technology supports packet based services only, however 3GPP does specifies fallback for

circuit switched services as well. To achieve this LTE architecture and network nodes require

additional functionality, this blog is an attempt to provide overview for same.

In LTE architecture, the circuit switched (CS) fallback in EPS enables the provisioning of voice

and traditional CS-domain services (e.g. CS UDI video/ SMS/ LCS/ USSD). To provide these

services LTE reuses CS infrastructure when the UE is served by E UTRAN.

See Understanding CS Fallback in LTE for more details.

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How does LTE Security works?

The following are some of the principles of 3GPP E-UTRAN security based on 3GPP Release 8

specifications:

The keys used for NAS and AS protection shall be dependent on the algorithm with which they are used.

The eNB keys are cryptographically separated from the EPC keys used for NAS protection (making it

impossible to use the eNB key to figure out an EPC key).

The AS (RRC and UP) and NAS keys are derived in the EPC/UE from key material that was generated by a

NAS (EPC/UE) level AKA procedure (KASME) and identified with a key identifier (KSIASME).

The eNB key (KeNB) is sent from the EPC to the eNB when the UE is entering ECM-CONNECTED state (i.e.

during RRC connection or S1 context setup).

See LTE Security Principles for more details.

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How does measurements work in LTE?

In LTE E-UTRAN measurements to be performed by a UE for mobility are classified as below

Intra-frequency E-UTRAN measurements

Inter-frequency E-UTRAN measurements

Inter-RAT measurements for UTRAN and GERAN

Inter-RAT measurements of CDMA2000 HRPD or 1xRTT frequencies

See Measurements in LTE E-UTRAN for details.

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What is Automatic Neighbour Relation?

According to 3GPP specifications, the purpose of the Automatic Neighbour Relation (ANR)

functionality is to relieve the operator from the burden of manually managing Neighbor

Relations (NRs). This feature would operators effort to provision.

Read Automatic Neighbour Relation in LTE for more details.

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How does Intra E-UTRAN Handover is performed?

Intra E-UTRAN Handover is used to hand over a UE from a source eNodeB to a target eNodeB

using X2 when the MME is unchanged. In the scenario described here Serving GW is also

unchanged. The presence of IP connectivity between the Serving GW and the source eNodeB, as

well as between the Serving GW and the target eNodeB is assumed.

The intra E-UTRAN HO in RRC_CONNECTED state is UE assisted NW controlled HO, with HO

preparation signalling in E-UTRAN.

Read LTE Handovers - Intra E-UTRAN Handover for more details.

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What is SON & how does it work in LTE?

Self-configuring, self-optimizing wireless networks is not a new concept but as the mobile

networks are evolving towards 4G LTE networks, introduction of self configuring and self

optimizing mechanisms is needed to minimize operational efforts. A self optimizing function

would increase network performance and quality reacting to dynamic processes in the network.

This would minimize the life cycle cost of running a network by eliminating manual

configuration of equipment at the time of deployment, right through to dynamically optimizing

radio network performance during operation. Ultimately it will reduce the unit cost and retail

price of wireless data services.

See Self-configuring and self-optimizing Networks in LTE for details.

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How does Timing Advance (TA) works in LTE?

In LTE, when UE wish to establish RRC connection with eNB, it transmits a Random Access

Preamble, eNB estimates the transmission timing of the terminal based on this. Now eNB

transmits a Random Access Response which consists of timing advance command, based on

that UE adjusts the terminal transmit timing.

The timing advance is initiated from E-UTRAN with MAC message that implies and adjustment

of the timing advance.

See Timing Advance (TA) in LTE for further details.

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How does LTE UE positioning works in E-UTRAN?

UE Positioning function is required to provide the mechanisms to support or assist the

calculation of the geographical position of a UE. UE position knowledge can be used, for

example, in support of Radio Resource Management functions, as well as location-based

services for operators, subscribers, and third-party service providers.

See LTE UE positioning in E-UTRAN for more details.

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How many operators have committed for LTE?

List of operators committed for LTE has been compiled by 3GAmericas from Informa Telecoms

& Media and public announcements. It includes a variety of commitment levels including

intentions to trial, deploy, migrate, etc.

