CSFB Procedures

13

1. IntroductionEvolved Packet Core (EPC), the

core network of next-generation mobilecommunications, can accommodatemultiple radio access systems like 3G,Long Term Evolution (LTE), and wire-less LAN in an integrated manner, andcan provide mobility management*1

seamlessly among such systems [1].Here, LTE, which is the new upcomingradio technology, provides all servicesusing IP without existing CircuitSwitched (CS) domain functions.

Accordingly, services like voice callsand the Short Message Service (SMS)that have traditionally been providedover the CS domain will be replaced, inprinciple, by VoIP, for example. Thiswill eventually require the deploymentof the IP Multimedia Subsystem(IMS)*2 as a service control platform[2].

However, at the beginning of LTEdeployment, it may take some timebefore IMS and VoIP services can beprovided due to the size of the targetcoverage area, the time required for

facility planning, etc. As a conse-quence, a scenario in which voice ser-vices are provided in a step-by-stepapproach has been discussed at 3GPP.Yet, a mobile terminal capable of LTEcannot utilise the 3G radio access sys-tem simultaneously while camping onLTE and, as a result, it would have nomeans of receiving a mobile terminat-ing call from the 3G CS domain. Thus,for the time that IMS is not provided onLTE, the terminal would not be able tooriginate or terminate any voice calls atall.

*1 Mobility management: Management of ter-minals which provides transmission, receptionand continuous communication even if termi-nals move.

*2 IMS: A communications system that integratesservices provided by CS using Internet tech-

nology such as Session Initiation Protocol(SIP).

NTT DOCOMO Technical Journal Vol. 11 No. 3

Special Articles on SAE Standardization Technology

EPC Voice over LTE Circuit Switched Services

CS Fallback Function for Combined LTE and 3G Circuit Switched Services

Itsuma Tanaka1

Takashi Koshimizu0

Katsutoshi Nishida0

The 3GPP, an international standardization body for creat-ing a next-generation mobile communications system, hascompleted technical specifications for CS Fallback, a func-tion for combining EPC supporting LTE and CS services like3G voice calls. CS Fallback will enable early provision ofvoice terminals having LTE capabilities. This function con-sists of three elemental technologies: technology for notifyinga mobile terminal in an LTE cell that a call request is beingmade from a 3G-CS system, technology for enabling themobile terminal receiving the call request to switch radioaccess systems, and technology for LTE/3G combined mobil-ity management.

Core Network Development Department

The 3GPP, an international standardization body for creating a next-generation mobile communications system, has completed technical specifications for CS Fallback, a function for combining EPC supporting LTE and CS services like 3G voice calls. CS Fallback will enable early provision of voice terminals having LTE capabilities. This function consists of three elemental technologies: technology for notifying a mobile terminal in an LTE cell that a call request is being made from a 3G-CS system, technology for enabling the mobile terminal receiving the call request to switch radio access systems, and technology for LTE/3G combined mobility management.

14


Nevertheless, voice is one of themost important services in mobile com-munications, and it is important thatvoice services can still be provided insome way even during the time that theLTE platform is providing high-speeddata communications services withoutdirect provision of voice by IMS.

In addition, the early provision ofLTE that is not limited to data-card ter-minals but also includes voice-capablehandheld-type terminals will mean evenmore convenient services for users.

Thus, after agreeing upon the abovescenario for the stepwise provision ofvoice services, it was decided at 3GPPto standardize a mechanism for switch-ing to 3G on originating or mobile ter-minating voice calls [3]. This function,called CS Fallback, enables voiceservices using existing 3G-CS domainfunctions to be provided to users even ifVoIP services are not being directlyprovided over LTE.

This article describes the functionalrequirements and technical realizationof CS Fallback

2. Overview of CS Fallback

2.1 CS Fallback at a Glance

The basic concept of CS Fallback isshown in Figure 1. Given a mobile ter-minal camping on LTE, a mobile termi-nating voice call arrives at the terminalfrom the existing CS domain via EPC.On receiving a paging message, themobile terminal recognises that the net-

work is calling the mobile terminal forCS-based voice and therefore switchesto 3G. The response confirming theacceptance of a call request is then sentfrom the mobile terminal to the 3G-CSsystem, and from that point on, all callcontrol for the voice service is per-formed on the 3G side.

2.2 Architecture

The CS Fallback consists of a func-tion to notify a mobile terminal of a callrequest from the CS domain and com-bined mobility management functionsbetween CS domain and EPC for thatpurpose. The network architecture ofCS Fallback is shown in Figure 2.

