Circuit-Switched
Fallback
Ultra-Flash CSFB
White Paper
HUAWEI TECHNOLOGIES CO., LTD.
Ultra-Flash CSFB White Paper
Senior Researcher: Xiaobo Wu, Wireless MBB Research Department, Wireless Network Research Department, Huawei
Area of expertise: LTE Voice, CSFB, VoLTE/SRVCC
Xiaobo Wu has over eight years of experience in telecommunications. As a leading technical member of Huawei’s Wireless
Service and Network Evolution Research team, he is responsible for voice solution research and its standardization, e.g.
CSFB, VoLTE/SRVCC.
Xiaobo Wu is also a delegate representing Huawei in 3GPP’s working group for system architecture. His expertise in LTE
voice is broadly acknowledged by the 3GPP standards community and he is recognized as an outstanding delegate of that
3GPP working group.
1
Ultra-Flash CSFB White Paper
LTE Voice Solutions Introduction
Long Term Evolution (LTE) has become a globally deployed
standard. However, as an all-IP, data-only transport
technology utilizing packet switching, LTE and LTE-capable
terminals introduce new challenges for meeting the
quality of service expectations established by
circuit-switched mobile telephony.
3GPP developed two approaches for providing voice
services with LTE: Circuit-Switched Fallback (CSFB) and
Voice over LTE (VoLTE), which is supported by Single Radio
Voice Call Continuity (SRVCC).
A voice interruption of shorter than 300 ms is mandatory
for commercial voice services, which requires excellent
synchronization between SRVCC IRAT Handover and
SRVCC Session Transfer procedures.
This synchronization is one of the biggest challenges for
SRVCC. 3GPP consequently introduced enhanced SRVCC
(eSRVCC) in Rel-10 for better synchronization, which
comes with a new Access Transfer Control Function
(ATCF)/Access Transfer Gateway (ATGW) in the IMS
signaling/media path.
Figure 1 CSFB Architecture
As shown in Figure 1, to enable CSFB, the MME (Mobile
Management Entity) connects to the MSC (Mobile
Switching Center) Server via a SGs interface enabling the
UE to be both CS- and PS-registered, which enables to fall
back from LTE to circuit switched, i.e. doing CSFB, when
needed for a call.
VoLTE is a voice telephony solution that delivers voice
services over LTE access using 3GPP’s IP Multimedia
Subsystem (IMS).
However, LTE coverage is not necessarily the same as
2/3G coverage, especially in early LTE deployments. For
scenarios where the UE leaves VoLTE coverage, SRVCC is
carried out to handover the VoLTE call to a 2/3G CS call.
This voice call continuity is accomplished by the following
two steps, as shown in Figure 2:


SRVCC Inter-RAT (IRAT) Handovers are performed like
traditional handovers between 2G and 3G, whereby a
handover sends the user’s device from LTE radio
access to GERAN/UTRAN radio access.
SRVCC Session Transfers are performed as a new
mechanism to move IMS access control and voice
media from LTE/IMS to legacy CS core network.
Figure 2 SRVCC Architecture
Ultra-Flash CSFB White Paper
CSFB Challenges
VoLTE, i.e. IMS together with SRVCC, is clearly the means
for providing voice services via LTE. Deployment schedules,
however, may differ for different networks.
CSFB, as an interim on the way to VoLTE, has been
launched commercially in several markets worldwide, and
has already become the predominant global solution for
voice in early LTE handsets.
Furthermore, CSFB will remain in place for many years as a
principal LTE voice roaming solution, and as a principal LTE
emergency call solution even when VoLTE is deployed.
Compared to a native 2/3G CS call, a main drawback of
legacy CSFB is the amount of steps that are added for
switching from LTE to 2/3G networks before the voice call,
which incurs longer call-setup times, especially in case of
LTE to GSM CSFB, as shown in Figure 3.
The industry has already invested a considerable amount
of effort in speeding up the switching. However, results
have remained limited and call-setup times continue to be
not satisfactory compared to native 2/3G CS calls.
Furthermore, some of these efforts require CSFB-specific
network updates, which do not necessarily provide any
value when evolving the network to support
VoLTE/SRVCC.
However, there is still strong interest in improving the
CSFB performance, but preferably via network updates,
which are also useful when evolving network to support
VoLTE/SRVCC.
Besides long call-setup times, there are other difficulties
inherent to CSFB deployment and its evolution towards
VoLTE/SRVCC in the future. Specifically, CSFB requires
strict mapping between the Tracking Area (TA) and
Location Area (LA) as well as the upgrading of all MSC
Servers surrounding the LTE coverage.
Figure 3 CSFB Call Delay
Also, since mapping between TA and LA cannot be 100%
accurate, operators have to do a lot of CSFB-specific
network planning/configuring, like adjusting existing 2/3G
Location Areas (LA) for better mapping to LTE Tracking
Areas (TA).
This is important as inaccurate TA/LA mapping may result
in Mobile Terminated call failure, forcing operators to
employ Mobile Terminated Roaming Retry (MTRR) or
Mobile Terminated Roaming Forwarding (MTRF), which
requires updating the entire CS core network.
Ultra-Flash CSFB White Paper
Ultra-Flash CSFB Solution
Instead of simply speeding up the switching process,
Ultra-Flash CSFB performs some CS call-related
procedures in parallel to the switching from LTE to 2/3G,
i.e. it triggers the SRVCC IRAT Handover procedure during
the CSFB procedure, which results in obvious benefits of
being able to provide an equivalent or even shorter
call-setup time than that of a native 2/3G CS call.
Figure 4
Ultra-Flash CSFB Architecture
As shown in Figure 4, compared to legacy CSFB, the MME
also connects to the MSC Server via the Sv interface,
enabling the SRVCC IRAT Handover during CSFB. For this,
the eNB triggers a SRVCC IRAT Handover during the CSFB
procedure.
As shown in Figure 5, with a default LTE data network
connection in operation, the UE triggers a mobile
originating (outgoing) CS voice call by sending an Extended
Service Request message to the MME.
Figure 5 Starting a CSFB call
As shown in Figure 6, the MME indicates to the eNB to
start a CSFB procedure and the eNB initiates a SRVCC IRAT
Handover procedure to switch the UE from LTE to 2/3G
rather than initiating the PS handover or Redirection
procedure of the legacy CSFB.
Once a handover to 2/3G has begun, the UE follows the
legacy CS call setup procedures except:
 Skipping the CS Radio Access Bearer (RAB) setup
procedure, as the RAB is pre-allocated during the
SRVCC IRAT Handover procedure.
 Skipping some NAS procedures, as the MSC Server
has already obtained some key information for the CS
call.
These two steps bring major gains for the call setup time,
even with the longer CSFB-specific time needed for
switching the RAT.
Mobile Terminated (incoming) voice calls follow the same
procedure except that paging happens as an additional
step.
Figure 6 SRVCC Handover to GERAN/UTRAN
Ultra-Flash CSFB White Paper
allocation procedure after the UE switches to 2/3G
for performing the call-setup procedure.
Call-setup time for Ultra-Flash CSFB

