The IP Multimedia Subsystem (IMS)

(1)

The IP Multimedia Subsystem (IMS)

Miguel A. Garcia Martin

Nokia Siemens Networks Application and Services in Internet

Helsinki University of Technology

October 24, 2007

(2)

• Brief Introduction to IMS

• IMS Architecture

• IMS signalling flows

(3)

Circuit-switched network

(2G or 3G)

The need for IMS

The circuit-switched network can establish

connections between terminals

Packet-switched network

(GPRS or 3G)

IP connections with GPRS or 3G also exist...

…but also a standardised way of using IP services

BETWEEN TERMINALS is needed

?

(4)

Circuit-switched network

(2G or 3G)

Packet-switched network

(GPRS or 3G)

Person-to-person IP connection

Any IP connection (e.g. GPRS, EDGE,

WCDMA, WLAN)

IP-based services possible between

terminals!

IP Multimedia Subsystem

Find and connect

SIP SIP

IP Multimedia Core finds the other

terminal and connects

the terminals with IP

(5)

Circuit-switched network

(2G or 3G)

Packet-switched network

(GPRS or 3G)

Person-to-Group IP connection

Any IP connection (e.g. GPRS, EDGE,

WCDMA, WLAN)

IP-based services possible between

terminals!

IP Multimedia Subsystem

Find and connect

SIP SIP

IP Multimedia Core finds the other

terminal and connects

the terminals with IP

(6)

SIP proxy

Packet-switched network

(GPRS or 3G)

Person-to-Machine IP connection

Any IP connection (e.g. GPRS, EDGE,

WCDMA, WLAN)

IP-based services possible between

terminals!

Find and connect

SIP SIP

IP Multimedia Core finds the other

terminal and connects the terminals with IP

IP Multimedia

Subsystem

(7)

Multimedia Communication – Rich Calls

IMS enables IP connections between terminals over different access networks, and even connections to circuit switched terminals

Rich communication is possible between IP terminals and can include e.g.:

• Voice

• Video

• Content sharing

• Application sharing

• Text

• etc.

… within one session, combined flexibly during the session

Sessions may have multiple participants

(8)

IMS in a nutshell

• IMS is a Multimedia (Call/) Control Service over packet networks

• IMS builds a common architecture for offering new services in the packet domain

– Including single login (authentication)

– Including access charging, service charging and content charging

• IMS allows two or more users to establish multimedia sessions.

– Including Quality of Service

• IMS introduces enhanced network services

– Including Presence, Conferencing, Push, Chat, Push-to-talk,

…

• Signalling plane is separated from user plane – Signalling plane always routed home

• IMS is access agnostic

(9)

IMS architecture

(10)

IP Multimedia Subsystem (IMS) architecture

PS

UE

SGSN

Internet

IMS

GGSN

Application Server

SIP phone Media Server

Gi/Mb

Mw Mg

Mb

Mn

TDM

MGW

ISUP Mb

Mb

SGW – Signaling Gateway PDG – Packet Data Gateway

Mp

PSTN

CPE

ISC

Radio Network

Cx

UE PDG

Go

SGW

Signaling

CSCF — Call Session Control Function MGW — IM-Media Gateway

MGCF — Media Gateway Control Function

PS

Cx

SIP WLAN

MRF HSS I-CSCF

P-CSCF WLAN

AP

SIP phone

Other TCP/IP network

SIP phone

ADSL/cable

S-CSCF

Mw

P-CSCF

(11)

Node classification (1)

P-CSCF

I-CSCF

HSS

SLF P-CSCF

BGCF

Mj Mk

Cx Dx

Cx

Dx

Mp Sh Mr

Sh Si

ISC ISC ISC

Mn Mw

Mw

Mw Mw

Mw

Mi

SIP-AS OSA-SCS IM-SSF

S-CSCF

MGCF SGW

MGW MRFP MRFC

• HSS (Home Subscriber Server)

• SLF (Subscription Locator Function)

Database elements

• S-CSCF – Serving Call/Session Control Function

• P-CSCF – Proxy Call/Session Control Function

• I-CSCF – Interrogating Call/Session Control Function

IMS control elements

• MGCF - Media Gateway Control Function

• BGCF – Breakout Gateway Control Function

• SGW – Signaling Gateway

• MGW – Media Gateway

Interworking Elements

(12)

Node classification (2)

P-CSCF

I-CSCF

HSS

SLF P-CSCF

BGCF

Mj Mk

Cx Dx

Cx

Dx

Mp Sh Mr

Sh Si

ISC ISC ISC

Mn Mw

Mw

Mw Mw

Mw

Mi

SIP-AS OSA-SCS IM-SSF

S-CSCF

MGCF SGW

MGW MRFP MRFC

• MRFC – Media Resource Function Controller

• MRFP – Media Resource Function Processor

Media server

• SIP AS – Session Initiation Protocol Application Server

• OSA-SCS AS – Open Service Architecture Service Capability Server

• IM-SSF AS – IP Multimedia Service Switching Function

Application Servers

(13)

The HSS and SLF

P-CSCF

I-CSCF

HSS

SLF P-CSCF

BGCF Mj Mk

Cx Dx

Cx

Dx

Mp Sh Mr

Sh Si

ISC ISC ISC

Mn Mw

Mw

Mw Mw

Mw

Mi

SIP-AS OSA-SCS IM-SSF

S-CSCF

MGCF SGW

MGW MRFP MRFC

• Main user database

• Stores IMS user related information

• Authentication vectors

• Service profile

• Considered an evolution of the GSM HLR

• Acts as a Diameter server

• Protocol: Diameter

Home Subscriber Server (HSS)

• Required when there is more than one HSS

• Looks up the user identity and finds its HSS

• Acts as an enhanced Diameter relay

• Protocol: Diameter

Subscription Location Function (SLF)

(14)

The S-CSCF

P-CSCF

I-CSCF

HSS

SLF P-CSCF

BGCF Mj Mk

Cx Dx

Cx

Dx

Mp Sh Mr

Sh Si

ISC ISC ISC

Mn Mw

Mw

Mw Mw

Mw

Mi

S-CSCF

MGCF SGW

MGW MRFP MRFC

• Central control plane node

• Provides session control

• Always located in the home network

• Acts as a SIP proxy and SIP registrar

• Acts as a Diameter client towards the SLF/HSS

• Performs authentication of the user

• Triggers services (but do not execute them)

• Applies network and user related policy

• Is dynamically allocated at registration time

• Generates charging records

• Protocols: SIP and Diameter

Serving Call/Session Control Function

(15)

The I-CSCF

P-CSCF

I-CSCF

HSS

SLF P-CSCF

BGCF Mj Mk

Cx Dx

Cx

Dx

Mp Sh Mr

Sh Si

ISC ISC ISC

Mn Mw

Mw

Mw Mw

Mw

Mi

S-CSCF

MGCF SGW

MGW MRFP MRFC

• First entry node in the home network

• Selects a S-CSCF for a user, when required

• Acts as a SIP proxy server

• Acts as a Diameter client towards the SLF/HSS

• Queries the HSS to find out the S-CSCF allocated to the user

• Generates charging records

• Protocols: SIP and Diameter

Interrogating Call/Session Control Fction.

(16)

The P-CSCF

P-CSCF

I-CSCF

HSS

SLF P-CSCF

BGCF Mj Mk

Cx Dx

Cx

Dx

Mp Sh Mr

Sh Si

ISC ISC ISC

Mn Mw

Mw

Mw Mw

Mw

Mi

S-CSCF

MGCF SGW

MGW MRFP MRFC

• First signaling point of contact from the UE

• Acts as an outbound SIP proxy server

• Located either in the visited or home network

• Performs compressions/decompression of SIP and SDP

• Applies local policy to media streams

• Interacts with the policy elements to authorize media streams

• Establishes an IPsec connection to the IE

• Generates charging records

• Protocols: SIP, IPsec, Sigcomp, Diameter

Proxy Call/Session Control Function

(17)

The PCRF

• Policy Control Decision

• Gets dynamic session information from the P- CSCF

• Gets subscriber policy information from the SPR (Subscription Profile Repository)

• Performs service data flow detection, gating, QoS and service flow charging

• Decides how a service data flow is treated at the GGSN

• Authorizes QoS resources

• Protocols: Diameter

Policy Control and Charging Rules Ftion.

