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Reduction (ISR) feature allows the UE to move between LTE and
2G/3G without performing Tracking Area (TA) or Routing Area
(RA) updates once it has been activated. A pre-requisite for ISR
activation is that the UE, SGSN, MME, Serving GW and HSS all support
ISR. At the first attach to the network, ISR is not activated. ISR
can only be activated when the UE has first been registered in an
RA on 2G/3G and then registers in a TA or vice versa.
If the UE first registers
on GERAN/UTRAN and then moves into an LTE cell, the UE initiates
a TA update procedure. In the TA update procedure, the SGSN, MME
and Serving GW communicate their capabilities to support ISR, and
if all the nodes support ISR, the MME indicates to the UE that ISR
is activated in the TAU accept message.
(CSFB) is an alternative solution to using IMS and SRVCC to
provide voice services to users of LTE. The IMS is not part of the
solution, and voice calls are never served over LTE. Instead, the
CSFB relies on a temporary inter-system that switches between LTE
and a system where circuit-switched voice calls can be served.
The ISR feature must
be enabled for the CSFB feature to work, the ISR feature is a license controlled
The LTE terminals
'register' in the circuit switched domain when powered and attaching
to LTE. This is handled through an interaction between the MME and
the MSC-Server in the circuit-switched network domain over the SGs
Consider the following
Voice calls initiated
by the mobile user: If the user makes a voice call, the terminal
switches from a LTE system to a system with circuit-switched voice
support. Depending on where the UE latches on after completion of
the voice call:
The packet-based services
that are active on the end-user device at this time are handed over
and continue to run in a system with circuit-switched voice support
but with lower data speeds.
The packet-based services
that are active on the end-user device at this time are suspended
until the voice call is terminated and the terminal switches back
to LTE again and the packet services are resumed.
Voice calls received
by the mobile user: If there is an incoming voice call to an end-user
that is currently attached to the LTE system, the MSC-Server requests
a paging in the LTE system for the specific user. This is done through
the SGs interface between the MSC Server and the MME. The terminal
receives the page, and temporarily switches from the LTE system
to the system with circuit-switched voice support, where the voice
call is received. Once the voice call is terminated, the terminal
switches back to the LTE system.
To support CS
fallback, existing procedures are modified and some additional CS fallback
specific procedures added to the EPS. Additions are done to the "Attach" and
"TA update" procedures which activate an interface called the SGs. This
interface is between the MME and MSC. It is used by the MSC to send paging
messages for CS calls to the UE on the LTE system.
Example of a CS
Table 1. Steps in a CS
receives an incoming voice call and sends a CS page to the MME over a SGs
The MME uses
the TMSI (or IMSI) received from the MSC to find the S-TMSI (which is used as
the paging address on the LTE radio interface).
forwards the paging request to the eNodeB in the TAs where the UE is
registered. The eNodeBs perform the paging procedures in all the cells in the
message includes a special CS indicator that informs the UE that the incoming
paging is for a terminating CS call.
the paging message, the UE performs a service request procedure which
establishes the RRC connection and sends a Service Request to the MME. The
Service Request message includes a special CS Fall-back indicator that informs
the MME that the CS fallback is required.
triggers the MME to activate the bearer context in the eNodeB with an
indication to perform fallback to GERAN or UTRAN.
selects a suitable target cell, by triggering the UE to send measurements on
the neighbour cells, and initiates a handover or cell change procedure. The
selection between handover or cell change procedure is based on the target cell
capabilities and is configured in the eNodeB.
target cell is a UTRAN cell, then MME can do subscriber context transfer using
Forward Relocation Req / Rsp / Complete / Complete Ack messages and set up the
radio contexts in UTRAN a-priori. However if the target cell is GERAN, then the
SGSN currently does not support PS handover procedure and hence transfer of
radio context from MME to 2G SGSN through Fwd reloc req / rsp
/complete/complete ack procedure is not possible in the current release. In
this scenario, CSFB is performed through a RRC release at the eNodeB and then a
Suspend Request is sent to the SGSN.
handover or cell change procedure, the UE detects the new cell and establishes
a radio connection and sends a page response to the MSC, through the target
page response arrives at the MSC, a normal mobile terminated call setup
continues and CS call is activated towards the UE.
