This document discusses LTE CS Fallback features which allow LTE networks to reuse CS infrastructure to provide voice and other circuit switched services. CS Fallback enables LTE terminals to redirect to 2G/3G networks when initiating CS services like voice calls. The key aspects covered include the CS Fallback network architecture using the SGs interface, the combined attach procedure used for location updates, advantages/disadvantages of different CS Fallback mechanisms, and signaling flows for CS Fallback and paging.

1. LTE eRAN3.0 CS Fallback Feature
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2. LTE eRAN3.0 CS Fallback Feature
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3. LTE eRAN3.0 CS Fallback Feature
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4. LTE eRAN3.0 CS Fallback Feature
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5.  CS Fallback has a simpler network architecture compared with VoIP over IMS.
 SRVCC is short for Single Radio Voice Call Continuity
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6.  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 is redirected to 2G/3G network after it initiates a CS service
such as voice call .
 CS Fallback applies to scenarios in which the coverage area of the
UTRAN/GERAN/CDMA2000 1xRTT network is the same as or includes that of the E-UTRAN.
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7. LTE eRAN3.0 CS Fallback Feature
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8.  To support CS Fallback, the SGs interface is required, so as to let the MME perform a UE
location update over the SGs interface so that the core network of the UTRAN or GERAN
learns about the UE location.
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9.  After a UE is powered on in the E-UTRAN, it initiates a combined EPS/IMSI attach
procedure.
 If a UE is camping on an E-UTRAN cell, it periodically initiates a combined TAU/LAU
procedure, which allows for simultaneous UE location updates both in the MME and in the
core network of the UTRAN or GERAN.
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10.  The Combined EPS/IMSI Attach Procedure is shown in the previous slide:
 After the RRC connection setup, the UE sends an Attach Request message to the
MME, requesting a combined EPS/IMSI attach procedure. This message also
indicates whether the CS Fallback or SMS over SGs function is required.
 The MME allocates an LAI to the UE, and then it finds the MSC/VLR for the UE
based on the TAI-LAI mapping. If multiple PLMNs are available for the CS domain,
the MME selects a CS PLMN based on the selected PLMN information reported by
the eNodeB. Then, the MME sends the MSC/VLR a Location Update Request
message over the SGs interface so that the core network of the UTRAN or GERAN
learns about the UE location, which contains the new LAI, IMSI, MME name, and
location update type.
 The MSC/VLR performs the location update procedure in the CS domain.
 The MSC/VLR responds with a “Location Update Accept” message that contains
information about the VLR and temporary mobile subscriber identity (TMSI). The
location update procedure is successful.
 At last, the UE is informed that the combined EPS/IMSI attach procedure is
successful by RRC Connection Reconfiguration message. (If the network supports
SMS over SGs but not CS Fallback, the message transmitted to the UE contains the
information element (IE) SMS-only. The message indicates that the combined
EPS/IMSI attach procedure is successful but only SMS services are supported.)
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11.  This table lists the advantages and disadvantages of the preceding mechanisms in terms of
impacts on the UEs, impacts on the networks, CS service access delay, and PS service
interruption time.
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12.  Once a LTE UE initiates a voice request, MME informs the eNodeB to perform a CS Fallback
procedure, which is shown in the slide:
 The UE sends the MME an NAS message Extended Service Request to initiate a CS service.
 The MME sends an S1-AP Request message to instruct the eNodeB to initiate a CS Fallback
procedure (If the MME supports the LAI-related feature, the MME also delivers the LAI to
the eNodeB).
 The eNodeB determines whether to perform a blind redirection based on the setting of
BlindHoSwitch under the HoModeSwitch parameter, as follows:
 If BlindHoSwitch is turned off, the eNodeB instructs the UE to perform inter-RAT
measurements. Then, the UE sends measurement reports to the eNodeB.
 If BlindHoSwitch is turned on, the eNodeB does not instruct the UE to perform
inter-RAT measurements.
 The eNodeB sends an RRC Connection Release message to instruct the UE to perform a
redirection. The message contains information about a target UTRAN frequency. If flash CS
Fallback is available, the RRC Connection release message includes information about a
target UTRAN frequency,PSC and their associated system information, In this way, the UE
can quickly access the target UTRAN without the need to perform the procedure for
acquiring system information of the target UTRAN cell. Then, the UE can directly initiate a
CS service in the UTRAN cell.
 Then, the eNodeB initiates an S1 UE context release procedure.
 The UE may initiate an LAU, a combined RAU/LAU, or both an RAU and an LAU in the
target cell and initiates a CS call establishment procedure in the target UTRAN cell.
