UMTS Core Network
V. Mancuso, I. Tinnirello
GSM/GPRS Network Architecture
Radio access network
BSS
GSM/GPRS core network
VLR
BTS
MS
GMSC
BSC
HLR
PCU
AuC
SGSN
EIR
BTS
IP Backbone
GGSN
database
Internet
V. Mancuso, I. Tinnirello
PSTN, ISDN
MSC
3GPP Rel.’99 Network Architecture
Radio access network
UTRAN
RNC
Iu CS
GMSC
VLR
BS
UE
MSC
HLR
Iur
Uu
AuC
Iub
RNC
BS
SGSN
EIR
Iu PS
Gn
IP Backbone
GGSN
database
Internet
V. Mancuso, I. Tinnirello
PSTN
Iub
Core network (GSM/GPRS-based)
3GPP Rel
Rel.’99
.’99 Network Architecture
Radio access network
UTRAN
Iub
RNC
BS
UE
Iur
Uu
Iub
RNC
BS
V. Mancuso, I. Tinnirello
2G => 3G
MS => UE
(User Equipment), often also
called (user) terminal
New air (radio) interface
based on WCDMA access
technology
New RAN architecture
(Iur interface is available for
soft handover,
BSC => RNC)
3GPP Rel.’99 Network Architecture
Core network (GSM/GPRS-based)
MSC is upgraded to 3G
MSC
Iu CS
MSC
GMSC
VLR
SGSN is upgraded to 3G
SGSN
GMSC and GGSN remain
the same
AuC is upgraded (more
security features in 3G)
HLR
AuC
SGSN
EIR
Iu PS
Gn
IP Backbone
GGSN
Internet
V. Mancuso, I. Tinnirello
PSTN
Changes in the core
network:
3GPP Rel.4 Network Architecture
UTRAN
(UMTS Terrestrial Radio
Access Network)
Circuit Switched (CS) core network
MSC
Server
GMSC
Server
New option in Rel.4:
GERAN
(GSM and EDGE Radio
Access Network)
V. Mancuso, I. Tinnirello
MGW
SGW
MGW
PS core as in Rel.’99
PSTN
SGW
3GPP Rel.4 Network Architecture
MSC Server takes care
of call control signalling
The user connections
are set up via MGW
(Media GateWay)
RANAP / ISUP
SS7 MTP
IP
Sigtran
V. Mancuso, I. Tinnirello
MSC
Server
GMSC
Server
SGW
MGW
core
SGW
MGW
PS core as in Rel.’99
PSTN
“Lower layer” protocol
conversion in SGW
(Signalling GateWay)
Circuit Switched (CS)
network
3GPP Rel.5 Network Architecture
UTRAN
(UMTS Terrestrial Radio
Access Network)
CS core
SGSN
GGSN
PS core
V. Mancuso, I. Tinnirello
HSS
Internet
GERAN
(GSM and EDGE Radio
Access Network)
IMS (IP
Multimedia
System)
MGW
PSTN
New core
network part:
3GPP Rel.5 Network Architecture
Interworking with the
PSTN may be required
for some time ...
V. Mancuso, I. Tinnirello
MGW
IMS (IP
Multimedia
System)
SGSN
HSS
GGSN
PS core
Internet / other IMS
Call/session control
using SIP (Session
Initiating Protocol)
CS core
PSTN
The IMS can establish
multimedia sessions
(using IP transport)
via PS core between
UE and Internet (or
another IMS)
New Service Concept
Content provider
Content provider
Service provider
Service provider
Carrier provider
End user
V. Mancuso, I. Tinnirello
End user
all want to make
profit
OSA (Open Services Architecture/Access)
OSA is being standardised, so that services provided by
different service/content providers can be created and
seamlessly integrated into the 3G network (this is the
meaning of “open” architecture)
OSA means in practice:
Service Creation Environment (SCE)
API
API
3G network
V. Mancuso, I. Tinnirello
API
API = Application
Programming
Interface
(Standardised)
CAMEL (2G & 3G)
CAMEL (Customised Applications for Mobile network Enhanced
Logic) is a set of “IN” (intelligent network) type functions and
procedures that make operator-specific IN services available to
subscribers who roam outside their home network.
