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Coexistence of GSM, HSPA and LTE
August 25, 2011 | By 4G Americas
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Transcript
.
...



CONTENTSCONTENTS
EXECUTIVE SUMMARY .............................................................................................................................. 4


1 Introduction.................................................................................................................................... 5


1.1
Coexistence of GSM, HSPA/WCDMA AND LTE........................................................... 5
1.2
The System Architecture Evolution Aspects .................................................................. 5
1.3
Scope ............................................................................................................................. 6
Voice Solutions .............................................................................................................. 6
Mobility ........................................................................................................................... 6
Roaming Aspects ........................................................................................................... 6
Radio Network Design and Performance....................................................................... 7
2 Radio Access Network Considerations ....................................................................................... 8


2.1
Technology Coexistence ................................................................................................ 8
2.2
Spectrum Considerations ............................................................................................... 8
Multi-Standard Radio ..................................................................................................... 8
2.3
The UICC/USIM and Roaming ..................................................................................... 10
2.4
Coverage Triggered WCDMA/GERAN Session Continuity ......................................... 10
3 Core Network Considerations ..................................................................................................... 11


3.1
Core Network Overview ............................................................................................... 11
3.2
Migratory ASpects ........................................................................................................ 12
Pre-Rel-8 Gn/Gp Mobility ............................................................................................. 13
Rel-8 S3/S4 Based EPS Mobility ................................................................................. 14
3.3
Subscriber Data Aspects: USIM/ISIM.......................................................................... 15
3.4
Roaming Scenarios ...................................................................................................... 15
Home tunneling or home routed scenario .................................................................... 16
Home tunneling along with possibility of local breakout .............................................. 16
4 QoS in 3GPP ................................................................................................................................ 16


4.1
QoS classification and differences in 3GPP networks ................................................. 18
QoS Classification in LTE Rel-8 ................................................................................... 18
QoS classification in UMTS and GPRS networks ........................................................ 20
4.2
Network-initiated vs. Terminal-initiated QoS control paradigms .................................. 20
5 LTE Services ................................................................................................................................ 22


5.1
Services over LTE ........................................................................................................ 22
5.2
Circuit Switched Fallback ............................................................................................. 24
RRC Release with Redirect ......................................................................................... 25
Enhanced Release with Redirect, Automatic System info Exchange with RIM........... 26
PS Handover between LTE and GERAN for CSFB ..................................................... 27
Cell Change Order ....................................................................................................... 28
5.3
IMS Voice ..................................................................................................................... 28
Introduction to IMS ....................................................................................................... 28
Introduction to MMTEL ................................................................................................. 30
5.4
LTE To Circuit Switched Voice Call Continuity ............................................................ 32
3GPP Rel-9 Architecture enhancements ..................................................................... 32
3GPP Rel-10 Architecture enhancements ................................................................... 33
5.5
Roaming Considerations .............................................................................................. 34
5.6
IMS Centralized SERVICES (ICS) ............................................................................... 34
Architecture for ICS ...................................................................................................... 35
5.7
Messaging over LTE.................................................................................................... 35
SMS over SGS ............................................................................................................. 35
IMS Messaging ............................................................................................................ 36
UICC/USIM and IMS Messaging.................................................................................. 38
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6 Regulatory Issues ........................................................................................................................ 39


6.1
Lawful Intercept ............................................................................................................ 39


6.2
Emergency Services .................................................................................................... 40


6.3
Telecommunications Device for the Deaf (TDD) ......................................................... 41


6.4
Non-Voice Emergency Service (NOVES) .................................................................... 41


6.5
Priority Services ........................................................................................................... 42


6.6
Commercial Mobile Alert System (CMAS) ................................................................... 43


7 Devices for Seamless Migration ................................................................................................. 46


7.1
Multimode and Multiband Devices ............................................................................... 46
Antenna Separation and frequency band of operation ................................................ 50


7.2
Support for LTE UE Profile Defined by IR.92 for Voice ............................................... 52


8 Evolution to communication with devices ................................................................................ 54


8.1
Network Security .......................................................................................................... 56


8.2
Network Considerations ............................................................................................... 56


8.3
Device Implications (Man-Machine Interface of UE) .................................................... 56


8.4
MTC Implications for Coexistence of GSM, HSPA, LTE ............................................. 58


Conclusion ................................................................................................................................................ 59


Glossary .................................................................................................................................................... 60


Appendix ................................................................................................................................................... 62


Acknowledgements ................................................................................................................................. 63


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EXECUTIVESUMMARYEXECUTIVESUMMARY
This whitepaper covers the coexistence of GSM, HSPA and LTE and migratory aspects from Rel-7 to Rel8
and beyond based on the 3GPP specification. The topics covered include:

.
The Radio Access Network Aspects of the Coexistence

.
The Core Network Considerations

.
The Quality of Service (QoS) Requirements

.
The Future Of Voice/Messaging Services on LTE including CSFB, SRVCC and VoLTE and IMS
Services

.
Regulatory Aspects

.
Multimode and Multiband Devices and Machine Type Communications for Coexisting Networks

The chapter on Radio Access Networks discusses the spectrum considerations and the concept of Multi-
Standard Radios (MSR) that will be key to coexistence in the future. Impact on antennas is briefly
touched upon as well. Then the chapter discusses UICC/USIM roaming. It closes with coverage triggered
session continuity from LTE to WCDMA/GERAN Networks.

The chapter on core network considerations provides a core network overview i.e. the Evolved Packet
Core (EPC) followed by migratory aspects related to pre-Rel-8 mobility as well as Rel-8 based mobility.
Subscriber data aspects are touched upon related to HLR evolution to HSS. Finally, some roaming
considerations are presented based on Home Tunneling as compared to Home Tunneling with the
possibility of local breakout.

In this whitepaper, QoS in 3GPP is explained and the concepts key to delay sensitive packet data e.g.
GBR (guaranteed bit rate) and non-GBR are presented. The difference between pre-Rel-8 and Rel-8
functionality e.g. QoS classification for LTE vs. for WCDMA Networks is also presented.

The LTE Services addressed include Circuit Switched Fallback (CSFB) - the mechanism to provide voice
services on the GSM/WCDMA network before IMS enabled LTE voice service is available. Voice over
LTE (VoLTE) based on IMS is then presented and supplementary IMS Services are introduced. Then
moving out from IMS Voice capable LTE coverage to WCDMA/GSM only coverage, Single Radio Voice
Call Continuity SRVCC is explained based on both Rel-9 and Rel-10 architecture. Roaming
considerations also are briefly touched upon and the concept of IMS Centralized Services ICS is
introduced. The chapter finishes with a description of messaging services over LTE based on SMS over
SGS (specified in CSFB) and IMS Messaging.

