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Voice Call Handover Scheme between 3G
LTE and 3G CS Network
Kyungmin Kim
The Graduate School
Yonsei University
Department of Electrical and Electronic Engineering
Voice Call Handover Scheme between 3G
LTE and 3G CS Network
A Masters Thesis
Submitted to the Department of Electrical and Electronic
Engineering and the Graduate School of Yonsei University
in partial fulfillment of the
requirements for the degree of
Master of science
Kyungmin Kim
December 2007
This certifies that the masters thesis of Kyungmin Kim is approved.
___________________________ Thesis Supervisor: [Jaiyong Lee]
___________________________ [Seong-Lyun Kim]
___________________________ [Jang-Won Lee]
The Graduate School Yonsei University December 2007
i
List of Contents
List of Contents ......................................................................................................... i
List of Figures......................................................................................................... iii
List of Tables ........................................................................................................... v
Abbreviation ........................................................................................................... vi
Abstract ............................................................................................................... viii
Chapter 1 Introduction............................................................................................... 1
Chapter 2 Overview of Existing Approaches for SRSC ................................................... 3
2.1 Combinational approach ................................................................................ 3
2.2 Call Re-establish Approach ............................................................................ 5
2.2.1 3GPP R7 VCC Review ......................................................................... 5
2.2.2 Call Re-establishment Approach ............................................................ 8
2.3 Gateway Approach...................................................................................... 11
Chapter 3 FW_MME Scheme Description .................................................................. 14
3.1 Motivation................................................................................................. 14
3.2 Functionality of FW_MME .......................................................................... 14
3.3 LTE to CS Handover Procedure..................................................................... 15
3.3.1 LTE Originating Call flow .................................................................. 16
3.3.2 LTE to CS Handover Procedure ........................................................... 18
3.4 CS to LTE Handover Procedure..................................................................... 19
3.4.1 CS Originating Call flow .................................................................... 20
3.4.2 CS to LTE Handover Procedure ........................................................... 21
Chapter 4 Analysis and Evaluation............................................................................. 25
4.1 Models for Mathematical Analysis................................................................. 25
4.1.1 Radio Link Delay .............................................................................. 25
ii
4.1.2 Network Node Queueing Delay ........................................................... 26
4.1.3 Internet and Remote Network Delay ..................................................... 28
4.2 Mathematical Analysis of Service Interruption Time ......................................... 29
4.2.1 Analytic Expressions for Partial Procedures ........................................... 29
4.2.2 Analytic Expressions for Service Interruption time .................................. 31
4.3 Numerical Result ........................................................................................ 35
4.3.1 Service Interruption Time Comparison .................................................. 35
4.3.2 Service Interruption Time and Error Rate of Air...................................... 37
Chapter 5 Conclusion .............................................................................................. 42
References............................................................................................................. 44
iii
List of Figures
Figure 2.1 Combination of existing handover schemes .................................................... 3
Figure 2.2 Coverage assumption of combinational approach ............................................ 4
Figure 2.3 Conceptual operation of R7 VCC ................................................................. 5
Figure 2.4 R7 VCC handover signal flow: from PS to CS ................................................ 6
Figure 2.5 R7 VCC handover fignal flow: from CS to PS ................................................ 7
Figure 2.6 Conceptual operation of call re-establishment ................................................. 8
Figure 2.8 Call re-establishment handover signal flow: form CS to LTE........................... 10
Figure 2.9 Conceptual operation of gateway approach................................................... 11
Figure 2.10 Gateway approach handover signal flow: from LTE to CS............................. 12
Figure 2.11 Gateway approach handover signal flow: from CS to LTE............................. 13
Figure 3.1 Functionalities of FW_MME ..................................................................... 14
Figure 3.2 LTE originating call diagram ..................................................................... 16
Figure 3.3 LTE originating call flow .......................................................................... 17
Figure 3.4 LTE to CS handover signal flow ................................................................. 18
Figure 3.5 CS originating call diagram ....................................................................... 20
Figure 3.6 LTE originating call flow .......................................................................... 21
Figure 3.7 CS to LTE handover signal flow ................................................................. 22
Figure 4.1 M/M/1 queueing model of network node ..................................................... 26
Table 4.1 Arrival rate of each network node................................................................. 27
Table 4.2 Service rate of each network node ................................................................ 27
Figure 4.2 M/G/1 queueing model of network node ...................................................... 28
Figure 4.3 CS attachment signal flow ......................................................................... 29
Figure 4.4 PDP context activation.............................................................................. 30
Figure 4.5 IMS registration....................................................................................... 31
iv
Figure 4.6 Signal flow of call re-establishment: LTE to CS handover............................... 32
Figure 4.7 Signal flow of call re-establishment: CS to LTE handover............................... 33
Figure 4.7 Signal flow of gateway approach: LTE to CS handover .................................. 34
Figure 4.8 Signal flow of gateway approach: CS to LTE handover .................................. 34
Figure 4.9 Service interruption time for LTE to CS handover ......................................... 35
Figure 4.10 Service interruption time for CS to LTE handover........................................ 36
Figure 4.11 LTE to CS service interruption time vs. frame error rate................................ 37
Figure 4.12 CS to LTE service interruption time vs. frame error rate................................ 38
Figure 4.13 Service interruption time vs. propagation delay of air ................................... 39
Figure 4.14 Service interruption time vs. queueing delay variation of network node ........... 40
Figure 4.15 Service interruption time vs. queueing delay variation of network node: .......... 40
v
List of Tables
Table 4.1 Arrival rate of each network node................................................................. 27
Table 4.2 Service rate of each network node…….. ....................................................... 27
vi
Abbreviation
3G 3rd Generation
3G LTE 3rd Generation Long Term Evolution
3GPP 3rd Generation Partnership Project
BS Base Station (or Node B)
BSS Base Station Subsystem
CS Circuit Switched
CSCF Call Session Control Function
ENB Enhanced Node B (or BS)
I-CSCF Interrogating-CSCF
IMS IP Multimedia Subsystem
IMT International Mobile Telecommunication
IMPI IP Multimedia Private Identity
IMPU IP Multimedia Public User identity
IMSI International Mobile Subscriber Identity
IP Internet Protocol
MGCF Media Gateway Control Function
MGW Media GateWay
MIH Media Independent Handover
MME Mobility Management Entity
MSC Mobile service Switching Center
P-CSCF Proxy-CSCF
PDP Packet Data Protocol
PS Packet Switched
R7 Release 7
vii
RLP Radio Link Protocol
RRC Radio Resource Control
SAE System Architecture Evolution
S-CSCF Serving-CSCF
SIP Session Initiation Protocol
SRSC Single Radio Service Continuity
TMSI Temporal IMSI
UA User Agent
UE User Equipment
UMTS Universal Mobile Telecommunication System
VCC Voice Call Continuity
VoIP Voice over IP
viii
Abstract
Voice Call Handover Scheme between 3G LTE and 3G CS Net-work
Kyungmin Kim
Dept. of Electrical and
Electronic Engineering
The Graduate School
Yonsei University
Convergence of different network technologies is major trend of today's network evolution.
Therefore many network technologies coexist and each technology has different coverage and
different characteristic. In this environment, mobile node can move between heterogeneous
network technologies, and it can disrupt the continuity of ongoing session. Hence, the seam-
less handover scheme between different network technologies is necessarily required that is
called as inter-system handover. Inter-system handover has several difficulties that different
characteristic of network systems, network inter-working problem, and radio resource limita-
tion.
Voice call handover between 3G LTE and 3G CS network is a kind of inter-system hand-
over that today’s hot issue of 3GPP working group. To make a handover scheme between 3G
LTE and 3G CS, there are two critical problems that lack of interface between two network
systems and radio resource limitation to single radio. Moreover the characteristic of voice call
is very sensitive to service interruption time. Accordingly a special low latency handover
scheme is required to provide seamless service.