For latest info visit http://ltemaps.org/

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How does Location Service (LCS) work in LTE network?

In the LCS architecture, an Evolved SMLC is directly attached to the MME. The objectives of this

evolution is to support location of an IMS emergency call, avoid impacts to a location session

due to an inter-eNodeB handover, make use of an Evolved and support Mobile originated

location request (MO-LR) and mobile terminated location request MT-LR services.

Release 9 LCS solution introduces new interfaces in the EPC:

SLg between the GMLC and the MME

SLs between the E-SMLC and the MME

Diameter-based SLh between the HSS and the HGMLC

For details read LCS Architecture for LTE EPS and LTE UE positioning in E-UTRAN

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How does Lawful Interception works in LTE Evolved Packet System?

3GPP Evolved Packet System (EPS) provides IP based services. Hence, EPS is responsible only for

IP layer interception of Content of Communication (CC) data. In addition to CC data, the Lawful

Interception (LI) solution for EPS offers generation of Intercept Related Information (IRI) records

from respective control plane (signalling) messages as well.

See Lawful Interception Architecture for LTE Evolved Packet System for more details.

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What is carrier aggregation in LTE-Advanced?

To meet LTE-Advanced requirements, support of wider transmission bandwidths is required

than the 20 MHz bandwidth specified in 3GPP Release 8/9. The preferred solution to this is

carrier aggregation.

It is of the most distinct features of 4G LTE-Advanced. Carrier aggregation allows expansion of

effective bandwidth delivered to a user terminal through concurrent utilization of radio

resources across multiple carriers. Multiple component carriers are aggregated to form a larger

overall transmission bandwidth.

LTE Handovers - Intra E-UTRAN Handover

By LteWorld - Posted on 10 April 2010

Intra E-UTRAN Handover is used to hand over a UE from a source eNodeB to a target eNodeB

using X2 when the MME is unchanged. In the scenario described here Serving GW is also

unchanged. The presence of IP connectivity between the Serving GW and the source eNodeB, as

well as between the Serving GW and the target eNodeB is assumed.

The intra E-UTRAN HO in RRC_CONNECTED state is UE assisted NW controlled HO, with HO

preparation signalling in E-UTRAN.

To prepare the HO, the source eNB passes all necessary information to the target eNB (e.g. E-

RAB attributes and RRC context) and UE accesses the target cell via RACH following a

contention-free procedure using a dedicated RACH preamble.

The HO procedure is performed without EPC involvement, i.e. preparation messages are directly

exchanged between the eNBs. The figure below shows the basic handover scenario where

neither MME nor Serving Gateway changes:

Detailed explanation of above scenario is below.

The source eNB configures the UE measurement procedures according to the area

restriction information. UE sends MEASUREMENT REPORT by the rules set by i.e. system

information, specification etc.

Source eNB makes decision based on MEASUREMENT REPORT and RRM information to hand

off UE and issues a HANDOVER REQUEST message to the target eNB passing necessary

information to prepare the HO at the target side.

Admission Control may be performed by the target eNB dependent on the received E-RAB

QoS information to increase the likelihood of a successful HO. The target eNB configures

the required resources according to the received E-RAB QoS information.

Target eNB prepares HO with L1/L2 and sends the HANDOVER REQUEST ACKNOWLEDGE to

the source eNB. The HANDOVER REQUEST ACKNOWLEDGE message includes a transparent

container to be sent to the UE as an RRC message to perform the handover.

The UE receives the RRCConnectionReconfiguration message with necessary parameters (i.e.

new C-RNTI, target eNB security algorithm identifiers, and optionally dedicated RACH

preamble, target eNB SIBs, etc.) and is commanded by the source eNB to perform the HO.

The source eNB sends the SN STATUS TRANSFER message to the target eNB to convey the

uplink PDCP SN receiver status and the downlink PDCP SN transmitter status of E-RABs for

which PDCP status preservation applies (i.e. for RLC AM).

After receiving the RRCConnectionReconfiguration message including the

mobilityControlInformation , UE performs synchronisation to target eNB and accesses the

target cell via RACH.

The target eNB responds with UL allocation and timing advance.

UE sends the RRCConnectionReconfigurationComplete message (C-RNTI) to confirm the

handover to the target eNB to indicate that the handover procedure is completed for the UE.

