12. oea100007 csfb solution issue1.00

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Copyright © 2011 Huawei Technologies Co., Ltd. All rights reserved.

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Arial

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黑体

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CSFB Solution


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References

3GPP TS 23.272 Circuit Switched (CS) fallback in Evolved

Packet System (EPS)

3GPP TS 29.118 Mobility Management Entity (MME) –

Visitor Location Register (VLR) SGs interface specification

(Release 8)


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Training Objectives

After learning this course, you can:

Describe the fundamentals of the CSFB.

Describe the key procedures of the CSFB.

Describe the SGs interface and protocol.

Describe the feature data configuration of the CSFB.

Describe the network deployment of the CSFB.


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Contents

1. CSFB Fundamentals

2. Key Procedures

3. SGs Interface and Protocol

4. Feature Data Configuration

5. Network Deployment


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Origin of the CS Fallback Solution

MME

SAE-GW

IMS CN

GU CS

Key issues

How can telecom operators provide the voice and SM services before

the IMS is deployed at the initial stage of the LTE?

The UE cannot provide the IMS service in the LTE coverage scope.

After falling back to the existing 2G/3G network, the UE initiates and

completes the Circuit Switched domain (CS) service.


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Voice Service of the CS Fallback

The CSFB service must be deployed by using the current 2G/3G CS network. The service

provides a fallback technology to ensure that a subscriber can be registered to the LTE-SAE

network and the traditional CS network at the same time. To use the voice service, the

subscriber falls back to the CS through the LTE-SAE network first.


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PS and SMS Services in the CSFB

MSC Server

HSS

SGSNBS

eNB SAE-GW

SGs

S6a

S11S1-C

S1-U

Gn/S3

Iu/Gb

Iu-cs/AUm/Uu

Uu

• For the SMS service, the EPC network does not require the UE to fall

back to the traditional CS to send or receive SMs. SMs are transferred

directly through the LTE NA signaling on the EPC network. This

greatly improves the efficiency of the SMS service.

The SM service does not

need to fall back, but is

transmitted transparently

at the SGs interface. When the voice

service falls back,

the data service

is switched to the

2G/3G PS or

suspended.


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Features of the CSFB Solution

The MSC/VLR supports the SGs interface.

The MME supports the SGs interface.

The UE supports both the CS service and PS service.

Solution Strengths Weaknesses

CS Fallback Low terminal cost.

Supporting the SMS over

SGs SM solution.

Minor change to the

network; a fast and highly

efficient transition solution.

Requiring full-network

coverage of 2G/3G wireless

signals.

The 2G/3G CS network

must be upgraded.

The LTE/SAE network must

support the related functions.


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Contents

1. Fundamentals

2. Key Procedures





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Contents

2. Key Procedures

2.1 CS Mobility Management Procedure

2.2 Voice Service Procedure (MO)

2.3 Voice Service Procedure (MT)

2.4 SM Service Procedure


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Attach (combined EPS/IMSI Attach)

MSC Server

SGSNBSC/RNC

eNB SAE-GW

SGs

MME

3

1

1

3

2


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Combined EPS/IMSI Attach Procedure

Attach type

• Combined EPS/IMSI

Attach

• UE is capable to use

CSFB

Derive VLR NO.

TAI -> LAI -> VLR NO.

Create SGs association

The MSC/VLR stores the MME

Attach accept

1. Attach Request

3. Derive VLR number

4. Location Update Request

5. Create SGs association

7. Location Update Accept

UE MME HSS MSC/VLR

2. Step 3 to step 16 of the Attach procedure specified in TS 23.401

6. Location update in CS domain

8. Step 17 to step 26 of the Attach procedure specified in TS 23.401 The MME sends TMSI

and TAI

The UE registers with the CS network and EPC network at the same time by using the attach procedure.

Save the MME address to forward the paging message


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Attach Procedure Trace

After "CSFB prefer"

is set on the UE, the

UE initiates

combined attach.


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Attach Procedure Trace (Cont.)


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The MME Sends an LA Update Req

to the MSC

When receiving the

combined attach request,

the MME sends an LA

update message to the MSC.


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The MSC Answers the LA Update

Resp to the MME

The MSC saves the

MME information to

forward downlink

paging to the MME.


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Attach Accept Trace

After the combined location

update, the UE saves the

TA list, LAI, GUTI, and

TMSI at the same time.


