The document discusses various voice solutions for LTE networks, focusing on Circuit Switched Fallback (CSFB), Simultaneous Voice and LTE (SVLTE), Over-The-Top (OTT), and Voice over LTE (VoLTE). It highlights the advantages and disadvantages of each solution, particularly emphasizing the benefits of VoLTE, such as enhanced voice quality, richer multimedia services, and guaranteed end-to-end Quality of Service (QoS). Additionally, it outlines the evolution and deployment strategies for IMS-based voice services.

1. VOLTE Introduction

2. Commercial LTE Voice Solutions
Page2

3. CSFB Definition
• The circuit switched fallback (CSFB) service enables calls to fall back to the CS domain. The
LTE network provides only data services. When a voice call is initiated or answered, it falls
back to the CS domain for processing. Therefore, carriers only need to upgrade the MSC
server without deploying the IMS network.
• Advantage: Voice services can be provided quickly with little change on the live
network.
• Disadvantage: When subscribers use voice services, data services carried on the LTE
network are interrupted, handed over, or suspended, and voice calls are connected
slowly. This affects user experience.
• Application scenario:
• CSFB is a transition solution before the IMS network deployment.
• If the IMS network is not deployed on the visited network, CSFB can provide voice
services for roaming LTE subscribers.
Page3

4. CSFB Definition (Cont.)
• Originating side
• The UE sends a voice call request to the
MME. The MME then instructs the UE to
fall back to the 2G or 3G network.
• The UE falls back to the 2G or 3G network.
• The UE initiates a call based on the
normal originating flow on the 2G or 3G
network.
MME
SGs
LTE
MSC server
CS EPC
2G/3G
MME
LTE
MSC server
CS EPC
2G/3G
Fallback
3
SGs
1
4 2
⚫ Terminating side
 The MSC server sends a paging requestto the
MME over the SGs interface.
 The MME forwardsthepaging request
instructing the UE to fall back to the 2G or 3G
network.
 The UE falls back to the 2G or 3G network.
 The UE sends a paging responseto the MSC
server and answers thecall based on the
normalterminating flow.
1
2
Fallback
3
Page4

5. SVLTE Definition
• In the simultaneous voice and LTE (SVLTE) solution, dual-mode mobile phones work on the
LTE network and the CS network simultaneously. The LTE network provides data services,
while the CS network provides voice services.
• Advantage: SVLTE is a solution for mobile phones without requirements for the network.
• Disadvantage: The cost and power consumption of dual-mode mobile phones are high.
LTE
2G/3G
CS EPC
MSC server
MME
Data service
Voice service
Page5

6. OTT Definition
• Over-the-top (OTT) is a solution that uses APPs, such as the Skype, to provide voice
services for LTE subscribers.
• Advantage:
• Only the OTT server needs to be deployed to the live network and slight change is
required. The OTT can enhance user experience by offering rich multimedia services.
• Disadvantages:
• The OTT solution is not carrier-class-based and has low reliability.
• The OTT solution cannot provide E2E QoS guarantee.
• The OTT solution is hard to provide common services, such as emergency calls and
interception, and services that are used only by regulatory bodies.
• Roaming and interconnection are hard to be implemented due to lack of standards.
Page6

7. VoLTE Definition
Voice over long term evolution (VoLTE) is an IMS-based voice solution defined
by 3GPP. By deploying the IMS network, operators not only migrate traditional
voice and short message services on live networks but also integrate voice
services with various enhanced functions to diversify services.
VoLTE advantages
⚫ The VoLTE solution inherits all services provided by the CS network, including common services, such as
emergency calls and interception, and services that areused only by regulatory bodies.
⚫ The VoLTE solution can implement E2E QoS controland guaranteevoice call quality.
⚫ In the VoLTE solution, voiceservices are carried on IP networks. Therefore, subscriberscan enjoy richer
multimedia services through the IMS network, competewith OTT.
⚫ Subscriber identities (such as the MSISDN) arereserved, which maximizes carriers'resourceusageand
facilitates service profitgain.
⚫ The IMS architecture, supported by mature 3GPP standards and specifications, is considered to be the
next generation corenetwork by the communications industry. TheIMS architectureensures roaming
services and network interworking.
Page7

8. Three Forces to Drive
VoLTE Development
•HD voice
•HD video session
•Faster call connection
•Rich multimedia communication
experience (IM/Presence
service/enhancedaddressbook)
• The LTE network features high
bandwidth, low delay, all IP, and always
online, which facilitates OTT
application development.
• IMS-based multimedia voice services
and RCS serve as carriers' main
competitive sources.
• VoLTE provided by carriers outperforms
OTT in the following aspects:
• Telecommunication subscriber ID
and authentication
• Interconnection and interworking
• QoS guarantee
• Handover and roaming to 2G and 3G
networks
• Fee package bound to mobile
broadband
• Fault troubleshooting and customer
care
•The spectral efficiency of LTE voice
services increases over four times
compared with that of CS voice
services.
•Number of voice channels per sector
•Source: Qualcomm
The time for settingupaVoLTE call is
half thatfor settingupa 3G call.
1. Competing with OTT
(real driving force) 33.33%
3. Unifyingthe network and reducingTCO
(long-termdrivingforce) 26.42%
2. User experience
(permanent driving force) 30.19%
VoLTE/RCS driving force
investigation result
(159 participants in the IMS summit
in April 2011)
Faster connectionrate
Better communication
quality
Higher spectral
efficiency
.
Richer
services
Page8

9. Advantage of VoLTE – Better and Faster
SD and HD video size
compare
Faster connection
rate
Better
communication
quality
Higher spectral
efficiency
Richer
services
⚫ Fast connection:
 CS call establish duration is around 4-5s，
VoLTE is 0.8 -1.5s.
⚫ HD video:
 VoLTE typical video definition is 480*640
（VGA）, HD 720P and full HD 1080P is
possible（H.264/H.265）
 3G typical video definition is 176*144
（QCIF）
⚫ VoLTE can significantly reduce the call establish delay, provide high-
definition audio and video calls, greatly enhance the user experience

10. Advantage of VoLTE -- HD Voice Quality
20
Hz
20k Hz
Common ear listening range (CD/MP3 range)
HD voice: (Twice frequency rangethe AMR-NB)
• VoCS voice: AMR-NB 300~3400 Hz, sampling frequency 8kHz, 12.2Kbps。
• VoLTE HD voice: AMR-WB (G.722.2 ) : 50~7000 Hz, same as the G.711 in PSTN, sampling frequency
16kHz, 23.85Kbps.
• CD music: 20~20K Hz, sampling frequency 44.1K.
50
Hz
80
Hz
12k Hz
The frequency range of human speech
7000
Hz
HD voice (AMR-WB)
300
Hz
3400
Hz
CS
（AMR-NB)
Audio
range

11. Advantage of VoLTE -- QoS
Dedicate bearer for VoLTE
Best effort for OTT services
QCI Resource Pri. Delay Error rate service
2 GBR 4 150ms 10-6
VoLTEvideo
6 Non-GBR 6 300ms 10-3
OTT video
LTE
QCI: QoS class identifier GBR: guaranteed bit rate

12. Advantage of VoLTE –VoLTE with Native RCS
VoLTE HD voice and video RCS embedded
contact Multimedia
message
Convergent
communication

13. Comparison Among CSFB, SVLTE, VoLTE and OTT
Solution Feature Advantage Disadvantage
Deployment
Suggestion
VoLTE
The IMS and LTE
networks provide voice
services.The handover
betweenthe LTE
network and 2G/3G
network is supported.
Rich multimedia
services.
HD voice/video.
E2E QoS
The IMS network
needs to be deployed.
Large-scale LTE
coverage
OTT
Voice services are
implemented byAPPs
of OTT carriers.
Slight change is
required on live
networks.
Voice calls are not
reliable.
Roaming and
interworking are hard
to be implemented.
-
CSFB
UEs attach to the LTE
network. When a UE
initiates or receives a
call, the UE falls back to
the CS network.
Slight change is
required on live
networks.
New NEs do not need
to be deployed.
The call delay is long.
User experience is
affected.
Initial phase of
the LTE network
(transition
solution)
SVLTE
Terminals attach to both
the CS and LTE
networks. The CS
network provides voice
services.
The live network does
not need to be adjusted.
Costs of mobile
phones are high.
Advantages of the
LTE network cannot
be used.
Initial phase of
the LTE network
(transition
solution)
Page13

14. Key Technologies Involved in VoLTE
• Voice based on IMS
• VoLTE voice services are IMS-based.
• SRVCC/eSRVCC
• If VoLTE voice services need to be handed over the GSM or RAN network, SRVCC or
eSRVCC is used.
• ICS for VoLTE
• To ensure that subscribers have consistent user experiences on the LTE and CS
networks, ICS is used.
Page14

15. IMS-based Voice Services
E-UTRAN
I/S-CSCF
SBC
/P-CSCF
Handset
EPC
MME
S-GW/P-
GW
IMS Core
PCRF
Default bearer: IMS signaling streams
Dedicated bearer: Voice media streams
Voice media streams
SIP signaling streams
• A default beareris set up for IMS signaling streams when the UE attaches to the network.
• Afterthe UE initiates a call, SIP signaling streams for call connectionis transmitted over the
default bearer through the P-GW to the IMS Core.
• A dedicatedbearerfor IMS voice media streams is set up during call connection.
• Voice media streams of the calling and called parties are transmitted over the dedicated bearer.
• The PCRF assigns and delivers QoS rules for signaling and mediastreams during the setup of
default and dedicated bearers.This ensures quality of signaling and media streams.
Page15