CAMEL = IN technology + global mobility
CAMEL Service Environment (CSE) is a logical entity in the
subscriber’s home network which processes IN related
procedures
CSE ≈ SCP in home network
V. Mancuso, I. Tinnirello
The IMS
The IP Multimedia Subsystem
Vincenzo Mancuso, PhD
V. Mancuso, I. Tinnirello
The IMS
The Third generation networks aim to merge
two most valuable resources in communication
technology, along with local PSTN networks
Cellular Networks
The Internet
Use packet switching, IP
The IP Multimedia Subsystem or IMS is the solution to integrate all
the services that the internet provides with the cellular and other
networks
Triple Play: coordination of voice, video and data
V. Mancuso, I. Tinnirello
IMS ValueValue-added services
IMS also can provide integrated
service to the user
Third party developed services can be provided by
operators, thus developing value-added services
Appropriate charging for multimedia sessions,
according to the content and the service offered
V. Mancuso, I. Tinnirello
IMS Service Integration
Problem
Statement:
various
networks providing services have a
“vertical infrastructure”
no horizontal links between networks
The challenge: to integrate these
networks and to create new
applications that would provide
service
to
next
generation
networks
V. Mancuso, I. Tinnirello
Component #1: the media
transport
After many other services,
introduction of real time
voice/video on Internet
The 1st problem is easy: how
to transmit voice/video?
IP packets
Codec
RTP
UDP
IP
RTP (Real-Time Protocol) is a protocol which
transports an encoded multimedia stream as
pieces, with a timestamp on each piece, and sends
them using UDP/IP
Any encoding is permitted for voice/video with
a Codec: MP3, ACC, MPEG4, AMR…
Several streams (voice+video) can be sent in
parallel with the same timestamp
The timestamp is used by the receiver to play
the voice/video in a regular way for quality
RTP is the media transport
V. Mancuso, I. Tinnirello
Component #2: session initiation
Registration of Betty’s phone
and address
The 2nd problem is more
difficult: how to initiate the
call? how to know the IPaddress of the called phone?
It is necessary to have a table which translates a
symbolic name “Betty” into an IP address
Each time a terminal service is put on, it must
notify its presence to update the table
So, thanks to an access to this table, it is possible
to make the address resolution : name -> IP
address
Registration of John’s phone
and address
The control of the address table
creates a tough competition…
V. Mancuso, I. Tinnirello
Why IMS?
Solutions to make the address resolutions
MSN, Yahoo, AOL have
designed a calling
architecture hypercentralized: 1 table
worldwide, only 1 operator
Skype promotes a hyperdecentralized architecture:
1 table per terminal with
peer-to-peer update
between terminals
“The introduction and development of
solution like Skype is highly significant
as it alters the notion of
telecommunications operators by
making the process fully electronic“
IDATE - ART, 2004
Operators dislike such approaches, so they push a way
very similar to e-mail: centralization per domain. This
approach is named “Internet Protocol Multimedia
Subsystem”, IMS.
V. Mancuso, I. Tinnirello
IMS User Identities
Sip:zehan.zeb@newstore.com
tel:+17324567888
zzeb@tmobile.co
m
IMS Subscriber
Private User Identity
Sip:zehan.zeb@example.com
tel:+88028112347
Public User Identities
V. Mancuso, I. Tinnirello
User Identity
Private identity
Issued by home provider
Used for AAA
Saved on ISIM (not modifiable)
Public
User Identity 1
Private
User Identity 1
Public identity
IMS
Normal SIP address (URI or TEL)
Subscription
Identifies the user publicly
User has one or more identities
Used for routing
Can be grouped into implicit registration sets
If one of the set is registered then the others are as well
At least one is stored on ISIM
Public
User Identity 2
»
Public
User Identity 4
Private
User Identity 2
Public
User Identity 5
Public
User Identity 6
Use a temporary identity derived from USIM during initial registration (derived from
IMSI)
PIDs are then provided by the S-CSCF in its reply to the registration
V. Mancuso, I. Tinnirello
Implicitly
Registered ID
Set 1
Service
Profile 2
Public
User Identity 3