One of the remarkable characteris-


Switching radio

Mobile terminal S4S3

S1-MME

Gs

DSGs

U-Plane interfaceC-Plane interface

S11

S5S-GW P-GW

HSS

lu-psSGSN

MSC/VLR

S6a

S1-U

MME

Evolved Packet Core (EPC)

3G core network

lu-cs

3G LTE

Figure 2 CS Fallback network architecture

3G/LTEcore network

Actual voice-call data (U-Plane)Control signal (C-Plane)

(1) Mobile terminating voice call

(3) Response to mobile terminating voice call

(4) Voice call

(2) Switch to 3G

Mobile terminal

LTE3G

Figure 1 Overview of CS Fallback

15

tics of the EPC supporting CS Fallbackis that it connects the Mobile SwitchingCenter (MSC)*3 and Visited LocationRegister (VLR)*4 in the 3G CS domainwith the Mobility Management Entity(MME)*5, which provides EPC mobilitymanagement functionality. The inter-face connecting MSC/VLR and MMEis called an SGs reference point. Thisinterface is based on the concept of theGs*6 reference point that exchanges sig-nalling with MSC, which connects tothe Serving General Packet Radio Ser-vice Support Node (SGSN)*7, a 3Gpacket switch. The SGs provides nearlyall the functions provided by the exist-ing Gs.

The CS Fallback function uses thisSGs reference point to transfer themobile terminating call requests fromthe CS domain to LTE. It also providescombined mobility managementbetween the 3G CS domain and theEPC to enable this transfer to takeplace.

3. Combined MobilityManagementbetween CS Domainand EPC Network

3.1 Basic Policy

A mobile communications networkmust always know where a mobile ter-minal is located to deliver mobile termi-nating service requests to the mobileuser on the mobile terminating side. Theprocedure for determining terminallocation is called mobility manage-

ment. As a basic function of mobilecommunications, 3G and LTE each pro-vide a mobility management function.

To complete a call using the CSFallback function, the CS domain needsto know which LTE location registra-tion area the mobile terminal is current-ly camping on. To this end, the MMEmust correlate mobility managementcontrol of the CS domain with that ofEPC and inform MSC/VLR that themobile terminal is present in an LTElocation registration area.

The 3G core network already incor-porates a function for linking mobilitymanagement of the CS domain withthat of the Packet Switched (PS)domain providing packet-switchingfunctions. As described above, the CSdomain and PS domain functions areprovided via separate switches. Thus, ifcombined mobility management can beused, the mobility management proce-dure for the terminal only needs to beperformed once, which has the effect of

reducing signal traffic in the network.This concept of combined mobilitymanagement is appropriated by the CSFallback function. Specifically,MSC/VLR uses the same logic forreceiving a location registration requestfrom SGSN as that for receiving a loca-tion registration request from MME.This achieves a more efficient com-bined mobility management betweenthe CS domain and EPC while reducingthe development impact on MSC.

As described above, a mobile termi-nal using LTE cannot use 3G at thesame time. This implies that the MME,which contains the LTE location regis-tration area (Tracking Area (TA)), isunable to identify which MSC/VLR itshould send the mobility managementmessages to from the TA alone. Tosolve this problem, the mapping of TAsand 3G Location Areas (LA) withinMME has been adopted. The conceptbehind TA/LA mapping is shown inFigure 3. Here, MME stores a data-

*3 MSC: A logical node having CS functionsspecified by 3GPP.

*4 VLR: A logical node located between themobile terminal and Home Location Register(HLR)/HSS. It serves as a database storing andmanaging subscriber information and provides

functions such as terminal mobility manage-ment.

*5 MME: A logical node accommodating a basestation (eNodeB) and providing mobility man-agement and other functions.

*6 Gs: An interface for exchanging information

between MSC/VLR and SGSN (see *7).*7 SGSN: A logical node that performs packet

switching and transfer as well as mobility man-agement for mobile terminals.


MME

Mapping table

TATA1TA2TA3

LALA1LA2LA2

MSC/VLRMSC/VLR#1MSC/VLR#2MSC/VLR#2

TA1 TA2 TA3

LA1 LA2

Identifying LA and MSC/VLR from TA

MSC/VLR#1 MSC/VLR#2

Figure 3 Location registration area mapping

16

*8 IMSI: A number for identifying an individualuser in Global System for Mobile communica-tions (GSM) and 3G networks. The number isstored in a SIM card and is unknown to theuser.