The CS fallback is
primarily supports voice calls but it also supports other CS services. In the
case of SMS services the UE need not switch to other radio interfaces. The UE
can remain on LTE and still send and receive SMSes. The SMS messages are
tunnelled between the UE and the MSC through the MME NAS signalling and the SGs
When ISR is
activated the UE is simultaneously registered at both SGSN and MME. So any
paging for CS services occurs at both the SGSN and the MME. In a network if ISR
is activated for an UE and CSFB is used in the network, the SGSN has to support
additional call flows.
The CS Fallback feature
is inter-works with the Idle Mode Signaling Reduction (ISR) feature.
The CS Fallback feature is primarily for the EPS, but at the SGSN,
it plays a role in deciding when the ISR feature should be activated
or de-activated at the SGSN.
To enable ISR for
subscriber peer nodes, the MME and SGW must support ISR functionality.
How it Works
Listed below are the
scenarios where ISR with CSFB is impacted by the SGSN, these scenarios
are applicable to both 2G and 3G when ISR is enabled:
The ISR is de-activated
(by not sending ISR active status indication in RAU Accept message
sent to UE) in the following cases:
The SGSN will not
sent the ISR activated indication at combined RAU/LAU procedure
(As per 3GPP TS23.272, section 4.3.5 ,release 11.2)
When the UE sends
a combined RAU and LAU to a S4-SGSN, the SGSN checks the "Combined
EPS/IMSI Attach Capability" bit in the "MS Network Capability"
IE received. If that bit indicates CSFB and/or SMS over
SGs is enabled for this UE, then the SGSN de-activates the ISR by
not indicating the "ISR Activated" status in RAU Accept message
sent to the UE. The SGSN in a CSFB/SMS over SGs configuration
never indicates "ISR Activated" in combined RAU procedures for CSFB/SMS
over SGs enabled UEs.
If CS Paging Indication
is received from MME for an ISR activated subscriber, the SGSN
pages to the subscriber indicating that the paging is for a CS call.
When a Mobile Terminating call arrives at the MSC/VLR (via
the G-MSC) for a UE that is camped on an E-UTRAN (ISR is active
and the SGs interface is active between MSC and MME), the MSC/VLR sends
a Page Request (SGsAP-PAGING-REQUEST) to the MME.
As ISR is active and
the UE is in ECM_IDLE state, the MME forwards the CS paging message
received from the MSC/VLR to the associated SGSN. The MME
gets the SGSN information in the regular ISR activation process.
The MME builds a "CS Paging Indication" message, which is a GTPv2
message, from the SGsAP-PAGING_REQUEST to the correct SGSN.
The SGSN receives the CS Paging Indication message from the MME,
and sends paging messages to RNS/BSSs. This information
is described in detail in 3GPP TS 23.060.
In Receive and handle
"Alert MME Notification" and send "Alert MME "Acknowledge" scenarios.
When the SGSN sends
an UE Activity Notification message over the S3 interface, if the MME
sends an Alert MME Notification earlier for the same subscriber
and the SGSN detects any UE activity (like Iu connection established
and so on).
Handling the problem
of Mobile Terminated voice calls getting dropped due to NULL SGs or
SGs association at MSC/VLR, when the implicit detach timer
expires at MME. In this case, the flag "EMM Combined UE Waiting"
is set at the SGSN, this ensures waiting for a combined procedure
(Combined RAU). A Combined RAU is forced if we receive a normal
periodic RAU (non-combined) by sending an IMSI Detach request to
UE. When a MME detaches the UE locally from E-UTRAN (due to PTAU
timer expiry and no contact with UE at E-UTRAN till the implicit
detach timer expiry at MME) it sends a Detach Notification with
cause "local detach" to the SGSN. The SGSN sets the "EMM Combined
UE Waiting" flag if UE is CSFB capable and this flag will be reset
only after combined RAU is received.
The call flow below
depicts a CS Paging example:
Table 2. Steps in a CS
Terminating call arrives at MSC/VLR (via the G-MSC) for a UE which is camped on
If the ISR
is active and the SGs interface is active between MSC and MME, then the MSC/VLR
sends a Page Request (SGsAP-PAGING-REQUEST) to the MME.