In case no ISIM is provided
»
Service
Profile 1
Implicitly
Registered ID
Set 2
Implicitly
Registered ID
Set 3
Service
Profile 3
Service
Profile 4
UICC
Universal Integrated Circuit Card
Used to store data, including authentication
information
Contains one or more applications
SMS
Phonebook
…
SIM
GSM Subscriber Identity Module
USIM
UMTS SIM
ISIM
IMS SIM
Applications are independent
SIM, USIM and ISIM can coexist on the same UICC
… but SIM cannot be used for IMS access (for security reasons)
V. Mancuso, I. Tinnirello
IP Multimedia Subsystem
An enabler
of new
applications
Next-gen
network
architecture
A standard
IMS
SIP
Video Clips
Sports, News
IP Network
SIP
ServiceService-layer
ControlControl-layer
TransportTransport-layer
At Home
SIP
On the Move
Converged Services
Rich Communications
SIP
SIP
SIP
In the Office
IMS is access agnostic, cost reducing, and service
enhancing
IMS gives the customer & carrier choice
V. Mancuso, I. Tinnirello
Voice
PushPush-2-Talk
Push--2-View
Push
PushPush-2-X
Location-Presence, Location
based Services
Picture
Messaging,
MM Messaging
Streaming
Audio, Video
E-mail
IM
Games,
Music
Downloads
Interactive Services,
Interactive Gaming
IMS
Goals & Tools
V. Mancuso, I. Tinnirello
IMS goals
Combine latest trends in tech
Run fast, no time for standardization of services
Mobile/Nomadic internet
Create a platform for multimedia
services
… and their development
Exploit/allow mobile packetswitching networks
Not a mere circuit-switching replacement
V. Mancuso, I. Tinnirello
IMS requirements
Support for establishing IP Multimedia
Sessions
Audio, video, messaging…
Support for mechanisms to negotiate QoS
distinguish users
operators want to control QoS
Support for interworking
with Internet
with packet-switching networks
Support for roaming
Home and visited network
Inter-operators roaming
…
V. Mancuso, I. Tinnirello
IMS requirements (cont’d)
…
Support for user activity control
policies imposed by the operator (general policies and per-user policies)
accomplish to service agreements
Support for fast service creation
don’t require service standardization
Standardize service capabilities instead
Support for multiple access
Not only GPRS, UMTS
IP is independent on lower protocol layers
V. Mancuso, I. Tinnirello
Protocols in IMS
3GPP reuses protocol developed by other
standards development organizations
ETSI (European Telecommunications Standard Institute)
IETF (Internet Engineering Task Force)
ITU-T (International Telecommunications Union - Telecommunications)
3GPP interacts with standards
development organizations as for the
development of existing and new
protocols
Protocols
RTP for media transport
Session control protocols
AAA protocols
Other
V. Mancuso, I. Tinnirello
Session Control Protocol
SIP (Session Initiation Protocol, by
IETF- RFC2821)
Protocol to establish and manage multimedia session
over IP
SIP borrows some design principles from SMTP and
HTTP
Does not differentiate the user-to-network interface from
the network-to-network interface (unlike BICC and
H.323)
It follows the client-server model
Text-based protocol
Easy to debug, extend and reuse for service building
V. Mancuso, I. Tinnirello
AAA protocol: DIAMETER
one protocol for many interfaces
Authorization
What service can be used?
Diameter
Client Application
Diameter
Server Application
Authentication
Are you really who you say you
are?
Accounting
Session Management
Session Management
Routing Management
Routing Management
Connection
Management
Connection
Management
Base Protocol
Base Protocol
€$£
•
DIAMETER
–
–
–
–
by IETF RFC3588
Evolution of RADIUS
Base protocol
Diameter applications
• Diameter applications used to customize/extend the base protocol for different interfaces, environments, and
applications
• E.g.: interact with SIP session setup (Authorization and Authentication)
• E.g.: interact with the billing subsystem to control accounting
• E.g.: interact with routing entities
V. Mancuso, I. Tinnirello
IMS Architecture
V. Mancuso, I. Tinnirello
New services for mobile
and fixed networks
Open, standardised, operator friendly, NGN
multimedia architecture for mobile and fixed services
Based on SIP, DIAMETER and COPS controls
Supports legal interception, localisation, PSTN interworking, etc.
Access Independent
Applications
Internet
3G mobile
WLAN
WiMAX
DSL
Fibre
Ethernet
IMS Platform
PSTN
IP transport
V. Mancuso, I. Tinnirello
MGW
Functions and nodes
IMS standardizes functions
IMS does NOT standardize nodes
The IMS Architecture is a collection of
functions linked by standardized
interfaces
A function can be implemented through one or more nodes
Multiple functions can share a node
Commonly a function per node
V. Mancuso, I. Tinnirello
A standard architecture for
service delivery
Application
Layer
AS
AS
SIP
DIAMETER
Session
Control
Layer
SIP
SIP
HSS
MR
F
MGCF
MGW
Connectivity
Layer
H.248
SIP
IP Network
GGS
N
PSTN/PLMN
CPE
V. Mancuso, I. Tinnirello
Access to IMS
The user can connect to an IMS network in
variety of ways, all of which use the
standard Internet Protocol (IP) for packet
switching
e.g., IMS terminals can register directly
on an IMS network
e.g., mobile phones, personal digital assistants (PDAs) and computers
even when they are roaming in another network or country (the visited
network)
The only requirement is that they can use IPv6 (also IPv4 in early IMS)
and run SIP user agents
V. Mancuso, I. Tinnirello
Other access examples
Fixed access
- e.g., Digital Subscriber Line (DSL), cable modems, Ethernet
Mobile access
e.g., W-CDMA, CDMA2000, GSM, GPRS
Wireless access
e.g., WLAN, WiMAX
Other phone systems like plain old
telephone service (POTS -- the old
analogue telephones) or PSTN, H.323
and non IMS-compatible VoIP systems,
are supported through gateways
V. Mancuso, I. Tinnirello
IMS functional elements
V. Mancuso, I. Tinnirello
Infrastructure of SIP Proxies
(media do not traverse the IMS..)
V. Mancuso, I. Tinnirello
IMS Functional Elements
Session Management (SIP)
Routing
Databases
Network Interoperability Elements
Services and Support Components
Charging Components
V. Mancuso, I. Tinnirello
Nodes/Functions in the IMS
User databases
HSS (Home Subscriber Server)
SLF (Subscriber Location Function)
SIP servers
CSCF (Call/Session Control Function)
AS (Application Server)
MRF (Media Resource Function)
MRFC (MRF Controller)
MRFP (MRF Processor)
BGCF (Breakout Gateway Control Function)
PSTN/CS gateways, decomposed into:
SGW (Signaling Gateway)
MGCF (Media Gateway Controller Function)
MGW (Media Gateway)
Charging collection functions
V. Mancuso, I. Tinnirello
Databases (HSS,SLF)
HSS is an evolution of the HLR (Home
Location Register) of GSM
Contains the user-related subscription data (e.g., location,
authorization and authentication information)
More than one HSS is possible
If #HSS > 1 SLF required
SLF maps users’ address to HSSs
HSS and SLF use DIAMETER with an IMSspecific diameter application
V. Mancuso, I. Tinnirello
Home Subscriber Server (HSS)
HSS
Diameter
Presence, Location and Profile
End-User Identity
Private and Public End-User Information
Registration Information
Service Initiation Information
Subscriber Service Profile (SSP)
Downloaded to CSCF at Registration
V. Mancuso, I. Tinnirello
More on HSS
HSS is a master user database that
supports all the IMS network functions
that actually handle communications
contains the subscription-related information (user profiles)
performs authentication and authorization of the user
can provide information about the user's physical location
is similar to the GSM Home Location Register (HLR) and
Authentication Centre (AUC) together
V. Mancuso, I. Tinnirello
CSCF
SIP servers or proxies, collectively called Call
Session Control Function (CSCF), are used to
process SIP signaling packets/messages in
the IMS
P-CSCF (Proxy)
I-CSCF (Interrogating)
S-CSCF (Server)
V. Mancuso, I. Tinnirello
Call/Session Control Func.