The chapter on Regulatory Aspects covers Lawful Intercept, Emergency Services, Non-Voice Emergency
Services, Priority Services and Commercial Mobile Alert System.

Then in the chapter on devices for seamless migration the spectrum and technology requirements of
devices for coexistence are presented. Multimode and Multiband devices with MIMO capability are
discussed. The requirements for supporting Voice over LTE (IMS Voice) are also presented.

The final chapter presents an update on Machine Type Communications (MTC) as reflected by
standardization activities.

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1INTRODUCTION1INTRODUCTION
The whitepaper covers the end-to-end considerations for the successful coexistence of GSM, HSPA and
LTE.

1.1COEXISTENCEOFGSM,HSPA/WCDMAANDLTE
As shown in Figure 1, devices with the three technologies: GSM, UMTS and LTE will co-exist for the
foreseeable future. The objective of this whitepaper is to identify all considerations from the RAN, Core
and Service layer to ensure a seamless migration for the end-user.

LTE
LTELTELTE

0.001
0.0010.001

19
19199
993 1997 20
20199733 201997 0
001 20
2011 200
005 2009 20
20200955 202009 1
113
33 20
202015
1515

Figure 1: Timeline for GSM, HSPA/WCDMA, LTE Subscriber/Device Evolution

0.010.11101001,00010,000MillionsGSMWCDMA0.01
0.1
1
10
100
1,000
10,000
Millions
GSMWCDMAGSMGSM
WCDMAWCDMA
1.2THESYSTEMARCHITECTUREEVOLUTIONASPECTS
The broad objectives of the SAE (System Architecture Evolution now referred to as EPC . Evolved
Packet Core) were to evolve the 3G access technologies and their supporting GPRS core network by
creating a simplified All-IP architecture to provide support for multiple radio accesses, including mobility
between various access networks, both 3GPP and Non-3GPP standardized technologies.

The goal is to provide a system evolution aiming at improving the performance of the existing system,
high performance handover between 3GPP accesses, easy migration to EPS, and manageable impacts
on roaming infrastructure upgrades, while reusing some of the very strong 3GPP architecture principles
like clear separation of control and user plane operation, and All-IP.

Requirements for reducing the latencies and delays in the radio access have imposed that the RAN
adaptation loops be radically optimized latency-wise by moving some of the supporting functions from the
old centralized node RNC to the new eNB, in the end the RNC being completely eliminated from the RAN
architecture via the transfer of its functions to the new Core Network (CN): EPC. That is equivalent to
changing the borderline between the E-UTRAN and the EPC as compared to previous 3G systems.

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However, this revolutionary change has resulted in the benefits of a flat architecture of both E-UTRAN
and EPC, with plenty of opportunities to optimize the backhaul, transport, as well as the overall network
management for the operators.

One major characteristic of the new system architecture conceived for EPS is the fact that it adopted a
“network based mobility model” (with two flavors: GTP or PMIP), a choice that is essential in ensuring that
the UE does not act differently depending on the protocol used by the network entities.

This is one of the most powerful features of the EPC: the major interface in the Core Network, S5/S8,
between the only two types of nodes in the flat EPC, S-GW and PDN GW, is specified in two different
variants, one utilizes the GTP protocol which is used in the legacy GSM/GPRS and WCDMA/HSPA
networks and the other uses the IETF PMIPv6 protocol.

1.3SCOPE
The document focuses on the various important segments concerning the successful integration of GSM,
HSPA and LTE as listed below.

VOICE
SOLUTIONS


3GPP Voice over LTE (VoLTE) Solutions are discussed, which include CSFB (Circuit Switch Fall Back),
IMS based VoLTE and SRVCC (Single Radio Voice Call Continuity).

VOLGA is not considered as it is not a 3GPP Specification. VOLGA is based on the existing 3GPP
Generic Access Network (GAN) standard, with the purpose of extending mobile services over a generic
IP access network. The main concept in VOLGA is to connect the already existing Mobile Switching
Centers to the LTE network via a gateway and make use of existing GAN concepts.

MOBILITY


The document presents EPS mobility solutions. Mobility with non-3GPP is capability of LTE that is not
considered in this document.

ROAMING
ASPECTS


From a coexistence perspective for roaming services of devices between pre-Rel-8 networks and Rel-8
and beyond networks, the document presents the following issues:

.
The interfaces supported by the two networks to communicate and seamlessly transfer service
flows across the two networks desirably in both directions

.
The ability of devices to operate in diverse multiple bands around the world to help subscribers
roam

.
The ability of devices and subscribers to authenticate themselves with the roaming network

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RADIO
NETWORK
DESIGN
AND
PERFORMANCE


The radio link throughput and latency performance for HSPA, HSPA+ and LTE networks is discussed at
length in the 3G Americas whitepaper by Rysavy Research1 and therefore is not addressed in the current
whitepaper on coexistence. Another aspect related to coexistence and network design is antenna
migration for the radio network to support multiple frequency bands and technologies with MIMO
capability. That discussion is also covered in another 3G Americas whitepaper.2

1 Transition to 4G: 3GPP Broadband Evolution to IMT-Advanced, Rysavy Research/3G Americas, Sept 2010
2 MIMO and Smart Antennas for 3G and 4G Wireless Systems Practical Aspects and their Deployment, 3G Americas, May 2010

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2RADIOACCESSNETWORKCONSIDERATIONS2RADIOACCESSNETWORKCONSIDERATIONS
The Radio Network Considerations for coexistence of LTE, HSPA and GSM networks include the
spectrum requirements, roaming and coverage triggered handovers to WCDMA/GSM. In the section on
spectrum considerations the concept of Multi-Standard Radios (MSR) is presented.

2.1TECHNOLOGYCOEXISTENCE
It takes years to complete a nationwide build-out of a new technology. A potential scenario of LTE / HSPA
and GSM/EDGE Build out is illustrated in Figure 2.


Figure 2: Illustrative GSM/EDGE, WCDMA/HSPA and LTE Buildout

It makes sense to continue the evolution of WCDMA/HSPA (to HSPA+) so that the user experience does
not degrade when the user is out of LTE coverage.

2.2SPECTRUMCONSIDERATIONS
The FDD frequency bands relevant for GSM, HSPA and LTE coexistence in the Americas are highlighted
in the Appendix in Table 7.

MULTI.STANDARD
RADIO


Due to this lack of spectrum, operators must optimize utilization of existing spectrum. Multiradio solutions
enable operators to share some components like the baseband for several carriers of the same
technology, while leveraging frequency specific radios. The 3GPP definition of MSR in 3GPP, TS 37.104
is:

“Base Station characterized by the ability of its receiver and transmitter to process two or more carriers in
common active RF components simultaneously in a declared RF bandwidth, where at least one carrier is
of a different RAT than the other carrier(s). MSR implies different technologies on the same frequency
band.”

