- Circuit Switched Fallback (CSFB) allows LTE devices to use circuit-switched voice and SMS services through GSM or other circuit-switched networks, as LTE is an all-IP packet-based network incapable of supporting circuit-switched calls. When making or receiving a voice call or SMS, the LTE device "falls back" to the 3G or 2G network. - CSFB was specified in 3GPP Release 8 and requires upgrading operators' core and radio networks. It is considered an interim solution until Voice over LTE (VoLTE) is fully implemented for delivering voice services over LTE networks.

1. Circuit Switched FallBack
(CSFB)
Circuit Switched FallBack (CSFB) is a technology whereby voice and SMS services are
delivered to LTE devices through the use of GSM or another circuit-switched network.
Circuit Switched FallBack is needed because LTE is a packet-based all-IP network that cannot
support circuit-switched calls. When an LTE device is used to make or receive a voice call or
SMS, the device "falls back" to the 3G or 2G network to complete the call or to deliver the SMS
text message.
CSFB was specified in 3rd Generation Partnership Project (3GPP) Release 8. CSFB requires a
software upgrade of the operators core and radio network.
CSFB is often seen as an interim solution for LTE operators. Voice over LTE (VoLTE) is
considered to be the long-term goal for the delivery of voice services on LTE networks.
Circuit Switched Fallback (CSFB)
CS domain services are the services that can be offered today in GSM-UMTS networks. Examples of such
services are: voice and its supplementary services (e.g. call waiting, call forwarding), USSD, LCS, SMS,
E911, LI, and even CS DUI video, etc. This rich set of CS domain features and capabilities are the result of
years of standardization works in 3GPP and operators investments to their GSM-UMTS network.
In EPS, richer features/services can be offered to the end-user together with voice via IMS. While this is
the case for EPS, it is challenging for some operators to launch EPS with data and voice/IMS from day
one. Hence, these operators need a migration path to allow them to start from EPS with data only and
allow the reuse of CS domain services until they get to the point where IMS voice can be added to the
EPS.
Such migration path is possible with CS Fallback (CSFB) feature. CSFB is introduced in 3GPP Rel-8 to
allow an UE in EPS to reuse CS domain services by defining how the UE can switch its radio from EUTRAN
access to other RAT (e.g. GERAN/UTRAN/1xRTT access) that can support CS domain services. In addition,
CSFB specification TS 23.272 also defines how the SMS is transferred to the UE natively via EPS from the
MSC. It should be noted that this type of SMS delivery mechanism is defined in CSFB specification but
the UE is not falling back to GERAN/UTRAN/1xRTT access.

2. With CSFB, UE under EPS can enjoy the fast PS data access and can switch over to GERAN/UTRAN/1xRTT
access for CS domain services when needed. In addition, UE can also utilize the SMS feature supported
by CSFB architecture.
UE, which wants to use CSFB, must first register itself to the CS domain via EPS. For GSM-UMTS CSFB
feature, UE performs a combined EPS/IMSI Attach/TAU procedure. In the EPS Attach/TAU response
message, the network indicates back to the UE whether CSFB (including SMS) is supported, “SMS-only”,
“CSFB Not Preferred”, or none of these features are supported. “CSFB Not Preferred” is an indication to
allow data centric devices to continue reside in EPS and to allow CSFB (including SMS) features to be
used. On the other hand, a voice centric device receiving “CSFB Not Preferred” or “SMS-only” will
assume CSFB is not supported in this network and will try to reselect to other networks (i.e. 2G or 3G) to
obtain voice services. In 1xRTT CSFB features, the UE is aware that the network supports 1xCSFB by
examining the system information broadcast information over E-UTRAN access and performs the 1xCS
registration to the 1xRTT MSC via the CDMA2000 signaling tunnel between the UE (via EPS) and 1xCS
IWS. This 1xCS registration request and response is transparent to the EPS.
After the UE has successfully registered itself to the CS domain (and has received positive response from
MME that CSFB is possible in GERAN/UTRAN case), it can then request the MME to perform CSFB
procedures whenever it wants to use CS domain services (e.g. originating a voice call or answer to a
terminating voice call). Besides voice call, USSD, MO-LR, MT-LR, NI-LR, and call-independent
Supplementary Services procedures (e.g. activates CFB) can also trigger CSFB procedures. In the CS
terminating scenario, an active UE has the ability to reject terminating call request while it still resides in
EPS. This is particularly useful when the end-user is watching a streaming video under EPS and does not
want to answer a call from an unknown number to avoid any streaming disruption in the streaming
video due to unwanted CSFB procedures.
For the GSM-UMTS CSFB feature, EPS can perform the CSFB procedure with PS handover
procedure, RRC connection release with redirection information, or cell change order with NACC (for
GERAN only). This is based on network configuration and deployment option. For 1xRTT CSFB feature,
CSFB can be done with RRC connection release with redirection information or 1xSRVCC based signaling
(known as enhanced 1xCSFB). 1xRTT CSFB UE may also have dual-Rx/dual-Tx or Dual-Rx/Single-Tx
capability. Dual-Rx/dual-Tx 1xRTT CSFB UE can simultaneously transmit and receive on both EPS and 1x
at the same time. This allows the UE to obtain 1x voice service from 1xRTT system while maintaining the
data stream over EPS at the same time. This is also based on network configuration and deployment
option, and UE capability. Dual-Rx/Single-Tx 1xRTT CSFB UE allows simplification in EPS network
deployment because there is no coordination is required between the E UTRAN and 1xRTT network (i.e.
S102 is not required).