Func. ((CSCF)
CSCF)
Diameter
SIP
PCSCF
ICSCF
SIP
SIP
SCSCF
SIP
SIP
CSCF – Processes SIP Signaling
P-CSCF
First Point of User Contact (located in the visited domain)
Authenticates user
May Include Policy Functions
S-CSCF
Central Node of Control Plane
Acts as Registrar for User (located in the home domain)
Invokes Application Servers
Performs Primary Routing Function
I-CSCF
Located at Edge of Administrative Domain (contact point for inter-domain messages)
Is the Ingress Network Point Defined in DNS
Shields Network Topology from External Networks
V. Mancuso, I. Tinnirello
IMS Signaling Path
(1 domain, no roaming)
V. Mancuso, I. Tinnirello
IMS Signaling Path
(4 domain roaming, media goes directly)
Sweden
User
Irish
User
Sweden User
In roaming
Irish User
In roaming
V. Mancuso, I. Tinnirello
CSCF
P-CSCF (Proxy)
Outbound/Inbound proxy server
First server contacted by the user
Fixed while registered
Security functions (integrity protection, IPSec,…)
Authenticates the user and extend the authentication to
other nodes within IMS
Compress/decompress SIP messages
Generates charging info
V. Mancuso, I. Tinnirello
P-CSFC details
SIP proxy that is the first point of contact for the IMS
terminal
It can be located either in the visited network (in full IMS
networks) or in the home network (when the visited
network isn't IMS compliant yet)
The terminal discovers its P-CSCF with either DHCP
or it is assigned in the PDP Context (GPRS)
It is assigned to an IMS terminal during registration, and
does not change for the duration of the registration
It sits on the path of all signaling messages, and can
inspect every message
It authenticates the user and establishes an IPsec security
association with the IMS terminal
Other nodes trust the P-CSCF, and do not have to authenticate the user again
It can also compress and decompress SIP messages
it may include a Policy Decision Function (PDF), which
authorizes media plane resources
it also generates charging records
V. Mancuso, I. Tinnirello
PDF for QoS in IMS
(intermediary between the applicationapplication-level QOS and
the 3G networknetwork-level QoS)
QoS)
V. Mancuso, I. Tinnirello
CSCF
I-CSCF (Interrogating)
SIP proxy at the edge of a domain
Advertised by DNS
Interface to HSS and SLF for routing purposes
Optional: encryption of sensitive information about
the domain (THIG: topology hiding inter-network
gateway)
V. Mancuso, I. Tinnirello
I-CSCF details
It is another SIP function located at the edge of an
administrative domain
Its IP address is published in the DNS of the domain
remote servers can find it, and use it as a forwarding point (e.g. registering) for
SIP packets to this domain
The I-CSCF queries the HSS using DIAMETER to
retrieve the user location
then it routes the SIP request to its assigned S-CSCF
Up to Release 6 it can also be used to hide the
internal network from the outside world (encrypting
part of the SIP message)
in which case it's called a THIG (Topology Hiding Inter-network Gateway)
From Release 7 onwards this "entry point" function is removed from the ICSCF and is now part of the IBCF (Interconnection Border Control Function)
which is also a firewall and a nat.
V. Mancuso, I. Tinnirello
CSCF
S-CSCF (Server)
SIP server with session control functions
SIP registrar (maintains a mapping between user
location and public user identity)
SIP routing
PEP
Always located in the home network
Interfaced with the HSS (e.g., to download the
user’s profile)
V. Mancuso, I. Tinnirello
S-CSCF details
It is the central node of the signaling plane
SIP server + session control
It is always located in the home network
It uses DIAMETER to the HSS to download and
upload user profiles
All necessary information is loaded from the HSS.
It handles SIP registrations
bind the user location and the SIP address
It sits on the path of all signaling messages, and can
inspect every message
It decides to which application server(s) the SIP
message will be forwarded, in order to provide their
services
It provides routing services, typically using
Electronic Numbering (ENUM) lookups
There can be multiple S-CSCFs in the network for
load distribution and high availability reasons
It's the HSS that assigns the S-CSCF to a user, when
it's queried by the I-CSCF.
V. Mancuso, I. Tinnirello
Application Server (AS)
AS
AS
AS
Diameter
SIP
Contains Call Related Application Logic
Facilitates a Service Creation Environment
Queried by S-CSCF in Real Time to Execute Logic
Filters can be applied at S-CSCF in order to inspect the SIP messages and
decide whether involve or not the AS
Generally Specialized for Each Service
May Provide Gateway to Legacy Applications (e.g.
AIN)
Can Behave as a SIP Proxy or Terminal (and in this
case receive the media!)
V. Mancuso, I. Tinnirello
AS
Three different AS types
SIP AS
native IMS application server
OSA-SCS (Open Service Access – Service Capability Server)
Interface to Open Service Application (AS) framework application
server (developed by 3GPP for UMTS)
IM-SSF (IP Multimedia Service Switching Function)
Reuse/interface with CAMEL (GSM)
Gateway for GSM Service Control Function
An AS can be located in the home
network or in an external third-party
network
If located in the home network, it can query the HSS with the DIAMETER
Sh interface (for a SIP-AS) or the Mobile Application Part (MAP) interface
(for IM-SSF).