(18)

The PCEF

• Policy Enforcement Function, integrated in the GGSN, PDG, or similar node

• Performs gate enforcement:

• A data flow passes through the GGSN

• Performs QoS enforcement:

• Controls bandwidth usage and packet forwarding time

• Performs credit control

• A data flow passes through the GGSN if there is enough credit to pay for it

• Protocols: Diameter

Policy and Charging Enforcement Ftion.

GGSN (incl.

PCEF) PCRF P-CSCF Rx

Gx

Online Charging System Gy

Offline Charging System Sp

Gz

SPR

(19)

Media Resource Function

P-CSCF

I-CSCF

HSS

SLF P-CSCF

BGCF Mj Mk

Cx Dx

Cx

Dx

Mp Sh Mr

Sh Si

ISC ISC ISC

Mn Mw

Mw

Mw Mw

Mw

Mi

S-CSCF

MGCF SGW

MGW MRFP MRFC

• Provides additional media resources

• Media announcement, IVR, transcoding, conferencing, etc.

Media Resource Function (MRFC+MRFP)

• Acts as a SIP User Agent

• Controls resources in MRFP via H.248

• Generates charging records

• Protocols: SIP, H.248

Media Resource Function Controller

• Mixes media

• Offers media resources (e.g., announcements)

• Provides floor control

• Protocols: RTP, MSRP, H.248

Media Resource Function Processor

(20)

Interworking with CS networks

• Needed when the IMS boundary is crossed to a Circuit Switched network (e.g., PSTN, PLMN, etc.)

• May take place at a home or third party network

• Both control and user plane

ISUP/MTP ISUP/IP

SIP

H.248

RTP

PCM

BGCF

IMS PSTN

SIP

SGW

MGCF

MGW

Telephone

Switch

(21)

The BGCF

ISUP/MTP ISUP/IP

SIP

H.248

RTP

PCM

BGCF

IMS PSTN

SIP

SGW

MGCF

MGW

Telephone

Telephone Switch

• IMS originated call breaks to the PSTN

• One BGCF located in the home network

• Does B-number analysis

• Locates a CS gateway operator

• Another BGCF located in visited/3 ^rd party network

• Does B-number analysis

• Locates an MGCF

• Generates charging records

• Protocols: SIP, Diameter (for charging)

Border Gateway Control Function

(22)

The MGCF

ISUP/MTP ISUP/IP

SIP

H.248

RTP

PCM

BGCF

IMS PSTN

SIP

SGW

MGCF

MGW

Telephone

Telephone Switch

• Translates SIP to ISUP (or BICC) over IP

• Controls resources in a Media Gateway via H.248

• Generates charging records

• Protocols: SIP, ISUP/SCTP, BICC/SCTP, H.248

Media Gateway Control Function

(23)

The SGW and MGW

ISUP/MTP ISUP/IP

SIP

H.248

RTP

PCM

BGCF

IMS PSTN

SIP

SGW

MGCF

MGW

Telephone

Telephone Switch

• Bridges lower layers:

• ISUP/SCTP/IP <-> ISUP/MTP

• BICC/SCTP/IP <-> BICC/MTP

• Protocols: MTP, SCTP

Signaling Gateway

• Translates Real-Time Protocol (RTP) to Pulse Code Modulation (PCM) in a circuit

• Transcodes when needed

• Protocols: H.248, RTP, PCM

Media Gateway

(24)

S-CSCF ISC ISC

Sh Sh

Si

HSS OSA AS

OSA-SCS SIP AS IM-SSF

gsmSCF

ISC

Application Servers

• Services are executed in Application Servers

• The S-CSCF involves one more ASes in the session signaling

• Three types of Application Servers

• SIP AS: native to IMS

• OSA-SCS AS, IM SCF: legacy services

• All three types exhibit the same behavior towards the S-CSCF

• ASes interface the S-CSCF and the HSS

• Protocols: SIP, Diameter

Application Servers

(25)