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UE-Initiated Detach Procedure

Detach type

• Combined EPS/IMSI

Detach

• EPS detach

• IMSI detach

Remove SGs association

The MSC removes the MME

1. Detach Request

3a. IMSI Detach Indication

4. Remove SGs association 5. Detach Accept

UE MME HSS MSC/VLR

2. Step 2 to step 10 of the UE-initiated Detach procedure for E-UTRAN as specified in TS 23.401

6. Step 12 to step 14 of the UE-initiated Detach procedure for E-UTRAN as specified in TS 23.401

3b. EPS Detach Indication

The UE deregisters from the EPC or CS network by using the detach procedure.

• UE-initiated detach

procedure

• MME-initiated detach

procedure

• HSS-initiated detach

procedure


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Detach Request from UE

Type of detach (In the UE to

network direction):

(1) EPS detach

(2) IMSI detach

(3) Combined EPS/IMSI detach


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EPS Detach Indication on the SGs

Interface

The association between

the MME and MSC is

removed through this

message.


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Detach Accept


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MME-Initiated Detach Procedure

3.Remove SGs

association

1.MME-initiated Detach as specified in TS 23.401

UE MME MSC/VLR HSS

2.EPS/IMSI

Detach Indication

After the procedure ends, the MSC/VLR removes the SGs association of MME related to this UE.


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HSS-Initiated Detach Procedure

3. Remove SGs association

UE MME HSS MSC/VLR

1. Step 1a to step 7b of HSS-initiated detach as specified in TS 23.401

2. EPS Detach Indication

4. Step 8a to step 10a of HSS-initiated detach as specified in TS 23.401

When changing

the subscription

data, the HSS

initiates a detach

procedure.


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Combined TA/LA Update Procedure

2. TAU Request

4. Location Update Request

6. Location Update Accept

7. TAU Accept

UE new MME HSS MSC/VLR

5. Location update in CS domain

1. UE determines to perform TAU

old MME

3. Step 4 to step 19 of TAU procedure as specified in TS 23.401

8. TAU Complete


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Contents

2. Key Procedures






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CSFB MO Call in Active Mode - PS HO

Supported

UE/MS MME BSS/RNS MSC eNodeB SGSN Serving

GW

2. Optional Measurement Report Solicitation

4. A/Iu-cs message (with CM Service Request)

4. CM Service Request

Location Area Update or Combined RA/LA Update

5. CM Service Reject 5. CM Service Reject If the MSC is changed

3. PS HO as specified in 23.401 [2] (preparation phase and start of execution phase)

6. CS call establishment procedure

1a. Extended Service Request

1b. S1-AP Message with CS Fallback indicator

7. PS HO as specified in 23.401 [2] (continuation of execution phase)

1a.1b include CS fallback Indicator Step 1-2

CSFB

Step 4-6 call

service


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Extended Service Request (step1a)

The UE sends a CSFB indicator in

the extended service request.


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Initial Context Setup Req (step1b)

Instruct the

eNodeB to

execute CSFB


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Initial Context Setup Resp (step1b)

This message means eNodeB

receive CSFB indicator.

The IE in this message is

meaningless.


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CSFB MO Call in Active Mode - PS

HO Supported (Step 2–3)

UE Source eNodeB Target RNC Source MME Target SGSN Serving GW HSS

1. Handover Initiation

2. Handover Required 3. Forward Relocation Request

5. Relocation Request

5a. Relocation Request Acknowledge

8 . Create Indirect Data Forwarding Tunnel Request

7. Forward Relocation Response

PDN GW

8 a. Create Indirect Data Forwarding Tunnel Response

Uplink and Downlink User Plane PDUs

Target Serving GW

4. Create Session Request

4a. Create Session Response

6. Create Indirect Data Forwarding Tunnel Request

6a. Create Indirect Data Forwarding Tunnel Response

Source

The Ho From E-UTRAN Command message

contains the CS Fallback Indicator.

The eNodeB requests the UE to report the measurement result. The eNodeB selects the target cell.

If Step 4 fails, the CSFB procedure also fails.

Handover

preparation

Start of

execution

Handover preparation and start of handover execution is as follows:


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CSFB MO Call in Active Mode - PS

HO Supported (Step7)

The downlink packet

cached in the PGW is

forwarded between

the source eNodeB

and target RNC.

The target SGSN

updates the tunnel

address to the PGW.

The resource is

removed at the

E-UTRANside.