As ISR is
active and the UE is in ECM_IDLE state, the MME forwards the CS paging message
received from the MSC/VLR to the associated SGSN. The MME receives the SGSN
information in the regular ISR activation process. The MME builds a "CS Paging
Indication" message, which is a GTPv2 message, from the SGsAP-PAGING_REQUEST to
the correct SGSN.
receives the CS Paging Indication message from the MME, and sends paging
messages to RNS/BSSs.
forwards the CS Paging Indication message to the UE.
fallback or Cell re-selection process progresses.
process is complete, the UE sends a CS Paging response to the RNS/BSS.
forwards the CS Paging Response to the MSC/VLR.
For detailed information on
CS paging procedure refer to 3GPP TS 23.060.
Alert and UE
The call flow below
depicts an Alert and UE Notification scenario:
requests the MME to report activity from a specific UE. The MSC/VLR sends a
SGsAP Alert Request (IMSI) message to the MME where the UE is currently
attached to an EPS network. On receiving the SGsAP Alert Request (IMSI)
message, the MME sets a Non-EPS Alert Flag (NEAF). If NEAF is set for an UE,
the MME informs the MSC/VLR of the next activity from that UE (and the UE is
both IMSI and EPS attached) and clears the NEAF.
If ISR is
activated for this UE, an "Alert MME Notification" message (GTPv2) is created
based on above SGs message and sent on the S3 interface by the MME to the
associated SGSN, in order to receive a notification when any activity from the
UE is detected.
The SGSN sends an
"Alert MME Acknowledge" and sets the SSAF flag, the "Alert MME Acknowledge" is
a GTPv2 message to the MME in response to the Alert MME Notification message.
If any UE
Activity is detected (UE is active, after an Iu connection is established), the
SGSN sends a "UE Activity Notification message" to the MME over the S3
When the UE wants to
perform a combined RAU/LAU, the SGSN verifies the "combined EPS/IMSI attach
capability" bit in MS Network Capability and if it indicates that CSFB and/or
SMS over SGs is enabled, then the SGSN de-activates ISR. The SGSN does not
indicate that ISR is activated in the RAU Accept message.
Procedures for CSFB Capable UEs
If the MME clears a
subscriber then SGs association with the MSC is closed and leads to a drop of
voice calls from the MSC. To avoid this issue a few changes are done in SGSN to
establish the Gs association between the MSC and the SGSN on ISR de-activation.
Notification" is received from the MME with Detach Type set as "Local Detach"
and if the UE supports EMM Combined procedures then, the SGSN sends an IMSI
Detach request to the UE and sets the "EMM Combined UE Waiting" flag.
If the SGSN then
receives a Periodic RAU Request and the flag "EMM Combined UE Waiting" is set,
an IMSI Detach is sent to the UE in order to ensure that next time the UE
performs a Combined RAU. This enables Gs association between the SGSN and the
MSC/VLR and the MT voice calls are not lost.
If the SGSN
receives a Combined RAU Request when the flag "EMM Combined UE Waiting" is set,
then this flag is cleared and Gs association is activated.
Last PDN De-activation Procedure
The MS initiated
last PDN de-activation procedure is listed below:
The SGSN sends a
DSR with OI=1, the cause not set to ISR deactivated.
PDP is deleted
from the SGW and the PGW.
In SGSN all PDPs
are de-activated. The S4 association is cleared.
In SGW all PDPs
are de-activated. Both the S4 and S11 associations at the SGW are cleared.
The MME continues
to retain the S11 tunnel.
Both the SGSN and
MME retain the ISR and S3 tunnel active. The active S3 tunnel serves incoming
voice calls if SGs association is retained at the MME.
If MME has a SGs
association and if periodic TAU timer from UE expires, the MME performs the
starts an implicit detach timer. If voice call is received at MSC/VLR when this
timer is running then:
MSC/VLR sends a SGs page to the MME.
sends an S3 page to the SGSN.
pages the UE with the "CN Domain Indicator = CS domain", and if the UE responds
to the page by doing a cell re-selection to CS domain, the MSC/VLR stops
call is completed.
implicit detach timer expires:
sends an EPS Detach Notification (IMSI detach) to the MSC/VLR.
sends a Detach Notification with cause "Local detach" to the SGSN (Refer to
3GPP TS 23.272v10.08, section 5.3.2 point no. 3).