*9 TMSI: A number allocated within the networkfor identifying a mobile terminal.


base that manages the correspondencebetween physically overlapping TAsand LAs. This information is used todetermine which MSC/VLR to targetfor location registration.

3.2 Combined Mobility

Management Procedures

The combined TA/LA update pro-cedure for CS fallback is shown indetail in Figure 4. First, the mobile ter-minal sends to the MME a TrackingArea Update (TAU) request messageindicating a combined TAU and thecurrent TA in which the mobile termi-nal is currently present (Fig. 4 (1)). TheMME then performs a location updateprocedure towards Home SubscriberServer (HSS), which is a database usedfor managing subscriber profiles (Fig. 4(2)). Next, the MME uses the TA/LAcorrespondence database to identify thecorresponding LA and the MSC/VLRthat is managing that area, and uses theSGs reference point to send a LocationArea Update (LAU) request message tothe MSC/VLR together with the LA soidentified (Fig. 4 (3)). The MSC/VLRthat receives the LAU request messagestores the correspondence between theID of the MME originating the requestand an ID such as the InternationalMobile Subscriber Identity (IMSI)*8

that identifies the subscriber (Fig. 4(4)). This enables the MSC/VLR toknow which MME the mobile terminalis currently connected to and that themobile terminal is camping on LTE.

Following this, the MSC/VLR performsa location registration procedure withthe HSS (Fig. 4 (5)). Finally, theMSC/VLR informs the MME of tem-porary user identity (Temporary MobileSubscriber Identity (TMSI))*9, which isused at the time of a mobile terminatingcall in the CS domain, and indicatesthat location registration has been com-pleted. The MME then informs themobile terminal of the TMSI and of theLA that the mobile terminal has beenregistered with thereby completingcombined location registration (Fig. 4(6) (7)).

4. CS Fallback CallControl Procedures

4.1 Mobile Originating Call

To originate a voice call using theCS Fallback function, a mobile terminalin the LTE location registration areamust first switch (fall back) to 3G. The

mobile-originating voice call procedureis shown in Figure 5.

To originate a call, the mobile ter-minal begins by sending a CS fallbackservice request message to the MME(Fig. 5 (1)). Since a packet-communica-tions transmission path (bearer) mustalways exist in EPC for the purpose ofproviding an always-on connection [4],the bearer also has to be handed over to3G. To accomplish this, the MMEissues a handover command to themobile terminal in LTE and initiates ahandover procedure (Fig. 5 (2)). Themobile terminal changes its radio fromLTE to 3G during this procedure (Fig. 5(3)). On completion of handover, themobile terminal issues an originatingrequest for voice service to theMSC/VLR. A voice-call connection isthen established using an existing call-originating procedure on 3G and the CSFallback procedure is completed (Fig.


(1) TAU request(TA, combined mobility management)

(3) Location registrationrequest

(LA)

(4) Stores MME correspondence

(2) Location registration

(7) Location registration response(TMSI, LA)

LTE

Mobile terminal identification

IMSI#1

MME connection information

MME#1MSC/VLR

MME

(6) Location registration response(TMSI, LA)


HSS

Figure 4 Procedure of combined location registration

17

5(4)).

4.2 Mobile Terminating Call

The mobile terminating voice callprocedure using CS Fallback is shownin Figure 6. When the MSC/VLRreceives a message indicating theoccurrence of a mobile terminating call(Fig. 6 (1)), the MSC/VLR identifiesthe corresponding MME from the callinformation received (Fig. 6 (2)). Then,the MSC/VLR sends a paging message(Fig. 6 (3)) towards the MME. Next,the MME sends a paging message tothe mobile terminal in LTE (Fig. 6 (4)).This paging message includes an indi-cation that the call is a CS service, andon identifying the call as such, themobile terminal sends a CS fallbackservice request signal to the MME (Fig.6 (5)). Following this, a handover pro-cedure to 3G as described above takesplace (Fig. 6 (6), (7)). The mobile ter-minal that is now switched to 3G sendsa paging response message to theMSC/VLR at which it is registered(Fig. 6 (8)). Finally, an existing mobileterminating call procedure on 3G isexecuted and the CS Fallback proce-dure is completed (Fig. 6 (9)).

5. Technique forImproving Call-ter-mination SuccessRate

Capter 3 described how TA/LAmapping is performed in conjunctionwith mobility management. In actuality,

however, mapping errors can occur dueto physical radio factors, and sucherrors can prevent a mobile terminalfrom registering its location with theMSC/VLR (Figure 7) to which it

should actually be registered. As aresult, it may turn out that an originat-ing call or response to a paging mes-sage, for example, is made to an inap-propriate MSC/VLR.


