1 1 PART 5 Multimedia & Multimedia Networking MSCEG442 IP Multimedia Subsystem (IMS) IMS started as a technology for 3 rd Generation mobile networks (under the auspices of the 3 rd Generation Partnership Project (3GPP), but it is now spreading to next generation wireline networks and is going to be a key to Fixed Mobile Convergence (FMC). It builds upon Session Initiation Protocol (SIP) which has emerged as the crucial technology for controlling communications in IP-based next generation networks (NGN). IMS is about services and applications, enabling service providers to offer rich multimedia services across wireless, packet and traditional circuit switched networks. It is standards based and uses open interfaces and functional components that can be assembled flexibly into hardware and software systems to support real-time interactive services and applications. The basic set of standards for IMS implementation were released in 2004 and the first implementations are beginning in the European wireless markets. The standards organizations are heavily involved in developing standards to fill inevitable gaps and to add new capabilities. However IMS is still untested in real-life major carrier networks and its wide scale implementation is some years away. That being said, it is the most likely evolution path for next generation networks, including those for Emergency Services.
Transcript
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PART 5
Multimedia & Multimedia Networking
MSCEG442
IP Multimedia Subsystem(IMS)
IMS started as a technology for 3rd Generation mobile networks (under the auspices
of the 3rd Generation Partnership Project (3GPP), but it is now spreading to next generation wireline networks and is going to be a key to Fixed Mobile Convergence
(FMC). It builds upon Session Initiation Protocol (SIP) which has emerged as the crucial technology for controlling communications in IP-based next generation
networks (NGN).
IMS is about services and applications, enabling service providers to offer rich multimedia services across wireless, packet and traditional circuit switched
networks. It is standards based and uses open interfaces and functional components that can be assembled flexibly into hardware and software systems to
support real-time interactive services and applications.
The basic set of standards for IMS implementation were released in 2004 and the
first implementations are beginning in the European wireless markets. The standards organizations are heavily involved in developing standards to fill
inevitable gaps and to add new capabilities. However IMS is still untested in real-life major carrier networks and its wide scale implementation is some years away. That
being said, it is the most likely evolution path for next generation networks, including those for Emergency Services.
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Contents
• Current Standards Activity
• Overview of IMS Architecture
• IMS Core Network Elements
• Registration and Call Flow
• Overview of IMS in the context of Emergency Services
This presentation will illustrate the current standards relating to IMS, define IMS and
the show the functional elements that comprise it. It will explain how elements interact and illustrate example call flows.
Not much work has begun on relating Emergency Services to IMS. This presentation will describe what has been done in 3GPP and highlight areas where
additional attention is required.
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Where does IMS come from?
• In Wireline there has been a long term move towards a Voice over Packet (VoIP) replacement for the PSTN
• Drivers have always been to reduce OPEX in fixed line networks by optimising bandwidth usage, and replacing outdated TDM switching fabric— Compressed speech into less than 64kbps channels (G Standards)
— Further optimisation using VAD, Silence Suppression, ATM AAL2 or IP multiplexing yields significant reduction in core network bandwidth requirements.
• Drivers in 3GPP for Wireless are more service related— IMS is born out of a need for ‘feature rich’ services
— Bandwidth requirements and their impact on capacity for the Radio interface (GSM, GPRS, 3G).
• IMS provides the final piece in the business case for the NGN
IMS comes from the need to evolve the TDM networks into robust,
extensible networks that can take advantage of emerging technologies. The migration from circuit switching to packet switching has played a significant
role in the maturation of data networking. Taking advantage of these same concepts, the voice network can evolve into a multimedia network that allows
the combination and coordination of voice, video and data (sometimes called the Triple Play).
The drivers have to be business driven. In the wireline networks there is a
need to increase bandwidths offered to users and replace the outdated TDM switches.
In wireless networks one clear driver is the ability to introduce services rapidly and uniformly. All services should be available to the user whether
within the home network or roaming to another network.