If the UE
is "combined EPS/IMSI attach capable" (as derived from MS Network capability)
and if ISR is active, the SGSN sends an IMSI detach request to the UE on
receiving Detach Notification with cause "local detach".
sets a flag called "EMM Combined UE waiting" (Refer to 3GPP TS 23.272v10.08,
IMSI detach request reaches the UE, the UE performs a Combined RAU, the "EMM
Combined UE waiting" flag is cleared at the SGSN and Gs association is
established between SGSN and MSC/VLR. ISR is deactivated at the UE.
IMSI detach request does not reach the UE, then on next signaling from the UE
based on the "EMM Combined UE waiting" flag being set, following action is
If an UE
performs a periodic RAU or NAS Service Request, then the UE is forced to do an
IMSI detach so that the UE does a Combined RAU again to establish Gs
Last PDN De-activation Procedure
Listed below are the
sequence of events which occur, if an UE is "combined EPS/IMSI attach capable"
and the last PDN is de-activated due to PGW initiated de-activation or HSS
The SGW forwards
the DBR to both the SGSN and the MME.
Both MME and SGSN
de-activate the PDN, and locally de-activate ISR (Refer to 3GPP TS 23.401
v10.08, section 220.127.116.11 (Note 2 and 3) and 3GPP TS 23.060 v10.801, section
The MME need not
send a Detach Notification to the SGSN.
scenario, where the SGSN is aware that it is a PGW initiated last PDN
de-activation, the UE is "combined EPS/IMSI attach capable" (as derived from MS
Network capability) and ISR was active earlier, the SGSN performs the following
If the UE is
in a PMM-CONNECTED state at the SGSN, then SGSN sends an IMSI detach request.
The SGSN sets a flag called "EMM Combined UE waiting". If the UE receives this
IMSI detach request, it performs a combined RAU into SGSN and at that point the
Gs association is established and the "EMM Combined UE Waiting" flag is cleared
by the SGSN.
If the UE is
in an IDLE state at the SGSN, then the SGSN pages the UE to deliver the PDP
de-activation request. If paging fails, the SGSN sets the "EMM Combined UE
Waiting" flag. When this UE performs a combined RAU to SGSN at a later time or
attaches to the SGSN, this flag is cleared.
If the UE is in
an E-UTRAN coverage area then, the MME detaches the UE and the UE is
re-attached to the network. If the UE is not in an UTRAN/GERAN coverage area,
then the SGSN pages the UE prior to sending IMSI detach. This paging request
If the UE does
not receive an E-UTRAN detach request or a paging request from the SGSN, and at
a later point if the UE returns to the SGSN with a periodic RAU / NAS Service
Request, then the SGSN performs the following:
Combined UE waiting" flag is set, this forces the UE to perform a IMSI detach
so that the UE does a Combined RAU again to establish a Gs association.
If the UE
receives the IMSI detach request sent in step (4), the UE performs a Combined
RAU to establish Gs association. On receiving a Combined RAU, the SGSN clears
the "EMM Combined UE waiting" flag.
The Idle mode
signaling reduction complies with the following standards:
3GPP TS 23.060, version
3GPP TS 23.401, version
3GPP TS 23.272, version
3GPP TS 29.274, version
Configuring ISR with
Circuit Switched Fallback
commands are used to configure 3G paging cause for CSFB:
configcontext context_nameiups-serviceiups_service_namernc id rnc_id [default | no ] ranap paging-cause-ie mme-signalling paging_cause_value end
is used to set the Paging Cause IE value for paging
from MME due to Circuit Switch Fallback (CSFB). Listed below are the paging
cause values which can be set:
Terminating conversational call
Terminating streaming call
Terminating interactive call
Terminating background call
Terminating low priority signaling
Terminating high priority signaling
command resets the specific parameters value to default. In this case it is set
to "5 - Terminating high priority signaling".
The no form of
the command suppresses the Paging Cause IE so that it is not included in
responses to Paging Requests.
trouble-shooting the CSFB feature
can be verified by executing the show command show iups-service,
the following parameter is displayed on executing the command: