umts core network
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UMTS UMTS CoreCore NetworkNetwork
V. Mancuso, I. Tinnirello
UMTS UMTS CoreCore NetworkNetwork
GSM/GPRS core networkRadio access networkBSS PS
TN
, ISD
N
BTS
BSC
MSC
VLR
GMSC
HLRMS
GSM/GPRS NetworkGSM/GPRS Network ArchitectureArchitecture
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database
IP Backbone
Internet
PS
TN
, ISD
N
BTS
SGSNAuC
EIR
GGSN
PCU
Core network (GSM/GPRS-based)Radio access networkUTRAN
UE
Iu CS
IurUu
PS
TN
BS
RNC MSC
VLR
GMSC
HLR
Iub
3GPP Rel.’99 Network Architecture3GPP Rel.’99 Network Architecture
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Iur
Iub
Uu
GnIu PS
database
IP Backbone
Internet
BS
RNC SGSNAuC
EIR
GGSN
Radio access networkUTRAN
UE IurUu
BS
RNCIub
2G => 3G MS => UE (User Equipment), often also called (user) terminal
New air (radio) interface based on WCDMA access technology
3GPP 3GPP RelRel.’99 Network Architecture.’99 Network Architecture
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Iur
Iub
Uu
BS
RNCNew RAN architecture(Iur interface is available for soft handover,BSC => RNC)
Core network (GSM/GPRS-based)
Iu CS
PS
TNMSC
VLR
GMSC
HLR
Changes in the core network:
MSC is upgraded to 3G MSC
SGSN is upgraded to 3G
3GPP Rel.’99 Network Architecture3GPP Rel.’99 Network Architecture
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GnIu PS
IP Backbone
Internet
SGSNAuC
EIR
GGSN
SGSN is upgraded to 3G SGSN
GMSC and GGSN remain the same
AuC is upgraded (more security features in 3G)
Circuit Switched (CS) core networkUTRAN
(UMTS Terrestrial Radio Access Network)
PS
TN
MSC Server
GMSC Server
SGW SGW
3GPP Rel.4 Network Architecture3GPP Rel.4 Network Architecture
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PS
TN
New option in Rel.4:
GERAN
(GSM and EDGE Radio Access Network) PS core as in Rel.’99
SGW
MGW
SGW
MGW
Circuit Switched (CS) core network
PS
TN
MSC Server
GMSC Server
SGW SGW
MSC Server takes care of call control signalling
The user connections are set up via MGW (Media GateWay)
“Lower layer” protocol
3GPP Rel.4 Network Architecture3GPP Rel.4 Network Architecture
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PS
TN
PS core as in Rel.’99
SGW
MGW
SGW
MGW“Lower layer” protocol conversion in SGW (Signalling GateWay)
RANAP / ISUP
SS7 MTP IP Sigtran
CS core
PS
TN
MGW
HSSIMS (IP
Multimedia
UTRAN
(UMTS Terrestrial Radio Access Network)
New core network part:
3GPP Rel.5 Network Architecture3GPP Rel.5 Network Architecture
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SGSN GGSN
Intern
et
HSSMultimedia System)
PS core
GERAN
(GSM and EDGE Radio Access Network)
CS core PS
TN
Intern
et / oth
er IMS
HSS
The IMS can establish multimedia sessions (using IP transport) via PS core between UE and Internet (or another IMS)
IMS (IP Multimedia
MGW
3GPP Rel.5 Network Architecture3GPP Rel.5 Network Architecture
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SGSN GGSN
Intern
et / oth
er IMS
HSS
PS core
Call/session control using SIP (Session Initiating Protocol)
Interworking with the PSTN may be required for some time ...
Multimedia System)
Service provider Service provider
Content provider Content provider
New Service ConceptNew Service Concept
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End user End user
Carrier provider
all want to make profit
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:
OSA (Open Services Architecture/Access)OSA (Open Services Architecture/Access)
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3G network
API API API
Service Creation Environment (SCE)API = Application
Programming Interface
(Standardised)
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 (2G & 3G)CAMEL (2G & 3G)
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CAMEL Service Environment (CSE) is a logical entity in the subscriber’s home network which processes IN related procedures
CSE ≈ SCP in home network
The IMSThe IMS
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The IP Multimedia Subsystem
Vincenzo Mancuso, PhD
The IMSThe IMS
�The Third generation networks aim to mergetwo most valuable resources in communicationtechnology, along with local PSTN networks
�Cellular Networks
�The Internet
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�Use packet switching, IP
�The IP Multimedia Subsystem or IMS is the solution to integrate allthe services that the internet provides with the cellular and othernetworks
�Triple Play: coordination of voice, video and data
IMS ValueIMS Value--added servicesadded services
�IMS also can provide integratedservice to the user
�Third party developed services can be provided byoperators, thus developing value-added services
�Appropriate charging for multimedia sessions,
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�Appropriate charging for multimedia sessions,according to the content and the service offered
IMS Service IntegrationIMS Service Integration
�Problem Statement: variousnetworks providing services have a“vertical infrastructure”
�no horizontal links between networks
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�The challenge: to integrate thesenetworks and to create newapplications that would provideservice to next generationnetworks
Component #1: the media Component #1: the media
transporttransport
�After many other services, introduction of real time voice/video on Internet
�The 1st problem is easy: how to transmit voice/video?
IP packets
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� 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
CodecRTPUDP
IP
Component #2: session initiationComponent #2: session initiation
� The 2nd problem is more difficult: how to initiate the call? how to know the IP-address of the called phone?
� It is necessary to have a table which translates a symbolic name “Betty” into an IP address
Registration of Betty’s phone and address
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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
� The control of the address table creates a tough competition…
Registration of John’s phone and address
Why IMS?Why IMS?
Solutions to make the address resolutionsSolutions to make the address resolutions
MSN, Yahoo, AOL have designed a calling architecture hyper-centralized: 1 table worldwide, only 1 operator
Skype promotes a hyper-decentralized architecture:
“The introduction and development of solution like Skype is highly significant
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decentralized architecture: 1 table per terminal with peer-to-peer update between terminals
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.
IMS User IMS User Identities Identities
zzeb@tmobile.co
Sip:zehan.zeb@newstore.com
tel:+17324567888
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zzeb@tmobile.com
Sip:zehan.zeb@example.com
tel:+88028112347
Public User Identities
Private User IdentityIMS Subscriber
User IdentityUser Identity
� Private identity� Issued by home provider
� Used for AAA
� Saved on ISIM (not modifiable)
� Public identity� Normal SIP address (URI or TEL)
� Identifies the user publicly
� User has one or more identities
Private User Identity 1
Public User Identity 1
Public User Identity 2
Public User Identity 3
Implicitly Registered ID
Set 1
Public User Identity 4
Implicitly Registered ID
Set 2
Service Profile 1
Service Profile 2
Service Profile 3
IMS Subscription
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� 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� In case no ISIM is provided
» 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
Implicitly Registered ID
Set 3
Private User Identity 2
Public User Identity 5
Public User Identity 6
Service Profile 4
UICCUICC
Universal Integrated Circuit Card
� Used to store data, including authentication information
� Contains one or more applications� SMS
� Phonebook
� …
� SIM
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� 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)
IP Multimedia SubsystemIP Multimedia Subsystem
A standardAn enabler
of newapplications
Next-gen network
architecture
SIPSIPIMSIMSIMSIMS
PushPushPushPush----2222----TalkTalkTalkTalk
Converged ServicesConverged ServicesConverged ServicesConverged ServicesRich CommunicationsRich CommunicationsRich CommunicationsRich Communications
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ServiceServiceServiceService----layerlayerlayerlayer
ControlControlControlControl----layerlayerlayerlayer