The target eNB verifies the C-RNTI sent in the RRCConnectionReconfigurationComplete

message. The target eNB can now begin sending data to the UE.

The target eNB sends a PATH SWITCH message to MME to inform that the UE has changed

cell.

The MME sends an UPDATE USER PLANE REQUEST message to the Serving Gateway.

The Serving Gateway switches the downlink data path to the target side. The Serving

gateway sends one or more "end marker" packets on the old path to the source eNB and

then can release any U-plane/TNL resources towards the source eNB.

Serving Gateway sends an UPDATE USER PLANE RESPONSE message to MME.

The MME confirms the PATH SWITCH message with the PATH SWITCH ACKNOWLEDGE

message.

By sending UE CONTEXT RELEASE, the target eNB informs success of HO to source eNB and

triggers the release of resources by the source eNB. The target eNB sends this message

after the PATH SWITCH ACKNOWLEDGE message is received from the MME.

Upon reception of the UE CONTEXT RELEASE message, the source eNB can release radio and

C-plane related resources associated to the UE context. Any ongoing data forwarding may

continue.

Self-configuring and self-optimizing Networks in LTE

By LteWorld - Posted on 11 October 2009

Self-configuring, self-optimizing wireless networks is not a new concept but as the mobile

networks are evolving towards 4G LTE networks, introduction of self configuring and self

optimizing mechanisms is needed to minimize operational efforts. A self optimizing function

would increase network performance and quality reacting to dynamic processes in the network.

This would minimize the life cycle cost of running a network by eliminating manual

configuration of equipment at the time of deployment, right through to dynamically optimizing

radio network performance during operation. Ultimately it will reduce the unit cost and retail

price of wireless data services.

As per 3GPP standards, a typical operational objective is to optimize the network according to

coverage and capacity.

Providing optimal coverage requires that in the area, where LTE system is offered, users can

establish and maintain connections with acceptable or default service quality, according to

operators requirements. Coverage and capacity are linked, a trade-off between the two of

them may also be a subject of optimization.

To achieve these objectives, 3GPP suggests to implement following functions

Detection of unintended holes in the coverage (planned by the operator)

Perform coverage optimization, including DL/UL channel coverage a

Ability to balance the trade-off between coverage and capacity

Once solution is implemented, it would result in

Continuous, optimized and matched UL and DL coverage

Optimized DL and UL capacity of the system

Balanced tradeoff between coverage and capacity

Interference reduction

Controlled cell edge performance

Minimized human intervention in network management and optimization tasks

Energy savings

More details about solution and use cases are available in 3GPP technical report "Evolved

Universal Terrestrial Radio Access Network (E-UTRAN); Self-configuring and self-optimizing

network (SON) use cases and solutions".

Implementing self configuration and self optimization under multi vendor environment is

challenging task. For this purpose, It is of importance that measurements and performance data

of different vendors follow same standard. Especially when the interaction between self

configuring/optimizing networks and O&M has to be considered.

Timing Advance (TA) in LTE

By agaur - Posted on 01 September 2010

In GSM system MS sends its data three time slots after it received the data from the BTS. This is

ok as long as MS-BTS distance is small but increasing distance requires consideration of

propagation delay as well. To handle it Timing advance (TA) is conveyed by network to MS and

current value is sent to the MS within the layer 1 header of each SACCH. BTS calculates the first

TA when it receives RACH and reports it to the BSC and BSC/BTS passes it to UE during

Immediate Assignment.

In UMTS Timing Advance parameter was not used but in LTE Timing Advance is back.

In LTE, when UE wish to establish RRC connection with eNB, it transmits a Random Access

Preamble, eNB estimates the transmission timing of the terminal based on this. Now eNB

transmits a Random Access Response which consists of timing advance command, based on

that UE adjusts the terminal transmit timing.

The timing advance is initiated from E-UTRAN with MAC message that implies and adjustment

of the timing advance.

3GPP TA Requirements

Timing Advance adjustment delay

UE shall adjust the timing of its uplink transmission timing at sub-frame n+6 for a timing

advancement command received in sub-frame n.

