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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
EIMS - 2019/2020
EIMS - 2019/2020
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
EIMS - 2019/2020 3
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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6
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>
EIMS - 2019/2020 7
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
EIMS - 2019/20208
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
EIMS - 2019/2020 9
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
EIMS - 2019/202010
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
EIMS - 2019/2020 11
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)
EIMS - 2019/2020
12
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
EIMS - 2019/2020 13
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
EIMS - 2019/2020
14
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
EIMS - 2019/2020 15
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)
EIMS - 2019/2020 16
EIMS - 2019/2020
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
EIMS - 2019/2020 18
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
EIMS - 2019/2020 20
EIMS - 2019/202021
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
EIMS - 2019/2020 22
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
EIMS - 2019/2020 23
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)
EIMS - 2019/2020 24
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
EIMS - 2019/2020 25
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
EIMS - 2019/2020 26
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
EIMS - 2019/202027
EIMS - 2019/2020 28
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
EIMS - 2019/2020 29
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
EIMS - 2019/2020
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
EIMS - 2019/2020 31
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
EIMS - 2019/202032
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)
EIMS - 2019/2020 33
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
EIMS - 2019/2020 34
IMS Session Setup and Control 1/4
Source: T. Magedanz (TU Berlin / Fraunhofer FOKUS)
EIMS - 2019/2020 35
IMS Session Setup and Control 2/4
Source: T. Magedanz (TU Berlin / Fraunhofer FOKUS)
EIMS - 2019/2020
36
IMS Session Setup and Control 3/4
Source: T. Magedanz (TU Berlin / Fraunhofer FOKUS)
EIMS - 2019/2020 37
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