This document discusses different methods for providing voice services over 4G networks, including Circuit Switched Fallback (CSFB) where voice calls fall back to 2G/3G, VoIP over LTE (VoLTE) where voice is carried independently over a separate bearer, and Single Radio Voice Call Continuity (SRVCC) which allows voice calls to continue on 2G/3G networks when handing over from LTE. It also covers topics like the EPS network architecture and different quality of service classes for EPS bearers.

1. Voice in 4G: CSFB, VoIP
& VoLTE
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2. Did you hear this joke about 4G?
Now lets get back to the serious discussions
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3. ©3G4G
IMT-Advanced and 4G Technologies

4. ©3G4G
Different paths to 4G
4G
LTE / LTE-A
(FDD – TDD)
3GPP 3GPP2
1xEV-DO:Enhanced Voice-Data Optimized
CDMA ONE: Code Division Multiple Access One
EDGE: Enhanced Data rates for GSM Evolution
eHRPD: Enhanced High Rate Packet Data
GPRS: General Packet Radio Service
GSM: Global System for Mobile Communications
HSPA: High Speed Packet Access
HSPA+: High Speed Packet Access Plus
OFDMA: Orthogonal Frequency Division Multiple Access
SCDMA: Synchronous CDMA
TD: Time Division
TD-SOFDMA TD Scalable OFDMA
UMTS: Universal Mobile Telecommunication System

5. The ‘4G’ LTE Network Architecture
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BSC
BTS
MSC
Voice (PSTN)
Network
SGSN
Data (IP)
Network
RNC
Node BeNodeB
MME
GGSN
Access
Network
Core
Network
Air
Interface
MSUEUE
BSSRNS
S-GW
P-GW
2G 2.5G
3G 4G
EPC

6. Circuit Switched Fallback (CSFB)
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LTE
2G/3G
UE
CSFB
• CSFB has been designed as an interim solution until the LTE networks
are rolled out completely.
• In case of CSFB, the UE falls back to 2G/3G network when a voice call /
SMS needs to be initiated / sent or received.
• Since the UE has just one receiver, the data rates after CSFB would be
the data rates offered by the 2G/3G network.

7. CSFB Approaches
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• Redirection: When UE starts a call in Idle (RRC_IDLE) state
• Handover: When the UE already has an active data connection (in
RRC_Connected state). The active data connection is handed over to
2G/3G network

8. SVLTE (Simultaneous Voice and LTE)
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• Also known as Dual Radio
Concept
• Separate registration takes place
for the CDMA/EV-DO and LTE
networks
• Each network need not be
aware of what the UE is doing
with the other one.
• Since Voice and Data are
independent one of them does
not impact the other one.
LTE
CDMA/EV-DO
• CDMA/EV-DO registration
• Paging for Voice call
• MO/MT Voice call
• LTE registration
• Paging for Data call
• Data call
UE

9. SGLTE (Simultaneous GSM and LTE)
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• Same as SVLTE but for GSM
Network
LTE
2G – GSM
• 2G registration
• Paging for Voice call
• MO/MT Voice call
• LTE registration
• Paging for Data call
• Data call
UE

10. SRLTE (Single Radio LTE)
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• Similar to SVLTE but Single Radio
• Only Apple implemented it in
iPhone
• Separate registration takes place
for the 2G/3G and LTE networks
• Each network is not aware of
what the UE is doing with the
other one.
• Only Voice or Data can be done
at any time, not both.
• UE scans paging channel
regularly making it inefficient
LTE
CDMA/EV-DO
• CDMA/EV-DO registration
• Paging for Voice call
• MO/MT Voice call
• LTE registration
• Paging for Data call
• Data call
UE

11. SxLTE was strongly discouraged
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• Devices were more expensive
because of dual radios
• Higher Power Consumption
• Does not allow legacy networks
to be switched off
• Can have competitive
disadvantage
2G
WiMAX

12. ©3G4G
UE eNB S-GW P-GW
Peer Entity
Radio S1 S5 / S8 Gi
E-UTRAN EPC Internet
End-to-End Service
EPS Bearer External Bearer
E-RAB S5/S8 Bearer
S1 BearerRadio Bearer
Type Error Rate Delay PriorityQCI Examples
1
GBR
10-2 100 ms 2 Conversational voice
2 10-3 150 ms 4 Conversational video
3 10-3 50 ms 3 Real-time games
4 10-6 300 ms 5 Streaming video
5
Non – GBR
10-6 100 ms 1 IMS Signalling
6 10-6 300 ms 6 Steaming video, web, email
7 10-3 100 ms 7 Voice, video, games
8 – 9 10-6 300 ms 8 – 9 Streaming video, web, email

13. Voice over IP (VoIP)
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• Voice goes as data, the same way as any other IP data.
Default Bearer
eNB
Internet
UE
APN1

14. Over The Top (OTT) Apps
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15. Why have 4G Voice?
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• Better for MNO’s with no legacy Infrastructure
• Re-farm Spectrum
• Better Voice Quality
• Faster Data Rates while voice calling – other services are not impacted
• Simpler operation
• Better Battery Life

16. Voice over LTE (VoLTE)
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• Voice is independent of Data and is carried by different bearer
• An additional signalling and data bearer is used for VoLTE
VoLTE bearer
Default Bearer
eNB
Internet
UE
APN1
APN2

17. Question: Can Voice work Independently of Data?
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VoLTE bearer
Default Bearer
eNB
Internet
UE
APN1
APN2

18. Handing over from LTE to 2G/3G
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19. VoLTE to VoHSPA Handover
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• HSPA call is controlled by IMS Core
E-UTRAN
UTRAN
IMS
Call
IMS
Call
Telephony
server
Rich
Communication
Suites
SMS
server
MGW
IMS CORE
VoHSPA Handover

20. Single Radio Voice Call Continuity (SRVCC)
©3G4G
• The Voice call continues on CS Core
E-UTRAN
UTRAN GERAN
IMS
Client
UE
Telephony
server
Rich
Communication
Suites
SMS
server
MGW
IMS CORE
SRVCC – PS to CS

21. Voice path before and after SRVCC
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UE
E-UTRAN
UTRAN GERAN
HSS
S-GW P-GW
MSC
MME
SGSN
SV
S3
UE
E-UTRAN
UTRAN GERAN
HSS
MSC
SV
MME
SGSN
S3
SRVCC
PStoCS
S-GW P-GW

22. Voice path before and after SRVCC
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UE
E-UTRAN
UTRAN GERAN
HSS
S-GW P-GW
MSC
MME
SGSN
SV
S3
UE
E-UTRAN
UTRAN GERAN
HSS
MSC
SV
MME
SGSN
S3
SRVCC
PStoCS
S-GW P-GW

23. Thank You
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