IMS provides the architectural umbrella for wireline and wireless
convergence.
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IMS-Related Standards Initiatives
There are a number of organizations whose activities directly apply in defining IMS.
The initial concept came from 3GPP in Europe to be the evolution of GSM networks. 3GPP depended heavily upon the work of IETF in defining SIP and
related protocols. It became clear that IMS had broader appeal and organizations such as TISPAN began defining extensions needed for the wireline network.
ATIS has incorporated IMS as the key concept in their NGN project.
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IMS Releases
IMS is being defined in phases. The early phases focused specifically on GSM
evolution. Releases kept building features and functionality and expanding the scope of applicability beyond GSM. Release 7 is to incorporate Emergency
Services.
It is important that the U. S. Emergency Services Industry be involved in this effort to assure its needs are recognized. While IMS has global scope, the needs of
Emergency Services in Europe are different that those in the U.S. A common and coordinated approach to Emergency Services will benefit the global community.
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What is IMS?
• An Industry Standard Service Architecture (SA) and Core Network (CN) architecture
• An IP Multimedia Services Architecture
• Defined with Open Standards from 3GPP and ETSI
• Based on IETF Protocols (SIP, RTP, RTSP, COPS, DIAMETER, etc.)
• Designed for both Wireless and Wireline Networks and for Fixed and Mobile Convergence (FMC)
• A Solution for Service Transparency
• Capable of Interworking with PSTN (i.e. legacy IN-based services)
CSCF – Call Session Control FunctionHSS – Home Subscriber Server
Access & Transport
Plane
Core NetworkSession
Control Plane
Service ArchitectureApplications/Services
Plane
HSSCSCF
AccessNetwork
OtherNetworks
Web PortalWeb Portal
ApplicationApplication
ServersServers
Session Session
ControlControlCentralizedCentralized
DatabasesDatabases
Media Media
Control &Control &
GatewaysGateways
MediaMedia
ServerServer
• IMS is defined as a network architecture that defines functional elements. Each functional element does not have to relate one-to-one to a physical element. A number of functional elements can be
incorporated into a physical element depending upon a vendors implementation. The Service
Architecture defines standard methods for services to be introduced while the Core Network defines
the interactions between functional elements.
• IMS is multimedia. Therefore you need to think beyond a typical voice call. The context could be
voice, video or graphics; or a combination correlated between two or more parties.
• IMS begin with 3GPP and ETSI, both which have their roots in Europe. To introduce IMS in the U.S.
there are some adoptions that will be needed to accommodate U.S. nuances. It is likely that these
will be handled by TIA and ATIS.
• IMS relies heavily existing standards developed by the IETF. Organizations representing IMS are
also heavily influencing the IETF activity to evolve or develop new standards needed to complete the
IMS picture.
• FMC – Fixed Mobile Convergence is the new buzz word of the industry. IMS’s intended applicability
is across wireless, wireline, cable, enterprise and other networks.
• One of the major attributes, which may have more play in wireless, is the idea of service
transparency no matter what network you are in (e.g. the home or visited network).
• There is a recognition that IMS must evolve into the networks and that there needs to be interworking between existing functionality and the newly envisioned capabilities.
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AccessNetwork
OtherNetworks
Web PortalWeb Portal
ApplicationApplication
ServersServers
Session Session
ControlControlCentralizedCentralized
DatabasesDatabases
Media Media
Control &Control &
GatewaysGateways
MediaMedia
ServerServer
Access
Layer
Session
ControlLayer
Application Layer
Open Industry Standard Support for a Variety of
Applications:
• Speed Applications to market
Common Session Control Element to Provide Service Interworking
• Predictable interactions between multiple services
Common Subscriber Database with Open
Interfaces
Service Consistency Across Wireless,
Wireline and VoIP Endpoints:
•Retain ownership of the subscriber and their
services
•Ability to provide differentiated services
Distributed Session Control
• IMS flexibility and scalability reduce OPEX
• Support mobility/portability
Common support for CoS, QoS, security,
scalability, reliability, and performance
Common OAM&P Environment
• Ease integration into OSS/BSS/NMS
Common Common
OAM&P,Billing, OAM&P,Billing,
etc.etc.