In this paper, the performances of existing inter-system handover schemes are evaluated.
ix
And an enhanced voice call handover scheme between 3G LTE and 3G CS system is proposed.
Also service interruption times of each handover scheme are analyzed. The result of the analy-
sis shows that the proposed scheme has the smallest service interruption time and only pro-
posed scheme can satisfy the strict requirement of service interruption time of voice call hand-
over that less than 300 ms. And finally we conclude that we can provide seamless voice call
handover using proposed scheme.
.Key words : Voice-Call, Handover, 3G LTE, ,3G CS, Inter-system,
1
Chapter 1 Introduction
All IP convergence and inter-working between different network technologies are hot issues
of today’s network evolution. Since each network technology has different characteristic, to
couple different network technologies is difficult to effect. And many legacy telecommunica-
tion network systems do not support IP service, it is another problem of network evolution to
all IP convergence.
3G LTE system is one of a leading candidate for next generation network system. It assumes
IP based PS only network and voice call should be served by VoIP. Since 3G LTE system is
expected to have hot spot coverage, 3G system will exists as an infrastructure system of 3G
LTE system. Accordingly, voice call handover between 3G LTE system and 3G CS system is
necessarily required to provide seamless service. But complete handover scheme for 3G LTE
and 3G CS system does not exist.
To enable handover between 3G LTE and 3G CS, there are two critical problems. First prob-
lem is the absence of network interface between two network systems. For legacy intra-system
handover, network level backward handover can be executed by the inter-working of network
entities that exist in same network system. But for inter-system handover, network level
backward handover is hard to be achieved due to the absence of network interface. Hence lar-
ger handover latency is prospected compare to intra-system handover. The second problem is
the limitation of radio resource. For 3G LTE and 3G CS system, we can't use dual radio simul-
taneously because of spectrum interference and battery consumption issues. For this reason,
the voice call handover scenario between 3G LTE and 3G CS system is called as SRSC (Sin-
gle Radio Service Continuity). Therefore, for SRSC scenario, we can't use existing 'make be-
fore break' schemes like 3GPP VCC or IEEE 802.21 MIH that based on dual radio capability.
Thus we cannot but use 'break before make’ scheme that causes larger service interruption
time.
2
There are some approaches to solve the problem of SRSC. They are categorized as three
categories that combinational approach, call re-establishment approach, and gateway approach.
First combinational approach has many problems that inefficient and indirect handover proc-
ess, network coverage problem, and large service interruption time. Second call re-
establishment approach provides direct and provides simpler handover process compare to the
combinational approach. But it also has large service interruption time problem. The last gate-
way approach has the smallest service interruption time among the tree approaches. But still
the service interruption time is unacceptable because to satisfy the strict requirement of voice
call and detailed session immigration procedure does not defined. Also authentication and
security problems should be concerned to be a complete solution. Therefore a new advanced
handover scheme that can satisfy the requirement for voice call is necessarily required.
The remainder of this paper is organized as follows: Section 2, we study about the three ap-
proaches solving SRSC problems. In section 3, the proposed FW_MME scheme is presented
and also detailed handover procedure is explained. In section 4, the performance analysis of
schemes for SRSC is done. And from the analysis, we compare the performance and confirm
the superiority of proposed scheme. And finally at section 6, we conclude this paper.
3
Chapter 2 Overview of Existing Approaches for SRSC
Approaches that solving SRSC problem can be categorized as three categories that combi-
national approach [1] [2], call re-establishment approach [3], and gateway approach [4] [5] [6].
Each approach has its own pros and cons, but no one can completely solve the problem of
SRSC.
2.1 Combinational approach
3G LTE 3G UMTS PS 3G UMTS CS
< PS to PS Handover > < R7 VCC Handover >
Figure 2.1 Combination of existing handover schemes
This approach does not use any new scheme for SRSC and uses only existing handover
methods and signaling. Figure 2.1 presents the conceptual operation of combinational ap-
proach. In this approach, when a mobile executes handover between 3G LTE and 3G UMTS
CS network, 3G UMTS PS network is necessarily required as an intermediate system. Thus
when LTE to CS handover occurs, LTE to UMTS PS handover is preceded and then, UMTS
PS to UMTS CS handover is progressed and vice versa for UMTS CS to LTE handover direc-
tion.
4
3G UMTS CS
3G UMTS PS
3G LTE
Figure 2.2 Coverage assumption of combinational approach
By this inter-mediate network requirement, the network coverage of each network tech-
nologies is assumed as figure 2.2. To execute handover between LTE and UMTS PS system,
PS to PS handover method that will be included in 3GPP LTE standard is used. And For hand-
over between UMTS PS and UMTS CS, 3GPP R7 VCC method is used [7].
The advantage of this approach is that no modification to existing network system and no
requirement of new network entity or signaling. But the handover process is very complicated
and can't perform direct handover between two target network systems. Hence very large ser-
vice interruption time is prospected ant it will be too large to support voice call handover. Also
it has limitation of network coverage and many countries like North America can't satisfy this
requirement of coverage limitation.
5
2.2 Call Re-establish Approach
Second approach is call re-establish approach. The concept of this approach is based on R7
VCC and some modifications are added to solve the SRSC problem. Since the R7 VCC is the
foundation of call re-establishment approach, we need to review the R7 VCC and then we
study about call re-establishment approach.
2.2.1 3GPP R7 VCC Review
Figure 2.3 Conceptual operation of R7 VCC
R7 VCC is a kind of handover method between 3G UMTS PS and UMTS CS network. Dif-
ferently to SRSC scenario, R7 VCC is dual radio based scheme that UE can use dual radio
simultaneously. Therefore, when handover occurs, UE makes a new access leg with target
6
network domain while maintaining existing access leg which UE have kept. And by the func-
tion of domain transfer in VCC application server, the old access leg is switched to new access
leg. And then, by releasing of the old network resource the handover procedure completed. In
this case, 'make before break' method is used.
IMS RNC MSC/ MGW
P-CSCF I-CSCF I-CSCF S-CSCF
UE
CS
UMTS domain Visited IMS Home IMS
MGCF/MGW
VCC AS
IP Bearer
2. CC Setup
3. CAMEL
4. IAM5. INVITE
6. INVITE
7. reINVITE8. reINVITE
18. 200 OK19. 200
OK20. ACK
21. 200 OK22. 200 OK
23. ANM24. CC Connect
25. CC Connect ACK26. ACK
27. ACK
Remote End
IP BearerCS Bearer
10. 183 session progress11. 183
12. 18313. 183 session progress
14. PRACK15. PRACK
16. PRACK
17. PRACK
29. ACK28. ACK
Internet
Figure 2.4 R7 VCC handover signal flow: from PS to CS
7
IMS RNC MSC/MGW
GGSN P-CSCF S-CSCF
UE
CS
UMTS domain Home IMS
MGCF/MGW
VCC AS
IP Bearer
Remote End
SGSN
1. PDP Context Activation
2. IMS Registration
3. INVITE4. INVITE 5. INVITE
6. reINVITE7. reINVITE
18. 200 OK
1. 200 OK
20. 200 OK21. 200 OK22. 200 OK
23. ACK24. ACK 25. ACK
26. BYE27. BYE
32. 200 OK 33. 200 OK
CS Bearer
IP Bearer
28. REL29. RLC
30. Disconnect31. Release
8. 183 session progress
9. 183
10. 18311. 18312. 183 session progress
13. PRACK 14. PRACK
15. PRACK
16. PRACK17. PRACK
Internet
Figure 2.5 R7 VCC handover fignal flow: from CS to PS
Figure 2.4 presents the detailed signal flow of PS to CS handover and figure 2.5 presents
the detailed signal flow of CS to PS handover. Though some detailed procedures are different,
the basic operation of both directions is same in wide sense.
8
2.2.2 Call Re-establishment Approach
LTE CS
VCC AS
IMS
LTE domain CS domain
RemoteRemote Leg
PS access legCS access leg
Announcemnet
Figure 2.6 Conceptual operation of call re-establishment
Dissimilarly to R7 VCC, UE can't use dual radio in SRSC scenario. Thus ‘make before
break’ can't be used and ‘break before make’ is used alternatively. When handover occurs,
beforehand UE changes its access leg, IMS VCC application server makes bearer for an-
nouncement to remote party. The function of this announcing bearer is to announce the state of
UE that it is in handover state and service will be interrupted for a while. After that UE re-
leases its old access leg and makes a new access leg with target network. And afterward this
process, the announcing bearer between MRF and remote party is switched to new end-to-end
bearer. Figure 2.6 presents the conceptual operation of call re-establishment approach.
9
LTE CS MME MGCF IM-MGW MRF I-CSCF S-CSCF AS Remote
UE IMS
MSC
Notify (LTE to CS transition indication)
200 OK
CCCF request announcement port on MRF
UPDATE(SDP MRF endpoint )
UPDATE (SDP MRF endpoint)
200 OK200 OK
Bearer for announcemnet
Notify
200 OK
NotifyNotify ( PS to CS transition Preparation complete)
UE releases LTE radio and changes its domain to 3G CS
RRC setup and registration with MSC
Setup IAMINVITE INVITE
UPDATE(Offer MGCF)
UPDATE (Offer MGCF)
200 OK200 OK
200 OK200 OKANMConnect
IP Bearer
CS Bearer IP Bearer
Service Interruption
Figure 2.7 Call re-establishment handover signal flow: from LTE to CS
10
LTE CS MME MGCF IM-MGW MRF I-CSCF S-CSCF AS Remote
UE IMS
MSC
CS to PS transition Notification via USSD
CCCF request announcement port on MRF
UPDATE(SDP MRF endpoint )
UPDATE (SDP MRF endpoint)
200 OK200 OK
Bearer for announcemnet
CS to PS transition Prepare complete Notification via USSD
UE releases CS radio and changes its domain to LTERRC setup and registration
INVITE INVITE
UPDATE(Offer MGCF)
UPDATE (Offer MGCF)
200 OK200 OK
200 OK200 OK
CS bearer
IP Bearer
IP bearer
ACK ACK
Service Interruption
Figure 2.8 Call re-establishment handover signal flow: form CS to LTE
Figure 2.7 and 2.8 present the detailed signal flow of call re-establishment approach for
both directions of handover. To enable these operations, some modifications are required to R7
VCC application server.
The advantages of call re-establishment approach are that direct handover between two do-
mains can be achieved and little effect to existing network system. Also this approach can be
applicable to other handover scenarios. But the critical disadvantage is large service interrup-
tion time.
11
2.3 Gateway Approach
LTE CS
VCC AS
IMS
LTE domain CS domain
RemoteRemote Leg
PS access leg
CS access leg
Voice IWF
PrepareHandover
PrepareHandover
Figure 2.9 Conceptual operation of gateway approach
The last approach to solve SRSC is gateway approach. In this approach, a special signaling
gateway is introduced to enable inter-working between 3G LTE and 3G CS system. By the
function of this gateway, the two network system can execute network level backward hand-
over to each other. The main advantage of this approach is relatively small service interruption
time. Figure 2.9 presents the conceptual operation of gateway approach
12
LTE CS MME RNC MSC Gateway MGCF/
MGW CSCF AS Remote
UE IMS
ENB
IP Bearer
Measurement Report HO Required Handover Request
MAP HO RequestRelocation
RequestRelocation
Request ACK MAP HO Response
Handover Prepare ResponseHandover Command
RRC Setup
Handover Complete
Setup
Relocation Complete
IAM INVITE INVITEreINVITE
reINVITE200 OK
200 OK200 OK200 OK
ANMConnect
LTE CS
Service Interruption
IP BearerCS Bearer
Figure 2.10 Gateway approach handover signal flow: from LTE to CS
13
LTE CS MME RNC MSC Gateway MGCF/
MGW CSCF AS Remote
UE IMS
ENB
IP Bearer
Measurement Report HO Required Handover
Request
HO Request
HO Response
HO Prepare Response
Handover Command
RRC Setup
Handover Complete Relocation Complete
INVITE INVITEreINVITE
reINVITE200 OK
200 OK200 OK200 OK
LTE CS
Radio Resource Reservation
PDP Context Activation
IMS registration
Service Interruption
IP BearerCS Bearer
Figure 2.11 Gateway approach handover signal flow: from CS to LTE
Figure 2.10 and 2.11 present the detailed signal flow of gateway approach for both hand-
over directions. But still the service interruption time exceed 500 ms following our analysis
and this is unacceptable for voice call handover. Additionally, session level detailed immigra-
tion and authentication and security problems does not fully considered. Hence to be a com-
plete solution for SRSC, more research is required.
14
Chapter 3 FW_MME Scheme Description
3.1 Motivation
Since voice call is very sensitive to service interruption time, the service interruption time
should be smaller than 300ms to provide seamless service. But to satisfy this requirement,
SRSC has two critical problems that lack of interface between two target networks and single
radio limitation. And these problems make hard to achieve seamless handover. As we see
above, there are three approaches to solve the problem of SRSC, but they are not only incom-
plete but also can't satisfy the requirement of voice call. Therefore we need to propose a new,
handover scheme that has service interruption time less than 300 ms [8] and contains all de-
tailed procedure including session level immigration, authentication and security.
3.2 Functionality of FW_MME
Figure 3.1 Functionalities of FW_MME
To satisfy our goal that less than 300 ms service interruption time, network level fast hand-
over is essentially required and to enable network level fast handover, network interface be-
tween two target networks is necessarily required. Therefore we introduce a new network en-
tity FW_MME (Full Working Mobility Management Entity) and its major role is to support
15
inter-working of 3G LTE and 3G CS system. For 3G LTE system, MME manages the mobility
of UE and FW_MME is a kind of a MME that is placed in 3G LTE system. Differently to
MME, FW_MME has several additional functions to provide seamless voice call handover for
between 3G LTE and 3G CS system. Figure 3.1 presents the functionalities of FW_MME.
First, to support LTE-CS inter-working, FW_MME have MME and MSC functionality. And
then FW_MME have MGCF and MGW functionality to support seamless session immigration.
Additionally FW_MME has SIP UA functionality to support optimization of handover proce-
dure and to reduce service interruption time. Detailed operation of FW_MME and practical
usage of each function will be described with detailed handover process and signal flow.
3.3 LTE to CS Handover Procedure
Handover can occur for both directions that from LTE to CS, and from CS to LTE. First, we
study about the LTE to CS direction handover and at next section we study about CS to LTE
direction handover.
16
3.3.1 LTE Originating Call flow
Control Plane
User Plane
BSS ENB
MSC
MGW
SAE Gateway
UE-CS UE-LTE
RemoteEnd
FW_MME
CS Domain LTE Domain
IMS Domain
S-CSCFP-CSCF
AS
Figure 3.2 LTE originating call diagram
To understand the process of LTE to CS handover, first we need to define the architecture of
network systems and also modify the originating call flow. Figure 3.2 presents the architec-
tural diagram of originating call flow when UE initiate a call from 3G LTE system. Hence UE
has uses its LTE radio to make a new call. The control path formulated through ENB, IMS
CSCF and IMS application server. And the user bearer path formulated through ENB and SAE
gateway.
17
UE-CS UE-LTE BSS MSC/MGW ENB FW_
MMESAE
Gateway P-CSCF S-CSCF Remote End
2. INVITE
7. 200 OK
8. 200 OK
UE CS LTE IMS
AS(Proxy)
3. INVITE
4. INVITE
9. 200 OK
12. MESSAGE[call info.]
HLR
11. MME queryUA
5. INVITE
6. 200 OK
13. MESSAGE[call info.]
10. 200 OK
1. INVITE
IP Bearer
Figure 3.3 LTE originating call flow
Figure 3.3 presents the detailed call flow of LTE originated call. To describe the detailed
operation, we need to refer figure 3.3.
