IMS + VoLTE Overview - Overall Network Topology Review - History of IMS - What's the IMS, VoLTE, ViLTE? - Motivations of IMS - R99, R4 & R5 Mobile Network Architecture - IMS Network Structure - IMS Network Enitities - CSCF - IMS Interfaces and Protocols - SIP Overview - IMS Registration Flow - IMS Basic Session Establishment Flow - IMS Basic Call Flow Signaling - Conclusion

1. IMS + VoLTE
Overview
Hamid Reza Bolhasani
CS/PS/EPC/IMS Consultant
Bolhasani@gmail.com
Aug 2018

2. 2
Table of Contents
 2G/3G/4G Overview
 What is IMS/VoLTE/ViLTE?
 IMS / History & Motivation
 IMS Network Structure
 IMS Reference Points & Protocols
 IMS Basic Signaling
 VoLTE Call Flow
 CSFB (Circuit Switch Fall Back)
 SRVCC (Single Radio Voice Call Continuity)
 Question & Answer
 Conclusion
 Exam

3. 3
 2G/3G Overview
GSM /GPRS BSS
BTS
BSC
NodeB
RNC
PCU
UTRAN
SCP
SMS
SCE
PSTN
ISDN
Internet,
Intranet
MSC/VLR GMSC
HLR/AUC
SGSN
CG BG
GGSN
GPRS
Other PLMN
IPBB

4.  4G/LTE Network Topology

5.  2G/3G/4G Network Overview
2G
GSM
3G
UMTS
4G
LTE
RNC
BTS
BSC
BTS
NodeB
NodeB
eNodeB
eNodeB
MSC
MGW
CS
Domain
PS
Domain
SGSN
GGSN
HSS (HLR)
SMC
EIR
OCS OFCS
MME
MME
SGW PGW
PCRF
EPC
Domain

6.  2G/3G/4G Network Overview

7.  What is IMS?
IP Multimedia Service
Session Service：
Voice and Video Call
Conference
Message
Non-Session Service：
IPTV、Media、Web
Multiple Access Modes
Mobile Network：
WiMax
LTE/ SAE
GSM/ WCDMA/
CDMA/ TD-SCDMA
Fixed Network：
LAN、WLAN、xDSL
IMS is IP Multimedia Subsystem
 Base on IP bear network
 Use SIP protocol as core session control protocol
 Support access-independent and thus provide more competitive service packages

8.  IMS Motivation / All IP Convergent
Legacy Core Network Arch.
Bearer
Control
Service
Charging
Database
Internet
Bearer
Control
Service
Charging
Database
Mobile
Network
Bearer
Control
Service
Charging
Database
Fixed
Network
 Vertical network, overlap in
hardware, service and database
 Separated user and service
 Difficult to decrease OPEX/CAPEX
Multiple Application
IP Bearer
IMS Core
(Unified session control)
Unified Database/Charging
Converged Multi-service Arch.
Multiple Access
 Horizontal network, converge fixed,
mobile and data service network
 Unified database and service
 Access independent
Convergent
Overlapping Cost

9.  History of IMS
2000/03 2001/03 2002/03 2005/12 2006/X
3GPP R99
-CN: TDM
3GPP R4
- CN: MSC
server and
MGW
3GPP R6
- IMS QoS,
Billing, FMC,
etc
3GPP R7
- IMS
enhance
3GPP R5
- CN: IMS
introduced
TISPAN R1
-- NGN/FMC, etc
TISPAN R2
-- NASS/RACS,
etc
The IMS is introduced as part of 3GPP specifications at the R5 stage, act as
the subsystem of PS domain.

10.  Network Architecture of 3GPP R99 and R4
MSC
RAN
MSC server
MGW
R99 CS domain R4 CS domain
 In R4, MSC divided into MSC server and
MGW, call control is separated with the
media bearer function.
CS - MSC/VLR/gsmSSF
PS - GGSN/SGSN
3G Core NetworkAccess External Network
HLR/SCP
R99NetworkArchitecture
TUP/ISUP
TDM
TUP/ISUP/BICC
TDM/ATM/IP
 In R99, 3G Core Network is separated into CS
and PS Domain.