S-CSCF ISC ISC

Sh Sh

Si

HSS OSA AS

OSA-SCS SIP AS IM-SSF

gsmSCF

ISC

The SIP AS

• Native AS for newly developed IMS services

• Can act as either SIP proxy, User Agent, or Back-to-Back-User-Agent (B2BUA)

• May interfaces the HSS to retrieve user data

• Generates charging records

• Protocols: SIP, Diameter

SIP AS

(26)

S-CSCF ISC ISC

Sh Sh

Si

HSS OSA AS

OSA-SCS SIP AS IM-SSF

gsmSCF

ISC

The IM-SSF

• Interfaces IMS with legacy Intelligent Network (IN) services

• From the IN perspective, it acts as a Service Switching Function (SSF)

• From IMS perspective, it acts as a SIP node

• Executes legacy IN services

• May interface the HSS to retrieve user data

• Protocols: SIP, CAP (CAMEL Application Protocol), Diameter (charging)

IM-SSF AS

(27)

S-CSCF ISC ISC

Sh Sh

Si

HSS OSA AS

OSA-SCS SIP AS IM-SSF

gsmSCF

ISC

The OSA-SCS

• Interfaces IMS with legacy Open Service Access application servers

• OSA is capable of providing authentication for third party service providers

• From IMS perspective, it acts as a SIP node

• Executes legacy OSA services

• May interface the HSS to retrieve user data

• Protocols: SIP, Diameter

OSA-SCS AS

(28)

IMS-ALG and TrGW

• Provides IPv4/IPv6 interworking in the signaling path

• Acts as a SIP B2BUA

• Controls a TrGW through the Ix interface

• Interfaces the I- and S-CSCF

• Rewrites SDP and SIP to change IP addresses

• Protocols: SIP, non-standardized Ix interface protocol

IMS Application Layer Gateway

• Provides IPv4/IPv6 interworking in the media path

• Acts as a NAT-PT/NAPT-PT (Network Address Port Translator – Protocol Translator).

• Protocols: RTP, non-standardized Ix interface protocol

Transition Gateway

P-CSCF

I-CSCF

HSS

SLF P-CSCF

BGCF Mj Mk

Cx Dx

Cx

Dx

Mp Sh Mr

Sh Si

ISC ISC ISC

Mn Mw

Mw

Mw Mw

Mw

Mi

S-CSCF

MGCF SGW

MGW MRFP MRFC

IMS-ALG

TrGW Ix

Mx Mx

(29)

Visited / Home domain

UE P-CSCF I-CSCF S-CSCF

BGCF MGCF

Gm Mw Mw

SLF

Dx Cx

HSS AS

Cx

MGW

Mc Mj

Mi

Mr

MRFC

GGSN

Go

Sh

ISC

MRFP

SGW PDF

Visited network

Home network

SGSN

•P-CSCF and GGSN are always

located in the same network, in this

case, the visited network

(30)

Visited / Home domain

UE P-CSCF I-CSCF S-CSCF

BGCF MGCF

Gm Mw Mw

SLF

Dx Cx

HSS AS

Cx

MGW

Mc Mj

Mi

Mr

MRFC

GGSN

Go

Sh

ISC

MRFP

SGW PDF

Visited network

Home network

Third party network

SGSN

•P-CSCF and GGSN are located in the home network

•SGSN is located in the visited

network

(31)

SGW

3GPP Rel 5/6 IMS Main Protocols

UE P-CSCF I-CSCF

MGCF

Gm Mw Mw

SLF

Dx Cx

HSS AS

Cx

MGW

Mc Mj

Mi

SIP SIP Diameter

H.248 Mr

MRFC

GGSN

Go Diameter for Policy Control

Sh

ISC

MRFP

H.248 SIP-ISUP Compression

S-CSCF

BGCF

(32)