V. Mancuso, I. Tinnirello
AS as a SIP Proxy
(e.g. find a taxi)
V. Mancuso, I. Tinnirello
AS as a SIP Terminal
(e.g. web server: receive media data!)
V. Mancuso, I. Tinnirello
Example of Filter Rule
(at SS-CSCF)
User A wants to divert all calls from boss
to an answering machine (i.e. the AS)
(method=INVITE) AND
(P-Asserted-Identity = boss@vodafone.com) AND
(Session Case = Terminating) ->message is addressed to
AS
V. Mancuso, I. Tinnirello
Multimedia Resource Function (MRF)
SIP
MRFC
MS
MS
Offers Services Such as Conferencing
MRFC – SIP User Interface toward S-CSCF
MRFP – Controls the Media Server (MS)
V. Mancuso, I. Tinnirello
MRF
The MRF (Media Resource Function) provides a
source of media in the home network
It provides media related functions such as media
manipulation (e.g. voice stream mixing, media
trans-coding) and playing of tones and
announcements.
Each MRF is further divided into
Media Resource Function Controller (MRFC) – signaling plane
Media Resource Function Processor (MRFP) – media plane
The MRFC acts as a SIP User Agent to the S-CSCF,
and controls the MRFP with a H.248 interface
The MRFP is a media plane node that implements
all media-related functions
V. Mancuso, I. Tinnirello
Multiparty Calls
(MRF as a special AS)
REFER:
Ask other
to join
V. Mancuso, I. Tinnirello
Multiparty Call
V. Mancuso, I. Tinnirello
Multiparty Call
V. Mancuso, I. Tinnirello
BGCF
The Breakout Gateway Control Function is a
SIP server that includes routing
functionality based on telephone numbers
used when calling from the IMS to a phone in a circuit switched
network
e.g., IMS to Public Switched Telephone Network (PSTN) or to
Public Land Mobile Network (PLMN)
BGCF routes to the appropriate (remote)
circuit switching domain
BGCF selects the (local) PSTN/CS gateway
V. Mancuso, I. Tinnirello
PSTN Gateway
SGW
ISUP
SIP
BGCF
SIP
SIP
MGCF
H.248
MGW
TDM
BGCF – Routes to Gateway Based Upon Telephone
Number
MGCF – Controlling Function for SGW and MGW
SGW – Provides Signaling Conversion Between SIP
and ISUP
MGW – Provides Conversion between RTP and TDM
V. Mancuso, I. Tinnirello
Home Network - Functional Elements
Home Subscriber Server
Domain Name Server
• Centralized DB
• HLR successor
• User profile
• Filter criteria (sent to S-CSCF)
• Which applications
• Which conditions
Application Servers
• Push-to-talk
• Instant messaging
• Telephony AS
• 3rd party or IMS Vendor
Media Resource Function Controller
• Pooling of Media servers (e.g. conference)
Home Network
DNS
ENUM
AS
AS
AS
HSS
P-CSCF
Media Gateway
Control Function
Diameter
SIP
P-CSCF
UA/UE
I-CSCF
SIP
S-CSCF
SIP
SIP
MRFC
MS
SIP
SIP
UA/UE
SIP
MS
• Interfaces to PSTN/PLMN by
• Converting SIP <-> ISUP
• Interworking RTP to circuit
• H.248 control of MGW
SIP
BGCF
SIP
MGCF
ISUP
SIP
Call Session
Control Function
H.248
• SIP registration
• SIP session setup
RTP
MGW
SS7
TDM
Serving CSCF
Proxy CSCF
• 1st contact point for UA
• QoS
• Routes to S-CSCF
Visited
Network
• Registrar
• Session control
• Application Interface
Interrogating CSCF
• Entry point for incoming calls
• Determines S-CSCF for Subscribers
• Hides network topology
V. Mancuso, I. Tinnirello
Breakout Gateway Control Function
• Selects network (MGCF or other BGCF)
in which PSTN/ PLMN breakout is to occur
PSTN
Network--toNetwork
to-Network Connectivity
SIP
UA/UE
Access
DNS
ENUM
RTP
RTP
Visited
Network
HSS
Diameter
SIP
Backbone
Packet
Network
AS
AS
AS
SIP