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Mobile Originating call in Active Mode –

PS HO Not Supported

UE/MS MME BSS/RNS MSC eNodeB


9. CS MO call

10b. Location Area Update

10a. Service Reject In case MSC is changed

10c. CS MO call

6. LA Update or Combined RA/LA Update or RA Update or LA Update and RA Update

3a. NACC,

5. S1 UE Context Release


S-GW

4. S1-AP: S1 UE Context Release Request

1b. S1-AP message with CS Fallback indicator

7a. Suspend (see 23.060)

8. Update bearer(s)

SGSN

7b. Suspend Request / Response

11. Routing Area Update or Combined RA/LA Update

3b. RRC connection release 6. If the RA/LA changes, the

location update procedure

may be triggered.

7a. If the target network does not

support DTM, the PS

service is suspended. The

MME deactivates the GBR

bearer and keeps the non-

GBR bearer.

Step 1-2

CSFB

Step 3-5

RRC

release

Step 7 PS

suspend

Step 9-10

call service


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Mobile Originating Call in Idle Mode

If the UE is in the idle state, the UE must change to the

EMM-CONNECTED activation state before initiating a

call. The UE first sends a service request message to set

up an S1 connection.

After the UE is activated, the Mobile Originating call in

Active Mode procedure starts.


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Contents

2. Key Procedures






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CS Fallback MT Procedure

MSC Server

SGSNBSC/RNC

eNB

SGs

MME

3

1

2

6

5

3

4

4

5

5

• Phase 1: The MME receives a Paging message from the SGs interface and pages the UE.

• Optional procedure: The subscriber returns to the connected state from the idle state.

• Phase 2: The subscriber is relocated to the target cell (through reselection or handover).

• Phase 3: The paging response is initiated at the CS.


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Callee Service Procedure in the Connected

State - Supporting PS Handover

The UE falls

back to the CS

4a. Location Area Update or Combined RA/LA Update

4b. Paging Response 4b. A/Iu-cs message (with Paging Response)

5b. Signalling Connection Release 5b. Connection Reject If the MSC is changed

5b. Location Area Update or Combined RA/LA Update

5b. CS call establishment procedure


5a. Establish CS connection

Option 1: MSC is not changed

UE/MS MME BSS/RNS MSC eNodeB SGSN Serving GW


3. PS HO as specified in 23.401 [2] (preparation phase and start of execution phase)

1b. Extended Service Request

1d. S1-AP Message with CS Fallback indicator

1a. Paging Request 1a. CS Paging Notification

1c. CS Paging Reject

1a. Service Request


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Service Procedure of the Callee in the Connected

State - Not Supporting the PS Handover

1a. CS Paging Notification 1a. Paging Request

SGSN



BSS/RNS


1a. Service Request

9a. Establish CS connection

9c. Roaming Retry for CS Fallback (clause 7.5)

Option 2: MSC is changed

Option 1: MSC is not changed

9b. Connection Reject

UE/MS MME MSC

3a. NACC, 3b. Signalling connection release

eNodeB

2. Optional Measurement Report

S-GW

1b. Extended Service Request

1d. S1-AP message with CS Fallback indicator

1c. CS Paging Reject

7a. Suspend (see TS 23.060)

8. Update bearer(s)


9b. Signalling Connection Release

9. Paging Response

Suspend the

PS service


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Service Procedure of the Callee in the

Idle State

7b. Initial UE Context Setup

7a. Extend Service Request

8c. Location Area Update

2. SRI procedure

8a. PS handover

8b. eNB assisted

cell change

UE eNodeB HSS GMSCMME RNC/ BSC MSC

1.IAM

3. IAM

4. Paging5. Paging

6. Paging

8d. Paging Response

The UE first changes from the ECM–idle state to the ECM–connected state.


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Paging from MSC


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Paging from MME to eNodeB

The MME indicates that the subscriber is

paged in the specified TA.


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Extended Service Req


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The MME Sends the Initial UE Context

Setup Message


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Contents

2. Key Procedures






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CS Fallback MO SMS over SGs

MSC Server

SGSNBSC/RNC

eNB SAE-GW

SGs

MME

5

1、2

1、2

4

3

5


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CS Fallback MO SMS over SGs

Page46


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CS Fallback MT SMS over SGs

MSC Server

SGSNBSC/RNC

eNB SAE-GW

SGs

MME

4

3

3

1

2、5

4

6

6


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CS Fallback MT SMS over SGs

Page48


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The following figure shows the system implementation of the SMS:

When the Short Message Service (SMS) is provided, the MME functions as an

intermediate forwarding node to forward the messages between the UE and the MSC.