16. VoLTE Basic
Call
• A dedicated
bearer for IMS
voice media
streams is set up
during call
connection
Page16

17. SRVCC Definition
• Single radio voice call continuity(SRVCC) is an IMS-based VoLTE.
• VCC describes how a voice call can be continuedwhen a UE moves between the target CS domain and the
LTE domain supporting VoIP services.
• SRVCC describes how a voice call can be continuedwhen a Single Radio UE moves between LTE/Pre-LTE and
2G/3G networks, that is, when a Single Radio UE switches an call session between IMS/LTE and CS.
Page17

18. SRVCC Definition (Cont.)
• Calls are set up in the LTE over VoIP / VoIMS.
• When a subscriber moves to an area uncovered by the LTE network while holding a call,
the call is switched to the CS domain. SRVCC interprets a switchover process.
Page18
⚫ SRVCC solution: The media
handover point is a peer
device. Such as a peer UE.
⚫ E2E signaling and bearer are
re-established.
⚫ The IMS network sends the
updated media information to
the remote network, and a
session is interrupted around
1s.

19. eSRVCC Definition
• Target: The handover delay is less than 300 ms
• Policy: The remote session update process is optimized. The media handover point is
near to the local side.
⚫ Method: Media and signaling
planes are anchored at the
ATCF or ATGW to avoid the
IMS session transfer process.
When an eSRVCC handover is
performed, only the bearer
between the UE and the
ATGW needs to be set up.
⚫ The original bearer is reused to
transmit media streams
between the peer device and
the ATGW.
Page19

20. eSRVCC Vs SRVCC
First eSRVCC testing (Vienna 201210)
before
after
SRVCC
E2E media re-establish
LTE
RAN
G/U
RAN
IMS/SBC
> 1.5s
Remote
side
eSRVCC
Local media establish
LTE
RAN
G/U
RAN
IMS/SBC
（ATCF & ATGW）
~ 0s
Remote
side
< 230ms
LTE to GSM HO time ~ 230ms (Beijing 201404)

21. ICS Definition
• IMS centralized services (ICS) describe how services of subscribers who access the CS domain or IP-CAN
using different terminalsare managed on the IMS network in centralized mode.
• With the ICS feature, all services are centralized to IMS. Therefore, subscribers can have consistent user
experiences on the LTE and CS networks.
• By deployingan MSC server that supports the ICS feature on a network, ICS subscribers can use services
providedby IMS during the initialphase of LTE in the following scenarios:
• CS subscribers who have subscribed to the ICS feature wish to use IMS services without changing their numbers.
• VoLTE subscribers who roam to or experience an SRVCC handover to a CS network wish to continue to use the IMS
services.
Page21

22. ICS Definition (Cont.)
• The ICS solution becomes the
best choice for evolution from the
mobile softswitch to IMS. The
eMSC server functions as a SIP UA
and connects subscribers to the
IMS Core.
• MSC server enhancements for ICS,
the MSC server that supports the
ICS feature, is called mobile
access gateway control function
(mAGCF). In this document, the
MSC server enhancements for ICS
is called eMSC server.
IMS Core
MGW
AS
mAGCF
LTE
2G/3G
LTE subscriber
MSC server
AS AS
Service layer
Centralized service
trigger point
Access layer
ICS Architecture
xPONand
FBB
CS subscriber
Page22

23. VoLTE Phase-based Deployment
Policy
LTE
LTE
2G/3G
LTE
2G/3G
LTE
2G/3G
Phase 2:
Continuous LTE coverage
Phase 1:
Hot spot LTE coverage
Phase 3:
Full LTE coverage
Pre-VoLTE Initial commercial use Large-scalecommercial
deployment
⚫ Terminals:
 Initial smartphone
 LTE data card and LTE CPE
⚫ Deploymentsuggestions:
 CSFB
 CPEs subscribe to IMS-
based VoIP voice services.
 Softclients subscribe to rich
media RCS services.
⚫ Terminals:
 Smartphone
 LTE data card and LTE CPE
⚫ Deploymentsuggestions:
 IMS-based VoLTE voice
services
 eSRVCC
 Convergentdatabase
SingleSDB
 LTE-based rich media RCS
services
⚫ Terminals:
 Smartphone
 LTE data card and LTE
CPE
⚫ Deploymentsuggestions:
 The LTE network becomes
the mainstream.
 ICS
 IMS roaming and
interworking

24. Network Evolution Solution
Page24

25. Evolution to IMS/SRVCC/ICS
The live network
solution evolves to ICS
in the following ways:
• Evolution from
deployment of the
overlay eMSC server
to ICS
• Direct Evolution to
ICS
Live network MSC servers
MSC servers
Deploy the SRVCC IWF/MGCF.
Enhanced with the mAGCF/SRVCC IWF
Page25