11.  3GPP R4 Network Architecture
Control
Layer
Bearer
Layer
MSC
H.248

12.  3GPP R5 Network Architecture

13.  IMS Network Structure
Gaming Conference
3rd Party
ApplicationsSCP
MGCF
I-CSCF
AGCFP-CSCF
S-CSCF
HSS
Bearer Control Layer
AG
IAD
PSTN/PLMN
MGW
SIP H.248
xDSL/LAN access
WiFi access
AP
SGSN
2G/3G RAN
SIP Terminals
SIP
SIP
Bearer Core NetworkGGSN AC
OMS
OSS
BGCF
IM-SSF OSA Presence PTT IM Group
MRFC
MRFP
Bearer
Control and
Access
Session
control
Service
capability
Application
A-RACF
CCF
SPDF
PCRF
H.248
NACF CLF

14.  IMS Network Layers Architecture
Application
Session Control
Service capability
Access
 Access and bearer
 User access-- GPRS/UMTS/CDMA/
WiFi/xDSL/LAN
 Connects the IMS to the current network
-- PSTN/PLMN/H.323/SIP VoIP
 Controls the bearer resource
 Session control layer
 Functions -- Call control, User
management, Service triggering,
Resources control, Interworking with
current network
IP Bearer
P/I/S-CSCF, MRFC
AGCF/MGCF
ARACF/ SPDF/ PCRF

15.  IMS Network Entities
The Network Elements of IMS in Session Control Layer are:
Function NE Function NE
Call control
P－CSCF
Network
interworking
MGCF
I－CSCF IM－MGW
S－CSCF BGCF
User
management
HSS
Media
resource
MRFC
SLF MRFP

16.  CSCF - Call Session Control Function
CSCF Key Functions
P-CSCF
(Proxy)
1: The 1st contact point to the IMS network in the
visiting domain or home domain
2: Access network control
3: QoS control, NAT control and security control
I-CSCF
(Interrogating)
1: First entry to the IMS network of a carrier
2: S-CSCF assignment and session routing
3: Topology hiding
S-CSCF
(Serving)
1: User registration authentication
2: Session route control (Normal, interworking,
Emergency call)
3: Service trigger
SCP SIP AS
HSS
database
S-CSCF
P-CSCFAGCF
MSAN
SIP Phone
MGCF
I-CSCF
Broadband WIFI
Legacy
network
3rd party AS
IP Bearer
NASS Subsystem RACS Subsystem

17.  Simple Model for IMS Call Procedure
While two IMS users call each other, the P/I/S-CSCF can handle the whole
signaling routing procedures.
S-CSCF I-CSCF
P-CSCFP-CSCF
S-CSCF
Caller part
Callee part
signaling media
Media
HSSDNS/ENUM

18.  IMS Interworking Nodes
NE Key Functions
MGCF Media Gateway Control Function
1:Controls the IMS-MGW for establish/modify/delete media channels
2: Selects the I-CSCF for incoming calls from PSTN/CS.
3: Performs protocol conversion between ISUP and SIP.
IM-MGW IM-MGW-- IMS-Media Gateway Function
1: Terminate bearer channels from a switched circuit network and
media streams from a packet network
BGCF Breakout Gateway Control Function
1: Select a proper MGCF for interworking with the PSTN/CS domain
SCP SIP AS
HSS
S-CSCF
PCSCFAGCF
MSAN
SIP Phone
MGCF
I-CSCF
Broadband WIFI
Legacy
network
3rd party AS
IP Bearer
NASS Subsystem RACS Subsystem
MGWBGCF

19.  Connection to Legacy Networks
While an IMS user call a PSTN/CS user
PLMN/PSTN
S-CSCF MGCFBGCF
While an IMS user call a PSTN/CS user which belongs to other operator
PLMN/PSTN
S-CSCF BGCF 2BGCF1 MGCF

20.  Database Function – HSS and SLF
NE Key Functions
HSS
Home
Subscriber
Server
1: User Identification, Numbering and addressing information;
2: User Security information: Network access control information
for authentication and authorization;
3: User Location information at inter-system level;
4: User profile information (iFC, etc).
SLF
Subscription
Locator
Function
When an operator has more than one HSS,
SLF is used to select the related HSS, and usually SLF is
combined with the HSS
SCP SIP AS
HSS/SLF
database
S-CSCF
P-CSCFAGCF
MSAN
SIP Phone
MGCF
I-CSCF
Broadband WIFI
Legacy
network
3rd party AS
IP Bearer
NASS Subsystem RACS Subsystem