Timing Advance adjustment accuracy

The UE shall adjust the timing of its transmissions with a relative accuracy better than or equal

to 4* TS seconds to the signalled timing advance value compared to the timing of preceding

uplink transmission. The timing advance command is expressed in multiples of 16* TS and is

relative to the current uplink timing.

Maintenance of Uplink Time Alignment

The UE has a configurable timer timeAlignmentTimer which is used to control how long the UE

is considered uplink time aligned

when a Timing Advance Command MAC control element is received then UE applies the

Timing Advance Command and start or restart timeAlignmentTimer.

when a Timing Advance Command is received in a Random Access Response message then

one of following action is performed by UE

- if the Random Access Preamble was not selected by UE MAC then UE applies the Timing

Advance Command and starts or restarts timeAlignmentTimer.

- else if the timeAlignmentTimer is not running then UE applies the Timing Advance Command

starts timeAlignmentTimer; when the contention resolution is considered not successful then

UE stops timeAlignmentTimer.

- else ignore the received Timing Advance Command.

when timeAlignmentTimer expires UE flushes all HARQ buffers, notifies RRC to release

PUCCH/SRS and clears any configured downlink assignments and uplink grants.

Timing Advance Command MAC Control Element

The Timing Advance Command MAC control element is identified by MAC PDU subheader with

LCID value = 11101 (Timing Advance Command) .

It has a fixed size and it consists of a single octet as show below.

Timing Advance Command MAC control element has following fields.

R: reserved bit, set to "0"

Timing Advance Command: This field indicates the index value TA (0, 1, 2 63) used to

control the amount of timing adjustment that UE has to. The length of the field is 6 bits.

LTE UE positioning in E-UTRAN

By LteWorld - Posted on 03 January 2010

UE Positioning function is required to provide the mechanisms to support or assist the

calculation of the geographical position of a UE. UE position knowledge can be used, for

example, in support of Radio Resource Management functions, as well as location-based

services for operators, subscribers, and third-party service providers.

Positioning functionality provides a means to determine the geographic position and/or velocity

of the UE based on measuring radio signals. The position information may be requested by and

reported to a client (e.g., an application) associated with the UE, or by a client within or

attached to the core network. The position information is reported in standard formats, such as

those for cell-based or geographical co-ordinates, together with the estimated errors

(uncertainty) of the position and velocity of the UE and, if available, the positioning method (or

the list of the methods) used to obtain the position estimate.

Several design options of the LTE E-UTRAN system (e.g., size of cell, adaptive antenna

technique, pathloss estimation, timing accuracy, eNode B surveys) would allow the network

operator to choose a suitable and cost-effective UE positioning method for their market.

Positioning the UE involves two main steps:

- signal measurements

- Position estimate and optional velocity computation based on the measurements.

The signal measurements may be made by the UE or the eNode B.

The standard positioning methods supported for E-UTRAN access are:

- network-assisted GNSS (Global Navigation Satellites Systems) methods

- downlink positioning

- enhanced cell ID method.

Hybrid positioning using multiple methods from the list of positioning methods above is also

supported.

E-UTRAN UE Positioning Architecture

Above figure shows the architecture in EPS applicable to positioning of a UE with E-UTRAN

access.

The MME receives a request for some location service associated with a particular target UE

from another entity (e.g., GMLC, eNB, or UE) or the MME itself decides to initiate some location

service on behalf of a particular target UE (e.g., for an IMS emergency call from the UE). The

MME then sends a location services request to an E-SMLC. The E-SMLC processes the location

services request which may include transferring assistance data to the target UE to assist with

UE-based and/or UE-assisted positioning and/or may include positioning of the target UE. The

E-SMLC then returns the result of the location service back to the MME (e.g., a position estimate

for the UE and/or an indication of any assistance data transferred to the UE). In the case of a

location service requested by an entity other than the MME (e.g., UE, eNB, or E-SMLC), the MME

returns the location service result to this entity.

The SLP is the SUPL entity responsible for positioning over the user plane.

source : 3GPP 3605-900

LTE Protocols & Specifications

In LTE architecture, core network includes Mobility Management Entity (MME), Serving Gateway

(SGW), Packet Data Network Gateway (PDN GW) where as E-UTRAN has E-UTRAN NodeB (eNB).

