Whitepaper Ultra-Flash CSFB

Circuit-Switched

Fallback

Ultra-Flash CSFB

White Paper

HUAWEI TECHNOLOGIES CO., LTD.

Ultra-Flash CSFB White Paper

1

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.


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).

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.

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

Figure 2 SRVCC Architecture


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.

Figure 3 CSFB Call Delay

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.

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 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.

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.

Figure 4 Ultra-Flash CSFB Architecture

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.

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 5 Starting a CSFB call

Figure 6 SRVCC Handover to GERAN/UTRAN


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.

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

allocation procedure after the UE switches to 2/3G for performing the call-setup procedure.

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.

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:

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.)

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.

UTRAN Mobile Originating Native UTRAN

CS Call

Redirection-

Based CSFB

PS HO-

Based 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


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.


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

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.

Figure 8 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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Whitepaper Ultra-Flash CSFB