3. After the UE is redirected to GERAN/UTRAN/1xRTT access via one of the above procedures, the existing
CS setup procedure is taken over for the remaining of the call.
In Rel-9, IDLE mode camping mechanism is enhanced in the EPS and GPRS to allow the network
to influence the UE’s RAT camping policy so that a CSFB UE will select GERAN/UTRAN access when it is in
IDLE condition. The intention is to minimize the occurrence of CSFB procedure from EPS to allow the UE
to invoke the CS domain services directly from GERAN/UTRAN as much as possible. On the other hand,
this requires additional intelligence in the cell reselection policy in the GERAN/UTRAN access in order to
move the UE in active state to EPS to enjoy the fast PS access when appropriate. There are also
optimization enhancements to Rel-9 for speeding up the overall CSFB procedure.
As indicated earlier, SMS delivery via CS Domain is also defined as part of the CSFB feature. UE can
utilize this feature after it has successfully attached itself to the CS domain. It should be noted that EPS
has the option to support only the SMS feature and not the CSFB feature which redirect the UE to
another RAT. For GERAN/UTRAN CSFB, MME can indicate this condition by having an SMS-only indicator
to the UE during their combined EPS/IMSI Attach/TAU procedure. For 1xRTT CSFB, this indication is not
specified, as the 1xCS registration procedure is transparent to the EPS. UE receiving the “SMS-only”
indicator will not invoke the CSFB request and should not expect any CS paging coming from EPS.
When interworking with a 3GPP MSC, SMS is delivered via the SGs interface. For MO-SMS, UE
first establishes a NAS tunnel to transfer the SMS PDU to MME. MME then transfer these SMS PDU over
to MSC via the SGs. MT-SMS works the same way by having the MME establish a NAS tunnel to UE
over E-UTRAN access.
When interworking with 1xMSC, the UE establishes a CDMA2000 tunnel with the 1xCS IWS via EPS
and SMS is delivered via that tunnel. EPS is transparent to this process.
3GPP also defines the CSFB UE in voice-centric and data-centric mode of operation in TS 23.221.
Voicecentric CSFB UE will always attempt to find a RAT where voice services can be supported. In the
example of UE receiving an SMS-only or “CSFB Not Preferred” indication from the network during
combined EPS/IMSI attach procedure, the voice-centric UE will autonomously switch to UTRAN/GERAN
access if coverage is available so voice service is possible to this user. With a data-centric mode of
operation, the CSFB UE will not switch to UTRAN/GERAN given the same scenario with the SMS-only

4. indication from the network and will forgo the voice services or CS domain services altogether. This is
because the data-centric mode UE wants the best possible PS access and voice is not the determining
factor to move away from EPS.
Circuit Switched Fallback (CSFB): A Quick Primer
I have explained CSFB with basic signalling here and there is a very interesting Ericsson whitepaper
explaining all Voice issues in LTE here.
The following CSFB details have been taken from NTT Docomo Technical Journal:
The basic concept of CS Fallback is shown in Figure 1. Given a mobile terminal camping on LTE, a mobile
terminating voice call arrives at the terminal from the existing CS domain via EPC. On receiving a paging
message, the mobile terminal recognises that the network is calling the mobile terminal for CS-based
voice and therefore switches to 3G. The response confirming the acceptance of a call request is then
sent from the mobile terminal to the 3G-CS system, and from that point on, all call control for the voice
service is performed on the 3G side.

5. The CS Fallback consists of a function to notify a mobile terminal of a call request from the CS domain
and combined mobility management functions between CS domain and EPC for that
purpose. The network architecture of CS Fallback is shown in Figure 2.
One of the remarkable characteristics of the EPC supporting CS Fallback is that it connects the Mobile
Switching Center (MSC) and Visited Location Register (VLR) in the 3G CS domain
with the Mobility Management Entity (MME), which provides EPC mobility management functionality.
The interface connecting MSC/VLR and MME is called an SGs reference point. This
interface is based on the concept of the Gs reference point that exchanges signalling with MSC, which
connects to the Serving General Packet Radio Service Support Node (SGSN), a 3G
packet switch. The SGs provides nearly all the functions provided by the existing Gs.