Procedure 1~5: UE sends SIP INVITE message to initiate a VoIP session and the INVITE
message transferred to IMS P-CSCF. Then the INVITE message transferred to S-CSCF and
application server. Then the application server sends the INVITE message to remote party. At
this point, every originating call for UE is anchored at IMS application server and it manages
the call state and provides session immigration capability when handover occurs.
Procedure 6~10: 200 OK response is transferred to UE through the network entities in re-
verse order of procedure 1~5
Procedure 11~13: IMS application server queries the address of FW_MME and registers
FW_MME as the SIP UA of UE and send the call information to FW_MME. Using thie in-
formation, FW_MME act as SIP UA of UE in the handover process.
18
3.3.2 LTE to CS Handover Procedure
UE-CS UE-LTE BSS MSC/MGW ENB FW_
MMESAE
Gateway P-CSCF S-CSCF Remote End
UE CS LTE IMS
AS
IP Bearer
1. Measurement Report 2. HO Required
3. Prep HO Request4. HO Request
5. HO Request
Ack 6. Prep HO Response
7. IAM
8. ACM
MSC
UA
9. INVITE w/ replaces 10. INVITE w/ replaces
11. INVITEw/ replaces
12. INVITE w/ replaces
13. 200 OK
14. 200 OK
16. 200 OK
MSC
27. HO Command
21. BYE 20. BYE
18. BYE
19. BYE
17. BYE
26. 200 OK
24. 200 OK
25. 200 OK
23. 200 OK22. 200 OK
28. Radio Setup
29. HO Complete 30. HO Complete
IP Bearer
IP BearerCS Bearer
15. 200 OK
CS Bearer
Figure 3.4 LTE to CS handover signal flow
Figure 3.3 presents the whole signal flow for LTE to CS handover. The detailed operation of
the handover process is described below.
Procedure 1~2: UE periodically measures the signal strength of its neighboring cells and
sends about the measurement report to supporting ENB. And if handover is required, the ENB
19
decides whether execute handover or not. When handover occurs, ENB sends ‘Handover Re-
quired’ message to FW_MME.
Procedure 3~8: When FW_MME receives the ‘Handover Required’ message, FW_MME
can classify which network domain the handover target cell belongs. And if the target cell be-
long to CS domain that not in LTE network, and using its MSC function, executes CS domain
inter-MSC handover process with target MSC [14]. By this inter-MSC handover process, the
CS bearer between FW_MME and handover target MSC in CS domain is established.
Procedure 9~16: After procedure 3~8, FW_MME sends SIP INVITE message with replace
header using its UA functionality. The function of this INVITE message is to change the exist-
ing UE-to-remote SIP session to FW_MME-to-remote session. And by the use of MGCF and
MGW functionalities, the FW_MME connects the IP bearer that exists between FW_MME
and remote to the CS bearer that will be established between FW_MME and CS domain.
Procedure 17~26: The remote party release the former SIP session with UE. Remote party
sends SIP BYE message to UE and UE response by the 200 OK message.
Procedure 27~30: These procedures are similar to CS inter-MSC handover. After session
immigration process (procedure 9~16) finished, FW_MME sends ‘Handover Command’ to
UE. Then, UE changes its radio to CS and executes radio setup with CS network [18], and
after radio link setup process, UE sends ‘Handover Complete’ to CS MSC and then this mes-
sage transferred to FW_MME. After this message transfer, the LTE to CS handover process
is completed.
3.4 CS to LTE Handover Procedure
In this section, we study about the CS to LTE handover procedure. Similar to section 3.3,
we first study about the call origination at CS domain and then we study about the handover
procedure.
20
3.4.1 CS Originating Call flow
Control Plane
User Plane
BSS ENB
MSC
MGW
SAE Gateway
UE-CS UE-LTE
RemoteEnd
FW_MME
CS Domain LTE Domain
IMS Domain
S-CSCFP-CSCF
AS
Figure 3.5 CS originating call diagram
Figure 3.5 presents the architectural call originating diagram from 3G CS system. UE uses
its CS radio to initiate call. To support seamless handover form CS to LTE domain, some spe-
cial process are added. Like R7 VCC, all originating call from CS is connected as a VoIP call.
At R7 VCC, IMS MGCF takes the place of CS-PS inter-working. Similar to this, the MGCF
and MGW functions of FW_MME take the place of CS-PS inter-working. Hence, FW_MME
act as IMS MGCF for CS originating call, and by CAMEL procedure, every originating call at
CS network is routed to FW_MME. As a result of call originating, the control path passes
21
through CS MSC, FW_MME, IMS CSCF and IMS application server. And user bearer path
formulated pass trough CS MGW, FW_MME, and SAE gateway.
UE-CS UE-LTE BSS MSC/MGW ENB FW_
MMESAE
Gateway P-CSCF S-CSCF Remote End
1. SETUP
5. INVITE
9. 200 OK
10. 200 OK11. 200 OK
UE CS LTE IMS
AS(B2BUA)
3. IAM
6. INVITE
12. ANM
2. CAMEL Procedure
4. INVITE
13. CONNECT
CS Bearer
MGCF
8. 200 OK
7. INVITE
IP Bearer
Figure 3.6 LTE originating call flow
Figure 3.6 presents the originating call flow for CS domain. This procedure is similar to R7
VCC except for the position of MGCF that FW_MME take the place of IMS MGW.
3.4.2 CS to LTE Handover Procedure
Network attachment procedure for LTE PS network is more complex than CS network be-
cause of PDP context activation and IMS registration. Therefore, CS to LTE handover is much
complicated than LTE to CS handover. Since voice call handover is very sensitive for service
interruption time, some optimization process is required to reduce service interruption time
[26]. Therefore we simplify the PDP context and IMS registration procedure by powerful
function of FW_MME.
22
UE-CS UE-LTE BSS MSC/MGW ENB FW_
MMESAE
Gateway P-CSCF S-CSCF Remote End
UE CS LTE IMS
AS
1. Measurement Report 2. HO Required 3. Prep HO Request
4. Bearer Setup
6. Make registration Info
7. REGISTER 8. REGISTER9. 200 OK10. 200 OK
11. Store registration result
12. Prep HO Response13. HO Command
14. Radio Setup
15. PDP Context Request
16. PDP Context Accept w/ registration info.
17. HO Complete
18. HO Complete
19. INVITE 20. INVITE 21. INVITE22. Re-INVITE
23. Re-INVITE
24. 200 OK
25. 200 OK
26. 200 OK27. 200 OK28. 200 OK
MSC
UA
MSC
MME
MSC
IP Bearer
5. Set IP Address for UE
Figure 3.7 CS to LTE handover signal flow
Figure 3.7 presents the whole signal flow for CS to LTE handover. The detailed operation of
the handover process is described below.
Procedure 1~2: UE periodically measures the signal strength of its neighboring cells and
sends about the measurement report to supporting BSS. And if handover is required, the BSS
sends ‘Handover Required’ message to MSC and the MSC decides whether executes handover
or not.
Procedure 3~4: When MSC decides handover, MSC executes inter-MSC handover proce-
dure with FW_MME. In this case, FW_MME is considered as another MSC to CS MSC.
23
Hence, MSC sends ‘Prepare Handover Request’ message to FW_MME. When FW_MME
receives this message, FW_MME prepare bearer with ENB.
Procedure 5: After bearer preparation, FW_MME executes PDP context activation proce-
dure with SAE gateway using its UA functionality and receives the IP address of UE and store
it.