A scenario for MSR radio is depicted in Figure 3. GSM Technology is utilized in the allocated frequency
band. After some time there is a mixed configuration of both GSM and WCDMA within the same
frequency band. Further along the time line the entire frequency band is allocated to WCDMA with no
change in hardware.

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Operator spectrum allocation O
OOp
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or spor spectr
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Common RF
GSM
Common RF
WCDMA GSM
Common RF
WCDMA
Figure 3: Example of MSR the same radio hardware is used for GSM, GSM/WCDMA and finally
WCDMA only


The practical advantage of MSR is that for a given band the same investment can potentially ease the
migration from GSM to WCDMA/LTE. An MSR capable radio for band category 2 can enable this
migration.

The scenario is that GSM is reaching maturity for an operator and investing further in “GSM only” capable
radios is not cost-effective. On the other hand, WCDMA is in growth mode with a greater lifespan. Also
LTE will potentially be introduced at some point. An MSR radio can be used for GSM to start with and
then be converted to WCDMA/LTE later on as shown in Figure 4.

+ +
GSM only WCDMA/LTE and GSM WCDMA/LTE

Figure 4: Migration from Spectrum used for GSM Only to WCDMA/LTE and GSM to finally
WCDMA/LTE


MSR requirements are applicable for band definitions and band numbering as defined in the
specifications TS 45.005 [2], TS 25.104 [3], TS 25.105 [4] and TS 36.104 [5]. For the purpose of defining
the MSR BS requirements, the operating bands are divided into three band categories as follows:

.
Band Category 1 (BC1): Bands for E-UTRA FDD and UTRA FDD operation
.
Band Category 2 (BC2): Bands for E-UTRA FDD, UTRA FDD and GSM/EDGE operation
.
Band Category 3 (BC3): Bands for E-UTRA TDD and UTRA TDD operation


Finally the specification defines MSR as:
.
“Single RAT” mode where a radio transmits only one radio access technology (Rel-8)

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.
“Multiple radio access technologies” are transmitted on same radio amplifier (Rel-9)

In summary MSR enables operators to meet current demand with existing spectrum based on one
technology with the option to reuse the same hardware for another technology in the future.

2.3THEUICC/USIMANDROAMING
Industry pundits hope that by the time LTE will be widely deployed worldwide, common radio bands will
be agreed upon so that new multi-bands handsets will enable seamless roaming in a large number of
countries. However, during a transition period, not all the handsets will support all the possible radio
bands. Therefore, there will be a need to change handsets when roaming in some countries. The USIM
will allow users to switch to a more appropriate handset when necessary, in order to get his home
operator services.

The ability to define LTE roaming preferences has been introduced from USIM specification 3GPP 31.102
Rel-8. In order to optimize roaming charges, operators need to upgrade their UICC to a Rel-8 or beyond
profile, and to properly configure the USIM. To offload network traffic efficiently, LTE network shall be set
as higher priority in the operator controlled PLMN selector with Access Technology.

If a legacy UICC/USIM is inserted into the device, this will not prevent the user from accessing LTE
network since backward compatibility is guaranteed. However, as the legacy UICC/USIM is not
configured with LTE, by default access to the LTE network will occur with lowest priority.

2.4COVERAGETRIGGEREDWCDMA/GERANSESSIONCONTINUITY
The Coverage Triggered WCDMA/GERAN Session Continuity feature uses the Event A2 (serving cell
becomes worse than threshold) measurement process3.

The UE measurements are reported to the serving RAN to make the final determination on redirection to
the WCDMA network. Types of measurements used in the handover evaluation process include the
following:

.
Reference Signal Received Power (RSRP), representing the mean measured power per
reference signal


.
Reference Signal Received Quality (RSRQ), providing an indication of the reference signal quality

The UE indicates to the serving RAN when it has poor LTE coverage. The poor coverage release can be
set to trigger on the RSRP value, RSRQ value, or both. The measurement reports sent by the UE to the
serving RAN contain either one or both of these values.

The RAN determines whether to release the UE with a redirection to a WCDMA network, depending on
the UE capabilities, and other parameters. If the UE is released with a redirection to a WCDMA network,
the release message contains the UMTS Absolute Radio Frequency Channel Number (UARFCN), to help
the UE find a suitable WCDMA cell.

3 Further details on Coverage Triggered WCDMA and GERAN (GSM) Session Continuity can be found in: 3GPP TS 36.300 Overall
description; Stage 2; 3GPP TS 36.304 User Equipment (UE) procedures in idle mode; 3GPP TS 36.331 Radio Resource Control
(RRC); Protocol Specification

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3CORENETWORKCONSIDERATIONS3CORENETWORKCONSIDERATIONS
This section deals with the phased migration from Rel-7 and earlier versions for legacy packet core
interfaces for IRAT mobility with LTE, HSPA and GSM.

3.1CORENETWORKOVERVIEW
The EPC architecture as defined in Rel-8 has enhanced the core network by addressing session
management issues along with a comprehensive policy framework. At the same time the definition has
enabled it to be used as a unified core network for legacy network. This section will provide an overview
of the EPC architecture, both for 3GPP operators that may evolve a Rel-7 and an earlier version of
deployed network to LTE as well as for operators to interwork with other LTE networks in a roaming
arrangement.


Figure 5: Evolved Packet Core
The main principles of the LTE-EPC architecture include:

.
IP-based protocols on all interfaces between network elements

.
Separation of the control and user plane. This entails an optimized architecture for the user plane

including a reduction in the number of network elements that are traversed by data packets

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.
A common anchor point known as PDN-GW for various versions of 3GPP defined access
technologies

.
Ability to assign independent IP addresses and other attributes to multiple flows for a given user
that can be assigned to different APN

.
A comprehensive policy framework that interworks with the user and control plane network
elements to provide appropriate QoS for flows

EPC architecture has defined the following network elements:

.
The PDN gateway (P-GW) serves as an anchor point for LTE. In a network with multiple access
technologies, it can also serve as a single common anchor point for all those technologies. Two
different mobility protocols have been standardized, based on GTP and Proxy Mobile IP. The
expectation is that GTP based protocol would be used where a legacy 3GPP network is being
evolved to LTE and Proxy MIP would be used where a non-3GPP network has to interwork with
LTE

.
The Serving Gateway (S-GW) is the anchor point that connects the eNodeB to the core network

.
The Mobility Management Entity (MME) handles control signaling for a given session. The MME
functionality is kept separate from the gateways to facilitate network deployment, independent
technology evolution, and fully flexible scaling of capacity

.
Policy Charging and Rules Function (PCRF) completes the policy framework for LTE network.
Even though the genesis of PCRF can be traced to IMS, its current usage has broadened to
encompass policy based QoS for both IMS and non-IMS based applications running over the
network

S6a is the interface that is used between the MME and the HSS.