The MME assists the UE and MSC in completing the SM function.

Different from the voice function, the SM function does not require the UE to fall back to

the GERAN/UTRAN. The E-UTRAN is directly used to provide the UE with the SMS.

System Implementation of the SMS


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SM Procedure of the Caller in the

Idle State

MS/UE MME MSC/VLR HLR/HSS SMS-

IWMSC SC

1. EPS/IMSI attach procedure

3. Uplink NAS Transport

4. Uplink Unitdata

5. Forward Short Message

6. Message transfer

7. Delivery report 8. Delivery report

9. Downlink Unitdata

10. Downlink NAS Transport

2. UE triggered Service Request

4a. Downlink Unitdata

4a. Downlink NAS Transport

11. Uplink NAS Transport

12. Uplink Unitdata

13. Release Request


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SM Procedure of the Caller in the

Activated State SM procedure of the caller of the CSFB in the activated state

The SM procedure of the caller in the activated state is similar to that in the idle

state. For the SM procedure of the caller in the activated state, the UE does not

need to initiate a service request procedure, but interacts with the MME directly on

the existing signaling connection.

Procedure of multiple SMs of the caller

When the first caller's SM has not been sent completely, the UE sends another SM.

The UE first sends the Uplink NAS Transport message to the MME, but does not

the acknowledgement message in Step 11 in the preceding figure. This avoids

reestablishment of the air interface and S1 interface connection after the air

interface and S1 interface are released.


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SM Procedure of the Callee in the Idle

State

SMS -

2. Message transfer

3. Send Routeing Info For Short Message

4. Forward Short Message 5. Paging

6. Paging 7. Paging

9b. Downlink NAS Transport

9c. Uplink NAS Transport

13. Delivery report 12. Delivery report

8. Service Request

MS/UE eNodeB MSC/VLR HLR/HSS SMS -

MME SMS-

GMSC SC

1. EPS/IMSI attach procedure

8a. Service Request

9d. Uplink Unitdata

10. Uplink NAS Transport 11. Uplink Unitdata

14. Downlink Unitdata 16. Release Request

15. Downlink NAS Transport

9a. Downlink Unitdata


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SM Procedure of the Callee in the

Connected State

SM procedure of the callee in the connected state

The SM procedure of the callee in the connected state is similar to that in the

idle state. For the SM procedure of the callee in the activated state:

- After receiving the SGsAP Paging Request message in Step 5 in the

preceding figure, the MME queries the UE context and knows that the current

UE is connected. The MME does not page the UE, namely, Step 6 to Step 8 in

the previous figure are skipped, and Step 8a is executed directly. The MME

sends an SGsAP Service Requestmessage to the MSC/VLR, indicating that

the NAS message can be transferred between the UE and the MME.


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Procedure of Multiple SMs of the

Callee

Procedure of multiple SMs of the callee

When the MSC requires to send more than one SM to the UE, the

MSC does not send the SGsAP Release Request message in Step

16 in the preceding figure. The MSC first sends an SGsAP Downlink

Unitdata message to transfer the new SM to the MME. This avoids

re-paging of the UE after the air interface and S1 interface are

released.


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Contents

1. Fundamentals

2. Key Procedures





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Newly Added SGs Interface Among the voice features of the CS Fallback, the most important interface is

the SGs interface. The SGs interface between the MME and MSC server is

used to process the mobility management and voice service paging

procedure between the EPS and the CS. In addition, the SGs interface also

provides the SMS transmission function (SMS over SGs).

The transport layer of the SGs interface uses SCTP to guarantee

transmission reliability.


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Message Functions of SGs

Message Name Function

SGsAP-LOCATION-UPDATE-

REQUEST

Attach, Combined LA/TA: The MME sends a

location update message to the MSC.

SGsAP-EPS-DETACH-INDICATION

message

EPS detach.

SGsAP-IMSI-DETACH-INDICATION IMSI detach.

SGsAP-SERVICE-REQUEST SMT: The MME sends a service request as

the response message of paging.

Service indicator: CS call indicator

or SMS indicator.

SGsAP-PAGING-REQUEST MT: The MSC sends a Paging message to

the MME.

SGsAP-UPLINK-UNITDATA SMO: The MME sends an SM to the MSC.

