3GPP IMS architecture

1

control

transport

Visited network Home network

P-CSCF

HSS

S-CSCF BGCF MGCF

MGW

PSTN

H.248

SIPSIP SIP

SIPISUP/IP

AS

Diameter

IP corenetwork

Accessnetwork

PCM

I-CSCF

Diameter

SIP

Diameter

HSS – Home Subscriber Server

CSCF – Call Session Control Function

BGCF – Breakout Gateway Control Function

MGCF – Media Gateway Control Function

VoIP – RTP – transport of media streams

Service plane

Control plane

Transport plane

SGW ISUP/MTP

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2

IMS layers: transport, session control, applications

Transport/user layer

Application layer

Control layer

S-CSCF

HSS

SIP AS SIP AS

MRFC

B-GW

SIP Diameter PSTN

RTP

I-CSCFP-CSCF

MRFP

H.248 / MEGACO

Parlay ASCAMEL

CSE

OSA GW IM SSF

Parlay API CAP API

ISC interface

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Three layer architecture of IMS (1/3)

• Unified architecture that supports a wide range of services enabled by the flexibility of Session Initiation Protocol (SIP)

• Transport and Endpoint Layer– Initiates and terminates SIP signaling to set up sessions and provide bearer services such as conversion of

voice from analog or digital formats to IP packets using Real time Transport Protocol (RTP)

– Provides the media gateways for converting the VoIP bearer streams to the PSTN TDM format.

– The media server provides many media related services including conferencing, playing announcements, collecting in-band signaling tones, speech recognition, speech synthesis, etc.

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Three layer architecture of IMS (2/3)

• Session Control Layer– Call Session Control Function (CSCF)

• provides the registration of the endpoints and routing of the SIP signaling messages to the appropriate application server

• interworks with the transport and endpoint layer to guarantee QoS across all services

– Home Subscriber Server (HSS) database• maintains the unique service profile for each end user

• By centralizing this information, applications can share information to create unified personal directories, multi client type presence information and blended services.

– Media Gateway Control Function (MGCF)• Interworks SIP signaling with the signaling used by the media gateway

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Three layer architecture of IMS (3/3)

• Application Server Layer

– Contains application servers, which provide the end-user service logic

• Telephony Application Server

• IP Multimedia – Services Switching Function (IM-SSF)

• Supplemental Telephony Application Servers

• Non Telephony Application Servers

• Open Service Access – Gateway (OSA-GW)

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IMS architecture

UE

P-CSCF I-CSCF S-CSCF

BGCF MGCF

HSS

Visited network Home network

AS

Gm

Mw

ISC

Mj

Mi

MRF

Cx

Sh

<SIP>

<DIAMETER>

<SIP>

<SIP>

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Terminal - User Equipment (UE)

• SIP UA

• Establishes GPRS PDP context for– signaling

– media transport

• ISIM for authentication– Public and Private User identity

– User Network address

– Security algorithms and keys

– At least USIM

• Corellation of session control with QoS reservation

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IMS components

UE

P-CSCF I-CSCF S-CSCF

BGCF MGCF

HSS

AS

Gm

Mw

ISC

Mj

Mi

MRF

Cx

Sh

<SIP>

<DIAMETER>

<SIP>

<SIP>

Home networkVisited network

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Proxy Call Session Control Function - P-CSCF

• P-CSCF (Proxy Call Session Control Function) – entry point of user terminal into the IMS world

– acting as an outbound/inbound SIP proxy server

– an IMS terminal communicates with a single P-CSCF during the registration

– establish IPsec security associations toward the IMS terminal

– verify the correctness of SIP request sent by the IMS terminal

– compressor and a decompressor of SIP messages

– may include PDF (Policy Decision Function) to authorize media plane

– generate charging information

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IMS components

UE

P-CSCF I-CSCF S-CSCF

BGCF MGCF

HSS

AS

Gm

Mw

ISC

Mj

Mi

MRF

Cx

Sh

<SIP>

<DIAMETER>

<SIP>

<SIP>

Visited network Home network

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Interrogating Call Session Control Function (I-CSCF)

• A SIP proxy located at the edge of an administrative domain

• The address of the I-CSCF is listed in the DNS records of the domain

• I-CSCF has an interface to the SLF and HSS and route the SIP request to the appropriate destination (typically S-CSCF)

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IMS components

UE

P-CSCF S-CSCF

BGCF MGCF

HSS

AS

Gm

Mw

ISC

Mj

Mi

MRF

Cx

Sh

Visited network Home network

I-CSCF

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Serving Call Session Control Function S-CSCF

• S-CSCF (Serving Call Session Control Function)

– Central node of the signaling plane

– Acts as a SIP server and a SIP registrar

•Maintains binding between the user location (e.g. IP address of the terminal) and the user’s SIP address of record (Public User Identity)

– Diameter interface to the HSS

•To download the authentication vectors of the user

•To download the user profile including service profile

•To inform the HSS of S-CSCF allocated to the user

– All the SIP signaling the IMS terminals sends and receives traverse the allocated S-CSCF