21.  Database Function – HSS and SLF
IM CN subsystem
CxC ShGr GcD
MSC / VLRGMSC
CS Domain
SGSN GGSN
PS Domain
CSCF
HSSMobility Management
User security info. generation
User security support
Service Provisioning support
Identification handling
Service authorization support
Access authorization
Session establishment support
Si
IM-SSF
OSA-SCS
Application Services Support
CAMEL Services Support
SIP AS
IMS Domain

22.  IMS Multimedia Resource Function
NE Key Functions
MRFC Multimedia Resource Function Controller
1: Controls the media stream resources in the MRFP.
2:Interprets information coming from an AS and S-CSCF (e.g
session identifier) and control MRFP accordingly.
MRFP Multimedia Resource Function Processor
1: Provides resources to be controlled by the MRFC.
2: Mixes incoming media streams (e.g. for multiple parties).
3: Processes media streams (e.g. audio transcoding, media analysis).
SCP SIP AS
HSS/SLF
database
S-CSCF
P-CSCF
MSAN
SIP Phone
MGCF
I-CSCF
Broadband WIFI
Legacy
network
3rd party AS
IP Bearer
NASS Subsystem RACS Subsystem
MRFC
MRFP

23.  Interfaces of IMS Network
UTRANUE
BGCF
BGCF
P-CSCF I-CSCF S-CSCF MRFC MRFP
HSSSLF
OSA-SCSIM-SSFSIP AS
SCP 3rd party AS
GGSN
IM-MGW
MGCF
PCRF
SGSN
SIP Terminal
MrMw Mw Mp
Mi
Mk
Mg
Mj
Mn
Ut
Rx
Gx
Cx CxDx ISC ISCISC
Sh
Si Sh
Dh
Other IP/IMS
Network
PLMN/PSTN
SBC
Gm
Mx
Gm
CCF
Rf
Rf
Mg

24.  IMS Interface and Protocols
NE A NE B Interface
Application
Layer Name
Transport
Layer Protocol
UE P-CSCF Gm SIP IPSec / UDP
PGW P-CSCF SGi IP IP
P-CSCF I-CSCF Mw SIP UDP
S-CSCF MGCF Mj SIP UDP
S-CSCF ATS ISC SIP UDP
MGCF I-CSCF Mg SIP UDP
SRVCC IWF ATS I2 SIP UDP
P-CSCF PCRF Rx Diameter SCTP
I-CSCF HSS Cx Diameter SCTP
S-CSCF HSS Cx Diameter SCTP

25.  SIP & H.248 / Reference Points
Reference point (SIP) Endpoints
Gm UE，PCSCF
Mw P-CSCF, I-CSCF ,S-CSCF
ISC I-CSCF, S-CSCF,AS
Mg S/I-CSCF,MGCF
Mi S-CSCF, BGCF
Mj BGCF,MGCF
Mk BGCF,BGCF
Mx SBC,P-CSCF
Mr S-CSCF,MRFC
Reference point (H.248) Endpoints
Mn MGCF, IM-MGW
Mp MRFC, MRFP

26.  Diameter / Reference Points
Reference point Endpoints
Cx I-CSCF, S-CSCF, HSS
Dx I-CSCF, S-CSCF, SLF
Rx P-CSCF,PCRF
Gx GGSN，PCRF
Rf CCF,S-CSCF,AS,MRFC
Sh HSS, AS
Dh SLF, AS
Diameter
P-CSCF
AS
SIP
SIP
SIP
SIP
HSS
Diameter
I-CSCF S-CSCF
SIP
OCS
Diameter
CCF
Diameter
AGCF
POTS
AG
POTS
AG
Diameter

27.  IMS Evolving Target Network
HG
2G/3G
MGW
mAGCF
SBC
MSAN
AGCF
Application
Server
HSS
IMS
AS1 AS2 AS3 AS4 AS...
SIP
SIP
SIP
CSCF…
MGW
H.248 H.248
SIP
MGCF
PSTN/PLMNM
SIP ISUP/BICC
SIP
SIP
SIP NarrowbandBroadband

28.  SIP Protocol
 The SIP was originated in Mbone (Multicast Backbone) experiment in 1996.
 IETF Standard defined by RFC 3261. (July, 2002)
 The Session Initiation Protocol (SIP) is an application layer protocol (Signaling) for creating,
modifying and terminating sessions with one or more participants.
 Can be used for Voice, Video, Instant Messaging, Gaming, etc.
 SIP is Text based like HTTP, Request / Reply Protocol and mostly used on UDP.