3.2MIGRATORYASPECTS
We can envision two different scenarios of LTE deployment. The first is where an operator already has a
combination of a GSM, UMTS, and HSPA network deployed and would like to evolve it to an LTE network
for seamless interworking across all access types. The second scenario is where LTE is deployed as a
green field network. This section discusses the migration from Rel-7 architecture with Gn/Gp to S4/S12
(3GPP Rel-8) interfaces. In either case, the operator also has to consider enabling its users with roaming
capabilities across other 3GPP operators’ networks.

As with any technology evolution, the question is how to address the impact of new network elements,
defined for LTE, on the legacy network. On one hand, the option of upgrading all RNC and SGSNs can
be a costly proposition and on the other hand, seamless interworking between the two networks is an
imperative.

In case the 2G/3G RNC and SGSN are not being upgraded, one could use GTP based Gn/Gp interfaces
to communicate with the EPC elements. Gn interface is used between the 2G/3G SGSN and LTE defined
MME and P-GW. Note that the use of Gn and Gp interface mandates that the GTP based mobility
protocol is supported by the P-GW. Similarly, for roaming scenarios Gp interface is used when an SGSN
is located in a vPLMN that communicates with a P-GW located in the hPLMN. This is addressed in the
subsection, “Pre-Rel-8 Gn/Gp Mobility” below.

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In the case that the 2G/3G SGSN has been upgraded to support LTE defined S3, S4, and S12 interfaces,
one could use either of GTP or PMIP mobility protocols and use the P-GW as the common anchor for all
the 3GPP access technologies. This is addressed in the subsection, “Rel-8 S3/S4 based EPS Mobility”
below.

The following subsections address EPS mobility. They compare the Gn/Gp Architecture with the S3/S4
Architecture (Rel-7 and Rel-8 respectively).

PRE.REL.8
GN/GP
MOBILITY


This mobility type supports mobility between LTE and already installed (legacy) 3GPP WCDMA/GSM
GPRS networks using the existing interfaces and mobility mechanisms. In fact, the existing GPRS nodes
will see the LTE/EPC nodes as other GPRS nodes. Hence, all adaptations for interworking are made in
the LTE/EPC side alone, including transfer and mapping mechanisms.

The MME and the PDN-GW both support the Gn interface towards the pre-Rel-8 SGSN for IRAT mobility.
Sessions from an LTE capable UE are always anchored on the PDN-GW. Inter-access session mobility is
possible when the UE moves between GERAN/UTRAN and LTE coverage with the help of the Gn
interface between the SGSN and the MME and the PDN GW. Various solutions exist for providing HLR
and HSS functionality in the network with consistent subscription data across all access technologies.

The benefit is that no upgrades are required in the existing networks (besides the user management
parts), and it is viable until the operator decides to upgrade to S3/S4 networks.

Figure 6 shows the LTE-WCDMA mobility. Also, the 3G Direct-Tunnel (between the RNC and the PDNGW)
may be used to offload the SGSN from payload, as illustrated by the Iu-U interface.


Figure 6: IWF for reuse of HLR by MME

An operator that has both EPC and pre-Rel-8 packet core elements in the network have the following
restrictions imposed on session mobility and bearer setup:

.
The Idle Mode Signaling Reduction (ISR) mechanism (which reduces the frequency of LTE
tracking area and 2G/3G Routing Area Updates cannot be used between the MME and pre-Rel-8
SGSNs since the Gn/Gp SGSNs do not support ISR procedures

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.
3GPP Rel-8 introduces the concept of dual-stack bearers, whereas pre-Rel-8 SGSNs do not
understand the concept of dual stack PDP contexts. Thus, separate EPC bearers for IPv4 and
IPv6 must be created such that both IP addresses can be preserved when the UE moves from
LTE to 2G/3G coverage

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PDNPDN
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S1-U
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Gn
S5
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Gr
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Figure 7: Pre-Rel-8 Gn/Gp Based Mobility between LTE and WCDMA/GSM

REL.8
S3/S4
BASED
EPS
MOBILITY


This mobility type supports mobility between LTE and upgraded WCDMA/GSM access networks, using
the 3GPP EPC “S”-interfaces and related functions for interworking and mobility. Inter-access mobility is
enabled by the S3 (SGSN-MME) and S4 (SGSN-SGW) interfaces.

The benefits of this solution (in comparison to the Gn type mobility) are, in brief:

.
SGW is the common session anchor for roaming and non-roaming traffic for all 3GPP radio
technologies which leads to a simplified network architecture

.
Allows for usage of ISR mechanisms which target increased terminal battery life through
decreased signaling while in idle mode

.
Allows for usage of 3G Direct Tunnel also for roaming users, further offloading the SGSN payload
plane

.
Allows for mobility between 3GPP and non-3GPP accesses while retaining a common PDN GW
and hence maintaining an active session without changing terminal IP address

Figure 8 shows the LTE-WCDMA mobility. Also the 3G Direct-Tunnel may be used to offload the SGSN
payload, in the picture illustrated by the S12 interface.

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rgerget:
t:t: WC
WCWCDMA a
DMA aDMA acce
cceccess a
ss ass and SGSN
nd SGSNnd SGSN

S1-MMES11LTES1-USource: LTE access and EPCS3S5WCDMAUES4IuS12HSSS6dS6aS16eNodeB
MME
S-GW
S1-MME S11LTE
PDNPDN
GW
RNC
S1-U
Source: LTE access and EPC
SGSN
S3
S5
WCDMA
UE
S4
Iu
S12
HSS
S6d
S6a
S16
Figure 8: Rel-8 S3/S4 Based Mobility between LTE and WCDMA/GSM

3.3SUBSCRIBERDATAASPECTS:USIM/ISIM
Coexistence of LTE with WCDMA/HSPA and GSM Network requires a smooth transition of home location
register HLR towards HSS based on 3GPP specifications. This also helps in providing next generation
Data Layered Architecture that addresses operator’s needs regarding centralization of subscriber data
used for authentication across multiple layers of the same session.

The SIM/USIM is the security token in 2G/3G networks for authenticating a subscriber in the operator’s
HLR. The USIM will continue to provide user authentication function at the access level to the LTE
network.

However, it is necessary to define a mechanism for service level authentication as well, for services such
as the IMS. The ISIM (an application on the UICC) is defined to hold both the user’s access level
credentials and the IMS Private User Identity that is stored in the HSS. The ISIM enables the user to
authenticate to the LTE operator’s IMS network and access its services.

3.4ROAMINGSCENARIOS
There are two roaming scenarios based on home tunneling compared to home tunneling with the
possibility of local breakout as shown in Figure 9. The 2G/3G networks refer to legacy CS or PC based
accesses like GSM/WCDMA in the network.