User Plane in IMS

• User plane in IMS is generally sent end to end (e.g., UE to UE, UE to MGW, or UE to MRFP)

• Audio and video protocols are sent over the Real-time Transport Protocol (RTP)

• The companion Real-time Transport Control Protocol (RTCP) is used to get feedback of the media reception

• Mandatory codecs in IMS:

– AMR narrowband

– Telephone event (RFC 2833) to send DTMF signals

– If wideband is supported, then AMR wideband is mandatory

– If video is supported, then H.263 baseline is mandatory; H.263 version 2 profile 3 and MPEG-4 visual are optional

– If text telephony is supported, then T.140 is mandatory

• Lately, the Message Session Relay Protocol (MSRP) has been added to carry

instant messages

(33)

Home Network A HSS

Visited Network A

1. INVITE

Home Network B HSS Visited Network B

1 1 . I N V IT E

7. Diameter LIR/LIA

8 . I N V IT E

12. INVITE

S-CSCF

I-CSCF S-CSCF

I-CSCF

P-CSCF P-CSCF

AS

4. INVITE 5. INVITE

6. INVITE

10. INVITE 9. INVITE

3 . I N V IT E 2 . I N V IT E

Session Setup (1)

(34)

Visited Network A Visited Network B

P-CSCF P-CSCF

INVITE INVITE

Home networks

200 OK 200 OK

SGSN GGSN

RTP

• Signalling traverses proxies and home network

• User plane is sent end to end

Session Setup (2)

(35)

Identities in IMS

IMS defines three types of identities:

• Public User Identity

• Private User Identity

• Public Service Identity

(36)

Public User Identity

• One or more public user identities are allocated to each user

• Takes the form of a SIP URI or TEL URI – sip:miguel.garcia@nsn.com

– tel:+358-50-480-4585

• Allocated by the home operator

• Used for triggering services

• Not used for authentications

• One public user identity can be stored in the UICC (aka SIM card)

• The user can register to the network one more public user identities

simultaneously,

– Procedure: Implicit public user identity

registration

(37)

Private User Identity

• One private user identities is allocated to each user

• Takes the form of a Network Access Identifier (NAI, RFC 2486)

– miguel@operator.net

• Allocated by the home operator

• Not known by the user

• Used for authentication purposes

• The private user identity is stored in the UICC (aka SIM card).

– It is unique, since the SIM is un-

clonable

(38)

Public/private user identity relation

• Assumption: one terminal, one UICC, one private user identity

• Different public user identities can be registered from different terminals

• The same public user identity can be be registered from different terminals

Private User Identity - 1

Private User Identity - 2

Public User Identity - 1

Public User Identity - 2

Public User Identity - 3 IMS

Subscription

(39)

Public Service Identity

• Similar to public user identities, but allocated to services rather than users

• Take the form of a SIP URI or TEL URI

– sip:conf34@operator.net

– tel:+358-800-555-2321

• Typically hosted by an Application Server

• Public service identities are not authenticated

– No corresponding private user

identity

(40)

IMS signaling flows

(41)

Preconditions for registration

UE SGSN GGSN HLR HSS

1. GPRS level registration

2. PDP Context Activation 3. P-CSCF discovery

4. IMS level registration

S-CSCF

(42)

Home Network A HSS

Visited Network A

1. REGISTER

S-CSCF I-CSCF P-CSCF

AS

4 . R E G IS T E R 2 . R E G IS T E R

IMS registration (1)

3. Diameter UAR/UAA

6 . 4 0 1 U n a th . 7 . 4 0 1 U n a th .