26. Evolution from CSFB to ICS
The live network solution evolves from CSFB to ICS in the following ways:
• Evolution from the standard CSFB to ICS
• Evolution from the CSFB proxy to ICS
MSC servers
MSC servers
MSC servers MSC servers
VoLTE (overlay eMSC server)
Enhanced with CSFB
Deploy MSC servers to serve as CSFB proxies. Enhanced with the SRVCC IWF/MGCF

27. VoLTE Network Architecture

28. Operation Support Layer and Service Layer
Operationsupport layer
⚫ Operation support layer
 Functions: This layer provides various functions,
such as network management, subscription data
storage, unified operations on the web portal,
charging, and equipment management.
 NEs: DMS, SPG, CCF, and EMS
Service layer
⚫ Service layer
 Functions: This layer consists of different
application servers and resourceservers to provide
services, such as instant messaging, conferences,
games, as well as service capabilities, such as group
and media resourcecapabilities.
 NEs: ATS and RCS
ATS
(MMtelAS/SCC AS/IP-SM-
GW/IM-SSF/AnchorAS)
Page28

29. Core Layer
Core layer
⚫ The core layer is divided into the followingthree
parts:
 IMS domain: NEs in the IMS domain implement
such functions for LTE subscribersas registration,
authentication, session path control, servicetrigger,
routeselection, resourcecontrol, inter-domain
interworking, and access resourcecontrol.
 CS domain: NEs in the CS domain implement such
functions for LTE subscribersas mobility
management and voice services, including
handovers and CSFB.
 SingleSDB: The SingleSDB provides the functions of
the USCDB, HLR, IMS-HSS, SAE-HSS, PCRF,
DNS/ENUMfor centralized storageof VoLTE
subscribers'servicedata. The data can be used by
the CS, IMS, and EPC domains.
Page29

30. Access Layer
Access layer
⚫ Access layer
 Functions:
 The access layer controls LTEsubscriber access
and provides mobility management for LTE
subscribers.
 When an LTE subscriber is in the LTE coverage, the
subscriber can access an IMS network through the
EPC.
 When a subscriber moves outof the LTE coverage,
the subscriber can access an IMS network through
a 2G/3G network using ICS.
 If the 2G/3G network does not supportICS, a
subscriber accesses a legacy CS network.
 NEs: eMSC server (mAGCF), S-GW, P-GW, and MME
Page30

31. Terminal Layer
Terminal layer
⚫ Terminal layer
 LTE terminals are classified into VoLTE terminals
and non-VoLTE terminals.
 VoLTE terminals are those that rely on an IMS or
LTE network to usevoice services. They include
Single Radio terminals, LTE data cards, and
CPEs.
 Non-VoLTEterminals are thosethat rely on a CS
network to usevoice services and that connect
to an LTE network to usedata services. They
include Dual Radio and CSFB terminals.
 Types: Single Radio terminals (SRVCCterminals),
LTE data cards, CPEs, DualRadio terminals (SVLTE
terminals), and CSFBterminals
Page31

32. NEs at the Operation Support Layer
NE Name Function
SPG
Provides a unified service provisioning interface and the web portal
and sends the service provisioning commands received from the
BSS to the HSS, ENUM, and AS.
CCF
Collects ACR messages from IMS charging NEs, such as the CSCF,
ATS, and MGCF, processes the ARC messages to generate CDRs,
and sends the CDRs to a BC specified by carriers.
EMS Interworks with the BSS to manage NEs in a centralized manner.
Page32

33. NEs at the Service Layer
NE Name Function
Common
telephony
application
server
(ATS)
⚫ MMTel AS: provides basic and supplementary multimedia
telephony services.
⚫ SCC AS: interworks with the SRVCC IWF and ATCF/ATGW to
provide the eSRVCC function, and provides the T-ADS function
to select a domain for the called party.
⚫ IP-SM-GW: provides short message interworking between IMS
and CS domains.
⚫ IM-SSF: triggers IN services in the CS domain based on the
subscription information provided by the S-CSCF and the local
configuration on the IM-SSF.
• Anchor AS: Routes VoLTE subscribers' calls to the IMS
domain to trigger convergent services by IN-based number
redirection of the anchor AS, when the VoLTE subscribers
originate or receive a call in the CS domain.
RCS AS
Provides the Open Mobile Alliance Instant Message (OMA IM),
Presence, XML document management server (XDMS), and data
synchronization (DS) functions to implement the IM, Presence, and
Network Address Book services.
Page33