Procedure 6~11: At procedure 6, FW_MME generates the IMS registration information us-
ing the information contained at ‘Prepare Handover Request’ message that is sent from MSC.
Using IMSI of UE, FW_MME generates IMPI and IMPU. Using these, FW_MME executes
IMS registration procedure as an UA. And then, at procedure 11, FW_MME stores the regis-
tration information. By this special operation, handover latency caused by IMS registration
can be greatly reduced.
Procedure 12~13: After PDP context activation and IMS registration finished, FW_MME
sends ‘Prepare Handover Response’ message to MSC. And then MSC sends ‘Handover Com-
mand’ message to UE.
Procedure 14: UE changes its radio to LTE and execute radio link setup with ENB The de-
tailed operation does not defined yet.
Procedure 15~16: After radio link setup, UE sends ‘PDP Context Request’ message to
FW_MME. When FW_MME receives this message, FW_MME sends ‘PDP Context Accept’
message with IP address of UE. Also this ‘PDP Context Accept’ message contains the IMS
registration information for UE. Therefore, by the ‘PDP context Accept’ message, UE receives
both of PDP context activation response and IMS registration response. Thus UE does not
need to execute additional PDP context activation or IMS registration process. By these spe-
cial treatments, much portion of the handover latency can be reduced.
Procedure 17~18: UE sends ‘Handover Complete’ message to FW_MME and FW_MME
reports it to CS MSC.
24
Procedure 19~28: UE sends INVITE message to translate preceding call to new end-to-end
VoIP call. When IMS application server receives this INVITE message, IMS application
server changes it to Re-INVITE message and sends it to remote party. And the 200 OK re-
sponse message is transferred to UE pass through IMS application and CSCFs. And finally,
the handover process is completed.
25
Chapter 4 Analysis and Evaluation
4.1 Models for Mathematical Analysis
The elements that consist of handover delay can be divided into four basic elements that ra-
dio link delay, network node queueing delay, internet delay, and remote network delay. Each
delay element has different analytic model and takes different portion of handover delay [20]
[26].
4.1.1 Radio Link Delay
We have two kinds of radio resources that analysis about delay of radio link. The one is 3G
UMTS and the other is 3G LTE radio. First, for UMTS radio link, RLP is used for efficient
radio link control and to enhance the performance. Therefore equation (4.1) is the analytic
expression of delay caused by RLP [22]. For 3G LTE radio link, the detailed operation does
not defined yet, but RLP is expected to be used. Thus we assume RLP model for 3G LTE radio
link.
26
]i)τ)2
1)j(j()(2jTP(C[P
p))(1k(P1)τ(kTT
p))p(p(21P
ionretranmissjth at the framesion retransmisith
thebeing ,detination at thecorrectly received framefirst the)P(CGPRS)for 20msorder (typically timeinterframeτ
interfaceair over thedelay n propagatio end toend Tion trialsretranmissn for RLPin y probabilit seccessP
linkair in theerror in being frame RLP a ofy probabilitpssion trialretransmis RLP of# n
frames of #k
n
j
j
itransij2
f
ftransRLP
21)n(n
f
ij
trans
f
∑∑ ++
+−−
+−+=
−−=
==
=====
+
(4.1)
3 micro second end to end propagation delay [24] and 3 retransmission trials are assumed.
And by 3GPP specification, 20 ms and 10 ms inter frame times are assume for UMTS and
LTE. And the data rate For UMTS CS is 9.6Kbps~128Kbps and for 3G LTE is
1Mbps~100Mbps.
4.1.2 Network Node Queueing Delay
Network NodeArrival Departure
Arrival Departure
λ : Rate Arrival μ : Rate Service
Network Node
Arrival
Departure
Arrival
λ : Rate Arrival
μ : Rate Service
Departure
Figure 4.1 M/M/1 queueing model of network node
Each network node has different characteristic and the delay caused by a network node can
be represented as a mathematical model using M/M/1 queueing model. Figure 4.1 shows the
27
M/M/1 queuing model for network node and Equation (4.2) is the analytic expression [25].
,)ρ(1λ
ρT
μλρ rate, arrival messageλ rate, service :μ
NodeNetwork NodeNetwork
NodeNetwork odeqNetwork_N −
=
== (4.2)
Table 4.1 presents the arrival rate of each network node and table 4.2 present the service
rate of each network node. And if the characteristic of a network node change, we can follow
the change by controlling these parameters [36].
Table 4.1 Arrival rate of each network node
λUE λNodeB λRNC λMSC λHLR λUE
50 100 200 300 300 50
λCSCF λMGCF λApplication λENB λMME λCSCF
500 500 500 100 900 500
Table 4.2 Service rate of each network node
μUE μNodeB μRNC μMSC μHLR μUE
2500 2500 5000 5000 5000 2500
μCSCF μMGCF μApplication μENB μMME μCSCF
5000 5000 5000 5000 5000 5000
28
4.1.3 Internet and Remote Network Delay
Network Node
Arrival by UEDeparture
Arrival by UEDeparture
etcSIP λ,λ : Rate Arrival
μ : Rate Service
Network Node
Departure
μ : Rate Service
Departure
Arrival by others
Arrival by others
Arrival by UE
etcSIP λ,λ : Rate Arrival
Arrival by others
Arrival by UEArrival by others
Figure 4.2 M/G/1 queueing model of network node
Delay caused by internet or remote network is very unstable and has much variation. Hence
for more accurate analysis, we use M/G/1 queueing model for internet and remote network
delay. Figure 4.2 presents the models of internet and remote delay and equation (4.3) is the
analytic expression of them [25]. For this case, ρetc = 0.5 and ρinternet = 0.4 and ρSIP=50 are
used.
setc2s
2etc
2sipsip
2etcetcInternet
sipetcetc
Internetsipetcs
qRemoteqInternet/
μ,μ ofmoment second theare X,X
,XλXλR
)ρρ(1)ρ(1
R)ρρ(1μ1
T
μλρ rate, arrival messageλ rate, service :μ
+=
−−+−
+−−=
==
(4.3)
29
4.2 Mathematical Analysis of Service Interruption Time
From the signal flows of handover in chapter 2 and 3, we can classify the service interrup-
tion period. And from the service interruption period, we can derive the equation of service
interruption time by applying the analytic model that is described in chapter 4.1.