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13.  UE Uplink Transfer Message:
 The message is sent by UE to MME to request a CS service.
 The exact request information is in the NAS PDU.
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14.  UE Context Modification Request:
 The message is sent by MME to eNodeB to request a CS Fallback.
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15.  UE Context Release Request:
 The message is sent by eNodeB to MME to release the UE from EUTRAN.
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16.  RRC Connection Release:
 The message is sent by eNodeB to UE to request a “Redirection”, that is to execute a CS
Fallback.
 In the release message, eNodeB instructs the UE to a Inter-RAT system(UMTS in this case).
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17.  Flash CS Fallback is defined in 3Gpp R9 .With this function, SIB can be included into
the ”RRC connection Release” during the redirection procedure. This is achived by the RIM
procedure. with RIM, eNodeB can get information from GERAN/UMTS.
 The RIM procedure supports two information exchange modes: Single Report and Multiple
Report. In Single Report mode, the source sends a request, and then the target responds
with a single report. In Multiple Report mode, the target responds with a report after
receiving a request from the source, and it also sends a report to the source each time the
system information changes.
 The Multiple Report mode is enabled or disabled by setting the RimSwitch parameter,
while the Single Report mode is not configurable
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18.  The RIM procedure and parameter settings are described as follows:
 If a neighboring UTRAN cell supports the Multiple Report mode and
UTRAN_RIM_SWITCH under the RimSwitch parameter is turned on, the RIM
procedure in Multiple Report mode is performed as follows: After an E-UTRAN cell
is set up, the eNodeB sends a request for system information to neighboring
UTRAN cells. After a neighboring UTRAN cell receives a request or the system
information changes, this cell sends the system information to the eNodeB.
 If an eNodeB supports flash CS Fallback, it requires the system information of
neighboring UTRAN cells to perform a redirection. If the serving cell does not have
that information, the eNodeB must initiate an RIM procedure in Single Report
mode to acquire the system information.
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19.  Once a LTE UE initiates a voice request, MME informs the eNodeB to perform a CS Fallback
procedure, which is shown in the slide:
 The UE sends the MME an NAS message Extended Service Request to initiate a CS
service.
 The MME sends an S1-AP Request message to instruct the eNodeB to initiate a CS
Fallback procedure (If the MME supports the LAI-related feature, the MME also
delivers the LAI to the eNodeB).
 The eNodeB determines whether to perform a blind redirection based on the
setting of BlindHoSwitch under the HoModeSwitch parameter, as follows:
 If BlindHoSwitch is turned off, the eNodeB instructs the UE to perform
inter-RAT measurements. Then, the UE sends measurement reports to the
eNodeB.
 If BlindHoSwitch is turned on, the eNodeB does not instruct the UE to
perform inter-RAT measurements.
 The eNodeB initiates the preparation phase for a PS handover. If the preparation is
successful, the eNodeB instructs the UE to perform a handover.
 After the handover, the UE may initiate a CS call establishment procedure with an
LAU or combined RAU/LAU procedure in the UTRAN.
 The UE’s context in EPS is released.
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20.  Once a LTE UE initiates a voice request, MME informs the eNodeB to perform a CS Fallback
procedure, which is shown in the slide:
 The MSC sends a Paging Request message from the CS domain to the MME over
the SGs interface. Then, either of the following occurs:
 If the UE is in idle mode, the MME sends a Paging message to the eNodeB.
Then the eNodeB sends a Paging message over the Uu interface to inform
the UE of an incoming call from the CS domain.
 If the UE is in active mode, the MME sends the UE an NAS message to
inform the UE of an incoming call from the CS domain.
 The UE sends an Extended Service Request message containing a CS Fallback
Indicator after receiving the paging message from the CS domain.
 The MME instructs the eNodeB over the S1 interface to perform CS Fallback.
 The subsequent steps are similar to the originated CS Fallback to UTRAN. The
difference is that the UE sends a Paging Response message from the UTRAN cell.
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21.  This table lists the advantages and disadvantages of the preceding mechanisms in terms of
impacts on the UEs, impacts on the networks, CS service access delay, and PS service
interruption time.
 The procedures of CS Fallback to GERAN are similar with those of to UTRAN, just the
CCO/NACC is particularly for GSM.
 During CS Fallback based on CCO/NACC, the eNodeB receives a CS Fallback Indicator from
the MME, and then it sends a Mobility From EUTRA Command message to the UE over the
Uu interface. The message contains information about the operating frequency, ID, and
system information of a target GERAN cell. The UE searches for the target cell based on
the information it received, and then it performs initial access to the cell to initiate a CS
service.