34. NEs at the Service Layer - ATS
• The ATS provides traditional
mobile services and
multimedia services.
Business
service
Mobile
special
service
TISPAN
PSTN
service
3GPP
MMTel
service
IMS network
Mobile terminal Soft
phone
SIP
terminal
CPE
MBB FBB PSTN
POTS POTS
ATS
(MMTel + SCC AS + IP-SM-GW+ IM-
SSF + anchor AS)
⚫ The ATS integrates the functions of the MMTel AS, IM-SSF, SCC AS, Anchor AS, and IP-SM-
GW to reduce the number of attempts to trigger iFC during calls and realize service
resources sharing.
Page34

35. NEs at the Service Layer - SCC AS (ATS)
⚫ The service centralizationand continuityapplication server
(SCC AS) implementshandoversin the IMS domainand
updatesthe media information about the remote UE. In
details, the SCC AS does the following:
EPC
CS
IMS Core
ISC
eMSC server
(SRVCC IWF)
LTE
GERAN
UMTS
Calledparty
access
SCC AS
Calling
connection 1
Calling
connection2
Calledconnection3
Mediaconnection1
Mediaconnection2
Handover
Call signaling anchorpoint
SCC AS
▪ Analyzes information
required by a handover.
▪ Associates handover
requests and anchors
sessions.
▪ Implements handovers
between the LTE network
and the UMTS network.
Page35

36. NEs at the Service Layer - T-ADS (SCC AS)
• T-ADS
• As defined in 3GPP specifications, domain selection by the network is implemented by the terminating access
domain selection (T-ADS).
• The T-ADS function is integrated with the SCC AS. The SCC AS queries the convergent HLR/HSS for the UE type and
access domain information and implements domain selection based on the carrier policies. Based on the T-ADS
information, the SCC AS determines whether the current call is routed to the IMS or CS network.
Page36

37. NEs at the Service Layer - IP-SM-GW (ATS)
⚫ The IP short message gateway (IP-SM-GW)
provides short message interworking between
IMS and CS domains.
IMS Core
ISC
IP-SM-GW
UE
OCS CGF/CDF
HSS
•SMS-GMSC/
•SMS-IWMSC
SMSC
SME
Ro Rf
Sh
E/Gd
1 SIP message
2 Translates instantmessages to shortmessages.
3 Sends short
messages to UEs.
⚫ After the UE sends a SIP message, the IP-SM-GW converts the instant message (SIP
message) to a text message. When the UE registers with IMS, the IP-SM-GW downloads
the SMSC address to a local directory.
Page37

38. NEs at the Service Layer - Anchor AS
(ATS)
The anchor AS anchors calls from the CS
network to the IMS network as follows:
⚫ Routes VoLTE subscribers' calls to the
IMS domain to trigger convergent
services by IN-based number redirection
of the anchor AS, when the VoLTE
subscribers originate or receive a call in
the CS domain.
⚫ Routes VoLTE subscribers' calls to the
IMS domain by IN-based number
redirection of the anchor AS, when the
VoLTE subscribers receive calls from CS
subscribers.
CS MSC server
IMS
Anchor AS
CS MSC server
IMS
Anchor AS
VoLTE subscriber
LTE
VoLTE
subscriber
AS
Page38

39. NEs at the Service Layer - RCS AS
• The RCS AS provides the OMA IM, Presence, XDMS, and DS functions to
implement the IM, Presence, and Network Address Book services.
RCS
Capability discovery
RCS-e
IM (1-1 and 1-M)
File transfer
Image/Video share + Voice
Offline storage and forwarding
Presence
NAB/CAB
VoLTE integration
Page39

40. NEs at the Core Layer
NE Name Function
IMS-HSS
(FE)
Controls subscriberroaming and stores the following information about a subscriber
in the IMS network: subscriberIDs,authentication data, service data, access
parameters,service trigger information, and roaming information.
SAE-
HSS/HLR
(FE)
Stores service-related data of subscribers,manages subscriptioninformation and
location information, and combines the following NEs:
• HSS in the EPC network
• HLR in CS and PS domains in the core network of the mobile telecommunication
system
USCDB
(BE)
Stores subscriberdata and implements the functions of adding, deleting, updating,
and querying data at the requestof the FE.
DNS/ENUM Implements functions provided by the DNS and the ENUM server.
eMSC
server
(SRVCC
IWF/CSFB
proxy)
Combines the following NEs:
• SRVCC IWF:provides eSRVCC to ensure call continuity when terminals move
from an LTE network to a 2G/3G network.
• CSFB proxy: moves LTE subscribers to a CS domain to provide voice services for
them.
MGCF
Realizes interworking between an IMS network and a PSTN or PLMN network.
If fixed network optimization is required or the live network provides a lot of fixed
network services,the UGC3200 is used as the MGCF. In other scenarios,the
MSOFTX3000is used as the MGCF.
Page40