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2G/3G
SGi
IP networks
. Basic case: home tunnelling
Other
accesses
S8
PDN GWHome
PLMN
Visited
PLMN
LTE
Serv GW
S7S9SGiSGiS7accesses.Advanced case: bothhome tunnellingand local
breakout possibleS8hPCRF
S7PDN
GW
Serv GW
vPCRF
S9
SGi
IP networks
SGi
IP networks
PDN GW
S7
2G/3G Other
accesses
. Advanced case: both home tunnelling and local
breakout possible
S8
LTE 2G/3G
SGi
IP networks
. Basic case: home tunnelling
Other
accesses
S8
PDN GWHome
PLMN
Visited
PLMN
LTE
Serv GW
S7S9SGiSGiS7accesses.Advanced case: bothhome tunnellingand local
breakout possibleS8hPCRF
S7PDN
GW
Serv GW
vPCRF
S9
SGi
IP networks
SGi
IP networks
PDN GW
S7
2G/3G Other
accesses
. Advanced case: both home tunnelling and local
breakout possible
S8
LTE
Figure 9: Home Tunneling (Routed) vs. Home Tunneling with possibility of local breakout

HOME
TUNNELING
OR
HOME
ROUTED
SCENARIO


In this scenario, a subscriber that roams into a partner network, referred to here as Visited PLMN is still
served by its Home PLMN. That is, the subscriber’s IP address is assigned by the P-GW located in the
Home PLMN. This enables all traffic to be routed to/from the Home PLMN. Similarly, the policies for
setting QoS priority as well as other charging are directly set by the hPCRF.

HOME
TUNNELING
ALONG
WITH
POSSIBILITY
OF
LOCAL
BREAKOUT


In this scenario, the idea is that some services such as VoIP that are delay sensitive need not be home
routed. Similarly, besides delay sensitive flows, other flows that need not be seen by the home network
could also be routed directly from the P-GW located in the Visited PLMN through the mechanism called
the “local breakout.” The idea is that depending on whether a flow needs to be home routed, it will be
assigned an IP address from different P-GWs, one located in the Visited PLMN and another located in the
Home PLMN.

Corresponding to the hPLMN and vPLMN the standard has also defined hPCRF and vPCRF respectively.
This section will explain the S7 and S9 interfaces and details around vPCRF and hPCRF. S7 interface is
used between the P-GW and PCRF in general. However, hPCRF communicates its policies for a given
subscriber that will be assigned an IP address from the vPLMN for local breakout by using S9 interface
between hPCRF and vPCRF.

4QOSIN3GPP
QoS is an important area for the successful growth of wireless networks. Applications have various
requirements from the transport network in terms of delay, bandwidth and error rate that they desire for
optimal performance or user experience. This poses a challenge for deployment of wireless networks.

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However, it poses an even greater challenge when different access networks with varying capabilities
have to coexist and a user tries to move seamlessly across these access networks with an expectation
that there is no impact to the service or application one is using. LTE Rel-8 has made significant strides
in defining QoS4 capabilities and a policy framework to support those capabilities. It also allows multiple
IP-CAN sessions similar to multiple PDP contexts in pre-Rel-8 systems. In both cases, IP addresses are
bound to IP-CAN sessions or PDP contexts.

Devices in an LTE system are assigned a data bearer and attach to the system. This is unlike previous
mobile broadband technologies, where data bearers were assigned at request from the device.
Therefore, LTE is considered an “always-on” technology.

Each IP-CAN session can support multiple IP-CAN bearers which in turn can support multiple service
data flows. Each IP-CAN bearer is considered to be an independent bearer with separate QoS Class
Identifier (QCI) and other defining attributes. Broadly speaking there are two types of bearers that have
been defined in the LTE Rel-8 network: Guaranteed Bit Rate and Non-Guaranteed Bit Rate.

Guaranteed Bit Rate (GBR) is where network resources equivalent to a certain bit rate are reserved at the
time of the creation of the bearer. This type of bearer requires active management of its attributes through
the lifetime of the bearer and hence a need to support dynamic management of policy rules that govern
such bearers. Also, this type of bearer is typically used for conversational or streaming applications where
certain bit rate has to be maintained for the duration of the session. It is important to note that even
though GBR bearers guarantee network resources by reserving them well in advance, including
bandwidth for successful delivery of packets it does not guarantee against deterioration of radio
conditions due to the geographical environment. Treatment of a bearer in such conditions is determined
by the policies in a specific deployment.

The second type of bearer is Non-Guaranteed Bit Rate (Non-GBR) bearer and is typically used for
interactive applications such as IMS signaling, progressive video streaming, web browsing, chat, etc., or
background applications such as FTP and email. For this type of a bearer there is no reservation for predefined
bandwidth or bit rate. As a result for such bearers, congestion could lead to dropped packets or
delay in delivery of packets which would be considered as an expected behavior.

One significant difference between Rel-8 networks and pre-Rel-8 networks is in the way bearer flows are
established. In Rel-8, at the time of session initiation a default bearer is established per IP address
assigned to a session which is always defined as a Non-GBR bearer. However, subsequently if an
application or flow requires specific attributes for a flow, an additional dedicated bearer can be
established that is either a GBR or Non-GBR bearer. A similar capability exists for pre-Rel-8 networks
where a dedicated bearer can be seized after establishing a secondary PDP context.

One of the primary characteristics of a default bearer is that unless a bearer other than a default bearer
can be identified for a given packet it will flow over the default bearer. This also implies that in case
different QoS treatment is to be given to packets from two different applications, one or more dedicated
bearers need to be established that can distinguish the two flows separately from these two applications.
However, if for some reason the dedicated bearer is removed, all the packets identified with that bearer
will from then onwards flow over the default bearer.

From an interoperability and coexistence perspective, this underscores the need for the legacy networks
to support secondary PDP contexts otherwise during handovers from an LTE network to UMTS or GPRS

4 QoS is defined in 3GPP specification TS 23.203 and TS 23.107

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networks the dedicated bearers in an LTE network would be carried over the primary PDP context. These
primary PDP contexts will not be able to distinguish separate classification that individual dedicated
bearer packets would require. This may especially be an important issue to address since many legacy
networks may not have enabled secondary PDP context in their networks (even though the standard may
support it) which would be essential for the transfer of dedicated bearers to such networks.

4.1QOSCLASSIFICATIONANDDIFFERENCESIN3GPPNETWORKS
In Rel-8 LTE network, QCI information is explicitly signaled across the network. In comparison, in the
GPRS Rel-8 network, QCI is signaled as a vector of the pre-Rel-8 QoS parameters. As a result, this has
some impact on coexistence of networks; especially for roaming users where except for a few QoS
parameters such as Allocation Retention Priority (ARP), GBR and MBR most other parameters are
proprietary and hence will be dependent on the network operator.