8. 401 Unauthorized

5. D iam ete r M A R

/M A A

(43)

Home Network A HSS

Visited Network A

1. REGISTER

S-CSCF I-CSCF P-CSCF

AS

4 . R E G IS T E R 2 . R E G IS T E R

IMS registration (2)

3. Diameter UAR/UAA

6 . 2 0 0 O K 7 . 2 0 0 O K

8. 200 OK

10. 200 OK 9. REGISTER

5. D iam ete r M A R

/M A A

3 ^rd party registration:

Informs the AS about the user

registration

(44)

IMS Terminal

(1) REGISTER

P-CSCF

(10) 401 Unauthorized

S-CSCF

(2) REGISTER

(9) 401 Unauthorized

(11) REGISTER

(20) 200 OK

(12) REGISTER

(19) 200 OK

I-CSCF HSS

(3) Diameter UAR (4) Diameter

UAA

(5) REGISTER

(8) 401 Unauthorized

(6) Diameter MAR (7) Diameter

MAA

(13) Diameter UAR (14) Diameter

UAA

(15) REGISTER

(18) 200 OK

(16) Diameter SAR (17) Diameter

SAA

Registration flow

(45)

Home Network A HSS

Visited Network A

1. INVITE

Home Network B HSS Visited Network B

1 1 . I N V IT E

7. Diameter LIR/LIA

8 . I N V IT E

12. INVITE

S-CSCF

I-CSCF S-CSCF

I-CSCF

P-CSCF P-CSCF

AS

4. INVITE 5. INVITE

6. INVITE

10. INVITE 9. INVITE

3 . I N V IT E 2 . I N V IT E

Session initiation (1)

(46)

IMS Terminal #1

(1) INVITE

P-CSCF

(18) 183 Session Progress

P-CSCF

(3) INVITE

(17) 183 Sesssion Progress (19) 183

Session Progress

S-CSCF S-CSCF

(7) Diameter LIR (8) Diameter

LIA

(15) 183 Session Progress

I-CSCF HSS IMS

Terminal #2

(5) INVITE (2) 100

Trying (4) 100

Trying

(6) 100 Trying

(9) INVITE (10) 100 Trying

(11) INVITE (12) 100

Trying (13) INVITE (14) 100

Trying

Originating

Visited Network

Originating Home Network

Terminating Home Network

Terminating Visited Network

(16) 183 Session Progress (20) 183

Session Progress

Pre-alert user Evaluation of

initial filter criteria

Evaluation of initial filter criteria

Session initiation flow (1)

(47)

IMS

Terminal #1 P-CSCF P-CSCF

(21) PRACK

S-CSCF S-CSCF

(22) PRACK

(26) 200 OK (27) 200 OK

(37) 200 OK

(25) PRACK (24) PRACK

(32) UPDATE

(36) 200 OK (30) 200 OK

(31) UPDATE

(40) 200 OK

I-CSCF HSS IMS

Terminal #2 Originating

Visited Network

Originating Home Network

Terminating Home Network

Terminating Visited Network

(20) 183 Session Progress

(29) 200 OK

Resource Reservation Resource Reservation

(34) UPDATE

(35) UPDATE

(38) 200 OK (39) 200 OK

Alert user (33) UPDATE

(28) 200 OK (23) PRACK

Session initiation flow (2)

(48)

Session initiation flow (3)

IMS Terminal #1

(63) ACK

P-CSCF

(44) 180 Ringing

P-CSCF

(64) ACK

(43) 180 Ringing (45) 180

Ringing

(47) PRACK

S-CSCF S-CSCF

(41) 180 Ringing

(48) PRACK

(52) 200 OK (53) 200 OK

(58) 200 OK

(51) PRACK (50) PRACK

(57) 200 OK (56) 200 OK

(61) 200 OK

I-CSCF HSS IMS

Terminal #2

(65) ACK

(66) ACK

Originating

Visited Network

Originating Home Network

Terminating Home Network

Terminating Visited Network

(42) 180 Ringing

(46) 180 Ringing

(55) 200 OK

(67) ACK (59) 200 OK

(60) 200 OK

Alert user

(54) 200 OK (49) PRACK

Accept session

(62) 200 OK

Media plane (RTP)

(49)

Home Network A Visited Network A

1. INVITE

CS interworking network

7 . I N V IT E

S-CSCF MGCF

BGCF P-CSCF

AS

PSTN switch

3. INVITE 4. INVITE

6. INVITE

2 . I N V IT E

Call to Circuit Switched (CS) networks

BGCF

5 . I N V IT E

SGW

8. H.248 MGW

9 . I S U P I A M

10. ISUP IAM

HSS

(50)