(1) mobile terminating call

(7) Handover+ radio switch

(8) Paging response

(9) Call termination + connection establishment

(4) Paging (CS)

(5) CS service request

(6) Handover command

LTE

Transmission path after handover

LTE-packet transmission path

MSC/VLR

MME

S-GW P-GW

SGSN

Packet- transmission path

(2) MME selection

Mobile terminal identification

IMSI#1

MME connection information

MME#1

OriginatingMSC

3G

(3) Paging

Figure 6 Mobile terminating call procedure

(1) Call-originatingrequest

(3) Handover+ radio system switch

(4) Call originating + connection establishment

(2) Handover command

LTE

3G

Transmission path after handover

LTE-packet transmission path

MSC/VLR

MME

S-GW P-GW

SGSN

Packet transmission path

Figure 5 Mobile originating call procedure

18


In short, normal communicationscannot be performed if an originating ormobile terminating call is handled viaan MSC/VLR with which the mobileterminal has not been appropriately reg-istered. To solve this problem, theRoaming Retry procedure has beenadopted in CS Fallback.

Using a mobile terminating call asan example, the Roaming Retry proce-dure is shown in Figure 8. Afterreceiving a mobile terminating call andswitching to 3G, the mobile terminalsends a paging response to anMSC/VLR with which location regis-tration has not been performed (here-inafter referred to as old MSC/VLR).The old MSC/VLR cannot determinewhat that response is for and thereforereturns a rejection to the mobile termi-nal in response to that request (Fig. 8(1) (2)). This reject message triggers themobile terminal to resend an LAU mes-sage. This LAU message is sent to theMSC/VLR that accommodates the LAin which the mobile terminal is current-ly located (hereinafter referred to asnew MSC/VLR) (Fig. 8 (3)).

The new MSC/VLR now performslocation update procedures with theHSS. Upon reception of the locationupdate request message, HSS deletesthe location registration information inthe old MSC/VLR (Fig. 8 (4) (5)). Thisdeletion of location registration infor-mation now triggers the old MSC/VLRto submit a resend request to the MSCthat originated the call request (Fig. 8

(6)). Finally, the originating MSCresends the call request to the newMSC/VLR, and from that point on, themobile terminating call is executed onthe 3G side (Fig. 8 (7)).

This procedure enables a connec-tion to be made without losing a mobileterminating call even if mapping oflocation registration areas between LTEand 3G has not been correctly per-formed.

6. ConclusionThis article described the CS Fall-

back function as one method of provid-ing voice services on LTE by combin-ing 3G and EPC mobility managementduring the initial LTE rollout periodprior to providing direct voice serviceslike VoIP. The CS Fallback functionmakes possible the early provision ofvoice terminals using LTE serviceswithout having to wait for the comple-tion of VoIP/IMS facilities while alsomaking LTE services more convenient


MME

Mapping table

TATA1TA2TA3

LALA1LA1LA2

MSC/VLRMSC/VLR#1MSC/VLR#1MSC/VLR#2

TA1 TA2 TA3

LA1 LA2

MSC/VLR#1 MSC/VLR#2

Although TA2 and LA2 actually overlap, TA2 corresponds to LA1 in the database

The mobile terminal is actually located in LA2

Figure 7 Mapping error problem

Old MSC/VLR

New MSC/VLR

3

OriginatingMSC

HSS

(1) Paging response

(2) Paging rejection

(5) Deletion of location registration information

(6) Resend request

(3) LAU request

(7) Resend call request


Figure 8 Roaming Retry procedure

19

for users. Going forward, future issuesinclude improving the CS Fallbackfunction by shortening connection time,enhancing the handover protocol, etc.

References[1] M. Yabusaki: All IP Mobile Networks,

First Edition, First Printing, 2009,

Ohmsha, Ltd. (In Japanese).

[2] 3GPP TS23.228 V.8.9.0: IP Multimedia

Subsystem (IMS); Stage 2, 2009.

[3] 3GPP TS23.272 V.8.4.0: Circuit

Switched Fallback in Evolved Packet Sys-

tem; Stage 2, 2009.

[4] K. Nishida et. al: Basic SAE Management

Technology for Realizing All-IP Network,

NTT DOCOMO Technical Journal,

Vol. 11, No. 3, pp.4-12, Dec. 2009.


CSFB Procedures

Documents

Transcript of CSFB Procedures