Third revision of IMS signaling course. The lecture was part of the communication protocols class 2014 delivered to students from FIIT STU Bratislava, Slovakia and University Zilina, Slovakia.
2. Outline
Summary “SIP & IMS Basics”
IMS and its Services
Protocols
Service Triggering
Q/A
3. Note!
These slides only summarize the lecture. Take notes.
Fixed agenda: IMS Signaling
Besides that:
Ask questions (how is it done in real-world, how did Slovak
Telekom do it)
Interrupt (I don’t understand, can you provide samples, can
we skip that)
Contribute (I’ve heard/read that…, I’m interested in…)
Discuss…
8. Technology Trends
Services
Data/IPNetworks
MobileNetworks
PSTN/ISDN
CATV
Access Transport & Switching Networks
Wireless
Access
Wireline
Access
IP Backbone
Existing and newly
emerging services
Service & Network Control
(QoS, Security, IP Mobility)
Too costly, per-service network
architecture
Single/simple/cost-effective network
infrastructure for existing & new services
9. Access & Transport
Plane
Core
Network
Session
Control Plane
Service Architecture
Applications/Services
Plane
HSSCSCF
Access
Networ
k
Other
Networks
Web Portal
Application
Servers
Session
Control
Centralized
Databases
Media
Control &
Gateways
Media
Server
IMS: Simplified Concept
10. Recapitulation
IMS is an open architecture for mobile and fixed services
The core and its services are independent from the access
Layered architecture
Transport, session control, applications
Transparency through standard interfaces
Session Control Layer
End point registration, authentication
Session establishment, routing, interconnect
Application Layer
Service Logic
11. Recapitulation ctd.
Service Control Layer
SIP: P/I/S-CSCF, (BGCF, I-BCF, MRFC, AS)
Diameter: HSS, (RACS/NASS, PCRF)
Application Layer
SIP/Diameter interface towards service control layer
SIP/XCAP interface (based on HTTP) towards UE
Call related application logic
IMS service (e.g. Presence, PoC)
Service Creation Environment
Northbound integration through service APIs
12. IMS entities
(Wiley, The IMS Concepts and Services)
Session management and routing family (CSCFs)
Databases (HSS, SLF)
Services (e.g. AS)
Support functions (PDF, SEG, THIG)
Charging
Interworking functions (BGCF, MGCF, IMS-MGW, SGW)
13. Core nodes
CSCF components separate logical functionality
P-CSCF eq. SIP Proxy. It acts as access point for UE towards the
IMS core.
I-CSCF eq. Inbound Proxy. It is placed on the borders of two
IMS domains. Entry point for served home users from visited
networks.
S-CSCF eq. SIP Registrar/Location Server. It also acts as an
anchor point for IMS service control (ISC) and service
invocation (based on iFCs).
HSS contains all subscriber and service related data
No special entity in basic SIP (simply: user database)
14. Session Management & Routing
Proxy-CSCF – User contact point with the IM CN
SIP compression, IPSec association, PDF interaction
Interrogating-CSCF – Subscriber contact point
Next-hop lookup from HSS, S-CSCF assignment and
routing, THIG functionality
Serving-CSCF – Service profile internal procedures
Handling registration, challenging UE, routing decisions
Responsible for Registration and Session Establishment,
Charging Data Generation, Media content check
15. Databases
HSS
Data storage for all subscriber and service-related data
SLF
Find HSS address for multiple HSS environment
16. SIP Application Server
Stand-alone AS
Northbound integration using various protocols possible,
e.g., HTTP REST, Parlay X
Open Service Access (OSA) gateway
Connect northbound to OSA Parlay based AS
IM Service Switching Function (SSF)
Connect northbound the AS layer to legacy services using
IN protocols (e.g. INAP, CAMEL)
Application Server
17. Other real-world components
Resource and Admission Control Subsystem (RACS)
Mechanisms for applications to request and reserve the
resources from access network (Session Admission Control,
resource reservation)
Network Attachment Subsystem (NASS)
Registration and initialization of CPE for access to IMS services
(IP addresses and configuration, user authentication)
Policy and Charging Control (PCC) framework
Policy and Charging Rules Function (PCRF)
Policy and Charging Enforcement Function (PCEF)
18. Signaling
SIP (signaling protocol)
SDP (embedded in SIP, describes the session, negotiation)
Media
RTP (end-to-end media delivery (audio, video))
MSRP (messaging, file transfer)
DNS
Diameter (AAA)
IPSec (secure communication)
MEGACO (media gateway control)
Protocols (extract)
20. User Identities
User identities
Private User Identity (user@realm)
Authentication and Subscription identification
Not used for routing
Public User Identity (sip:user@domain.tld or tel:+1234567890)
Contact to be reached by others
SIP URI or tel URI
Implicit set of public user identities for grouping
registration
Services and other network entities can be addressed
using a SIP URI
User identities are part of the user profile
22. IMS Registration
Required before a user can access services or perform
calls
Precondition: UE has IP address & knows IMS entry point
All CSCF are used
P-CSCF (home/visited): Entry point, determines I-CSCF
I-CSCF (home): Determines S-CSCF
S-CSCF (home): Authenticates the subscriber, registers IMS
subscriber, interacts with service layer
User assigned to one S-CSCF after successful registration
Knows user profile until de-registration
24. Domain Name Service
Link IP addresses with domain names
Support in locating SIP servers (NAPTR, SRV, A/AAAA)
NAPTR resolves the preferred protocol and the DNS
string to locate the service
ngnlab.eu. 7200 IN NAPTR 10 50 "s“ "SIP+D2T“ _sip._udp.ngnlab.eu.