Capable of
Interworking with the
PSTN (i.e. legacy IN-
based services)
IP Multimedia Subsystem (IMS)Key Attributes
• Media Gateways are the interface between IMS and the legacy PSTN world. This allows calling between the richly featured multimedia IMS environment to the existing voice networks.
• The interworking between legacy endpoints and the IMS network allow those endpoints to access the features and functions that IMS can provide. Of course, this functionality may be limited by the capabilities of the endpoint. For example, network based features may be available to the legacy endpoint, but not multimedia.
• IMS defines common support for Classes of Service, Quality of Service, security and other attributes. However, standards are still evolving in many of these areas.
• Since IMS is built upon IP, it supports the flexibility and scalability to support mobility, portability, service creation, etc. All of these together may provide operational and financial improvements beyond existing networks.
• IMS defines how to develop subscriber databases which include User Profiles that enumerate identity, services, security levels, etc.
• IMS defines common session control that applies to any media. So the way a voice call is set up is identical to how a video call is set up. Based upon the class of service, different resources may be allocated in the network.
• IMS specifies common OAM&P environment that allows the evolution of operation support systems.
• IMS, in its self, does not define services. However, it defines how services are accessed. These services may be inherent in the IMS network or can provide gateways to existing service platforms.
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RNCMSC(Server)
SGSNGGSN
CNCN
MGW
BSC
UMTS/GPRSUMTS/GPRS
WLANWLAN
CorporateCorporate
P-CSCF
I-CSCF
MRF
MGW
MGCF
IMSIMS
S-CSCF
SIP ApplicationServersSIP Application
Servers
HSS
CDMA 2000CDMA 2000DSL/Cable ModemDSL/Cable Modem
DSLAM/CMTS
IMS Access Network Independence
IMS is designed to be applicable to the evolution of all types of networks. The major
wireless carriers have committed to IMS as their next generations network. All of the U.S. major wireline carriers have embraced IMS in their evolution path. Cable
network companies have not embraced IMS as of yet since Cable Labs has just defined IP-based network topology that was pre-IMS. It is thought that as cable
networks evolve, they too will embrace IMS.
One major advantage of IMS’s commonality is that carriers from each discipline can
purchase equipment based upon the same standards, thereby potentially decreasing the cost to provide duplicate networks for different media services.
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Clie
nts
Clie
nts
De
vic
es
De
vic
es
Ac
ce
ss
Ac
ce
ss
Tra
nsp
ort
Tra
nsp
ort
Se
ssio
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essio
n
Co
ntr
ol
Co
ntr
ol
Ap
plic
ati
on
sA
pp
lic
ati
on
s
Managed Core IP Network Media GW
Signaling GW
MRFCMRFCCSCFCSCF
Service BrokerService BrokerHSSHSS
Pres.Data
Loc.Data
SubsData
Micro BillingMicro Billing
Integrated Integrated PrePre--paid & paid &
PostpaidPostpaid
AAAGroupLists
MediaServer
POTSphone
GGSNPDSN
MSCCellular
AccessPoint
WiFi/802.11x Local Loop
VoIPhandset
ISPBXIP-PBX
IP-phonesISDN-phones
IP-phones
DSL/CableIP LAN
WirelessRouter
BaseStation
LTE
(IP-Centrex)
Office/Hotel HotspotHome On the Road Home/Office
Dual-modeWiFi phoneSoft-phones
PSTN SS7
SIP Clients
Ckthandset
Multiple user interfaces with common look and feel
IMS is being defined as the convergence vehicle for all types of access
connectivity. IMS allows typical Centrex customers to migrate to IP-based Centrex services and provides direct connectivity from the myriad of IP
PBXs that are being deployed. IMS provides a natural evolution as LECsdeploy their broadband access to the home as well as providing cable
providers a standardized approach for network evolution. This theme extends to the introduction of WiFi or WiMax technologies that are inherently
IP based.