Call flow to CS networks (1)

BGCF 2

S-CSCF BGCF 1

Originating

Network Home Network

Evaluation of initial filter criteria

(3) INVITE (4) 100 Trying (1) INVITE

(2) 100 Trying

(5) INVITE (6) 100 Trying

(15) 183 Session Progress (16) PRACK

(21) 200 OK

MGCF

CS interworking Network

(11) 183 Session Progress (7) INVITE (8) 100 Trying (13) 183 Session

Progress (14) 183 Session

Progress

(17) PRACK

MGW

(20) 200 OK

SGW

(18) H.248 MODrq (19) H.248 MODrp

(12) ISUP: IAM

(9) H.248 ADDrq

(10) H.248 ADDrp

(51)

Call flow to CS networks (2)

BGCF 2

S-CSCF BGCF 1

Originating

Network Home Network

MGCF

CS interworking Network

MGW SGW

(22) UPDATE

(25) 200 OK

(28) 180 Ringing (29) 180 Ringing

(32) PRACK

(30) 180 Ringing

(35) 200 OK

(41) 200 OK

(39) 200 OK (40) 200 OK

(43) ACK

(23) UPDATE

(38) H.248 MODrs

(44) ACK (24) 200 OK

(33) PRACK (34) 200 OK

(26) ISUP: COT

(27) ISUP: ACM

(31) 180 Ringing

(36) ISUP: ANM (37) H.248 MODrq

(42) 200 OK

Media plane (RTP)

(52)

Home Network A Visited Network A

CS interworking network

6 . I N V IT E

S-CSCF MGCF

BGCF P-CSCF

AS

PSTN switch

1. ISUP IAM

8. INVITE

2 . I S U P I A M

Call from Circuit Switched (CS) networks

I-CSCF

SGW

3. H.248 MGW

9 . I N V IT E

HSS

4. I N V IT E

5. Diameter

LIR/LIA

7. INVITE

10. INVITE

(53)

Call flow from CS networks (1)

I-CSCF

S-CSCF HSS

Terminating

Network Home Network

Evaluation of initial filter criteria

(7) Diameter LIA (8) INVITE

(5) 100 Trying (6) Diameter LIR

(10) INVITE (11) Trying

MGCF

CS interworking Network

(9) 100 Trying

(4) INVITE

MGW SGW

(2) H.248 ADDrq (3) H.248 ADDrp

(1) ISUP: IAM

(12) 183

Sess. Progress (13) 183

Session Progress (16) PRACK

(14) H.248 MODrq (15) H.248 MODrp (17) PRACK

(18) 200 OK

(19) 200 OK

(54)

Call flow from CS networks (2)

I-CSCF

S-CSCF HSS

Terminating

Network Home Network

(22) UPDATE

MGCF

CS interworking Network

(21) UPDATE

MGW SGW

(20) ISUP: COT

(25) Ringing

(26) Ringing (30) PRACK

(28) H.248 MODrq (29) H.248 MODrp (31) PRACK

(23) 200 OK

(24) 200 OK

(27) ISUP: ACM

(32) 200 OK

(33) 200 OK

(34) 200 OK

(35) 200 OK

(40) ACK

(38) H.248 MODrq (39) H.248 MODrp (41) ACK

(37) ISUP: ANM (36) 200 OK

Media plane (RTP)

(55)

Triggering services

• Services are triggered in the S-CSCF (but executed in ASes)

• Central to triggering services is the initial Filter Criteria (iFC)

– The iFC is a collection of triggers set for a given public user identity

– The iFC defines the Application Servers to be contacted when the conditions are fulfilled

– The iFC is stored in the HSS embedded in the user profile.

– The S-CSCF downloads the user profile when the user registers.