41. NEs at the Core Layer (Cont.)
NE Name Function
MRFP
Provides resourcesforannouncement playing and digit collection,voice conferences,
SD video conferences.
I-CSCF/S-
CSCF/BGCF
/MRFC
Combines the following NEs to provide call control, service triggering, and routing
functions:
• I-CSCF:As the entrance to the IMS home network, the I-CSCF assigns or queries
the HSS to selectan appropriate S-CSCF to serve subscribers.
• S-CSCF:The S-CSCF is located on the home network as the central IMS node. It
implements subscription,authentication, session,routes, and service triggering.
• BGCF:The BGCF is located on the home network to selectan MGCF for calls
betweenan IMS network and a PSTN or PLMN network. It is integrated into the S-
CSCF to reduce the need for standalone BGCFs.Therefore,call connectiondelay
is shorten and the reliability is enhanced.
• MRFC: The MRFC controls the MRFP to play announcements and collectdigits,
and provide resources forvoice conferencesand SD video conferences.
Page41

42. NEs at the Core Layer (Cont.)
NE Name Function
P-
CSCF/SBC/ATC
F/ATGW
• P-CSCF:located on the visited network as the first contact point for
subscriberswithin an IMS domain. The P-CSCF forwards SIP signaling
betweenSIP subscribers and their home networks.
• SBC: provides the core bordergateway function (C-BGF) and interconnection
bordergateway function (I-BGF).The C-BGF is deployedat the edge of an
access network to provide attack protectionand NAT traversal. The I-BGF
provides mediachannels for an IMS network to interwork with other networks
that use SIP or H.323 signaling.
• ATCF/ATGW:located betweenthe P-CSCF and I-CSCF/S-CSCF and
interworks with the SCC AS to provide eSRVCC.
MGW/IM-MGW
Provides audio media channels for interworking between an IMS network and a
PSTN or PLMN network.
Page42

43. NEs at the Core Layer - SRVCC IWF
⚫ The SRVCC IWF is collocated
with an MSC server or deployed
as a standalone entity. If the
SRVCC IWF is deployed as a
standalone entity, it functions
as a proxy of the MSC server
and performs conversion for
signaling messages between
the MSC server, SBC, and MME.
EPC
CS
IMS Core
Sv
eMSC server
(SRVCC IWF)
SCC AS
MME
MSC
UE
SRVCC IWF
Handover request
MSC
MSC
MSC
1
Selects target MSC
2
⚫ The SRVCC IWF ensures call continuity when terminals move
from an LTE network to a 2G/3G network. When receiving the
location update request from a UE, the SRVCC IWF changes the
UE access location from the MME to the MSC server based on
the target cell ID contained in the location update request.
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44. NEs at the Core Layer - ATCF/ATGW
⚫ The ATCF/ATGW is locatedbetween the P-CSCF and I-CSCF/S-CSCF and
interworks with the SCC AS to provide eSRVCC.
• During a call, the ATCF or ATGW determines whether to anchor a call on the media
plane based on carriers' policies and terminal capabilities.
• During an eSRVCC handover, the ATCF or ATGW correlates a handover request
initiated by the SRVCC IWF with the anchored session, updates session bearer
information, and initiates a handover request to the SCC AS.
Signaling before a
handover
Bearer before a
handover
Signaling aftera
handover
Bearer after a
handover
Local handover duration: < 300 ms
PS
CS
SBC
IMS Core
Handover
SCC AS
SRVCC
IWF
2G/3G
LTE
Remote UE
ATCF
ATGW
Change is not required.
Page44

45. NEs at the Access Layer
NE Name Function
eMSC server
(mAGCF)
Helps ICS subscribers in a CS domain to register with an IMS domain
and provides basic and supplementary services for them.
S-GW/P-GW
• S-GW in an EPC domain: serves as an anchor point for the user
plane between access networks in 3GPP. The S-GW is used as an
interface to screen different access networks in 3GPP.
• P-GW in an EPC domain: serves as an anchor point between a
3GPP access network and non-3GPP access network. It provides
an interface for interworking between a 3GPP access network and
external PDN.
MME
Implements mobility management for LTE subscribers, including
subscriber context and mobility status management, and temporary
identity distribution.
Page45

46. Interfaces in the IMS and CS domains and
Interworking
Page46

47. Interfaces for Accessing
Page47

48. Interfaces and Protocols on the VoLTE
Network
Protocol Purpose
SIP SIP is used to control calls between IMS or CS NEs.
Diameter
Diameter is used to exchange the subscriptionand authentication information
betweenthe CSCF, the MME, and HSS. It is also used to exchange charging
information between the CCF/OCS and IMS NEs.
H.248
H.248 is used to exchange messagesbetweenthe media gateway controller
(such as the MSC server, MGCF, and MRFC) and the media gateway (such as
MGW, IM-MGW, and MRFP).
MAP
MAP defines a means of information exchange among NEs to realize the
roaming function and SMS of UEs.
CAP
CAP is used between the SSP/IM-SSF and SCP to allow CS or IMS subscribers
to use traditional IN services.
ISUP
ISUP is used forsignaling interworking betweenMSC servers,and between the
MSC server/anchorAS and the MGCF.
GTP-C
GTP-C is used to exchange messages betweenthe MME, SRVCC-IWF,and S-
GW.
SGsAP
SGsAP is used to perform mobilitymanagement and paging processesof short
message servicesbetweenan EPC domain and a CS domain.
HTTP HTTP is used to exchange messagesbetweenterminals and DMSs.
Page48