SGsAP-DOWNLINK-UNITDATA SMT: The MSC sends an SM to the MME.


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Contents

1. Fundamentals

2. Key Procedures





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7. CSFB Feature Related Configuration (1)

Operation procedure

1. Establish inter-device communication between the MME and MSC:

- Run the ADD SCTPPARA command to add the SCTP parameter template of association

path between the MME and MSC.

- Run the ADD VLR command to add the SCTP association information from the MME to

the specified MSC.

- Run the DSP VLR command to query whether the SCTP association state between the

MME and MSC is normal.

2. Activate the CSFB service:

- Run the ADD LAIVLR command to add an LA of MSC coverage.

- Run the ADD TAILAI command to add a TA range of LA coverage.

- Run the SET SGSPARA command to set the service running parameters of the SGs

interface.

Note: The CSFB feature requires license support. Run the DSP LICENSE command to view

whether the current license supports the CSFB feature.


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Establish inter-device communication between the MME and MSC.

[USN MML]: Add the SCTP parameter template of association path between

the MME and MSC.

ADD SCTPPARA: SCTPPARAINDEX="SCTP Protocol Parameter Index";

[USN MML]: Add the SCTP association information from the MME to the

specified MSC.

ADD VLR: VLRNO = "VLR No.", IPTYPE = IPV4, VLRIPV4_1 = "VLR IPv4

Address 1", VLRIPV4_2 = "VLR IPv4 Address 2", SUBRACKNO =

"Subrack No.", SLOTNO = "Slot No.", LOCALIPV4_1 = "Local IPv4

Address 1", LOCALIPV4_2 = "Local IPv4 Address 2", SCTPINDX = "SCTP

Protocol Parameter Index";

[USN MML]: Query whether the SCTP association state between the MME and

MSC is normal.

DSP VLR: VLRNO = "VLR No.";


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Step 2 Activate the CSFB service.

Add LAs of MSC coverage.

[USN MML]: ADD LAIVLR: BGNLAI = "Beginning LAI",

ENDLAI = "End LAI", VLRNO = "VLR No.";

Add a TA range of LA coverage.

[USN MML]: ADD TAILAI: BGNTAI = "Beginning TAI",

ENDTAI = "End TAI", LAI = "LAI";


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Step 3 Set the service running parameters of the SGs

interface.

[USN MML]: SET SGSPARA: TS6_1 = "Location Update Timer",

TS8 = "EPS Separation Timer", TS9 ="Display IMSI Separation

Timer", TS10="Implicit IMSI Separation Timer", NS8 = "EPS

Separation Retransmission Times", NS9 = "Display IMSI

Separation Retransmission Times", NS10 = "Implicit

IMSISeparation Retransmission Times";


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Contents

1. Fundamentals

2. Key Procedures





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CS Fallback Deployment

Solution 1 Strengths 1. The live network is upgraded

on a small scale. Only one to

two MSC servers need to be

upgraded or newly deployed

for each MSC pool.

2. The solution can resolve the

problem of Roaming Retry.

Weaknesses The function of MSC pool must be

supported.

Solution 2 Strengths The function of MSC pool does not

need support.

Weaknesses All the MSCs in the LTE coverage of

the live network must be upgraded.

In addition, the network must be

upgraded to support the Roaming

Retry function.

1. Deploy the MSC server and MME that support the SGs interface. Ensure that other network entities of the SAE are not affected.

2. Ensure the mapping between the TA and the LA to avoid execution of unnecessary LAU and Roaming Retry procedures during

CS Fallback.

3. For solution 1, you are advised to deploy a new pair of MSC servers of high capacity and high integration to provide the CS

fallback service.

4. For solution 2, the software is upgraded for only the MSC in the common coverage area of the LTE and 2G/3G networks where

the CS fallback service will be provided. You are advised to use the active plane and standby plane to upgrade the software in

turn and minimize the service interruption time.

Deployment principles

MSC Pool

……

MSC Server

MME Pool

GW Pool

HSS

Solution1: Depoly SGs MSC server per pool

Solution2: Upgrade all MSCs


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Impact Factors of CSFB Performance

The TAI List of the LTE network is inconsistent with the LAC of the CS network. The location

update procedure is added to the caller procedure. The roaming retry procedure is added to the

callee procedure. The CSFB procedure latency is added.