• Every SIP request that initiates a SIP dialog (e.g., INVITE,

SUBSCRIBE) , or every standalone SIP request (e.g., REGISTER, OPTIONS), make the S-CSCF to evaluate the iFC

• Example of Filter Criteria semantics:

– All INVITE requests addressed to the user

– Where the calling party is a given identity

– Are sent to a given AS

(56)

Initial Filter Criteria

Application Server Trigger Point Trigger Point

Service Point Trigger

SIP URI

Default Handling 1 to n Priority

Service Point Trigger Service Point Trigger

Request-URI SIP Method SIP Header Session Case Session Description 0 to 1

Service Information

User Profile

Service Profile n

Service Profile 2 Service Profile 1

Public Identification n 1 to n

Public Identification 2 Private User Identity

1 to n

Public Identification 1

Shared Initial Filter Criteria n

Shared Initial Filter Criteria 2 Shared Initial Filter Criteria 1

0 to n

Initial Filter Criteria n

Initial Filter Criteria 2 Initial Filter Criteria 1

0 to n

Structure of the User Profile / Filter Criteria

(57)

AS acting as a SIP UA (1)

S-CSCF AS

P-CSCF

1. INVITE 2. INVITE

3 . IN V IT E 4 . 2 0 0 O K

5. 200 OK 6. 200 OK

Service provided to an originating user

(58)

AS acting as a SIP UA (2)

S-CSCF AS

I-CSCF

1. INVITE 2. INVITE

3 . IN V IT E 4 . 2 0 0 O K

5. 200 OK 6. 200 OK

P-CSCF

Service provided to a terminating user

(59)

AS acting as an originating UA

S-CSCF AS

P-CSCF

3. INVITE 2. INVITE

1 . IN V IT E 6 . 2 0 0 O K

5. 200 OK

4. 200 OK

(60)

AS acting as a SIP proxy

S-CSCF AS

P-CSCF

1. INVITE 2. INVITE 3 . IN V IT E 4 . I N V IT E

9. 200 OK 10. 200 OK

5. INVITE 6. 200 OK 7 . 2 0 0 O K 8 . 2 0 0 O K

Service provided to an originating user

(61)

AS acting as a SIP proxy

S-CSCF AS

I-CSCF

1. INVITE 2. INVITE 11. 200 OK 12. 200 OK

P-CSCF

3 . IN V IT E 4 . I N V IT E 9 . 2 0 0 O K 1 0 . 2 0 0 O K

5. INVITE 6. INVITE 7. 200 OK 8. 200 OK

Service provided to a terminating user

(62)

AS acting as a redirect server

S-CSCF AS

I-CSCF

1. INVITE 2. INVITE

3 . IN V IT E

P-CSCF

4 . 3 0 2 M o v e d T e m p o ra ril y

5. 302 Moved Temporarily 6. 302 Moved

Temporarily

Service provided to a terminating user

(63)

AS acting as a B2BUA

S-CSCF AS

P-CSCF

1. INVITE A 2. INVITE A 3 . IN V IT E A 4 . 2 0 0 O K

5. 200 OK 6. 200 OK

8. INVITE B 9. 200 OK 1 0 . 2 0 0 O K 7 . I N V IT E B

Service provided to an originating user

(64)

AS acting as a B2BUA

S-CSCF AS

I-CSCF

1. INVITE A 2. INVITE A 5. 200 OK 6. 200 OK

P-CSCF

3 . IN V IT E A 4 . 2 0 0 O K 1 2 . 2 0 0 O K 7 . I N V IT E B

8. INVITE B 9. INVITE B 10. 200 OK 11. 200 OK

Service provided to a terminating user

(65)

IMS bibliography

Two books on the market describe IMS

“The 3G IP Multimedia Subsystem: Merging the Internet and the Cellular Worlds, 2 ^nd edition”.

• Gonzalo Camarillo and Miguel-Angel Garcia-Martin

• Published by John Wiley and Sons Ltd.

• ISBN: 0470018186

• December 2005

“The IMS: IP Multimedia Concepts and Services in the Mobile Domain, 2 ^nd edition”.

• Miikka Poikselkä, Georg Mayer, Hisham Khartabil, and Aki Niemi.

• Published by John Wiley and Sons Ltd.

• ISBN: 0470019069

• March 2006

(66)

The IP Multimedia Subsystem (IMS)