It is recognized that not all end points will be SIP enabled. Therefore, legacy
systems can take advantage of the IMS services by entering the network through signaling and media gateways.
As shown in the wireless cloud, a network can evolve such that it can take advantage of existing access techniques while evolving to IP connectivity
that can natively interconnect to the IMS core.
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IMS Functional Elements
• Session Management (SIP)
• Routing
• Databases
• Network Interoperability Elements
• Services and Support Components
• Charging Components
•IMS is the only emerging concept that provides architectural consistency for an
evolving telecommunications network. It encompasses the full range of capabilities required to evolve and eventually replace the legacy TDM network. Not only does it
deal with call delivery, but registration, billing, operations and administration.
•IMS is built upon SIP. In using SIP, IMS is able to take advantage of the work of the IETF. In fact, much of the effort in IETF today is as a result of the IMS influence.
•IMS defines routing elements to include ingress from users, routing within networks and routing between networks. It also acknowledges the need to interwork between
IMS networks and the PSTN.
•Databases are a key component of the IMS structure ranging from those that home subscriber information to those that provide services.
•Inherent in the design of IMS is the concept of interoperability.
•One of the initial concepts of IMS was to define the structure that would allow new
services to be introduced easily and seamlessly. That is, no matter where the user is they should have access to all of the services to which they have subscribed.
• And once services are used, IMS provides the architecture to allow recording and
charging.
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IMS Functional Elements
• Application Server (AS)• Breakout Gateway Control Function (BGCF)• Call Session Control Function (CSCF)• Home Subscriber Server (HSS)• Media Gateway Function (MGW)• Media Gateway Control Function (MGCF)• Multimedia Resource Function Controller (MRFC)• Multimedia Resource Function Processor (MRFP)• Security Gateway (SEG)• Serving/Gateway GPRS Support Node (SGSN/GGSN)• Signaling Gateway (SGW)• Subscription Locator Function (SLF)• Service Capability Interaction Manager (SCIM)• Policy Decision Function (PDF) – “Bandwidth Manager” (CoS & QoS)
This slide shows the functional elements of IMS. Those listed in red are covered in
this presentation. The others are important to complete the picture, but are best left for discussion related to the specific topic, e.g. security.
The complete solution for the support of IP multimedia applications consists of terminals, IP-Connectivity Access Networks (IP-CAN), and the specific functional elements of the IMS subsystem.
In the 3GPP IMS specifications GPRS/UMTS is only one example of an IP-Connectivity Access Networks. 3GPP defines the link (QoS, charging, etc) between IMS and several IP-CANs:
3GPP R5 -> only GPRS/UMTS access
3GPP R6 -> also WLAN
3GPP R7 (in co-operation with TISPAN R1) -> DSL access
The IP-CAN maintains the IP connection while the user moves and hides these moves from the IMS subsystem. Each IP-CAN exposes one anchor point (GGSN, BRAS, WLAN PDG) towards the IMS.
GGSN = Gateway GPRS Serving Node (IP Edge, puts external IP packets to/from terminal on/from GPRS tunnel from/to the Backbone)
The IMS specific functional elements are: CSCFs (SIP session control servers), HSS/SLF (user database), AS (Applications), PDF (QoS/Charging enforcer), BGCF/MGCF/SGW (interworking with legacy circuit switched networks), IMS-ALG/Tr-GW (interworking between IPv6 and IPv4 networks), MRF (conferencing control etc).
3GPP & TISPAN standards only define a “logical / functional”architecture, not a “physical” one. Manufacturers can each chose how to combine the 3GPP functional blocks into physical products.