29.  SIP Protocol
 Use URIs for addressing – Single Communications Identity
 mailto:dbaron@MIT.edu for email
 xmpp:dbaron@MIT.EDU for instant messaging
 sip:dbaron@MIT.EDU for voice and video
 Username replaced by numbers for telephone applications.
◇The URI in a SIP request is used for call routing or subscriber
location. The URI consists of the following parts:
Sip:28970808@huawei.com:5061;user=phone?Subject=foo
• Scheme
• Username(optional parameter)
• Password(optional parameter)
• Host name
• Port(optional parameter)
• URI parameter
• Header and Body(optional parameter)

30.  SIP Components
 User Agents
 Cleints – Make Request
 Servers – Accept Requests
 Server Types
 Redirect Server
 Proxy Server
 Registrar Server
 Proxy Server
 Location Server
 Gateways
DNS
Server
Location
Server
Terminating
User Agent
Outbound
Proxy
Originating
User Agent
DNS
SIP
SIP
SIP SIP
RTP
Registrar
Inbound
Proxy
SIP

31.  Simplified SIP Network Architecture
SIP Request SIP Request
Media Stream (RTP)
User Agent
(Server)
Receives SIP
Requests
Request Proxy Server
Determines Where to Send
the Signaling Messages
User Agent
(Client)
Sends SIP
Requests
SIP ResponseSIP Response

32.  Logical Entities of SIP
Logical SIP entities
User Agents (UA)
 User Agent Client (UAC):
Initiates SIP requests
 User Agent Server (UAS):
Returns SIP response
 Both UAC and UAS can
terminate a call
 Register: Maintains the location of SIP
users, sip clients needs to update the
location using register request
 Proxy: decides next hop and forwards
request
 Redirect: Accepts SIP request and
translates to new address
Network Servers

33.  SIP Methods
Method Description
INVITE Requests a session
ACK Final Response to the INVITE
OPTIONS Ask for server capabilities
CANCEL Cancels a pending request
BYE Terminate a session
REGISTER Send user’s address to server

34.  SIP Responses
Response Type Sample
1XX Provisional 100 Trying
2XX Successful 200 OK
3XX Redirection 302 Moved Temporarily
4XX Client Error 404 Not Found
5XX Server Error 504 Server Timed-Out
6XX Global Failure 603 Decline

35.  SIP Flows - Basic

36.  SIP Flows - Basic
 Session setup
Client A Proxy A Proxy B Client B
INVITE INVITE
INVITE100 TRYING
100 TRYING
180 RINGING
180 RINGING
180 RINGING 200 OK
200 OK
200 OK
 Registration
Client A Proxy A
Register
401 unauthorized
Register
200 OK
100 TRYING

37.  SIP Message Structure
 SIP is a Text based protocol and
comprised by 3 parts:
 Request-Line
 Header
 Body

38.  SIP Request / Response Example

39.  SDP Example
SDP
Parameter
Parameter
Name
Remarks
v Version number v=0
o Origin containing
name
o=<user name> <session id> <version> <network
type> <address type> <address>
s Subject
c Connection Connection IP address(10.216.6.108)
t Time t=<start time> <stop time>
m Media Media format (audio); Port number(17368)
a Attribute Media encoding (PCM A Law); Sample rate (8000Hz)
v=0
o=HuaweiSoftX3000 868 868 IN IP4 10.216.9.200
s=Sip Call
c=IN IP4 10.216.6.108
t=0 0
m=audio 17368 RTP/AVP 8
a=rtpmap：8 PCMA/8000

40.  IMS User Registration Procedure
Home
network
(4)
(6)
401
S-CSCF
HSS
(3)
S-CSCF assignment
(1)
Register
(2)
Register
(7)
401
(8)
401
P-CSCF
I-CSCF
Home
network
(4)
(6)
401
S-CSCF
HSS
(3)
S-CSCF assignment
(1)
Register
(2)
Register
(7)
401
(8)
401
P-CSCF
I-CSCF
Visit
network