Similarly, operator policies decide (for lack of any standardized mapping) the mapping between the QCI
defined by 3GPP and Diff Serve Code Point (DSCP) markings used in the IP transport network. Since in
Rel-8 existing bearers can be modified, the Gateways should be appropriately able to modify
corresponding DSCP markings in the downlink and RAN elements such as eNodeB should be able to
modify corresponding uplink packets markings.

QOS
CLASSIFICATION
IN
LTE
REL.8


For each of the GBR and Non-GBR type of bearers, several QCI are defined to further characterize these
bearers with attributes such as priority, packet delay budget, and packet error loss rate. In all, nine QoS
classes have been defined in the Rel-8 network as shown in Table 1.

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Table 1: Standardized QCI characteristics for EPC Rel-8 Network5


To provide traffic separation in the uplink, an operator-configurable mapping is offered between QCIs and
Logical Channel Groups (LCGs), in addition to multiple bearers. LCGs are also referred to as radio bearer
groups.

Service prioritization is enabled by mapping QCIs to logical channel priorities used by the User
Equipment (UE) for uplink rate control. The following table maps the QoS class identifier (QCI) to different
bearers.

From a coexistence standpoint, LTE Rel-8 QoS definitions are not the same as the corresponding preRel-
8 descriptions. For that reason some level of mapping between the corresponding QoS classes is
required to be standardized for bearers that move between the two types of access networks. UMTS

5 3GPP 23.203, Policy and charging control architecture

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defines four QoS classes: conversational, streaming, interactive best effort, and background best effort.
They have been mapped to the nine QCI classes defined under the Rel-8 PCC architecture for GPRS.

QOS
CLASSIFICATION
IN
UMTS
AND
GPRS
NETWORKS


In UMTS, there are four basic traffic classes that had been defined6. They are: conversational, streaming,
interactive, and background. To enable seamless mobility of bearers across UMTS and GPRS access
networks, a one-to-one mapping was defined as shown in Table 2.

Table 2: Recommended mapping for GPRS QoS Class Identifier to/from UMTS QoS parameters

GPRS
QoS
Class
Identifier
value

1

2

3

4

5

6

7

8

9

Traffic Class

Conversational

Conversational

Streaming

Streaming

Interactive

Interactive

Interactive

Interactive

Background

UMTS QoS parameters

Traffic
Handling
Priority


n/a

n/a

n/a

n/a

1

1

2

3

n/a

Signalling
Indication


n/a

n/a

n/a

n/a

Yes

No

No

No

n/a

Source
Statistics
Descriptor


Speech
(NOTE)

Unknown

Speech
(NOTE)

Unknown

n/a

n/a

n/a

n/a

n/a

NOTE: The operator\'s configuration should reserve QCI values that map to \"speech\" for
service data flows consisting of speech (and the associated RTCP) only.

4.2NETWORK.INITIATEDVS.TERMINAL.INITIATEDQOSCONTROLPARADIGMS
There are two distinct paradigms to request established dedicated bearers in wireless networks. They are
terminal-initiated and network-initiated QoS control paradigms. The earlier 3GPP networks only supported
terminal-initiated QoS control. With the definition of Rel-7 GPRS and then later in Rel-8 EPC, network-
initiated QoS control was introduced.

In a terminal-initiated QoS control paradigm, a UE is responsible for requesting for specific bearer
attributes. This, in turn, implies that applications running on a UE be able to access radio channel
capabilities as well as be able to request specific bearer attributes at the time of flow establishment.
These in turn would have to be communicated to the radio access network (RAN). Typically, such QoS
requests are performed using terminal vendor-specific application programming interface (API) calls and
could vary for each terminal type.

6 Quality of Service (QoS) concept and architecture in UMTS is described in 3GPP TS 23.107

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On the other hand, a network-initiated QoS control paradigm relies on the QoS request being generated
from the network side. This request could be generated by the PCEF located at the gateway or even from
the policy infrastructure such as PCRF. As more and more smartphones are being introduced by the
operators, this paradigm has become even more attractive. Traditionally, web browsing has mostly been
operated over best effort bearers. However, with the advent of specialized applications running on
smartphones, the network operator can initiate and provide specialized QoS to an individual bearer that
meets the requirements of the application.

Network-initiated QoS control lessens the role of terminal for QoS and policy control. This is applicable
both for operator controlled applications such as IMS based voice, streaming TV, etc., and other third
party application and content providers (ACP). Some of the advantages of network-initiated QoS control
are:

.
An application provider can request specific QoS for a given bearer from the network which could
be access agnostic and independent of the terminal type. Such requests would typically be from
specialized applications that communicate with their corresponding network-based servers rather
than for simple web browsing. This is turn will foster a richer set of applications due to ease in
development over multiple platforms

.
The above also helps where an application may run on a UE that is separate from the radio
modem such as laptop etc.

.
It enables more consistent exception-handling procedures that can be deployed by the network
operator. For example, in case there is not enough bandwidth or other resources available at the
time of request, the network can subsequently fulfill the request based on network policies without
the application generating another request. The same can be accomplished for any modifications
to the QoS of dedicated bearers based on network conditions as result of other bearers in the
APN

As can be seen, both QoS control paradigms can be useful. However, as the set of applications running
on today’s wireless networks become more complex and have richer QoS requirements, network-initiated
paradigm has distinct advantages over the other.

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5LTESERVICES5LTESERVICES
This chapter gives an overview of the different standards and industry initiatives to provide telephony over
LTE access. The recommendations are to use 3GPP standards when introducing VoLTE access. The
reason for this is to avoid fragmentation on the terminal market as well as to optimize the integration
efforts by introducing IMS from start in core instead of taking intermediate steps before IMS Voice is
introduced.

5.1SERVICESOVERLTE
Multiple services are envisioned for continuity of end-user experience as LTE is introduced to an existing
3GPP network. 3GPP has evaluated multiple solutions for voice over mobile broadband, including voice
over HSPA+. The result has been a standard defining the minimum requirements for VoLTE, as defined
in IR.92. Table 3 below describes 3GPP defined standards for voice and SMS over EPS as well as the
transition solution in areas where LTE coverage is not available.