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VisitedVisited
NetworkNetwork
IMS Home Network - Functional Elements
HSSHSSDNSDNS
ENUMENUM
II--CSCFCSCF SS--CSCFCSCF
Home Network
ASASASASASAS
Home Subscriber Server• Centralized DB• HLR successor• User profile• Filter criteria (sent to S-CSCF)
• Which applications• Which conditions
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 Vendor
Application Servers• Push-to-talk• Instant messaging• Telephony AS• 3rd party or IMS VendorDomain Name ServerDomain Name Server
This slide shows a single network topology using IMS. A call originating from a SIP User Agent in the IMS network may go to another SIP UA or egress to the PSTN.
• The HSS contains all of the subscriber information. In the wireless network it is the
evolution of the HLR. In the wireline network it is the equivalent of customer records provisioned on switches.
• Application Servers are where the application reside. There may, for example, be originating services or terminating services. The filtering criteria is loaded into the S-
CSCF when the subscriber registers with the network.
• DNS is used to identify elements use in the session set up.
• The CSCFs manage the session control: registration, set up, tear down, feature
activation.
• The P-CSCF is first point of interaction with the User Agent. It also manages Quality of
Service and other conditions specific to a UA.
• The I-CSCF is used in network to network signaling. The I-CSCF hides the network
topology from an external network.
• The S-CSCF is the primary signal processing engine in IMS. It manages registration, checks for triggers for services and performs routing (although routing instructions may
come from Application Servers).
• Media Resources may be conference services, IVRs or other network services.
• If a call must egress to the PSTN the BGCF selects the appropriate Media Gateway that
can be used.
• Media Gateways control the conversion from IP to PSTN TDM signaling. Media
Gateway Control Functions control the signaling between IMS and the PSTN (e.g. IP to
SS7).
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Home Subscriber Server (HSS)
• 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
HSSHSS
Diameter
The HSS is the central repository for user-related information. In wireless networks
it is the evolution of the HLR. The HSS contains all the user-related subscription data required to handle multimedia sessions. These data include, among other
items, location information (not the physical location), security information (including authentication and authorization), user profile information (including the services
that the user is subscribed to) and the S-CSCF that is allocated to the user.
A network may contain more that one HSS in the case the number of subscribers is too high to be handled by a single HSS. All data related to a particular user are
stored within a single HSS. The HSS is typically implemented using a redundant configuration.
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Application Server (AS)
• Contains Call Related Application Logic
• Facilitates a Service Creation Environment
• Queried by S-CSCF in Real Time to Execute Logic
• Generally Specialized for Each Service
• May Provide Gateway to Legacy Applications (e.g. AIN)
ASASASASASAS
SIP
Diameter
Application Servers host and execute services. Depending upon the application they
may operate as a SIP Redirect Server, Proxy, User Agent or Back to Back User Agent (B2BUA).
There may be three categories of AS: SIP AS, OSA-CS and IM-SSF.
SIP AS – This is a native Application server that hosts and executes IMS services
based upon SIP. New IMS applications will be developed in the SIP AS.
OSA-SCS (Open Services Access – Service Capability Server) – This Application
Server provides an interface to the OSA framework Applications Server. In essence it provides a gateway function.
IM-SSF (IMS Service Switching Function) – This is a specialized Applications
Server that alls reuse of CAMEL (Customized Applications for Mobile network Enhanced Logic) services. These services are specific to GSM and European
networks. A similar function will provide access to U.S. Advanced Intelligent Network (AIN) services.
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Call/Session Control Function (CSCF)
• CSCF – Processes SIP Signaling• P-CSCF
— First Point of User Contact— Authenticates user— May Include Policy Functions
• C-CSCF— Central Node of Control Plane— Acts as Registar for User (Downloads SSP from HSS)— Invokes Application Servers— Performs Primary Routing Function
• I-CSCF— Located at Edge of Administrative Domain— Is the Ingress Network Point Defined in DNS— Shields Network Topology from External Networks
II--CSCFCSCF SS--CSCFCSCFPP--CSCFCSCFSIP
SIP
SIPSIP
SIP
Diameter
In general CSCF provide the SIP routing logic in the IMS network.