– Provides SIP routing (e.g. translation from phone number to SIP URI)

– Enforces the policy of the network operator

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IMS components

UE

P-CSCF I-CSCF S-CSCF

BGCF MGCF

HSS

AS

Gm

Mw

ISC

Mj

Mi

MRF

Cx

Sh

Visited network Home network

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Breakout Gateway Control Function (BGCF)

• Selects PSTN/CS domain to forward a call– local MGCF

– another BGCF

• MGCF selection process not defined– configuration

– TRIP or a similar solution

Media Gateway Control Function (MGCF)

• Gateway to PSTN

– SIP - PSTN mapping

– Establishes appropriate bearer capabilities

– Transcoding

– Acts as UA (registration not required)

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17

IMS components

UE

P-CSCF I-CSCF S-CSCF

BGCF MGCF

HSS

AS

Gm

Mw

ISC

Mj

Mi

MRF

Cx

Sh

Visited network Home network

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Media Resource Function (MRF)

• Supports conference services and announcements

• Multimedia Resource Control Function (MRCF)

– Interprets information from S-CSCF andAS

• e.g. conference reservation

– controls MRPF

• Multimedia Resource Processor Function (MRPF)

– Sets bearer capabilities based on MRFCrequests

– Mixing and distribution of media

– Media streaming for user interaction

• Uses H.248 (MEGACO)

MRCF

MRPF

SIP

H.248

Media

Interworking with PSTN – PSTN/CS Gateway

• PSTN/CS = MGCF + SGW + MGW

• MGCF (Media Gateway Control Function)

– ISUP-SIP mapping

– MGW control for media transcoding

– Selection of I-CSCF for calls to PSTN

• SGW (Signalling Gateway Function)

– Supports interworking between SS7 and MGCF

– Transport of ISUP messages in SIP messages to MGCF MGCF (SIP-T orBICC)

– ISUP/BICC/MTP – ISUP/BICC/SCTP/IP

• MGW (Media Gateway)

– Termination of media streams

• RTP - PCM

• AMR - G.711 EIMS - 2019/2020 19

BGCF - Breakout Gateway Control Function

• SIP server – routing based on telephone numbering– Using of info delivered by TRIP protocol

• IMS – PSTN/PLMN calls

• Selection of an interworking network

• Selection of an appropriate PSTN/CS gateway (MGCF) ifinterworking refers to the same network in which BGCF islocated

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Application Server (AS)• An SIP entity that hosts and executes

services

• Four modes of operation:

– AS acting as terminating UA, or redirect server

– AS acting as originating UA

– AS acting as a SIP proxy

– AS performing third party call control/ B2BUA mode

• ISC - SIP Interface to S-CSCF

• Implemetation variants of ASs– SIP AS

– OSA-SCS (Open Service Access-Service Capability Server)

– IM-SSF (IP Multimedia Service Switching Function)

S-CSCF

AS

ISC

1

2 3

4

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Implementation variants of IMS AS

CSCFS-CSCF

SIP Application ServerSIP AS

HSSHSSOSA service

capability server(SCS)

OSA servicecapability server

(SCS)

IM-SSFIM-SSF

Camel Service

Environment

CAMEL ServiceEnvironment

OSA

application OSA AS

ISC

Cx ISC

ISC

CAP

MAP

Parlay OSA API

SCIM

Sh

Si

MRFCMRFC

Mr

AS AS

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Parlay OSA application servers – third parties

Online &Offline

Charging (ECF, CCF)

Service Delivery Platform

(Application server) Parlay App 1

Service Platform Trigger PointsParlay API (MPCC, MMM, Presence, Charging ...

SIP Interface

HSS

S-CSCF

SPT

SPT Filtering criteria

ISC

SIP

SIP-Server

Sh =

Diameter

Cx =

DiameterRo + Rf =

Diameter

Diameter

Parlay X GWParlay App 2

Parlay App 3

Parlay X App 1Parlay X App 2

Other accessnetworks

Source: T. Magedanz (TU Berlin / Fraunhofer FOKUS)

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IMS components

UE

I-CSCF

BGCF MGCF

HSS

AS

Gm

Mw

ISC

Mj

Mi

MRF

Cx

Sh

Visited network Home network

I-CSCF S-CSCF

AS

MRFI-CSCF

AS

P-CSCF S-CSCF

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Home Subscription Server (HSS)

• HSS (Home Subscriber Server)– Technically, an evolution of the HLR– Central repository for user-related information

• location information• security information (authentication and authorization)• user profile information (the services that the user is

subscribed to)• S-CSCF allocated to the user

• SLF (Subscriber Location Function)– Simple database that maps users’ addresses to

HSSs

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HSS – Home Subscriber Server