P/SP/S-CSCF
SIP
I-CSCF
SIP
S-CSCF
P-CSCF
SIP
SIP
SIP
MRFC
MS
SIP
SIP
MS
SIP
BGCF
SIP
MGCF
ISUP
RTP
Home
Network
Proxy/Serving CSCF
•Manages call origination
•Selects destination network
• Routes to I-CSCF
Interrogating CSCF
• Entry point for incoming calls
• Determines S-CSCF for Subscribers
• Hides network topology
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H.248
MGW
SS7
TDM
PSTN
Home and Visited Networks
Inherited from 2G, 3G
Most of IMS nodes and functions are located
in the Home network
P-SCFC can be either in the home or visited
network
If GPRS access P-CSCF in the same network of the GGSN
simplifies the operator’s management
allows roaming to non-IMS GPRS CANs (Connectivity Access networks)
Drawback: trombone effect when GGSN is in the home network (GGSN is
in the media plane)
In a long term perspective the P-CSCF will be located in the visited
network
Media servers can be out of both
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IMS architecture
Examples
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John is calling Betty
Introducing the HSS and the S
S--CSCF
HSS
When the phones
get connected they
register their
name/IP to the HSS
The HSS is the table user/address
The S-CSCF is a SIP proxy which works
on messages to provide users
(consumers, enterprises) with calling
services including registration being a
mediation SIP2DIAMETER
SIP
S-CSCF
SIP
Changes the SIP message replacing “Betty” by its IP
address found in the HSS
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John is calling a taxi to meet Betty
Introducing the Application Server (AS)
In addition to the basic name/address translation,
the S-CSCF routes SIP messages to:
The network of Betty, if different
The applications such as: Push-To-Talk, Instant Messaging, Advance Call Control,
Voice/video mailbox, nearest Taxi… running on AS, a SIP proxy application server
…
nearest Taxi application (location, fleet…)
AS
S-CSCF
Changes the SIP message replacing “taxi” by
the IP address of the nearest available taxi
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John’s and Betty’s phones do not support a common
voice encoding
Introducing the MRFC and MRFP
Intercepting the SIP “invite” message, the S-CSCF/AS
detects a non compatibility between the codecs of the
phones : it forwards it to the MRFC (a SIP proxy).
The MRFC adjusts the SIP messages in order to orient the
RTP flow to the MRFP (a RTP proxy), for trans-coding
S-CSCF
SIP
MRFC
MGCP, H248 Megaco
RTP (codec: G729)
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MRFP
RTP (codec: AMR)
During its travel John is calling Betty
The operator has made a segmentation of its services offer
Introducing the P-CSCF, II-CSCF
The P-CSCF is the 1st SIP proxy seen by the terminal
It controls the bearer plan via COPS protocol
It adjusts the SIP message (e.g., compression) and forwards it to the I-CSCF of the home
network
The operator may have several S-CSCFs (e.g., offer
segmentation)
So it introduces, the I-CSCF SIP proxy as the entry point of its
network
also used as the entry point for calls from other operators
Visited Network
HSS
…
P-CSCF
S-CSCF (consumers)
I-CSCF
S-CSCF (enterprises)
Home Network
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John is calling Betty who has
a legacy phone
Introducing the MGCF and the MGW
At the border of the IMS network with the phone network,
an adaptation is necessary.