49. Interfaces and Protocols on the VoLTE
Network - SIP
Interface Location and Function Protocol
Mw
Located betweenthe P-CSCF,I-CSCF,and S-CSCF,the Mw interface
is used for message exchangebetweenCSCFs during IMS registration
and sessionprocessesand also functions as a proxy.
Located betweenthe ATCF and S-CSCF/SRVCC-IWF,the Mw interface
is used to control eSRVCC.
SIP
Mr
The Mr interface is located between the CSCF and the MRFC. By using
the Mr interface,the S-CSCF obtains network resource-relatedservices
from the MRFC, including announcement playing, digit collection,
conferencebridges,and video streams.
SIP
Mi
The Mi interface is located between the S-CSCF and the BGCF.When
the S-CSCF discoversthat a sessionneeds to be routed to the CS
domain, it uses the Mi interface to forward the sessionto the BGCF so
that the BGCF can selectan appropriate breakout to the CS domain.
SIP
Mj
The Mj interface is located between the BGCF and the MGCF. When
the BGCF receives sessioncontrol signaling over the Mi interface,it
selects an appropriate breakout to the CS domain. If the selected
breakout is in the same network as the BGCF,the BGCF uses the Mj
interface to forward the sessioncontrol signaling to the MGCF.
SIP
Page49

50. Interfaces and Protocols on the VoLTE
Network - SIP (Cont.)
Interface Location and Function
Protoco
l
Mg
The Mg interface is located between the I-CSCF and the MGCF. The
MGCF uses the Mg interface to forward sessioncontrolsignaling from
the CS domain to the I-CSCF so that the CS domain can interwork with
the IMS domain.
SIP
Mk
The Mk interface is located between BGCFs.This interface is used to
forward sessioncontrolsignaling from the BGCF that resides in the
same network as the originating S-CSCF to the BGCF that resides in
the same network as the MGCF.
SIP
Mm
The Mm interface is located betweenthe CSCF and other IP networks.
This interface supports IPv4 and IPv6 addresses.
SIP
ISC
The ISC interface is located betweenthe S-CSCF and the AS. Based
on the iFC template data obtained from the HSS and SIP service
requests sent from an IMS UE, the S-CSCF determineswhether to
trigger a service. If the service needs to be triggered,the S-CSCF
routes the sessionrequestover the ISC interface to a specific AS for
final processing.
SIP
Page50

51. Interfaces and Protocols on the VoLTE
Network - SIP (Cont.)
Interface Location and Function Protocol
Nc
The Nc interface is located between the MSC server/anchorAS and
the MGCF and between MSC servers.The main functions of this
interface are as follows:
•Manages mobility and handover and provides interworking for CS
subscribers.
•Allows CS subscribers to be anchored in an IMS network.
ISUP/SIP
I2
The I2 interface is located between the mAGCF and the ATCF/I-
CSCF/S-CSCF.This interface is used between a CS network and an
IMS network to allow CS subscribersto access an IMS network. This
interface is also used to convert messages sentfrom the A/Iu-CS
interface to IMS SIP signaling messages.
SIP
Gm
The Gm interface is located betweenan IMS UE and the P-CSCF for
registration and sessioncontrol forIMS subscribers.
SIP
Page51

52. Interfaces and Protocols on the VoLTE
Network - Diameter
Interfac
e
Location and Function Protocol
Cx
The Cx interface is located between the CSCF and the IMS-HSS. The
CSCF and the IMS-HSS use the Cx interface to perform the following
operations:
• The I-CSCF queries the IMS-HSS forinformation required for selecting
an S-CSCF.
• The CSCF queries the IMS-HSS for routing information.
• The CSCF queries the IMS-HSS for roaming authorization information.
• The CSCF downloads from the IMS-HSS security parameters required
for IMS subscriberauthentication.
• The IMS-HSS sends iFC data associated with the IMS sessionto the
CSCF.
Diameter
Dx
The Dx interface is located between the CSCF and the SLF. If multiple
HSSs are deployed onan IMS network, the CSCF uses the Dx interface
to query the SLF forthe HSS that stores subscriptiondata of a subscriber.
The Dx interface is not required when only one HSS is deployed.
Diameter
Sh
The Sh interface is located between the IMS-HSS and the SIP AS.The
SIP AS uses the Sh interface to query the IMS-HSS for value-added
service data and to synchronize data to the IMS-HSS.
Diameter
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53. Interfaces and Protocols on the VoLTE
Network - Diameter (Cont.)
Interface Location and Function Protocol
Dh
The Dh interface is located between the SLF and the SIP AS.The AS
uses the Dh interface to identify the HSS that stores subscriberdata
based on the subscriberidentity and home domain information.
Diameter
Rf
The Rf interface is located between the CCF and other entities such as
the CSCF,MRFC, BGCF,and AS.This interface is used for session-
related offline charging.
Diameter
Ro
The Ro interface is located between the OCS and other entities such as
the AS, MRFC, and OCG.This interface is used for session-related
online charging.
Diameter
Rx
The Rx interface is located between the PCRF and the application entity
such as the P-CSCF.The Rx interface is used forsessioninformation
exchange at the application layer. Based on the sessioninformation, the
PCRF determines the policies and charging control schemes to be used.
The PCRF exchanges the decided policies and charging control
schemes with the PCEF.
Diameter
Page53