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22.  The Cell Change order (CCO) procedure with Network Assisted Cell Change (NACC) is an
alternative to the RRC Connection Release with Redirection procedure used for CS Fallback.
The main difference is that the UE is moved to the target RAT whilst in RRC Connected
Mode, also MME can get some response(UE Context Required) from GSM so as to trigger
the UE context release procedure.
 In this CS Fallback procedure, the eNodeB sends a “Mobility From EUTRA Command”
message over the Uu interface to indicate the operating frequency and ID of the target
GERAN cell. If the source cell has the system information of the target cell, the system
information is also carried in the message.
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23. LTE eRAN3.0 CS Fallback Feature
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24. LTE eRAN3.0 CS Fallback Feature
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25.  The eNodeB’s configuration may affect following steps：
 1, Triggering phase
 After receiving a CS Fallback Indicator from the MME, the eNodeB decides whether
to perform CS Fallback in the form of a blind handover based on the UE capabilities
and the setting of the blind handover switch. Compared with measurement-based
handovers, blind handovers have shorter access delays but a lower handover
success rate.
 2, Measurement phase
 The eNodeB delivers the inter-RAT measurement configuration to the UE, and the
UE performs inter-RAT measurements as instructed.
 If CS Fallback fails to be performed for the UE in the form of a measurement-based
handover due to measurement timeout or exceptions, the eNodeB initiates a blind
handover.
 3, Decision phase
 The eNodeB evaluates the measurement results and generates a list of candidate
cells.
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26.  After a UE initiates a CS service in an E-UTRAN cell, the MME sends the eNodeB an S1-AP
Request message that contains a CS Fallback Indicator, notifying the eNodeB that the UE
should be transferred to the target network. The target networks to which CS Fallback can
be performed are specified by the HoAlgoSwitch parameter.
 After receiving the CS Fallback Indicator, the eNodeB checks the blind-handover switch
setting and UE capabilities for each RAT. It behaves as follows:
 If the BlindHoSwitch check box under the HoModeSwitch parameter is cleared,
blind handovers are prohibited. Under this condition, if the UE is capable of
measuring an RAT, the eNodeB delivers the inter-RAT measurement configuration
to the UE, instructing the UE to perform measurements on this RAT.
 If the BlindHoSwitch check box under the HoModeSwitch parameter is selected
or the UE is incapable of measuring an RAT, the eNodeB does not instruct the UE
to perform measurements on this RAT.
 If the eNodeB has not received any measurement reports within 3 seconds after
measurements are started for an RAT, or if a measurement-based handover to the
RAT fails to be performed for the UE due to exceptions, the eNodeB will instruct
the UE to perform a blind handover to another RAT that has not been measured.
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27.  BlindHoSwitch check box is under the Handover Mode switch parameter. If it is checked,
the eNodeB does not instruct the UE to perform measurements on this RAT, and eNodeB
selects the target for the CS Fallback.
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28.  The eNodeB checks all inter-RAT neighboring cells of the cell serving the UE and considers
the following three sets (in descending order of priority) based on the LAI:
− Set 1 consists of neighboring cells with the same LAI as the received one.
− Set 2 consists of neighboring cells with the same PLMN ID as that indicated in the
received LAI.
− Set 3 consists of neighboring cells located in the same PLMN as the serving PLMN of
the UE.
 The eNodeB selects the CS Fallback target cell from the set that has the highest priority
among all non-empty sets. If there are multiple neighboring cells in the highest-priority
non-empty set or the LAI is not indicated, the eNodeB selects the neighboring cell with the
highest blind handover priority as the CS Fallback target cell.
 The blind handover priority is based on the blind handover priority of the target RAT and
blind handover priority of the specific cell. The former takes precedence over the latter.
 If the three sets are empty, the eNodeB delivers the inter-RAT measurement configuration
to the UE. This CS Fallback will be performed in the form of a measurement-based
handover.
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29.  If the LAI is indicated by MME, The eNodeB delivers information about an inter-RAT
frequency in the measurement configuration to the UE if both the following conditions are
met:
− The PLMN ID of any inter-RAT neighboring cell on this frequency is the same as the
PLMN indicated in the LAI or the serving PLMN of the UE.
− The UE is capable of measuring this frequency.
 Note that only the frequencies that the UE is capable of measuring are delivered in the
measurement configuration.
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30.  MOD CSFALLBACKBLINDHOCFG
 By default, UTRAN is configured with the highest priority, GERAN is configured with the
second priority.
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31.  MOD UTRANNCELL
 MOD GERANNCELL
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32.  After having received the B1 measurement reports for CS Fallback, the way in which the
eNodeB determines the target cell for the CS Fallback varies depending on whether an LAI
has been received, as follows:
 If no LAI has been received, the eNodeB generates a list of candidate cells for CS
Fallback based on the received reports. It selects the cell with the highest signal
quality from this list.