Table 3: 3GPP defined standards for voice and SMS over LTE

Service
Capability
Prior
to
LTE
introduction
Potential
intermediary
step
at
LTE
introduction
On
LTE
Introduction
Basic
Voice
Service
Provided
by
CS
Domain
Provided
by
CS
domain
after
CS
Fallback
Provided
by
IMS
Supplementary
Services
Provided
by
CS
Domain
Provided
by
CS
domain
after
CS
Fallback
IMS
MMTEL;
possibility
for
IMS
MMTEL
for
2G/3G
access
using
IMS
Centralized
Services
SMS
Provided
by
CS
Domain
SMS
over
SGs
SMS
over
IP
or
SMS
over
SGs
Emergency
Calls
Provided
by
CS
Domain
Provided
by
CS
Domain
IMS
Emergency
Calls,
or
E911
provided
by
CS
Domain
using
CS
Fallback
or
by
UE
autonomously
RAT
switching
Service
Continuity
Provided
by
CS
Domain
Provided
by
CS
Domain
after
CS
Fallback
or
VoIP
over
PS
access
PS
Handover
for
mobility
within
LTE;
SRVCC
for
mobility
to
2G/3G
access.


CSFB: This is a network and device based mechanism by which devices active on LTE are required to
re-tune to 2G/3G. Therefore they perform CS (network) Fallback (CSFB) from LTE to access legacy CS
based services like voice and SMS. LTE/EPC provides a mechanism to page the subscriber over the
LTE access.

SRVCC: This is a network based mechanism by which a single radio voice call continuity (SRVCC)
feature ensures voice call continuity between IMS over PS access and CS access for calls that are

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anchored in IMS when the UE is capable of transmitting/receiving on only one of those access networks
at a given time.

To implement SRVCC in a 3GPP network7, from E-UTRAN to UTRAN/GERAN, MME first receives the
handover request from E-UTRAN with the indication that this is for SRVCC handling, and then triggers
the SRVCC procedure with the MSC Server enhanced with SRVCC via the Sv reference point. There is
no interworking function between the MME and the MSC.

Deployment options for Voice over LTE will depend on each operator’s specific scenario. Operators have
different starting points which will influence the deployment strategy, i.e., operators may start with:

.
Voice support in LTE using CSFB8 or

.
IMS Voice support in LTE including support for SRVCC; CSFB support for inbound roamers

Also the frequencies that will be used for the LTE build out will heavily influence the time it takes to
achieve mainly continuous or even full LTE coverage.

Figure 10 illustrates a scenario for overlaying of LTE over an existing 2G/3G network:


Figure 10: Illustrative Scenario of Voice over LTE migration over time

Initially LTE coverage is non-contiguous. For continuity of 2G/3G wireless services, it is required that all
CS based services use CS Fallback. This requires that devices re-tune away from LTE towards 2G/3G
and use legacy wireless and core networks for services.

In continuous LTE coverage, IMS based VoIP and Multimedia servers are used to replace legacy CS
voice with VoIP and Multimedia. As LTE coverage increases and LTE voice is supported by IMS, and to
provide contiguous service in some areas when UE moves out of LTE coverage, SRVCC feature in the
network ensure continuity in voice between PS based LTE and CS based 2G/3G networks in the LTE
contiguous areas. Outside the contiguous LTE coverage, CSFB is used to provide legacy services.

In case of mainly continuous LTE coverage, PS based handover (PS HO) and/or SRVCC is used to
maintain VoIP and legacy services between LTE areas.

Some operators consider upgrading 3G networks as well supporting IMS Voice by deploying IMS
Centralized Services (ICS). This provides communication services such that all services, and service
control, are based on IMS mechanism and enablers, complementing LTE coverage, either when

7 SRVCC: http://www.3gpp.org/ftp/Specs/archive/23_series/23.216/
8 CSFB: http://www.3gpp.org/ftp/Specs/archive/23_series/23.272/

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introducing IMS Voice on LTE or thereafter. In this case PS HO must be supported between LTE and 3G
for seamless service experience.

5.2CIRCUITSWITCHEDFALLBACK
CS Fallback is a concept for offering CS domain services together with LTE / E-UTRAN radio.

A CS Fallback capable UE, which is attached to E-UTRAN, may use GERAN or UTRAN to establish CS
services. The function has been standardized as part of 3GPP Rel-8.


Figure 11: EPS architecture for CS fallback and SMS over SGs

Figure 11 shows the interfaces related to support for legacy voice and SMS over different accesses.

The need for CSFB arises in multiple scenarios which include:

.
To provide voice services to LTE capable terminals before an operator has launched IMS Voice
over LTE

.
In initial stages, as a complement to IMS MMTel in order to support emergency calls and SMS
when they are not yet supported in EPC/LTE network

.
As an intermediate solution to retain current roaming relationship before IMS roaming
agreements are settled, since that will require some time

.
To steer (PS based) voice traffic from LTE to an overlay GSM or WCDMA network (with CS
based architecture), when LTE spectrum is not adequate to support both VoIP and high data
rates advertised in the market

Some operators also want to decouple the LTE rollout from IMS/MMTel and then CS Fallback is the only
standardized alternative.

For this purpose an interface is introduced between the MSC and the MME, called SGs-interface and is
based on the SCTP protocol. It provides the support for Mobility Management, Paging and SMS, as
shown in Figure 12: Circuit Switched Fall Back (CSFB).


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CS
CSCS V
VVo
ooi
iic
cce
ee

2G/3GCS CoreCSFBPhoneCALLWCDMA/GSMCS VoiceCS Voice2G/3G
CS Co r e
CSFB
Ph o n e CALL
WCDMA/GSMWCDMA/GSMCS VoiceCS Voice
EvolvedPacketCoreCSFBPhoneLTESGsPAGINGEvolvedPacketCoreEv o l v ed
Pa c k et
Co r e
CSFB
Ph o n e
LTE
SGs
PAGING
Figure 12: Circuit Switched Fall Back (CSFB)

A prerequisite for CS Fallback is that the UE is registered in the MSC while being attached to E-UTRAN
and registered in the MME. This is achieved by using combined Mobility Management procedures for
EPS and CS.

When originating a voice call and when receiving a page for CS voice the UE is moved to 2G/3G and the
voice is sent over one of these access types. The page response is then sent from the new RAT
supporting the CS. It can be done via IRAT PSHO or RRC Connection Release with Redirect from LTE to
WCDMA (UTRAN) or GSM (GERAN) and via CCO (with or without NACC) to GSM (GERAN). The UE will
return to LTE after call completion if LTE is preferred and coverage exists.

Additionally, 3GPP has specified a special SMS handling (SMSoSGs). SMS can be sent via the SGsinterface
without doing a fallback to CS. UEs that are interested in SMS but not in other CS services (e.g.,
Laptop cards) have the possibility to attach with an SMS only option to E-UTRAN. Such a UE is able to
use the special SMS handling without the need to support the fallback to CS.

There are three main CSFB procedures:

1. Release with Redirect (without or with automatic SI exchange)
2. Packet Switched HandOver (PSHO)
3. Cell Change Order (CCO)
Depending on the CSFB procedure different network elements are affected. The CSFB function is only
possible to realize in areas where LTE coverage is overlapped with GSM, WCDMA or coverage. CSFB
allows retaining the current CS roaming relationships between operators, since CS voice is still used.