TA3

TA1

TA2

TA6

TA6

TA4

TA3

TA1

TA2

TA3

TA1

TA2

LA 2

LA 1

MSC 1

BSC 1 BTS 1

MSC 2

BSC 2 BTS 2

The UE is powered

on at LA 1 and

registered with

MSC 1.

When the UE initiates a

caller service at LA 2, the

location must be re-

updated. MSC 2 is used to

initiate the caller service.

The coverage scope of

TA List covers LA 1

and LA 2.

Location update is not initiated when the

subscriber moves from LA 1 to LA 2.

LTE TA List


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Impact Factors of the CSFB

(Caller Procedure)

UE/MS MME BSS/RNS MSC eNodeB


9. CS MO call

10b. Location Area Update

10a. Service Reject In case MSC is changed

10c. CS MO call


3a. NACC,



S-GW


1b. S1-AP message with CS Fallback indicator

7a. Suspend (see 23.060)

8. Update bearer(s)

SGSN


11. Routing Area Update or Combined RA/LA Update

3b. RRC connection release

The TA List is

consistent with the LA

planning. As a result,

the LAU procedure is

initiated after CSFB.


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Impact Factors of the CSFB (Callee

Procedure)

GMSC HLR Old

VMSC/VLR New

VMSC/VLR MS

PRN (call ref.,GMSC@, Roaming retry) 2

PRN ACK (MSRN) SRI ACK

IAM (MSRN) Paging

LocUpdate

Authentication Procedure Update Location

Cancel Location 3

Cancel Location Ack

RCH (call reference, roaming retry)

Insert Subscriber Data (multiple)

Insert Subscriber Data (continued)

Update Location Ack

Further procedures related to location update. e.g. ciphering, TMSI reallocation. PRN

New VMSC/VLR may delay setup until

location update procedure finishes 8

PRN ACK (MSRN) 7

LocUpdate Accept

TMSI Cmplt

Normal MT call procedure follows.

Old MSC stops paging timer and inform GMSC

2nd SRI ACK (MSRN) 7

2nd SRI (B, basic call interrogation) 5

REL RLCs 4

ACK

HLR delays the sending of PRN until location update

procedure finishes 6

SAE/LTE

CSFallback

SRI (B, GMSC@,call Ref.,Roamingretry) 1

IAM (MSRN)

The TA List is

consistent with the LA

planning to avoid

initiation of the roaming

retry procedure after

the CSFB


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32pt

) :18pt

SMS over SGs Deployment

1. Deploy the MSC server and MME that support the SGs interface. Ensure that other network

entities of the SAE are not affected.

2. The telecom operator can deploy a small number of MSC servers as required. The minimum

number of MSC servers is one in the whole network.

3. Deployment of the SMS network is not affected, such as SM-SC.

Deployment principles

……

MSC Server

MME Pool

GW Pool

HSS

MSC

MSC


35pt

32pt

) :18pt

Voice Solution to UE Settings

UE is set to IMS

voice preferred,

CS voice

secondary

UE initiates EPS

attach procedure

(non combined)

UE checks for IMS

voice supported

Indication from

Network

UE uses IMS

Voice

Supported

UE performs

combined TAU for

CSFB as in TS

23.272

Not

supported

UE uses CSFB

Success with no “SMS-only”

and no “CSFB

Not Preferred” indication

UE checks for

voice centric or

data centric setting

UE stays in current

RAT

Data centric

Voice centric

UE reselects to

other RAT

TAU performed

Fail or Success with “SMS only”

indication or Success with

“CSFB Not Preferred” indication

UE is set to CS

Voice preferred,

IMS PS Voice

secondary

Success with no

“SMS only” indication

and no “CSFB Not

Preferred” indication

UE checks for

voice centric or

data centric setting

UE stays in current

RAT

Data centric

Voice centric

UE reselects to

other RATUE uses CSFB

UE checks for IMS

voice supported

Indication from

Network

UE uses IMS

Voice

Supported TAU performed

Not

supported

Fail or Success with

“SMS only” indication

or Success with

“CSFB Not Preferred”

indication

UE initiates a

combined EPS/

IMSI attach

procedure

IMS PS Voice preferred CS Voice preferred

Reference from 3GPP TS 23.221 Annex A.2 Reference from 3GPP TS 23.221 Annex A.3

1. A subscriber can set the voice mode as desired.

2. Adding the network capability enables the subscriber to enjoy uninterrupted voice service when

roaming between different networks.