SRV look-up for a NAPTR given address indicates the
domain and port the service listens on
_sip._udp.ngnlab.eu. 7200 IN SRV 0 0 5060 icscf.ngnlab.eu.
A/AAAA to find the IP address of the domain name
icscf.ngnlab.eu. 7200 IN A 147.175.103.213
26. Important SIP “additions”
P-Access-Network-Info includes port location/cell
From/To eq. IMPU
Path informs S-CSCF about routing destination for terminating requests
Collected during registration using INVITE (e.g. P-CSCF)
Populated to Route headers in in-registration terminating requests on S-CSCF
Authorization contains IMPI and other values
200 OK Service-Route to populate S-CSCF address to P-CSCF for
originating requests
Populated to Route headers in in-registration originating requests on UE
27. Important SIP “additions” – ctd.
Choose a registered IMPU for session establishment
UE – P-CSCF: P-Preferred-Identity
P-CSCF – I/S-CSCF : P-Asserted-Identity
P-Associated-URI informs client about registered
IMPUs
Event: reg after registration to inform UE about
events on S-CSCF (e.g. HSS-initiated deregistration)
28. After registration, subscriber is reachable through
public user identity for communication
IMS subscriber can access services now or perform
calls
P-CSCF (home or local)
Proxy, contacts assigned S-CSCF for the calling subscriber
S-CSCF (home)
Service control and logic
Contacts application or other party
I-CSCF
Entry point for communication from other domain
IMS Session Establishment
36. Application Layer Interaction
User profile contains service profile
Service Profile
Public Identification (assigned subscribers)
Initial Filter Criteria (triggering AS interaction)
Initial Filter Criteria (iFC)
Trigger points with service point triggers
(conditions when to interact)
Application server (SIP URI for interaction)
41. Filtering
Only initial SIP requests
Initial filter criteria (iFC) retrieved from HSS during
registration
Subsequent filter criteria (sFC) provided by
application server (beyond 3GPP R8)
Allows dynamic definition of trigger points during
application runtime
42. I/S-CSCF are interaction points with the service layer
I-CSCF for public service identities (PSI) explicit access
S-CSCF for services (of served users) implicit access
Applications have interface towards HSS
User profile information
Location information, service information
Complexity of security, authorization, access
interaction etc. all handled by the core
Application Routing
43. Application Routing ctd.
Application server (AS) can have different functions
Terminating AS (e.g., acting as user agent)
Originating AS (e.g., wake up service, click to dial)
SIP Proxy server (e.g., for SIP header manipulation)
Back-to-back user agent (e.g., for deeper modifications in
SIP dialog as supplementary service enabler)
45. Is anything still unanswered?
How do IMS services work?
Why is the IMS needed for some communications services?
Is it?
But I have heard of service X, why don’t they use the IMS?
Will we build all future services on top of IMS?
Are IMS services only those inherited from the Telco past?
Will Telco’s deploy multiple IMS? IMS in the cloud? Share an
IMS?
Will IMS bring in new revenues? Is it cheaper to deploy
services on the IMS compared to stand-alone deployments?
46. Deepen understanding of CSCF/HSS roles
Function of components
Routing within control layer and towards applications
Understanding IMS user identities
Service control, routing, application layer interaction
Many things omitted in this presentation
Network access layer, IMS reference points names
S-CSCF assignments in detail, SLF/HSS separation
QoS resource reservation
PSTN Breakout
Charging
Hopefully covered all open questions (last chance )
Summary
48. This lecture is available at the following link:
http://bit.ly/fiit-ims-signaling-3
Download
49. References
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