 If an LAI has been received, the eNodeB considers the candidate cells in sets 1, 2,
and 3. It sorts cells in each list by signal quality, and it selects a target cell from the
three lists in descending order of priority.
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33.  MOD CSFALLBACKHO
 The received signal code power (RSCP) threshold for CS Fallback to UTRAN is specified by
CS FallbackHoUtranB1ThdRscp.
 − The Ec/No threshold for CS Fallback to UTRAN is specified by CS
FallbackHoUtranB1ThdEcn0.
 − The received signal strength indicator (RSSI) threshold for CS Fallback to GERAN
is specified by CS FallbackHoGeranB1Thd.
 − The pilot strength threshold for CS Fallback to CDMA2000 1xRTT is specified by
CS FallbackHoCdmaB1ThdPS.
 The time-to-trigger for event B1 related to CS Fallback to UTRAN/GERAN/CDMA2000
1xRTT is specified by the CS FallbackHoUtranTimeToTrig, CS
FallbackHoGeranTimeToTrig, and CS FallbackHoCdmaTimeToTrig parameters,
respectively
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34.  The triggering of event B1 indicates that the signal quality is higher than a specified threshold in at least one
neighboring cell. 3GPP TS 36.331 defines the entering and leaving conditions of event B1 as follows:
 Entering condition: Mn + Ofn - Hys > Thresh
 Leaving condition: Mn + Ofn + Hys < Thresh
 Where:
Mn is the measurement result of the neighboring cell.
Ofn is the frequency-specific offset for the frequency of the neighboring cell. It is
contained in the associated measurement object IE in the measurement configuration.
Hys is the hysteresis for event B1. The hysteresis values for CS Fallback to UTRAN,
GERAN, and CDMA2000 1xRTT are contained in the measurement configuration.
Thresh is the threshold for event B1.Thresh are set based on the measurement
quantities for each RAT.
 If the entering condition of event B1 is continuously met during a period known as time-to-trigger, the UE
reports information about the cells that meet the condition to the eNodeB.
 The following parameters related to event B1 for CS Fallback are the same as those related to event B1 for
inter-RAT handovers:
 Frequency-specific offset for the frequency of the neighboring cell
 Hysteresis
 Reporting interval
 Maximum number of cells contained in one report
 Number of periodic reports
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35. LTE eRAN3.0 CS Fallback Feature
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36.  Parameter description
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MO Parameter ID Description
GeranNcell BlindHoPriority
Indicates the priority of the neighboring cell during blind
handovers. Blind handover is a process in which the
eNodeB instructs a UE to hand over to a specified
neighboring cell. There are 32 priorities altogether. The
priority has a positive correlation with the value of this
parameter. Note that the value 0 indicates that blind
handovers to the neighboring cell are not allowed.
UtranNCell BlindHoPriority
Indicates the priority of the neighboring cell during blind
handovers. Blind handover is a process in which the
eNodeB instructs a UE to hand over to a specified
neighboring cell. There are 32 priorities altogether. The
priority has a positive correlation with the value of this
parameter. Note that the value 0 indicates that blind
handovers to the neighboring cell are not allowed.
CSFallBack
Ho
CsfbHoGeranB
1Thd
Indicates the RSSI threshold for event B1 that is used in
CS fallback to GERAN. A UE sends a measurement
report related to event B1 to the eNodeB when the RSSI
in at least one GERAN cell exceeds this threshold and
other triggering conditions are met. For details, see
3GPP TS 36.331.
CSFallBack
Ho
CsfbHoGeranTi
meToTrig
Indicates the time-to-trigger for event B1 that is used in
CS fallback to GERAN. When CS fallback to GERAN is
applicable, this parameter is set for UEs and used in the
evaluation of whether to trigger event B1. When
detecting that the signal quality in at least one GERAN
cell meets the entering condition, the UE does not send
a measurement report to the eNodeB immediately.
Instead, the UE sends a report only when the signal
quality continuously meets the entering condition during
the time-to-trigger. This parameter helps decrease the
number of occasionally triggered event reports, the
average number of handovers, and the number of wrong
handovers, and thus helps prevent unnecessary
handovers. For details, see 3GPP TS 36.331.
CSFallBack
Ho
CsfbHoUtranB1
ThdEcn0
Indicates the Ec/No threshold for event B1, which is
used in CS fallback to UTRAN. When CS fallback to
UTRAN is required, this parameter is set for UEs and
used in the evaluation about whether to trigger event B1.