4.2.1 Analytic Expressions for Partial Procedures
RRC
IMS NodeB RNC MSC/VLR
UE
CS
UMTS Domain
RRC System Information (BCCH)
RRC Connection Request (CCCH)
NBAP : Radio Link Setup Response
Start RX
NBAP : Radio Link Setup Request
ALCAP : Establish Request
ALCAP: Establish CNF
DCH-FP : Downlink Synchronization
DCH-FP: Uplink synchronization
Start TX
RRC Connection Setup (CCCH)
NBAP : Radio link Restore Indication
RRC: RRC Connection Setup Complete (DCCH)
RRC: Initial Direct Transfer [CM Serviec Request ] RANAP :Initial UE Message [CM Service
Request ]
IMS NodeB RNC MSC/VLR
UE
CS
UMTS Domain
RANAP: Direct Transfer
[Authentication Request ]RRC: Downlink Direct Transfer [Authentication Request ]
RRC: Uplink Direct Transfer [Authentication Response ] RANAP: Direnct Transfer
[Authentication Response ]
RANAP: Security Mode CammandRRC: Security Mode Command
RRC: Security Mode CompletRANAP: Security Mode
complete
RANAP: Direct Transfer [TMSI
Reallication Cammand ]RRC: DL Direct Transfer [TMSI Reallocation Cammand ]
RRC: UL Direct Transfer [TMSI Reallocation Complete ] RANAP: Direct Transfer [TMSI
Reallocation Complete ]
RRC: Setup
RANAP: Setup
ETCENB ENBRadio
Link Delay
ENBλ ENBλ
RNC
RNCλRRC message
RadioLink Delay
qRNCqNodeBRLPRRC 1T2T2TT ++=
RRC
IMS NodeB RNC MSC/VLR
UE
CS
UMTS Domain
RRC System Information (BCCH)
RRC Connection Request (CCCH)
NBAP : Radio Link Setup Response
Start RX
NBAP : Radio Link Setup Request
ALCAP : Establish Request
ALCAP: Establish CNF
DCH-FP : Downlink Synchronization
DCH-FP: Uplink synchronization
Start TX
RRC Connection Setup (CCCH)
NBAP : Radio link Restore Indication
RRC: RRC Connection Setup Complete (DCCH)
RRC: Initial Direct Transfer [CM Serviec Request ] RANAP :Initial UE Message [CM Service
Request ]
IMS NodeB RNC MSC/VLR
UE
CS
UMTS Domain
RANAP: Direct Transfer
[Authentication Request ]RRC: Downlink Direct Transfer [Authentication Request ]
RRC: Uplink Direct Transfer [Authentication Response ] RANAP: Direnct Transfer
[Authentication Response ]
RANAP: Security Mode CammandRRC: Security Mode Command
RRC: Security Mode CompletRANAP: Security Mode
complete
RANAP: Direct Transfer [TMSI
Reallication Cammand ]RRC: DL Direct Transfer [TMSI Reallocation Cammand ]
RRC: UL Direct Transfer [TMSI Reallocation Complete ] RANAP: Direct Transfer [TMSI
Reallocation Complete ]
RRC: Setup
RANAP: Setup
ETCENB ENBRadio
Link Delay
ENBλ ENBλ
RNC
RNCλRRC message
RadioLink Delay
qRNCqNodeBRLPRRC 1T2T2TT ++=
Figure 4.3 CS attachment signal flow
To derive the equations for service interruption time, first we need to define some partial
procedures. Figure 4.3 presents the detailed signal flow of CS attachment procedure. Left part
of figure 4.3 presents the RRC setup procedure. And right part of figure 4.3 presents the re-
mained procedures that include authentication and TMSI allocation. From the signal flow, CS
30
attachment procedure can be modeled as equation (4.4) and (4.5). And for LTE attachment
procedure, similar procedure is assumed. Equation (4.6) and (4.7) is the model of LTE at-
tachment procedure. For both case, attachment procedure includes RRC setup procedure.
UERNCNodeBRLPUMTS_RRC 2T4T7T4TT +++= (4.4)
MSCRNCNodeBRLPCS_Attach 3T11T14T11TT +++= (4.5)
UEENBLTE_RadioLTE_RRC 2T7T4TT ++= (4.6)
MMEENBLTE_RadioLTE_Attach 3T14T11TT ++= (4.7)
LT E EN B MME
U E
C S
LT E D om ain
SAE Gatew at
D irect T ransfer : Activation PD P C ontext R equest
R AB Assignm ent R eauest
R esponse
ALC AP Iub D ata T ransfer Bearer Setup
R adio Bearer Setup
R adio Bearer Setup C om plete
C reate PD P C ontext R equest
R esponse
D irect T ransfer : Activation PD P C ontext R esponse
Figure 4.4 PDP context activation
31
UE ENB/ MME P-CSCF HSS S-CSCF
RegisterCx-Query/
Cx-Select-Pull
Cx-Query Resp/Cx-Select-Pull Resp
Register
Cx-put/Cx-PullCx-Put Resp/Cx-Pull Resp
200 OK200 OK
401 (Unauthorized)401 (Unauthorized)
RegisterRegister
Figure 4.5 IMS registration
Figure 4.4 and 4.5 present the detailed signal flow of PDP context activation and IMS regis-
tration. From the signal flows, we can derive equation (4.8) and (4.9) that the delay of PDP
context activation and IMS registration procedure for LTE system [27].
SAE_GWMMEENBLTE_RadioPDP 4T4T4T4TT +++= (4.8)
HSSCSCFMMEENBLTE_RadioIMS_Reg 3T8T4T4T4TT ++++= (4.9)
4.2.2 Analytic Expressions for Service Interruption time
Using above equations we can derive the analytic expressions of service interruption time
for call re-establishment approach. They are equation (4.10), (4.11) and figure 4.6 and 4.7
present the detailed signal flow of call re-establishment handover process. Since the delay of
RLP is depends on the size of message, TRLP_utu in equation (4.10) is the RLP delay with CS
user-to-user message. Therefore the value of TRLP_utu is calculated as equation (4.1) with CS
user-to-user message,
32
CS_AttachRLP_utuNodeBRNCMSCMGCF
LTE_RadioENBMMECSCFASInternetRemoteEtoCSCall_Re_LT
T2T2T2T2T2T
TTT9T4T3T3TT
++++++
++++++=(4.10)
LTE_AttachRLP_utuNodeBRNCMSCMGCFLTE_Radio
ENBMMECSCFAsInternetRemotetoLTECall_Re_CS
TTTT T T3T
3T3T9T4T3T3TT
+++++++
+++++=(4.11)
LTE CS MME MGCF IM-MGW MRF I-CSCF S-CSCF AS Remote
UE IMS
MSC
Notify (LTE to CS transition indication)
200 OK
CCCF request announcement port on MRF
UPDATE(SDP MRF endpoint)
UPDATE (SDP MRF endpoint)
200 OK200 OK
Bearer for announcemnet
Notify
200 OK
NotifyNotify ( PS to CS transition Preparation complete)
UE releases LTE radio and changes its domain to 3G CS
RRC setup and registration with MSC
SetupIAM
INVITE INVITE
UPDATE(Offer MGCF)
UPDATE (Offer MGCF)
200 OK200 OK
200 OK200 OKANM
Connect
IP Bearer
CS Bearer IP Bearer
Service Interruption
Figure 4.6 Signal flow of call re-establishment: LTE to CS handover
33
LTE CS MME MGCF IM-MGW MRF I-CSCF S-CSCF AS Remote
UE IMS
MSC
CS to PS transition Notification via USSD
CCCF request announcement port on MRF
UPDATE(SDP MRF endpoint )
UPDATE (SDP MRF endpoint)
200 OK200 OK
Bearer for announcemnet
CS to PS transition Prepare complete Notification via USSD
UE releases CS radio and changes its domain to LTERRC setup and registration
INVITE INVITE
UPDATE(Offer MGCF)
UPDATE (Offer MGCF)
200 OK200 OK
200 OK200 OK
CS bearer
IP Bearer
IP bearer
ACK ACK
Service Interruption
Figure 4.7 Signal flow of call re-establishment: CS to LTE handover
The analytic expressions of service interruption time for gateway approach are equation
(4.12), (4.13). And figure 4.8 and 4.9 presents the detailed signal flow of gateway approach.
RemoteInternetASCSCFMGCF
MSCRNCNodeBRLP_utuRRCinterruptLTE_to_CS_
2T2T2T4T2T
3T3T3T3TTT
+++++
++++= (4.12)
RemoteInternetASCSCFMME
ENBLTE_radioIMS_regPDPRRC_LTEinterruptCS_to_LTE_
2T2T2T4T3T
3T3TTTTT
+++++
++++= (4.13)
34
LTE CS MME RNC MSC Gateway MGCF/MGW CSCF AS Remote
UE IMS
ENB
IP BearerMeasurement Report HO
Required Handover Request
MAP HO RequestRelocation
RequestRelocation
Request ACK MAP HO Response
Handover Prepare ResponseHandover Command
RRC Setup
Handover Complete
Setup
Relocation Complete
IAMINVITE INVITE
reINVITEreINVITE200 OK
200 OK200 OK200 OK
ANMConnect
LTE CS
Service Interruption
IP BearerCS Bearer
Figure 4.7 Signal flow of gateway approach: LTE to CS handover
LTE CS MME RNC MSC Gateway MGCF/MGW CSCF AS Remote
UE IMS
ENB
IP Bearer
Measurement Report HO Required Handover
Request
HO Request
HO Response
HO Prepare Response
Handover Command
RRC Setup
Handover CompleteRelocation Complete
INVITE INVITEreINVITE
reINVITE200 OK
200 OK200 OK200 OK
LTE CS
Radio Resource Reservation
PDP Context Activation
IMS registration
Service Interruption
IP BearerCS Bearer
Figure 4.8 Signal flow of gateway approach: CS to LTE handover
35
And finally, the analytic expressions of proposed scheme are (4.14), (4.15). The detailed
signal flows are presented in previews chapter that figure 3.4 and 3.7.