TransportTransportTransportTransport----layerlayerlayerlayerIMS is access agnostic, cost reducing, and service IMS is access agnostic, cost reducing, and service IMS is access agnostic, cost reducing, and service IMS is access agnostic, cost reducing, and service
enhancingenhancingenhancingenhancingIMS gives the customer & carrier choiceIMS gives the customer & carrier choiceIMS gives the customer & carrier choiceIMS gives the customer & carrier choice
SIP
SIP SIPSIP
At HomeAt HomeAt HomeAt Home In the OfficeIn the OfficeIn the OfficeIn the OfficeOn the MoveOn the MoveOn the MoveOn the Move
IMSIMSIMSIMSIP NetworkIP NetworkIP NetworkIP Network
SIP
PushPushPushPush----2222----TalkTalkTalkTalkPushPushPushPush----2222----ViewViewViewView
PushPushPushPush----2222----XXXX
Presence, LocationPresence, LocationPresence, LocationPresence, Location----based Servicesbased Servicesbased Servicesbased Services
Interactive Services, Interactive Services, Interactive Services, Interactive Services, Interactive GamingInteractive GamingInteractive GamingInteractive Gaming
Games, Games, Games, Games, Music Music Music Music
DownloadsDownloadsDownloadsDownloads
Picture Picture Picture Picture Messaging, Messaging, Messaging, Messaging,
MM MessagingMM MessagingMM MessagingMM Messaging
VoiceVoiceVoiceVoice
Video ClipsVideo ClipsVideo ClipsVideo ClipsSports, NewsSports, NewsSports, NewsSports, News
EEEE----mailmailmailmailIMIMIMIM
Streaming Streaming Streaming Streaming Audio, VideoAudio, VideoAudio, VideoAudio, Video
IMSIMS
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Goals & Tools
IMS goalsIMS goals
�Combine latest trends in tech
�Run fast, no time for standardization of services
�Mobile/Nomadic internet
�Create a platform for multimedia services
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services
�… and their development
�Exploit/allow mobile packet-switching networks
�Not a mere circuit-switching replacement
IMS requirementsIMS requirements
�Support for establishing IP Multimedia Sessions� Audio, video, messaging…
�Support for mechanisms to negotiate QoS� distinguish users� operators want to control QoS
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� operators want to control QoS
�Support for interworking� with Internet� with packet-switching networks
�Support for roaming� Home and visited network� Inter-operators roaming
�…
IMS requirements IMS requirements (cont’d)(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
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� don’t require service standardization� Standardize service capabilities instead
�Support for multiple access� Not only GPRS, UMTS� IP is independent on lower protocol layers
Protocols in IMSProtocols 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
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development organizations as for the development of existing and new protocols
�Protocols� RTP for media transport� Session control protocols� AAA protocols� Other
Session Control ProtocolSession 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
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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
AAA protocol: DIAMETERAAA protocol: DIAMETERone protocol for many interfacesone protocol for many interfaces
�Authorization� What service can be used?
�Authentication� Are you really who you say you
are?
�Accounting� € $ £
DiameterClient Application
DiameterServer Application
Session Management Session Management
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Routing Management
Connection Management
Connection Management
Base Protocol Base Protocol
Routing Management
• 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
IMS ArchitectureIMS Architecture
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New services for mobile New services for mobile
and fixed networksand 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.
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Access Independent
MGW
3G mobile
PSTNPSTN
IP transport
IMS PlatformWLANWiMAX
DSLFibre
Ethernet
ApplicationsInternetInternet
Functions and nodesFunctions and nodes
�IMS standardizes functions
�IMS does NOT standardize nodes
�The IMS Architecture is a collection of functions linked by standardized interfaces
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�A function can be implemented through one or more nodes
�Multiple functions can share a node
�Commonly a function per node
A standard architecture for A standard architecture for
service deliveryservice delivery
ASApplication
Layer
Session Control HSS
SIPSIP
DIAMETER
AS
SIPMRF
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MGCFControlLayer
ConnectivityLayer PSTN/PLMNPSTN/PLMNPSTN/PLMNPSTN/PLMNIP Network
HSS
H.248
SIP
SIP
F
CPE
MGW
GGSN
Access to IMSAccess to IMS
The user can connect to an IMS network invariety 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
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� 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
Other access examplesOther access examples
�Fixed access
- e.g., Digital Subscriber Line (DSL), cable modems, Ethernet
�Mobile access
� e.g., W-CDMA, CDMA2000, GSM, GPRS
�Wireless access
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� 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
IMS functional elementsIMS functional elements
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Infrastructure of SIP ProxiesInfrastructure of SIP Proxies(media do not traverse the IMS..)(media do not traverse the IMS..)