54. Interfaces and Protocols on the VoLTE
Network - Diameter (Cont.)
Interface Location and Function Protocol
Gx
The Gx interface is located between the PCRF and the PCEF (such as
the P-GW).The PCRF uses the Gx interface to deliver charging policies
to the PCEF,and the PCEF uses the Gx interface to reportbearer events
to the PCRF.
Diameter
S6a
The S6a interface is located betweenthe MME and the SAE-HSS.This
interface is used to transmit subscriptionand authentication data for
authorized subscribersto access the EPC network.
Diameter
Page54

55. Interfaces and Protocols on the VoLTE
Network - H.248
Interface Location and Function Protocol
Mp
The Mp interface is located between the MRFC and the MRFP. The
MRFC uses the interface to control the MRFP for announcement
playing, conferences,and DTMF digit collection.
H.248
Mn
The Mn interface is located between the MGCF and the IM-MGW. The
MGCF uses the Mn interface to control mediastreams transmitted on
the IM-MGW and allocation of specialresources.
H.248
Mc
The Mc interface is located between the MSC serverand the MGW.The
MSC serveruses the Mc interface to control media streams transmitted
on the MGW and allocation of specialresources.
H.248
Page55

56. Interfaces and Protocols on the VoLTE
Network - MAP
Interface Location and Function Protocol
E/Gd
The E/Gd interface is located between the IP-SM-GW and the SMSC for
SMS interworking between an IMS network and a CS domain.
MAP
E
The E interface is located between the MSC server and the SMSC for
SMS interworking.
MAP
Si
The Si interface is located between the IM-SSF and the HLR and is used
to obtain IN service information about subscribers.
MAP
J
The J interface is located betweenthe IP-SM-GW and the HLR and is
used to obtain the routing information about short messages.
MAP
C/D
The C/D interface is located betweenthe MSC server/MGCF/SMSC and
the HLR. The MSC server/SMSC and the HLR use the C/D interface to
perform the following operations:
• The MSC server and SMSC query the HLR for routing information.
• The MSC server sends a location update requestto the HLR.
• The MSC server and the MGCF query the HLR for the roaming
numbers and subscriptioninformationabout subscribers.
• In the convergentHLR/HSS anchoring solution, the HLR uses this
interface to obtain the IMRN from the MGCF.
MAP
Page56

57. Interfaces and Protocols on the VoLTE
Network - GTP-C
Interface Location and Function Protocol
S11
The S11 interface is located between the MME and the S-GW.The MME
and S-GW use the S11 interface to transmit bearer setup,update, and
deletionmessages.Whena UE is in the ECM-IDLE state, the S-GW
instructs the MME to page the UE to restore the S1 bearer.
GTP-C
Sv
The Sv interface is located between the MME and the SRVCC IWFto
control the eSRVCC handover.
GTP-C
Page57

58. Interfaces and Protocols on the VoLTE
Network - CAP/HTTP/SGsAP
Interface Location and Function Protocol
CAP
The CAP interface is located betweenthe MSC server/IM-SSF/anchor
AS and the SCP.The CAP interface allows CS or IMS subscribers to use
traditional IN services and CS subscribers to be anchored in an IMS
network.
CAP
HTTP
The HTTP interface is located betweena UE and the DMS. The terminal
uses the HTTP interface to perform self-help upgrade and fault
diagnosis.
HTTP
SOAP
The SOAP interface is located between the SPG and the AS/HSS.The
SPG uses the SOAP interface to deliver commands that are run on the
SPG2800 web portal to the AS or HSS.
HTTP
SGs
The SGs interface is located betweenthe MME and the CSFB MSC
server/CSFB proxy.The SGs interface is used to perform mobility
management and paging procedures betweenan EPC domain and a CS
domain in the CSFB scenario.
SGsAP
Page58