As shown in Figure 7, an Ultra-Flash CSFB call-setup takes
only about 3.6 seconds, which is significantly shorter than
a native UTRAN call setup (4.85 seconds or 35% slower).
In other words, Ultra-Flash CSFB can provide an even
shorter call-setup time than a native 2/3G CS call. One
may wonder why this can happen.
The key factor is Ultra-Flash CSFB triggers the SRVCC IRAT
Handover during the CSFB procedure, which results in
faster call-setups even for scenarios involving switching
from LTE to 2/3G.
The Second Aspect
The MSC Server has already obtained some key
information for the CS call during the SRVCC IRAT
Handover procedure, so it can skip some NAS procedures
when the UE initiates the CS call via 2/3G after the
switching, specifically:
Faster call-setup times from Ultra-Flash CSFB are possible
due to the following three aspects:
The First Aspect
During a SRVCC IRAT Handover procedure, Ultra-Flash
CSFB performs some CS call-related procedures in parallel
to switching from LTE to 2G/3G, specifically:

During CSFB-triggered switching, the CS RAB is
already pre-allocated by the SRVCC IRAT Handover
procedure, so there is no need for the CS RAB
GERAN/UTRAN gets UE capabilities from E-UTRAN
via the SRVCC IRAT Handover procedure, so it does
not need to retrieve UE capabilities from the UE
after the UE switches to 2/3G.

Skipping the authentication procedure as the UE
and network generate a CS security key during the
SRVCC procedure.

Skipping IMSI/IMEI retrieval procedures as the MSC
Server gets them from MME.

Skipping or delaying TMSI Reallocation procedure.
The Third Aspect
In case of Ultra-Flash CSFB to GERAN, the call-setup
signaling exchange between UE and GERAN is very quick
with the pre-allocated radio resource, where all the
signaling is transmitted via a fast signaling channel that
uses the traffic channel.
Table1 Mobile Originated call-setup times for Ultra-Flash CSFB compared to native UTRAN CS calls, PS HO-based CSFB
and redirection-based CSFB. (All units are in milliseconds.)
UTRAN Mobile Originating
Native UTRAN
CS Call
RedirectionBased CSFB
PS HOBased CSFB
Ultra-Flash
CSFB
Service Request for CSFB
0
150
150
150
IRAT Measurement
0
0
200
200
Handover from LTE to UTRAN
0
0
500
500
Redirection from LTE to UTRAN
0
1100
0
0
CS Call-Setup Procedure
4850
5750
5650
2750
Total
4850
7000
6500
3600

Above data for CSFB to CSFB and Ultra-Flash CSFB to Ultra-Flash CSFB based on Huawei Lab test data. Call-setup
times are from “UE triggering CS call” to “UE receiving Alerting”.

Analysis shows Ultra-Flash CSFB to GERAN is similar to UTRAN. But legacy CSFB to GERAN is much worse than to
UTRAN as shown in Figure 3.