41.  IMS User Registration Procedure
2－2nd register message
(10)
Register
(12)
Register
(14)
Register
(15)
User profile
(16)
200 OK
(17)
200 OK
(18)
200 OK
S-CSCF-A
HSS-A
I-CSCF-A
P-CSCF-A
I-CSCF-A
(13)
Assign S-CSCF
UE
1－ Initial register message
(1)
Register
(3)
Register
(5)
Register
(6)
Authentication
data
(7)
401
(8)
401
(9)
401
S-CSCF-A
HSS-A
I-CSCF-A
P-CSCF-A
I-CSCF-A
(4)
Assign S-CSCF
UE
DNS
(2)
DNS query
(11)
DNS query

42.  IMS Registration Signaling Flow
Home Network
S-CSCFCaller UE P-CSCF DNS I-CSCF HSS
AS
1.REGISTER
2.DNS Query
3.DNS Query
Resp
4. REGISTER
7.REGISTER
8.Diameter: MAR
9.Diameter: MAA10. 401
Unauthorized
11. 401 Unauthorized12. 401
Unauthorized
13.REGISTER
14.DNS Query
15.DNS Query
Resp
16. REGISTER
19.REGISTER
25.REGISTER
26. 200 OK
22. 200 OK
24. 200 OK
23. 200 OK
5. Diameter: UAR
6. Diameter: UAA
17. Diameter: UAR
18. Diameter: UAA
20.Diameter: SAR
21.Diameter: SAA
Initiate
register
Send the
authentication
parameters to UE
S-CSCF
assignment
Download the
authentication
sets
S-CSCF
assignment
Download
service profile
（iFC etc..)
Authentication
response
3rd party
registration

43.  Related Diameter Messages
Message Explanation
UAR User Authorization Request
UAA User Authorization Answer
MAR Multimedia Authentication Request
MAA Multimedia Authentication Answer
SAR Server Assignment Request
SAA Server Assignment Answer

44.  Simple IMS Call Flow
S-CSCF I-CSCF
P-CSCFP-CSCF
S-CSCF
Caller part
Callee part
1.request
2.request
3.request 4.request
5.request
6.request
Signaling Media
media
HSS
MO S
S
MT
DNS/ENUM

45.  Basic Session Establishment between 2 IMS Users
Invite
Invite
Invite
Invite
Invite
Invite
183
183
183
183
183
183
media
S-CSCF-A
HSS-A
P-CSCF-A
I-CSCF-A
I-CSCF-B
HSS-B
S-CSCF-B
P-CSCF-B
DNS/ENUM
Calling party Called party

46.  IMS User Call PSTN User
IMS calling
party
Invite
S-CSCF-A
HSS-A
P-CSCF-A
UMG
I-CSCF-A
MGCF
Invite
Media
control
PST
N
PSTN called party
IAM
Invite
ENUM
Query
Tel
Fail
Invite

47.  PSTN User Call IMS User
I-CSCF-A
HSS-A
P-CSCF-A
UMG
S-CSCF-A
MGCF PSTN/PLMN
PSTN
calling
party
IAM
InviteInvite
Invite
Invite
IMS called party

48.  IMS Session Establishment Signaling Flow
The IMS user
performs
media
negotiation to
determine the
media IP
address,
media stream
type, and
coding/decodi
ng methods.
UE1/PCSCF1 SCSCF1/AS1 ICSCF2
INVITE
INVITE
183183
PRACK PRACK
200 OK (UPDATE)
180 Ringing
200 OK (INVITE)
ACK ACK
200 OK(PRACK) 200 OK(PRACK)
UPDATE UPDATE
PRACK
200 OK
200 OK (UPDATE)
180 Ringing
PRACK
200 OK
200 OK (INVITE)
The session is
established.
SCSCF2/AS2 PCSCF2/UE2
INVITE
INVITE
183 183
PRACK
200 OK(PRACK)
UPDATE
200 OK (UPDATE)
180 Ringing 180
Ringing
PRACK
200 OK
200 OK (INVITE) 200 OK (INVITE)
ACK

49. 49
Thanks ForYour Attention!
Hamid Reza Bolhasani
Bolhasani@gmail.com