The UE, MSC, and MME jointly implement the CSFB function.


35pt

32pt

) :18pt

CS Fallback Latency Analysis


1b. S1-AP Request message with CS Fallback indicator

1c. S1-AP Response message

UE/MS MME BSS/RNS MSC eNodeB SGSN Serving GW


4b. A/Iu-cs message (with CM Service Request)

4b. CM Service Request

Location Area Update or Combined RA/LA Update

5. CM Service Reject 5. CM Service Reject

If the MSC

is changed

3a. PS HO as specified in 23.401 [2] (preparation phase and start of execution phase)

6. CS call establishment procedure


4a. Location Area Update or Combined RA/LA Update

3c. Update Bearer(s) 3b. Suspend

P-GW/ GGSN

The UE initiates a call

Normal CS

procedure

Additional

latency

• During the CS fallback procedure, the CSFB needs additional latency (that is, PS

HO/NACC/RRC Release)


35pt

32pt

) :18pt

CS Fallback Latency Analysis

Latency (ms) GERAN

native

PS HO

case

Plain RRC

release RRC release

(SIB cache)

Idle to active (E-UTRAN)

0 100 100 100

Extended service req. 0 150 150 150

Measurement 0 2640 0 0

PS HO 0 250 0 0

RRC release 0 0 50 50

Camp/Access in GERAN

0 0 2600 600

GERAN RRC connection

setup

1000 0 1000 1000

NAS/E2E procedure (M-M)

2000-5000 2000-5000 2000-5000 2000-5000

Total 3000-6000 4890-7890 5900-8900 3900-6900

CSFB to GERAN

CSFB to UTRAN

Latency

(ms)

UTRAN

native

PS HO

case

RRC

Release

RRC

Release

(cached

SIBs)

Idle to active (E-

UTRAN)

0 100 100 100

Extended service

req.

0 150 150 150

Measurement 0 320 0 0

PS HO 0 250 0 0

RRC release 0 0 50 50

Camp/Access in

UTRAN

320 0 1530 250

UTRAN RRC

connection setup

750 0 750 750

NAS/E2E

procedure (M-M)

2000-5000 2000-5000 2000-5000 2000-5000

Total 3070-6070 2820-5820 4580-7580 3300-6300


35pt

32pt

) :18pt

CS Fallback Performance Optimization

Common optimization methods

Selective idle mode camping: In idle state, the UE camps in the

2G/3G cell. The method ensures that the call latency of the CSFB

is consistent with the native CS call latency, but the PS service

performance is not guaranteed.

Enhanced RRC redirection with SIB: The method is a R9 feature.

The RRC Release message contains the 2G/3G SIB information.

This prevents the UE from consuming time to read SIB on the

2G/3G network.


35pt

32pt

) :18pt

Comparison between the SRVCC and

CSFB

Highlight SRVCC CSFB

Coverage Supports LTE + IMS hot spot

coverage and provides voice and

data services.

Provides PS services within the

LTE coverage, and voice

services within the 2G/3G

coverage.

Trigger condition Triggered when a subscriber

moves from a location within the

LTE coverage to a location within

the 2G/3G coverage.

Triggered when the subscriber

uses the CS service.

The MME

addresses the MSC.

Queries the DNS according to the

Target ID.

Target ID -> hostname -> IP

Configures the MSC IP statically

according to the Target TAI.

Target TAI -> LAI -> VLR NO. ->

MSC IP

Voice indicator in

the extended

service request.

SRVCC indicator CSFB indicator

Attach type EPS attach Combined IMSI/EPS attach

UE settings SRVCC preferred CSFB preferred


35pt

32pt

) :18pt

Comparison between the SRVCC and

CSFB

Highlight SRVCC CSFB

Network

deployment

Requires the IMS, and uses

the IMS and CS to implement

voice services.

Uses the CS to implement

voice services.

Upgrades the MME/E-MSC to

support the Sv interface.

Upgrades the MME/E-MSC

to support the SGs interface.

Upgrades the HSS to support

theSTR-SN and C-MSISDN.

Null

• STR-SN handover number: The SRVCC/IWF uses the number as the

called number for routing to the SRVCC AS.

• C–MSISDN: The SRVCC AS uses the number to associate the CS

session with the original IMS session.


35pt

32pt

) :18pt

Summary

1. Fundamentals

2. Key Procedures




Thank you www.huawei.com

12. oea100007 csfb solution issue1.00

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