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IMS Functional ElementsIMS Functional Elements
�Session Management (SIP)
�Routing
�Databases
�Network Interoperability Elements
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�Services and Support Components
�Charging Components
Nodes/Functions in the IMSNodes/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)
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�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
Databases (HSS,SLF)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
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�If #HSS > 1 ���� SLF required
�SLF maps users’ address to HSSs
�HSS and SLF use DIAMETER with an IMS-specific diameter application
Home Subscriber Server (HSS)Home Subscriber Server (HSS)
�Presence, Location and Profile
�End-User Identity
HSSHSS
Diameter
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�Private and Public End-User Information
�Registration Information
�Service Initiation Information
�Subscriber Service Profile (SSP)
�Downloaded to CSCF at Registration
More on HSSMore 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
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�can provide information about the user's physical location
�is similar to the GSM Home Location Register (HLR) and Authentication Centre (AUC) together
CSCFCSCF
�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)
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�I-CSCF (Interrogating)
�S-CSCF (Server)
Call/Session Call/Session Control Control FuncFunc. (. (CSCF)CSCF)
� CSCF – Processes SIP Signaling� P-CSCF
� First Point of User Contact (located in the visited domain)
� Authenticates user
II--CSCFCSCF
SS--CSCFCSCF
PP--CSCFCSCF
SIP
SIP
SIPSIP
SIP
Diameter
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� 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
IMS Signaling PathIMS Signaling Path(1 domain, no roaming)(1 domain, no roaming)
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IMS Signaling PathIMS Signaling Path(4 domain roaming, media goes directly)(4 domain roaming, media goes directly)
Irish User
Sweden User
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Irish UserIn roaming
Sweden UserIn roaming
CSCFCSCF
�P-CSCF (Proxy)
�Outbound/Inbound proxy server
�First server contacted by the user
�Fixed while registered
�Security functions (integrity protection, IPSec,…)
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�Security functions (integrity protection, IPSec,…)
�Authenticates the user and extend the authentication to other nodes within IMS
�Compress/decompress SIP messages
�Generates charging info
PP--CSFC detailsCSFC 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
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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
PDF for PDF for QoSQoS in IMSin IMS(intermediary between the application(intermediary between the application--level QOS and level QOS and
the 3G networkthe 3G network--level level QoSQoS))
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CSCFCSCF
�I-CSCF (Interrogating)
�SIP proxy at the edge of a domain
�Advertised by DNS
�Interface to HSS and SLF for routing purposes
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�Optional: encryption of sensitive information about the domain (THIG: topology hiding inter-network gateway)
II--CSCF detailsCSCF 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
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� 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 I-
CSCF and is now part of the IBCF (Interconnection Border Control Function) which is also a firewall and a nat.
CSCFCSCF
�S-CSCF (Server)
�SIP server with session control functions
�SIP registrar (maintains a mapping between user location and public user identity)
�SIP routing
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�SIP routing
�PEP
�Always located in the home network
�Interfaced with the HSS (e.g., to download the user’s profile)
SS--CSCF detailsCSCF 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
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� 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.
Application Server (AS)Application Server (AS)
� Contains Call Related Application Logic
� Facilitates a Service Creation Environment
ASASASASASAS
SIPDiameter
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� 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!)
ASAS
�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)
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�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).
AS AS asas a SIP Proxya SIP Proxy(e.g. find a taxi)(e.g. find a taxi)
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AS AS asas a SIP Terminala SIP Terminal(e.g. web server: receive media data!)(e.g. web server: receive media data!)