6. The CS Fallback function uses this SGs reference point to transfer the mobile terminating call requests
from the CS domain to LTE. It also provides combined mobility management
between the 3G CS domain and the EPC to enable this transfer to take place.
Combined Mobility Management between CS Domain and EPC Network:
A mobile communications network must always know where a mobile terminal is located to deliver
mobile terminating service requests to the mobile user on the mobile terminating side. The procedure
for determining terminal location is called “mobility management". As a basic function of mobile
communications, 3G and LTE each provide a mobility management function.
To complete a call using the CS Fallback function, the CS domain needs to know which LTE location
registration area the mobile terminal is currently camping on. To this end, the MME must correlate

7. mobility management control of the CS domain with that of EPC and inform MSC/VLR that the mobile
terminal is present in an LTE location registration area.
The 3G core network already incorporates a function for linking mobility management of the CS domain
with that of the Packet Switched (PS) domain providing packet-switching functions. As described above,
the CS domain and PS domain functions are provided via separate switches. Thus, if combined mobility
management can be used, the mobility management procedure for the terminal only needs to be
performed once, which has the effect of reducing signal traffic in the network. This concept of combined
mobility management is appropriated by the CS Fallback function. Specifically, MSC/VLR uses the same
logic for receiving a location registration request from SGSN as that for receiving a location registration
request from MME. This achieves a more efficient combined mobility management between the CS
domain and EPC while reducing the development impact on MSC.
As described above, a mobile terminal using LTE cannot use 3G at the same time. This implies that the
MME, which contains the LTE location registration area (Tracking Area (TA)), is unable to identify which
MSC/VLR it should send the mobility management messages to from the TA alone. To solve this
problem, the mapping of TAs and 3G Location Areas (LA) within MME has been adopted. The concept
behind TA/LA mapping is shown in Figure 3. Here, MME stores a database that manages the
correspondence between physically overlapping TAs and LAs. This information is used to determine
which MSC/VLR to target for location registration.

8. The combined TA/LA update procedure for CS fallback is shown in detail in Figure 4. First, the mobile
terminal sends to the MME a Tracking Area Update (TAU) request message indicating a combined TAU
and the current TA in which the mobile terminal is currently present (Fig. 4 (1)). The MME then performs
a location update procedure towards Home Subscriber Server (HSS), which is a database used for
managing subscriber profiles (Fig. 4 (2)). Next, the MME uses the TA/LA correspondence database to
identify the corresponding LA and the MSC/VLR that is managing that area, and uses the SGs reference
point to send a Location Area Update (LAU) request message to the MSC/VLR together with the LA so
identified (Fig. 4 (3)). The MSC/VLR that receives the LAU request message stores the correspondence
between the ID of the MME originating the request and an ID such as the International Mobile
Subscriber Identity (IMSI) that identifies the subscriber (Fig. 4 (4)). This enables the MSC/VLR to know
which MME the mobile terminal is currently connected to and that the mobile terminal is camping on
LTE. Following this, the MSC/VLR performs a location registration procedure with the HSS (Fig. 4 (5)).
Finally, the MSC/VLR informs the MME of temporary user identity (Temporary Mobile Subscriber
Identity (TMSI)), which is used at the time of a mobile terminating call in the CS domain, and indicates
that location registration has been completed. The MME then informs the mobile terminal of the TMSI
and of the LA that the mobile terminal has been registered with thereby completing combined location
registration (Fig. 4 (6) (7)).

9. CS Fallback Call Control Procedures - Mobile Originating Call:
To originate a voice call using the CS Fallback function, a mobile terminal in the LTE location registration
area must first switch (fall back) to 3G. The mobile-originating voice call procedure is shown in Figure 5.
To originate a call, the mobile terminal begins by sending a CS fallback service request message to the
MME (Fig. 5 (1)). Since a packet-communications transmission path (bearer) must always exist in EPC for
the purpose of providing an always-on connection, the bearer also has to be handed over to 3G. To
accomplish this, the MME issues a handover command to the mobile terminal in LTE and initiates a
handover procedure (Fig. 5 (2)). The mobile terminal changes its radio from LTE to 3G during this
procedure (Fig. 5 (3)). On completion of handover, the mobile terminal issues an originating request for

10. voice service to the MSC/VLR. A voice-call connection is then established using an existing calloriginating
procedure on 3G and the CS Fallback procedure is completed (Fig. 5(4)).
CS Fallback Call Control Procedures - Mobile Terminating Call:
The mobile terminating voice call procedure using CS Fallback is shown in Figure 6. When the MSC/VLR
receives a message indicating the occurrence of a mobile terminating call (Fig. 6 (1)), the MSC/VLR
identifies the corresponding MME from the call information received (Fig. 6 (2)). Then, the MSC/VLR
sends a paging message (Fig. 6 (3)) towards towards the MME. Next, the MME sends a paging message
to the mobile terminal in LTE (Fig. 6 (4)). This paging message includes an indication that the call is a CS
service, and on identifying the call as such, the mobile terminal sends a CS fallback service request signal

11. to the MME (Fig. 6 (5)). Following this, a handover procedure to 3G as described above takes place (Fig.
6 (6), (7)). The mobile terminal that is now switched to 3G sends a paging response message to the
MSC/VLR at which it is registered (Fig. 6 (8)). Finally, an existing mobile terminating call procedure on 3G
is executed and the CS Fallback procedure is completed (Fig. 6 (9)).