The figures shown below provide mapping of protocols to corresponding specifications. To

find 3GPP LTE specification, click at the corresponding protocol in the images below.

Protocol structure of control plane in between UE & MME is shown

below.

This figure below shows protocol structure in between UE & P-GW user plane. GPRS Tunnelling

Protocol for the user plane (GTP-U) tunnels user data between eNodeB and the S-GW as well as

between the S-GW and the P-GW in the backbone

network.

The X2 interface is defined between two neighbour eNBs. This figure below shows the control &

user plane protocol stack of the X2 interface.

Understanding CS Fallback in LTE By LteWorld - Posted on 27 September 2009

LTE technology supports packet based services only, however 3GPP does specifies fallback for

circuit switched services as well. To achieve this LTE architecture and network nodes require

additional functionality, this blog is an attempt to provide overview for same.

In LTE architecture, the circuit switched (CS) fallback in EPS enables the provisioning of voice

and traditional CS-domain services (e.g. CS UDI video/ SMS/ LCS/ USSD). To provide these

services LTE reuses CS infrastructure when the UE is served by E UTRAN.

A CS fallback enabled terminal, connected to E UTRAN may use GERAN or UTRAN to connect to

the CS domain. This function is only available in case E UTRAN coverage is overlapped by either

GERAN coverage or UTRAN coverage.

The figure above provides architecture for CS fallback in EPS.

CS Fallback and IMS based services can co-exist in the same operators network. Although

its not very straight forward to support CS fallback, all participating elements i.e UE, MME, MSC

& E-UTRAN needs to support additional functionalities.

The support CS fallback in EPS a new interface SGs is added in LTE architecture. SGs interface is

the reference point between the MME and MSC server. SGs interface is used for the mobility

management and paging procedures between EPS and CS domain, and is based on the Gs

interface procedures.

The SGs reference point is also used for the delivery of both mobile originating and mobile

terminating SMS.

The CS fallback enabled network elements need to support the following additional functions:

UE

supports access to E-UTRAN/EPC as well as access to the CS domain over GERAN and/or

UTRAN.

Combined procedures for EPS/IMSI attach, update and detach.

CS fallback and SMS procedures for using CS domain services.

MME

Deriving a VLR number and LAI from the GUTI received from the UE or from a default LAI.

Maintaining of SGs association towards MSC/VLR for EPS/IMSI attached UE.

Initiating IMSI detach at EPS detach.

Initiating paging procedure towards eNodeB when MSC pages the UE for CS services.

Support of SMS procedures

Rejecting CS Fallback call request (e.g. due to O&M reasons)

Use of the LAI and a hash value from the IMSI to determine the VLR number when multiple

MSC/VLRs serve the same LAI.

MSC

Maintaining SGs association towards MME for EPS/IMSI attached UE.

Support of SMS procedures as provided in 3GPP specification

E-UTRAN

Forwarding paging request and SMS to the UE.

Directing the UE to the target CS capable cell.

At MME - MSC Server interface a new protcol SGsAP is being added to support CS fallback.

SGsAP protocol is based on the BSSAP+. Stream Control Transmission Protocol (SCTP) is used to

transport SGsAP signaling messages.

A CS Fallback and IMS capable UE would follow the procedures for domain selection for UE

originating session/calls according to 3GPP specification 23.221.

If a UE is configured to use SMS over IP services and it is registered to IMS then it would send

SMS over IMS, even if it is EPS/IMSI attached.

The home operator has option to activate/deactivate the UE configuration to use SMS over IP by

means of device management in order to allow alignment with HPLMN support of SMS over IP.

When UE is performing CS fallback procedure for Mobile Originating Call for the purpose of

emergency call, it needs to indicate to the MME that this CS fallback request is for emergency

purpose. MME also indicates to the E-UTRAN via the appropriate S1-AP message that this CS

fallback procedure is for emergency purpose.

Contents of this blog are mostly derived from 3GPP specification 23.272, for better and detailed

understanding, same should be referred.

Although there had been talks about another approach for CS Fallback by VoLGA which does

not require any enhancement in existing CS elements like MSC but for VoLGA another set of

additional nodes are needed. to know more about VoLGA refer one of our earlier blog LTE

needs VoLGA.