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Example of Filter RuleExample of Filter Rule(at S(at S--CSCF)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
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AS
Multimedia Resource Function (MRF)Multimedia Resource Function (MRF)
SIP
MSMS MSMS
MRFCMRFC
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�Offers Services Such as Conferencing
�MRFC – SIP User Interface toward S-CSCF
�MRFP – Controls the Media Server (MS)
MRFMRF
� 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
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� 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
Multiparty CallsMultiparty Calls(MRF as a special AS)(MRF as a special AS)
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REFER:Ask other
to join
Multiparty CallMultiparty Call
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Multiparty CallMultiparty Call
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BGCFBGCF
�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
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�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
PSTN GatewayPSTN Gateway
MGCFMGCF
H.248
ISUP
BGCFBGCFSIP
SIP
TDM
SGWSGW
SIP
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� 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
MGWMGWTDM
Home Network Home Network -- Functional ElementsFunctional Elements
HSSHSSDNSDNS
ENUMENUM
Home Network
ASASASASASAS
Home Subscriber 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 VendorDomain Name Server
SIP
SIP
SIP
Diameter
MRFCMRFC
Media GatewayControl Function• Interfaces to PSTN/PLMN by
• Converting SIP <-> ISUP• Interworking RTP to circuit
Media Resource Function Controller• Pooling of Media servers (e.g. conference)
PP--CSCFCSCFSIP
UA/UE
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VisitedVisitedNetworkNetwork
II--CSCFCSCF SS--CSCFCSCFPP--CSCFCSCF
MGCFMGCF
MGWMGWPSTNPSTN
H.248
ISUPBGCFBGCF
SIP
SS7SS7
SIPSIP
SIP
SIPSIP
SIP
RTP TDM
Call SessionControl Function• SIP registration • SIP session setup
MSMS MSMS
MRFCMRFC • Interworking RTP to circuit• H.248 control of MGW
Breakout Gateway Control Function• Selects network (MGCF or other BGCF)in which PSTN/ PLMN breakout is to occur
Proxy CSCF• 1st contact point for UA• QoS• Routes to S-CSCF
Interrogating CSCF• Entry point for incoming calls• Determines S-CSCF for Subscribers• Hides network topology
Serving CSCF• Registrar• Session control• Application Interface
SIP
UA/UE
NetworkNetwork--toto--Network ConnectivityNetwork Connectivity
HSSHSSDNSDNS
ENUMENUM
SIP
ASASASASASAS
BackboneBackbone
AccessAccessSIP
SIP SIP
SIP
SIP
DiameterRTP
RTP
MRFCMRFC
PP--CSCFCSCF SIP
SIP
UA/UE
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II--CSCFCSCF SS--CSCFCSCF
VisitedNetwork
Home Network
P/SP/S--CSCFCSCFBackboneBackbone
PacketPacketNetworkNetwork
MGCFMGCF
MGWMGWPSTNPSTN
H.248
ISUPBGCFBGCF
SIP
SIP
SS7SS7
SIPSIP
SIP
SIPSIP
RTP TDM
MSMS MSMS
MRFCMRFC
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
Home and Visited NetworksHome 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
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� 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
IMS architectureIMS architecture
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Examples
John is calling BettyJohn is calling BettyIIntroducing ntroducing the HSS and the Sthe HSS and the S--CSCFCSCF
� 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
HSS
When the phones get connected they
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SIP S-CSCF
get connected they register their name/IP to the HSS
Changes the SIP message replacing “Betty” by its IP address found in the HSS
SIP
John is calling a taxi to meet Betty John is calling a taxi to meet Betty Introducing Introducing the Application Server (AS)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
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S-CSCF
AS
…nearest Taxi application (location, fleet…)
Changes the SIP message replacing “taxi” by the IP address of the nearest available taxi
John’s and Betty’s phones do John’s and Betty’s phones do not support not support a common a common
voice encodingvoice encodingIntroducing the MRFC and MRFPIntroducing 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
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RTP flow to the MRFP (a RTP proxy), for trans-coding
MRFC
MRFP
MGCP, H248 Megaco
RTP (codec: AMR)
SIPS-CSCF
RTP (codec: G729)
During its travel John is calling BettyDuring its travel John is calling Betty
The The operator has made a segmentation of its services offeroperator has made a segmentation of its services offerIIntroducing ntroducing the the PP--CSCF, ICSCF, I--CSCFCSCF
� 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
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network
� also used as the entry point for calls from other operators
Home Network
Visited Network
P-CSCF
… S-CSCF (consumers)
S-CSCF (enterprises)I-CSCF
HSS
John is calling Betty who hasJohn is calling Betty who has
a legacy phonea legacy phoneIntroducing the MGCF and the MGWIntroducing 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
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MGCF
