SRVCC (Single Radio Voice Call Continuity) allows an ongoing voice call on an LTE network to handover or handoff seamlessly to a circuit-switched network such as GSM or UMTS when the UE moves out of LTE coverage or the voice call quality degrades in LTE. The key aspects are: 1) The LTE network triggers the handover when voice call quality deteriorates. 2) The MME coordinates the handover to the MSC via the Sv interface. 3) The call is transferred to the circuit-switched network while maintaining the voice call.

1. SRVCC (SingleRadioVoice Call Continuity)
Overview

2. SRVCC Overview | Commercial in confidence | 2015-06-09 | Page 2 (22)
LTE
SRVCCR10 Architecture
GSM / WCDMA
S1-Ue-Uu
Abis /
Iub
Um / Uu
S1-MME
Gm
S11
ISC
Mw /
Mg /
Mx
I2
Sv
A / IuCS
Mb
Iq
Mb
Mb
BTS
NodeB
eNodeB
MME
BSC
RNC
SCC AS
P-CSCF
ATCF
MME
MSS
ATGw
A
B
I-/S-
CSCF
Mw
Home NetworkServing Network
SGi
S&P
GW
SRVCC
handover
SRVCC
functionality
SR-
VCC
SR-
VCC
SR-
VCC
SR-
VCC
SR-
VCC
SR-
VCC
SR-
VCC
HLR /
HSS
SR-
VCC
MMTel
SR-
VCC
MTAS

3. SRVCC Overview | Commercial in confidence | 2015-06-09 | Page 3 (22)
IMS Telephony Voice to CS handover
– Handover triggered by LTE RAN.
Handover signalling from MME in EPC to MSC
(via the Sv reference point)
– MSC prepares handover and connects CS leg to IMS
– ATCF and SCC AS executes access transfer
MMTel
IMS
ATCF &
SCC AS
MSS
EPC
LTE
GSM / WCDMA
GERAN
UTRAN
LTE
B
SRVCC
handover
A
Sv
Access
transfer
Prepare
Handover
CS coverage
LTE SRVCC handover
Moving UE
SRVCCOverview

4. SRVCC Overview | Commercial in confidence | 2015-06-09 | Page 4 (22)
MMTel
LTE
GSM / WCDMA
BTS
NodeB
eNodeB EPG
MME
BSC
RNC
SCC AS
P-CSCF
ATCF
MME
MSS
MRS
A
I-/S-
CSCF
MTAS
SRVCCprocedures
Preparation (“STN-SR” flow)
B
HLR /
HSS
Register
Register
“STN-SR”
Push “STN-SR”
“STN-SR”“STN-SR”
Attach
UE SRVCC support

5. SRVCC Overview | Commercial in confidence | 2015-06-09 | Page 5 (22)
MMTel
LTE
GSM / WCDMA
SRVCC
handover
BTS
NodeB
eNodeB EPG
MME
BSC
RNC
SCC AS
SBG
P-CSCF
MME
MSS
MRS
A
I-/S-
CSCF
MTAS
SRVCCprocedures
Focuson media plane
B
“STN-SR”
SRVCC HO
SRVCC HO
“STN-SR”
Call setup
“STN-SR”
Handover
trigger to UE
eNodeB detects that UE
needs to perform
handover (based on bad
coverage or load in the
system).
MME prepares for
SRVCC handover and
informs MSS (including
STN-SR).
MSS first reserves
resources in 2G/3G.
MSS initiates SRVCC
handover by “dialing”
IMS using STN-SR and
sending HO msg to UE.
Based on the information
from MSS, SBG directs
MRS to redirect media to
MSS instead over LTE.
“STN-SR”

6. SRVCC Overview | Commercial in confidence | 2015-06-09 | Page 6 (22)
MMTel
LTE
GSM / WCDMA
SRVCC
handover
BTS
NodeB
eNodeB S&P
GW
MME
BSC
RNC
SCC AS
P-CSCF
ATCF
MME
MSS
ATGw
A
B
I-/S-
CSCF
Home NetworkServing Network
HO request
Signaling
Voice media
1. The eNodeB
decides to do
SRVCC HO
2. The MME
handles the
SRVCC HO via
the Sv interface
3a. The MSS and RAN
prepares to receive a
handover
3b. The MSS set up a call leg
to the ATCF using the STN-
SR number. And performs
HO towards Access
network
5. The ATGW is updated
with new media
destination for
transferred party
6. Media is
established
towards UE
4. The ATCF finds
the anchored call
and executes
session transfer
MTAS
SRVCCprocedures

7. SRVCC Overview | Commercial in confidence | 2015-06-09 | Page 7 (22)
SRVCCExecution
E-UTRAN MME
MSC Server
MGW
Target
MSC
2: HO Required
Target
BSS
3: PS to CS Req
(Target ID, STN-SR,
IMSI; C MSISDN...)
6: Prep HO Resp.
10: PS to CS Resp.
11: Handover Command
4: Prep HO Req
12: HO from E-UTRAN Command
8: Initiation of Session Transfer (STN-SR)
15: HO Complete
Decision for Handover
1: Measurement Reports
5: HO Req/Ack
7: Establish Circuit
13: UE tunes
to GERAN
14: Handover Detection
16: HO Complete
18: PS to CS Complete/Ack
IMS
9. Session transfer and
release of IMS access leg.
17: Answer
UE

8. SRVCC Overview | Commercial in confidence | 2015-06-09 | Page 8 (22)
SRVCCExecution

9. SRVCC Overview | Commercial in confidence | 2015-06-09 | Page 9 (22)
• Target cell SRVCC capability administration
• Parameter srvccCapability on ExternalUTRANCellFDD
• NOSUPPORT 0
• CS_ONLY_SUPPORTED 1
• PS_AND_CS_SUPPORTED 2
• Frequency priority for VoIP
• Parameter voicePrio, set on UtranFreqRelation and EutranFreqRelation
•Instead of connectedModeMobilityPrio used for non-voice connections, voicePrio is used
for all UEs with at least one QCI1-bearer, prioritizing voice traffic (QCI=1) separately of non-
voice traffic, independently of SRVCC feature state.
• Licensing
• SRVCC Handover to UTRAN license controls whether to perform SRVCC handover to WCDMA :
featureStateSRVCCtoUTRAN
• Feature featureStateServiceTriggeredMobility enable to further tune radio mobility events
(A1/B2/A5 etc.) for Voice Traffic. Events considered in the following examples are:
QciProfilePredefined=qci1,measReportConfigParams
• a1ThresholdRsrpPrimOffset / a2ThresholdRsrpPrimOffset
• b2Threshold1RsrpOffset / b2Threshold2RscpUtraOffset
• a5Threshold1RsrpOffset / a5Threshold2RsrpOffset
Srvccto UTRANConfig

10. SampleSRVCC DT Log

11. Beginning of Call, VOLTE call set up
*In this example, the call already granted ims signaling(qci5),only
coversational call(Qci1) set up involve when VOLTE call set uo

12. Start of SRVCC Flow
*In this example, the call already granted ims signaling(qci5),only
coversational call(Qci1) set up involve when VOLTE call set uo
Trigger for NW to initiate
SRVCC