RRC
RELEASE
WITH
REDIRECT


The basic CSFB option is RRC (Radio Resource Connection) Release with Redirect defined in Rel-8. The
impacted nodes are indicated by the CSFB in the circle in Figure 13.

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With RRC Connection Release with Redirect, it will be possible to introduce CSFB without any major
updates to current GSM (GERAN) and WCDMA (UTRAN) system. The UE will return to LTE (E-UTRAN)
after call completion.

MME
MSC
SGsSGs
GSM
WCDMA
LTE
CSFB
LTELTE
MSC
CSFB CSFB
CSFB
HLR
HSS
2G/3G
CSFB
Figure 13: Release with redirect

Impacted nodes are UE, eNB, MME and MSC. No impact on GSM or WCDMA. The main characteristics
of this procedure are:

.
Slower call set-up time because broadcast SI (System Information) needs to be read by the UE in
the target RAN

.
Long PS outage time for WCDMA or DTM (Dual Transfer Mode) GSM

ENHANCED
RELEASE
WITH
REDIRECT,
AUTOMATIC
SYSTEM
INFO
EXCHANGE
WITH
RIM


The enhanced option in Rel-9 may use the standardized RIM (RAN Information Management)
procedures9 to transfer System Information (SI) from UTRAN/GSM and WCDMA to LTE10. This requires
additional impact on the total network. 3GPP impacts the nodes indicated by RIM in the circle in Figure

14. It is recommended that RIM is used for GSM, and deferred measurement11 is used in WCDMA.
9 Currently, there are procedures defined on the Gb and Gn interfaces to enable signaling of GERAN SI/PSI ([Packet] System
Information) between BSSs. This RAN Information Management (RIM) mechanism was defined initially for the use of NACC,
although in a manner that could be extended for applications other than NACC. It consists of the following messages:

-RAN INFORMATION REQUEST - from Source BSS to Target BSS . requests GERAN SI/PSI.
-RAN INFORMATION . from target BSS to source BSS . analogous to the Information Exchange over Iur and
includes GERAN SI/PSI for one or more GERAN cells.
-RAN INFORMATION ACKNOWLEDGE . from Source BSS to Target BSS.
-RAN INFORMATION ERROR -to inform about e.g. message syntax errors.
10 http://www.3gpp.org/ftp/Specs/archive/36_series/36.410/
11
http://www.3gpp.org/ftp/specs/archive/25_series/25.331/
.
When
active,
the
UE
can
transmit
RRC
messages
on
RACH
and
receive
RRC
messages
commanding
it
to
enter
CELL_DCH
with
reading
a
subset
of
overhead
messages


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RIM needs to be rolled-out in the source and target RAN as well as for SGSN and MME. SGSN and MME
will be needed to route the RIM container to the correct target node.

MME
MSC
SGsSGs
GSM
WCDMA
LTE
CSFB
LTELTE
MSC
CSFB
CSFB
CSFB
HLR
HSS
RNC
CSFB
RIM procedures
SMS
SGSN
RIM procedures
RIM procedures
RIM RIM
RIM
RIM
BSC
RIM
Figure 14: Enhanced Release with redirect using RIM procedures

Additional Nodes impacted include RIM support in RNC, BSC, eNB, SGSN and MME. In addition a
network Impact is that RIM has to be rolled out in RAN and PCN. The benefit is faster call set-up times
compared to the non-enhanced solution.

PS
HANDOVER
BETWEEN
LTE
AND
GERAN
FOR
CSFB


Another alternative option to the RRC release is CSFB12 based on PSHO13. The impacted nodes are
indicated by the PS-HO in the circle in Figure 15. When the device originates a call or receives a page for
a terminating call, it checks if the underlying network is PSHO capable.

If the underlying network is PSHO capable, the UE requests a PS Handover (with indication of CSFB) to
the underlying UTRAN or GERAN network. On completion of the PSHO, the UE requests the network to
suspend the newly acquired data bearer. The UE then sets up a CS bearer to handle an originating voice
call, or respond to the MSC with a CS Paging response. The MSC continues with CS call setup over a
CS bearer. If the underlying network is not PSHO capable, procedures using Cell Change Order are
executed.

When PSHO is supported, the PS bearer is prepared first in the target cell before CSFB takes place. The
benefit is a shorter service interruption time.

12 Sec. 6 and Sec. 7 of http://www.3gpp.org/ftp/Specs/archive/23_series/23.272/
13 Description of PSHO: http://www.3gpp.org/ftp/Specs/archive/23_series/23.401/

4G
Americas
.
Coexistence
of
GSM.HSPA.LTE.May
2011
27



MME
MSC
SGsSGs
GSM
WCDMA
LTE
CSFB
LTELTE
MSC
CSFB CSFB
CSFB
HLR
HSS
CSFB
RN
PSHO
PSHO PSHO
SGSN
PSHO
BSC
PSHO MME
MSC
SGsSGs
GSM
WCDMA
LTE
CSFB
LTELTE
MSC
CSFB CSFB
CSFB
HLR
HSS
CSFB
RN
PSHO
PSHO PSHO
SGSN
PSHO
BSC
PSHO
Figure 15: PS Handover between LTE and GERAN for CSFB

Additional node impact is the support of PSHO with CSFB from LTE to BSC. The benefit includes shorter
PS outage time during CSFB IRAT handover.

CELL
CHANGE
ORDER


At reception of the request from the MME to page the UE, if the UE and network support inter-RAT cell
change order to GERAN and the target cell is GERAN, and PSHO is not supported to GERAN:

.
The eNodeB can trigger an inter RAT cell change order (optionally with NACC) to a GERAN
neighbor cell by sending an RRC message to the UE.

.
The inter-RAT cell change order may contain a CS Fallback Indicator which indicates to UE that
the cell change order is triggered due to a CS fallback request.

.
The UE moves to the new cell in GERAN. The UE uses the NACC information and/or receives
the broadcast System Information and when it has the necessary information to access the
GERAN cell, establishes a radio signaling connection.

5.3IMSVOICE
This section discusses the IMS Multimedia Telephony specified by 3GPP in order to provide a flexible
and enriched service. It includes traditional supplementary services that are enhanced as well as services
beyond voice like messaging, video, file sharing, etc. The supplementary services are specified in 3GPP
TS 22.173.

INTRODUCTION
TO
IMS


The IP Multimedia Subsystem (IMS) is an architectural framework for delivering Internet
Protocol (IP) multimedia services. To ease the integration with the Internet, IMS uses IETF protocols
wherever possible, e.g. Session Initiation Protocol (SIP). SIP is a text based protocol that is extensible
with parameters that describe the type of multimedia session being established.

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