RLP_utuMSCNodeBRRCUEENB
FW_MMEASCSCFSInternetRemoteinterruptLTE_to_CS_
TTTTTT
TT2TTTT
++++++
++++= − (4.14)
LTE_RadioUEENBFW_MME
ASCSCFSInternetRemoteinterruptCS_to_LTE_
5TT5T2T
2T6T2T2TT
+++
+++= − (4.15)
4.3 Numerical Result
4.3.1 Service Interruption Time Comparison
0 20 40 60 80 100 120 1400.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Data rate (Kbps)
Ser
vice
Inte
rrupt
ion
(sec
ond)
Call Re-establishGatewayFW-MME
Figure 4.9 Service interruption time for LTE to CS handover
Using the analytic models of service interruption time in section 4.2, we can compare the
service interruption time of three kinds of solution approach. Figure 4.9 presents the service
36
interruption time of LTE to CS handover for three kinds of approach. The result shows that
call re-establishment approach has the largest service interruption time. Gateway approach
reduces service interruption time using a signaling gateway. Thus it has smaller service inter-
ruption time than call re-establishment approach. Since our scheme introduces not only signal-
ing gateway but also optimizing methods, our scheme shows smallest service interruption time
among the three solutions. And if we look at the absolute service interruption time, only our
scheme can satisfy the requirement of voice call handover that less than 300 ms service inter-
ruption time.
0 1 2 3 4 5 6 7 8 9 100.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
Data rate (Mbps)
Ser
vice
Inte
rrupt
ion
(sec
ond)
Call Re-establishGatewayFW-MME
Figure 4.10 Service interruption time for CS to LTE handover
Figure 4.10 shows the service interruption time for CS to LTE handover. Similar to LTE to
CS handover, call re-establishment approach has largest service interruption time and gateway
approach has smaller service interruption time than call re-establishment approach. And our
scheme has smallest service interruption time. For CS to LTE handover, due to end-to-end SIP
session requirement, we can’t make a new access leg before radio transition. Therefore CS to
LTE handover has larger service interruption time than LTE to CS handover. Although our
scheme introduces some optimization method, our scheme has more than 300 ms service inter-
37
ruption. But about 500 ms service interruption time is reasonable in wide sense.
4.3.2 Service Interruption Time and Error Rate of Air
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.40.25
0.3
0.35
0.4
0.45
0.5
Frame Error Rate
Ser
vice
Inte
rrupt
ion
(sec
ond)
9.6 Kbps19.2 Kbps64 Kbps128 Kbps
Figure 4.11 LTE to CS service interruption time vs. frame error rate
In this section, more detailed analysis about our scheme is presented. Figure 4.11 presents
the service interruption time for LTE to CS handover corresponds to data rate and frame error
rate. The result shows two kinds of relation that the relation between service interruption time
and data rate and that between service interruption time and frame error rate. First, the service
time tends to decrease when data rate is high. But data rate higher than 64 Kbps does not
cause the decrease of service interruption time. This is because of the limitation of the size of
CS signaling message. And for second relation, service interruption time increases exponen-
tially as frame error rate increase. More than 30 % frame error rate can critically decrease the
performance.
38
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.40.5
0.51
0.52
0.53
0.54
0.55
0.56
0.57
0.58
0.59
Frame Error Rate
Ser
vice
Inte
rrupt
ion
(sec
ond)
0.5 Mbps1 Mbps2 Mbps10 Mbps
Figure 4.12 CS to LTE service interruption time vs. frame error rate
Figure 4.12 presents the service interruption time for CS to LTE handover correspond to
data rate and frame error rate. For this case, more than 1Mbps data rate does not effect to ser-
vice interruption time. The reason of this is that more than 1Mbps data rate is large enough to
transmit control signal packet in one data frame. Also the frame error rate more than 30 % can
critically decrease the performance
39
4.3.3 Service Interruption Time and Propagation Delay of Air Link
10-6 10-5 10-4 10-3 10-20.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
Propagation delay
Ser
vice
Inte
rrupt
ion
(sec
ond)
LTE to CS handoverCS to LTE handover
Figure 4.13 Service interruption time vs. propagation delay of air
In this section, more detailed analysis about the effect of propagation delay is presented.
Figure 4.13 presents the service interruption time variation that corresponds to the propagation
delay variation of air link. When the range of propagation delay is smaller than 10-3 second,
the increment of propagation delay does not cause much increment of service interruption
time. But when the range of propagation delay is larger than 10-3 second, the increment of
propagation delay causes much increment of service interruption time. Since the speed of elec-
tro-magnetic wave is very fast that close to the speed of light, the propagation delay of air link
does not exceed 10-3 second for general telecommunication system. Therefore, the variation of
propagation delay does not cause much effect to service interruption time.
40
4.3.4 Service Interruption Time and Queueing Delay Variation of Net-
work Node
10-4 10-3 10-2 10-10.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Order of Queueing delay
Ser
vice
Inte
rrupt
ion
(sec
ond)
LTE to CS handoverCS to LTE handover
Figure 4.14 Service interruption time vs. queueing delay variation of network node
10-4 10-3 10-20.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
Order of Queueing delay
Ser
vice
Inte
rrupt
ion
(sec
ond)
LTE to CS handoverCS to LTE handover
Figure 4.15 Service interruption time vs. queueing delay variation of network node:
Observe in reasonable range
41
In this section, more detailed analysis about the variation of queueing delay of network
node is presented. Figure 4.14 presents the service interruption time variation corresponds to
the queueing delay variation of network node. The result shows that increment of queueing
delay causes much increment of service interruption time. Since the general queueing delay of
network node is tens of millisecond [26], figure 4.15 presents for reasonable range of queue-
ing delay that order of 10-4~10-2 second. From the result of this section, the queueing delay of
network node is an effective factor of service interruption time.
42
Chapter 5 Conclusion
All IP convergence and inter-working between different network technologies is a major
trend of today’s network evolution. In this environment, mobile node can moves between dif-
ferent network technologies and the movement of mobile node can disrupt the continuity of
ongoing session. Thus handover between different network technologies is necessarily re-
quired that called inter-system handover. Since different network technologies have their own
characteristic, inter-system handover has difficult problem to be solved.
Voice call handover between 3G LTE and 3G CS system is one of inter-system handover and
it is a hot research topic of 3GPP working group. To provide handover between 3G LTE and
3G CS system, there are two critical problems that the absence of interface between two net-
work systems and radio resource limitation to single radio. Due to this single radio limitation,
the handover problem between 3G LTE and 3G CS system is called SRSC. Additionally the
characteristic of voice call handover that very sensitive to service interruption time makes the
problem more difficult. There are three kinds of approaches to solve this SRSC problem, that
combinational approach, call re-establishment approach and gateway approach. But no one
can provide complete handover solution and they can’t satisfy the strict service interruption
time requirement of voice call handover less than 300 ms.
In this paper FW_MME scheme has proposed to solve the SRSC problem, The FW_MME
scheme introduces a new powerful network entity called FW_MME and the FW_MME pro-
vides various functionalities to achieve seamless handover. Using the functionalities of
FW_MME first, we can achieve the inter-working between 3G LTE and 3G CS network. And
also we can achieve simplification of handover procedure especially on network attachment
procedure.