I-CSCF S-CSCF

Mw

SLF

Dx Cx

HSS AS

Cx

SIP

SIPDiameter

Sh

ISC

Diameter

GPRSaccess

MAP

SIP

Mw

The first registration

UAR: User-Authorization-RequestUAA: User-Authorization-AnswerMAR: Multimedia-Authentication-RequestMAA: Multimedia-Authentication-AnswerSAR: Server-Assignment-RequestSAA: Server-Assignment-AnswerRAND: Random challengeAUTN: Network Authorization TokenCK: Ciphering KeyIK: Integrity Key

13. UAR

1. Register

Visited network Home network

P-CSCF

2. Register

UA I-CSCF HSS

3. UAR

4. UAA

S-CSCF

Selection of r S-CSCF

5. Register

6. MAR

10. 401 UnauthorisedRAND||AUTN

12. RegisterRES

7. MAARAND||AUTN||XRES||CK||IK

9. 401 UnauthorisedRAND||AUTN

11. RegisterRES

Authenticationvector selection

8. 401 UnauthorisedRAND||AUTN

14. UAA

15. RegisterRES

22. OK20. OKCK||IK

21. OKCK||IK

Authentication

18. SAR

19. SAA

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Home Network of Mobile A Home network of Mobile B

HSS - A

I-CSCF - A S-CSCF - A

HSS - B

S-CSCF - B I-CSCF - B

P-CSCF - A

Network Visited by Mobile A Network Visited by Mobile B

P-CSCF - B

Mobile A Mobile B

Call Control

11

22

33

4 4

5 5

Example: registration

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Control

Bearer

Visited network

of callee

Home network of called userHome network of

calling user

Visited network of callerP-CSCF

HSS

S-CSCFSIP

SIP

Diameter

P-CSCF

AS

HSS

S-CSCF SIPAS

SIP

Diameter

I-CSCF

Diameter

DNS

Packet

CoreAccess Access

SIP INVITE1

1

Retrieving user’s profile2

2

3

3 Execution of a service scenario

Resolution of User B home network address and

INVITE forwarding

4

4

Identification of Registrar for user B and INVITE forwarding5

5

Retrieving user’s profile 6

6

7

7 Execution of a service scenario

INVITE forwarding to do User B 8

8

Ringing / Alerting10

10

SDP session negotiation /

Reservation of resources

9

9

RTP

Answer / call active11

11

IMS users session

Packet

Core

SIP UESIP UE

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30

Home Network of Mobile A Home network of Mobile B

HSS - A

I-CSCF - A S-CSCF - A

HSS - B

S-CSCF - B I-CSCF - B

P-CSCF - A

Network Visited by Mobile A Network Visited by Mobile B

P-CSCF - B

Mobile A Mobile B

Call Control

1

2

3

4

5

6

7

Example 2 – IMS mobile call

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Control

Bearer

Network vizited by the caller Home network of the caller

P-CSCF

HSS

S-CSCF

BGCFMGCF

MGW

H.248

SIP

SIP SIP

AS

BGCF

SIP

SIP

PSTN

ISUPDiameter

PacketCore

Access

SIP INVITE1

1

2 3

Selection of access to PSTN and selection of MGCF4

4

ISUP IAM7

75

Seizing of a link/trunk and determination of MGW capabilities5

PCM

SDP session negotiation / Reservation of resources6

6

RTP stream

Ringing / Alerting8

8 9

Answer / Connect9

IMS to PSTN call

Retrieving user’s profile 2

3 Execution of a service scenario

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P-CSCF - A

Network Visited by Mobile A

HSS - A

I-CSCF - A S-CSCF - A

BGCF - A

Home Network of Mobile A PSTN

MGW - B

MGCF - B

Mobile A

Gateway Control

User Profile

User Profile

1 2

3

4

5

Call to PSTN (CS domain)

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IMS Session Setup and Control

• Happens after user registration (and authentication!)

• Goals of IMS session setup– Capability and media negotiation (e.g. codecs, port numbers, addresses)

– Network resource reservation and the support of QoS preconditions

• Additional functions performed by the IMS during the IMS Session Setup:– Routing to the terminating IMS (= the IMS of the B-Party)

– Routing / Breakout to the PSTN / CS-Domain when the B-Party is not in the IMS, but in the PSTN / CS-Domain

– Service Control / Invocation of Application Servers to trigger the execution of Originating- and/or Terminating IMS Services

– Integrity/Confidentiality Protection of SIP Messages

– QoS/Media Authorization

– SIP Signaling Compression

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IMS Session Setup and Control 1/4

Source: T. Magedanz (TU Berlin / Fraunhofer FOKUS)

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IMS Session Setup and Control 2/4

Source: T. Magedanz (TU Berlin / Fraunhofer FOKUS)

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IMS Session Setup and Control 3/4

Source: T. Magedanz (TU Berlin / Fraunhofer FOKUS)

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IMS Session Setup and Control 4/4

Source: IMS Tutorial -T. Magedanz (TU Berlin / Fraunhofer FOKUS)

EIMS

Project is co-financed by European Union within European Social Fund38EIMS - 2019/2020