Call-setup time shown for legacy CSFB may even need to add another 1 to 2 seconds when the CSFB needs to
include a Location Area Update or MTRR/MTRF procedure.
Ultra-Flash CSFB White Paper
Figure 7 Mobile Originated call-setup Times for UTRAN (All units are in milliseconds)
Ultra-Flash CSFB Deployment
Ultra-Flash CSFB can be easily deployed by adding the
IRAT Handover functionality from SRVCC to CSFB
deployments:



Operators only need to update some (one at the
minimum) MSC Servers rather than all the MSC
Servers surrounding the LTE coverage, which
significantly minimizes the impact on legacy 2/3G
networks.
The network certainly knows which 2/3G cell is the
best target for switching over to from the LTE cell,
which means a strict TA/LA mapping is not needed
as TA/LA misalignment is resolved by SRVCC IRAT
Handover.
As the problem of TA/LA misalignments is no longer
relevant there is also no need for deploying MTRR
or MTRF.
One may also argue about deployment difficulties for
Ultra-Flash CSFB. Fortunately, Ultra-Flash CSFB only
requires a light SRVCC IRAT Handover rather than a
full-blown SRVCC deployment, which significantly
reduces deployment efforts compared to full SRVCC.
Differences to full SRVCC are:

Similar to legacy CSFB, Ultra-Flash CSFB still relies
on the legacy 2/3G CS domain to provide voice and
therefore doesn’t require deploying IMS,
which also means there is no need for the SRVCC
Session Transfer procedure.

Ultra-Flash CSFB is a call-setup procedure rather
than a VoLTE to 2/3G CS call handover procedure,
so the SRVCC question about voice interruption
time does not apply.
The above two key characteristics allow for an
unproblematic and easy Ultra-Flash CSFB deployment.
In summary, compared to legacy CSFB, Ultra-Flash CSFB
significantly improves CSFB call-setup times and comes as
an easy and future-proof deployment, which deploys a
subset of the full SRVCC functionality and has no impact
on terminals and GERAN/UTRAN RATs.
As Ultra-Flash CSFB only relies on deploying a light SRVCC
IRAT Handover, this allows for early or gradual
investments into full SRVCC while smoothly evolving to
VoLTE/SRVCC, which saves on operator investments.
Compared to standard SRVCC functionality, please note:

Ultra-Flash CSFB may require some light software
updates in eNB/MME/MSC servers.

Regarding Ultra-Flash CSFB to UTRAN, eNB could
have no impact but is only required to support PS
Handover based CSFB.
Ultra-Flash CSFB White Paper
Figure 8 below shows the major phases of Voice
evolution.
The initial phase in LTE voice evolution introduces CSFB,
which currently is under way. CSFB means all voice traffic
is handled by legacy Circuit-Switched (CS) networks, while
data traffic is preferably handled by LTE Packet-Switched
(PS) for LTE capable terminals.
During the next phase in LTE voice evolution currently
under way, Ultra-Flash CSFB based on SRVCC IRAT
Handover will be introduced. Transition to VoLTE/SRVCC
Figure 8
with IMS is gradual with some early deployments and
trials. In this phase, CS services such as voice and
emergency calls are still mainly delivered using Ultra-Flash
CSFB even when IMS is deployed.
The final phase of LTE voice evolution introduces native
VoLTE and full SRVCC functionality (including SRVCC IRAT
Handover and SRVCC Session Transfer). Please note that
Ultra-Flash CSFB is still heavily used at this stage especially
for roaming terminals from networks without IMS and
emergency call services.
LTE Voice Commercialization
Conclusions
Transition to VoLTE will be gradual and will not occur over
a short period of time. CSFB will remain in place and
co-exist with VoLTE for a long time.
This, however, does not change the fact that legacy CSFB
has a long call-setup time and some difficulties for
deployment as well as for evolution towards
VoLTE/SRVCC.
Compared to legacy CSFB, Ultra-Flash CSFB requires
deploying the IRAT Handover from SRVCC but has no
impact on LTE-capable terminals and GERAN/UTRAN.
Due to using the SRVCC IRAT Handover procedure,
Ultra-Flash CSFB can significantly improve the CSFB
call-setup time and comes as an easy and future-proof
deployment.
Furthermore, Ultra-Flash CSFB relies on only a subset from
the overall SRVCC and supports easy evolution to
VoLTE/SRVCC, which heavily saves operator investments.
It might, however, require some limited software updates
in the eNB (optional for UTRAN)/MME/MSC Server
compared to standard SRVCC functionality.
In the scope of this paper, it is assumed all that UEs
support SRVCC IRAT Handover. Ultra-Flash CSFB is also
possible with a non-SRVCC IRAT Handover-capable UE and
can provide decent call-setup improvements, but enabling
this requires some additional small updates for
GERAN/UTRAN RATs.
References
[1] 3GPP TS 23.216: "Single Radio Voice Call Continuity (SRVCC); Stage 2".
[2] 3GPP TS 23.272: "Circuit Switched (CS) fallback in Evolved Packet System (EPS); Stage 2".
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