MGW
H248 MGCP,Megaco
SIPLegacy Call control (SS7)
Phone transmission
…
Internet PSTN/PLMN network
RTP
IMS Communication FocusIMS Communication FocusRoaming IMS Mobile Users Roaming IMS Mobile Users
Access Network A
P-CSCFCSIP / SDP
S-CSCFA
P-CSCFD
Service Platform A(ASA)
S-CSCFBPDF
Gm
SIP / SDPSIP / SDP
Service Platform B(ASB)
Serving Network AServing Network B
Access Network B
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SGSN GGSN GGSN SGSN
UEBUEA
SIP/SDPinviting Go SIP/SDP
IP Backbone Network
Data- Path
PDP ContextSessionlevel(SIP/SDP signalling)Bearer level(PDP context activation / modification / Release)Interactionbetweensession andbearer level(COPS)
Gm Gm
PDP Context
I-CSCF (between P-CSCF and S-CSCF) not shown for simplicity
uey@homeb.com
Go
A Typical Example of an IMS CallA Typical Example of an IMS CallNetwork XNetwork X Network YNetwork Y
P-CSCF
I-CSCFI-CSCF
S-CSCF
S-CSCF
HSS
HSS
ASAS
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User B
DSL/Cable ModemDSL/Cable Modem
DSLAM/CMTS
RNC
GGSN
Network Z (UMTS/GPRS)Network Z (UMTS/GPRS)
User A
SGSN GRXGRXP-CSCF
The Signaling Plane The Signaling Plane
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SIP functionalitySIP 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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� A (possible) text-based way to describe the media session
SDPSDP
Session-level
version
user
subject
Start time
user IP
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Media-level
G711µ-law
H.261 codec
Start time
Stream direction
SIP entitiesSIP entities
�Registrar
�User Agent� SIP endpoints handled by users (also automatically, based on user-
defined rules)
�Proxy server� SIP routers
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� 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
SIP transactionsSIP transactionsTransaction 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)
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� e.g.: INVITE sip:alice@domain.com SIP/2.0
� Status line (in an answer)
� e.g.: SIP/2.0 180 Ringing
Start lineOne or more headers (name:value)
---------EMPTY LINE ---------Message body (OPTIONAL)
SIP message format
Method Request URI Protocol version
Protocol versionStatuscode
Humancode
SIP methodsSIP methods
ACK ack the final response for INVITEBYE terminate a sessionCANCEL cancel a pending requestINFO transport PSTN telephony signalingINVITE establish a sessionNOTIFY notify a UA about a particular eventOPTIONS queries a server about capabilities
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OPTIONS queries a server about capabilitiesPRACK ack for provisional responsePUBLISH upload UA’s info to a serverREGISTER map the current URI and location with
the public URISUBSCRIBE request to be notified about an eventUPDATE update session characteristicsMESSAGE instant message in the message bodyREFER instruct a server to send a request
SIP in the IMSSIP in the IMS
�In principle, no difference with the public Internet
�In practice, IMS imposes some constraints
�Wireless access
�Security
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�QoS
�Roaming
�3GPP/IETF defines a profile of utilizationof SIP in the IMS environment
�E.g., RFC 3261 (redefinition of SIP)
�RFC 3665 and 3666 (Best Current Practice)
PrerequisitesPrerequisitesTwoTwo--phase registrationphase registration
Inbound/Outbound
Offline subscription
Login to theIP access net
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Inbound/OutboundFixed Proxy
Online registration
IP access net
DHCP orLower levelmechanism
Entering the IP Connectivity Access Entering the IP Connectivity Access
NetworkNetwork�The access to GPRS includes the
assignment of servers and IP address
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PDF Context Activation/DePDF Context Activation/De--ActivationActivation
�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
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�When the session ends, all PDP contexts are deactivated
PP--CSCF discoveryCSCF 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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IMSIMS--level registrationlevel registration
�The user requests authorization to use the IMS
�The IMS authenticate and eventually authorize the user
�SIP REGISTER is mandatory
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�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
IMSIMS--level registrationlevel registration
UA
A: U
ser A
uth
. An
s.M
AA
: Mu
ltile
dia
Au
th.A
ns
SAA
:Ser
verA
ssig
nm
ent A
ns.
•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
Withoutauthentication
DL User Auth vector
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UA
R: U
ser A
uth
. Req
MA
R:M
ulti
med
iaA
uth
.Req
SAR
:Ser
verA
ssig
nm
ent R
eq.
Challenge the User
DL User Profile
authenticationvector included
Basic Session SetupBasic Session Setup
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Session Session
setupsetup
Many nodes and
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Many nodes and many messages involved in the setup
Different planes/layers•media•signaling•CAN•core
Session setup (Session setup (cont’edcont’ed))
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