This parameter indicates the Ec/No requirement for the
UTRAN cells to be included in the measurement report.
A UE sends a measurement report related to event B1 to
the eNodeB when the Ec/No in at least one UTRAN cell
exceeds this threshold and other triggering conditions
are met. For a cell with large signal fading variance, set
this parameter to a large value to prevent unnecessary
handovers. For a cell with small signal fading variance,
set this parameter to a small value to ensure timely
handovers. For details, see 3GPP TS 36.331.

37.  Parameter description
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MO Parameter ID Description
CSFallBack
Ho
CsfbHoUtranB1
ThdRscp
Indicates the RSCP threshold for event B1, which is
used in CS fallback to UTRAN. When CS fallback to
UTRAN is applicable, this parameter is set for UEs and
used in the evaluation about whether to trigger event B1.
This parameter indicates the RSCP requirement for the
UTRAN cells to be included in the measurement report.
A UE sends a measurement report related to event B1 to
the eNodeB when the RSCP in at least one UTRAN cell
exceeds this threshold and other triggering conditions
are met. For details, see 3GPP TS 36.331.
CSFallBack
Ho
CsfbHoUtranTi
meToTrig
Indicates the time-to-trigger for event B1 that is used in
CS fallback to UTRAN. When CS fallback to UTRAN is
applicable, this parameter is set for UEs and used in the
evaluation of whether to trigger event B1. When
detecting that the signal quality in at least one UTRAN
cell meets the entering condition, the UE does not send
a measurement report to the eNodeB immediately.
Instead, the UE sends a report only when the signal
quality continuously meets the entering condition during
the time-to-trigger. This parameter helps decrease the
number of occasionally triggered event reports, the
average number of handovers, and the number of wrong
handovers, and thus helps prevent unnecessary
handovers. For details, see 3GPP TS 36.331.
ENodeBAlg
oSwitch
HoAlgoSwitch
Indicates the collective switch used to enable or disable
handover and CS fallback algorithms.
Flash CS fallback to UTRAN: If the switch for CS fallback
to UTRAN is turned off, this switch does not take effect;
Flash CS fallback to GERAN: If the switch for CS
fallback to GERAN is turned off, this switch does not take
effect.
ENodeBAlg
oSwitch
HoModeSwitch
Indicates the switches corresponding to the inputs based
on which the eNodeB determines handover policies.
Note that EutranVoipCapSwitch will be removed in the
later versions. In this version, the setting of this switch is
still synchronized between the M2000 and the eNodeB,
but it is no longer used internally. Therefore, avoid using
this switch.
CSFallBack
BlindHoCfg
InterRatHighest
Pri
Indicates the high-priority system to be considered in
blind handovers. It is UTRAN by default.

38.  Parameter description
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MO Parameter ID Description
CSFallBack
BlindHoCfg
InterRatLowest
Pri
Indicates the low-priority system to be considered in
blind handovers. It is CDMA2000 by default. If this
parameter is set to NULL, the system with the low blind
handover priority is not specified. In this case, only blind
handovers to the high- and medium-priority systems are
applicable.
CSFallBack
BlindHoCfg
InterRatSecond
Pri
Indicates the medium-priority system to be considered in
blind handovers. It is GERAN by default. If this
parameter is set to NULL, the system with the medium
blind handover priority is not specified. In this case, only
blind handovers to the high-priority system are
applicable.
CSFallBack
Ho
LocalCellId
Indicates the local ID of the cell. It uniquely identifies a
cell within a BS.
ENodeBAlg
oSwitch
RimSwitch
Indicates the collective switch for the RAN information
management (RIM) function.
UTRAN_RIM_SWITCH: Indicates the switch used to
enable or disable the RIM procedure that requests
event-driven multiple reports from UTRAN cells.
If this switch is turned on, the eNodeB can send RAN-
INFORMATION-REQUEST protocol data units (PDUs) to
UTRAN cells to request multiple event-driven reports.
If this switch is turned off, the eNodeB cannot send RAN-
INFORMATION-REQUEST PDUs to UTRAN cells to
request multiple event-driven reports.
GERAN_RIM_SWITCH: Indicates the switch used to
enable or disable the RIM procedure that requests
event-driven multiple reports from GERAN cells.
If this switch is turned on, the eNodeB can send RAN-
INFORMATION-REQUEST PDUs to CERAN cells to
request multiple event-driven reports.
If this switch is turned off, the eNodeB cannot send RAN-
INFORMATION-REQUEST PDUs to GERAN cells to
request multiple event-driven reports.

39. LTE eRAN3.0 CS Fallback Feature
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