Analytic result shows that other approaches have more than 500 ms service interruption time
43
and they can’t satisfy the strict requirement of voice call handover that less then 300 ms. But
using proposed FW_MME scheme we can achieve strict voice call service interruption time
requirement that less than 300 ms. And finally, we can conclude that using proposed
FW_MME handover scheme we can provide seamless voice call handover between 3G LTE
and 3G CS system.
Further research is needed to reduce service interruption time for CS to LTE handover.
And research about the characteristic of internet and remote network with various parameters
also required to more accurate analysis.
44
References
[1] 3GPP S2-062718, “on CS Call continuity”, Aug 2006.
[2] 3GPP S2-062889, “RAN Impact for Single Radio VCC”, Sep 2006.
[3] 3GPP TR 23.806, “Voice Call Continuity between CS and IMS Study”, Dec 2006.
[4] 3GPP S2-063159, “Single Radio Voice Call Continuity”, Aug 2006.
[5] 3GPP S2-070735, “Single Radio VCC: Outline of Basic Scenarios”, Feb 2007.
[6] 3GPP S2-070745, “IMS-controlled Voice Call Continuity between CS Domain and the
LTE/SAE”, Feb 2007
[7] 3GPP TS 23.206, “Voice Call Continuity between Circuit Switched and IP Multimedia
Subsystem”, Sep 2006.
[8] 3GPP S1-99306 “Quality of Service”, May 1999
[9] 3GPP TS 23.002, “Network Architecture”, Dec 2005
[10] 3GPP TS 23.060, “General Packet Radio Service (GPRS); Service description; Stage 2”,
Jun 2006
[11] 3GPP TS 23.228, “IP Multimedia Subsystem”, Sep 2006.
[12] 3GPP TR 23.882, “3GPP System Architecture Evolution: Report on Technical Options
and Conclusions”, Nov 2006.
[13] 3GPP TS 24.228, “Signaling Flow for the IP multimedia call control based on Session
Initiation Protocol (SIP) and Session Description Protocol (SDP)”, Sep 2006.
[14] 3GPP TS 23.009, “Handover procedures”, Mar 2006
[15] 3GPP TS 25.413, “UTRAN Iu interface RANAP signaling”, Jun 2006
[16] 3GPP TS 29.002, “Mobile Application Part (MAP) specification”, Dec 2006
[17] 3GPP TS 43.129, “Packet-switched handover for GERAN A/Gb mode; Stage 2”, Jun
45
2006
[18] 3GPP TS 08.08, “Mobile-services Switching Centre - Base Station System (MSC - BSS)
interface; Layer 3 specification”, Sep 2003
[19] 3G/UMTS complete mobile originated circuit switched call setup, www.3g4g.co.uk/ZG
[20] Nilanjan Banerjee,Wei Wu , Kalyan Basu and Sajal K. Das, "Analysis of SIP-based
mobility management in 4G wireless networks“, Computer Communications Volume 27,
Issue 8 , May 2004, Pages 697-707, Advances in Future Mobile/Wireless Networks and
Services
[21] Dirk Pescha, Maria Isabel Pousa and Gerry Fosterb;, “Performance evaluation of SIP-
based multimedia services in UMTS”, Computer Networks, Volume 49, Issue 3 , 19 Oc-
tober 2005, Pages 385-403, Selected Papers from the European Wireless 2004 Conference
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VoIP in 3G Wireless Networks”, Mobile Computing, IEEE Transactions on Volume 5, Is-
sue 9, Sept. 2006 Page(s):1121 - 1132 Digital Object Identifier 10.1109/TMC.2006.135
[24] Esmael Dinan, Aleksey Kurouchkin, and Sam Kettani “UMTS Radio Interface System
Planning and Optimization”, Bechtel Telecommunication Technical Journal Volumue 1,
Number 1, December 2002.
[25] L. Kleinrock, QUEUEING STSTEMS vol.1 Theory, Wiley, New York, 1975 [19] McNair,
J.; Fang Zhu; “Vertical handoffs in fourth-generation multinetwork environments” Wire-
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Identifier 10.1109/MWC.2004.1308935
[26] 3GPP TR 25.853 v4.0.0 Delay Budget within the Access Stratum
[27] Kyungmin Kim, Hyunduk Jung, Jaiyong Lee, Eunhui Bae, and Sungho Choi, "Delay
Analysis of Voice Call Handover between UMTS CS, UMTS PS and WLAN," ITC-
46
CSCC 2007, Busan, Korea, TA1-2
[28] Jaeho Lee, and Jaiyong Lee, "Route Enhancement Scheme using HMIP in Heterogeneous
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Springer-Verlag, vol. 3961, pp. 21-30, Jan. 2006
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Journal of Communication and Networks (JCN), vol. 7, no. 2, pp. 178-191, June 2005
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IEICE Transactions on Communications, vol. E89-B, no. 3, pp. 731-738, March 2006.
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Number 1, December 2002.
47
국문요약
3G LTE 네트워크와 3G CS 네트워크 사이에서의 음성 호
핸드오버 방안
다양한 네트워크 기술들의 융합은 오늘날 네트워크 기술의 진화의 큰 흐름이다.
이 같은 변화에 따라 다양한 종류의 서로 다른 네트워크 기술은 공존하게 되며,
각각의 네트워크 기술들은 서로 다른 범위의 서비스 영역을 그리고 다양한 특성
을 가지게 된다. 이러한 환경에서 무선단말은 다양한 네트워크 기술들 사이를 이
동해 다닐 수 있으며 이 같은 단말의 움직임은 현재 사용하고 있는 세션의 연속
성을 유지하는데 문제를 일으킨다. 따라서 다양한 네트워크 기술들을 이동하는 단
말에게 끊김 없는 서비스를 제공하기 위한 핸드오버 기술이 요구된다. 그런데 이
러한 이종망간의 핸드오버를 제공하는 데에는 서로 다른 네트워크의 특성, 네트워
크 연동 문제, 라디오 자원의 제약 등으로 인한 어려움이 따른다.
3G LTE 네트워크와 3G CS 네트워크 사이에서의 음성호 핸드오버는 이 같은 이
종망 사이의 핸드오버 중 하나로 현재 3GPP의 주요한 연구주제 중 하나이다. 3G
LTE 네트워크와 3G CS 네트워크 사이에서 핸드오버를 제공 방안을 만드는 데에는
두 가지의 큰 문제점이 있다. 첫 번째 문제점은 두 네트워크 사이에 연동을 위한
인터페이스가 존재하지 않는다는 점이며 두 번째 문제점은 한번에 한가지 라디오
만 사용할 수 있다는 라디오 자원 활용 측면에서의 문제 점이다. 게다가 음성호의
경우 서비스가 끊어지는 시간에 매우 민감한 특성을 가지고 있어서 끊김 없는 핸
드오버를 제공하는데 더 많은 어려움이 따른다.
본 논문에서는 지금까지 제안된 3G LTE 네트워크와 3G CS 네트워크 사이의 핸
드오버 방안들을 평가하고 향상된 핸드오버 방안을 제시한다. 또한 제안된 핸드오
버 방안 및 기존의 핸드오버 방안들을 수학적으로 분석하여 각각의 성능을 비교
한다. 분석 결과는 제안된 방법이 가장 짧은 서비스 장애시간을 가지며, 또한 제
안된 방법만이 300 ms 이내라는 엄격한 음성 호 핸드오버의 요구조건을 만족시킬
48
수 있음을 보여준다. 이 같은 결과를 바탕으로 제안된 3G LTE 네트워크와 3G CS
네트워크 사이에서의 음성호 핸드오버 방안을 이용하여 끊김 없는 서비스를 제공
할 수 있음을 알 수 있다.
핵심이 되는 말 : 음성 호, 핸드오버, 3G LTE, 3G CS, 이종 망 간 핸드오버