The MGCF handles the control for the 2 worlds and drives
the MGW (Media gateway)
… controls circuits and MGW much like a VoIP softswitch
MGCF
Legacy Call control (SS7)
SIP
H248 MGCP,
Megaco
…
RTP
Phone transmission
Internet
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MGW
PSTN/PLMN network
IMS Communication Focus
Roaming IMS Mobile Users
Serving Network A
Serving Network B
Access Network A
Service Platform B
(ASB)
Service Platform A
(ASA )
P-CSCFC
PDF
Gm
SIP/SDP
inviting
uey@homeb.com
SIP / SDP
SIP / SDP
S-CSCFA
SIP / SDP
S-CSCFB
Access Network B
P-CSCFD
PDF
Gm
Go
Go
SGSN GGSN
Data- Path
SIP/SDP
GGSN SGSN
IP Backbone Network
UEA
UEB
PDP Context
PDP Context
Sessionlevel(SIP/SDP signalling)
Bearer level(PDPcontext activation / modification / Release)
Interaction betweensession andbearer level(COPS)
I-CSCF (between P-CSCF and S-CSCF) not shown for simplicity
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A Typical Example of an IMS Call
Network Y
Network X
AS
AS
S-CSCF
S-CSCF
HSS
HSS
I-CSCF
I-CSCF
P-CSCF
P-CSCF
SGSN
DSL/Cable Modem
GRX
Network Z (UMTS/GPRS)
GGSN
DSLAM/CMTS
RNC
User A
User B
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The Signaling Plane
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SIP functionality
SIP’s main goal is to deliver a session descriptor to a
user at his/her/its location
A session descriptor contains the information needed
for a remote user to join the session
IP address and port
Codecs
…
SDP (Session Description Protocol)
A (possible) text-based way to describe the media session
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SDP
version
user
subject
user IP
Session-level
Start time
G711µ-law
Stream
directio
n
H.261 codec
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Media-level
SIP entities
Registrar
User Agent
SIP endpoints handled by users (also automatically, based on userdefined rules)
Proxy server
SIP routers
Usually is co-located with the registrar, and always uses the information
on the registrar (through a location server) to route the calls
Forking proxy
Parallel or serial forking in the message routing
Redirect server
For routing, but no message delivery
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SIP transactions
Transaction structure
A request from the client
Zero or more provisional responses from the server
A final response from the server
Each message begins with a start line
Request line (in a request)
Method
Request URI
Protocol version
e.g.: INVITE sip:alice@domain.com SIP/2.0
Status line (in an answer)
e.g.: SIP/2.0 180 Ringing
Protocol version
Status Human
code code
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Start line
One or more headers (name:value)
---------EMPTY LINE --------Message body (OPTIONAL)
SIP message format
SIP methods
ACK
BYE
CANCEL
INFO
INVITE
NOTIFY
OPTIONS
PRACK
PUBLISH
REGISTER
ack the final response for INVITE
terminate a session
cancel a pending request
transport PSTN telephony signaling
establish a session
notify a UA about a particular event
queries a server about capabilities
ack for provisional response
upload UA’s info to a server
map the current URI and location with
the public URI
SUBSCRIBE request to be notified about an event
UPDATE
update session characteristics
MESSAGE
instant message in the message body
REFER
instruct a server to send a request
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SIP in the IMS
In principle, no difference with the public
Internet
In practice, IMS imposes some
constraints
Wireless access
Security
QoS
Roaming
3GPP/IETF defines a profile of utilization
of SIP in the IMS environment
E.g., RFC 3261 (redefinition of SIP)
RFC 3665 and 3666 (Best Current Practice)
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Prerequisites
Two--phase registration
Two
Offline subscription
Login to the
IP access net
Inbound/Outbound
Fixed Proxy
DHCP or
Lower level
mechanism
Online registration
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Entering the IP Connectivity Access
Network
The access to GPRS includes the
assignment of servers and IP address
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PDF Context Activation/DeActivation/De-Activation
A PDP context is “pre-setup” for the SIP
signaling
Different PDP contexts are then activated for
the media flows (e.g. according to the agreed
codec)
When the session ends, all PDP contexts are
deactivated
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P-CSCF discovery
The IMS terminal has to discover the IP address of the
P-CSCF
The procedure can be
stand alone (DHCP+DNS)
integrated with the access to the IP (e.g., with the PDP context)
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IMS--level registration
IMS
The user requests authorization to use
the IMS
The IMS authenticate and eventually
authorize the user
SIP REGISTER is mandatory
DIAMETER is used by the CSCF to contact the HSS
For security reasons, the user is challenged to show its
identity this require two SIP REGISTER messages
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Without
authentication
DL User Auth
vector
UAA: User Auth. Ans.
MAA: Multiledia Auth.Ans
SAA:Server Assignment Ans.
IMS--level registration
IMS
•The HSS tells the I-CSCF whether a S-CSCF
was already allocated to the user.
•If not, I-CSCF will choose based on
capabilities notified by HSS in the UAA
authentication
vector included
DL User
Profile
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UAR: User Auth. Req
MAR:Multimedia Auth.Req
SAR:Server Assignment Req.
Challenge the User
Basic Session Setup
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Session
setup
Many nodes and
many messages
involved in the setup
Different
planes/layers
•media
•signaling
•CAN
•core
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Session setup (cont’ed
(cont’ed))
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