P-CSCF – The P-CSCF is the first point of contact between the IMS terminal and the network . All signaling from/to the IMS terminal go through the P-CSCF. Thje P-
CSCF is allocated to the IMS terminal during registration and provides functions such as security, authentication, and the correctness of the SIP requests. The P-
CSCF may include a Policy Decision Function (PDF) that authorizes media plane resources and manages Quality of Service over the media plane.
I-CSCF – The is 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. When a SIP
server follows SIP procedures to find the next SIP hop for a particular message the SIP server obtains the address of an I-CSCF of the destination domain.
S-CSCF – The S-CSCF is the central note of the IMS signaling plain. It acts as a registrar in that when the IMS terminal registers the S-CSCF obtains SSP
information from the HSS. All signaling passes through a S-CSCF. The S-CSCF inspects every SIP message and determines whether the SIP signaling should visit
one or more Application Servers. Those ASs would potentially provide a service to the user.
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PSTN (Circuit Switched) Gateway
• 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
MGCFMGCF
MGWMGW
H.248
ISUP
BGCFBGCFSIP
SIP
TDM
SGWSGW
SIP
PSTNPSTN
SS7SS7
IMS networks must be able to deliver calls to and receive calls from the PSTN. In
order to do this there is a need to interwork signaling (e.g. SIP to ISUP) and bearer channels (e.g. RTP to TDM).
BGCF – The BGCF provides routing functionality based on telephone numbers. The BGCF is only used in a circuit switched network, such as the PSTN. Its basic
functions are 1) select an appropriate network where interworking with the circuit switched (CS) domain is to occur or 2) select an appropriate PSTN/CS gateway (i.e.
MGCF).
MGCF – The MGCF is the central node of the PSTN/CS gateway. It implements a
state machine that does protocol conversion and maps SIP to ISUP. It also controls the resources of the Media Gateway.
SGW – The Signaling Gateway performs the lower layer protocol conversion. In this presentation it is assumed part of the MGCF.
MGW – The Media Gateway interfaces to the media plane of the CS network. One
side the MGW is able to send and receive IMS media over RTP and on the other side the MGW uses one or more PCM time slots to connect to the CS network.
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Multimedia Resource Function (MRF)
• Offers Services Such as Conferencing
• MRFC – SIP User Interface toward S-CSCF
• MRFP – Controls the Media Server (MS)
SIP
MSMS MSMS
MRFCMRFC
The Media Resource Function provides a source of media in the IMS network. This
may be the ability to play announcements, mix media streams (for conferencing), transcode between different codecs, and do any sort of media analysis.
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IMS Network-to-Network Connectivity
HSSHSSDNSDNS
ENUMENUM
II--CSCFCSCF SS--CSCFCSCF
Visited
Network
Home
Network
SIP
ASASASASASAS
P/SP/S--CSCFCSCFBackboneBackbone
PacketPacket
NetworkNetwork
AccessAccess
MGCFMGCF
MGWMGWPSTNPSTN
H.248
ISUPBGCFBGCF
SIP
SIP
SIP
SS7SS7
SIP
SIP
SIP
SIP
SIPSIP
Diameter
RTP TDM
RTP
RTP
MSMS MSMS
MRFCMRFC
PP--CSCFCSCF SIP
SIP
Proxy/Serving CSCF•Manages call origination•Selects destination network• Routes to I-CSCF
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
Interrogating CSCF• Entry point for incoming calls• Determines S-CSCF for Subscribers• Hides network topology
UA/UE
This slide illustrates the IMS network to network connectivity. A call in the
Visited network goes to the home network and may be terminated to a SIP UA within the network or egress to the PSTN.
• Within the Visited network the P-CSCF and S-CSCF process the
origination of the call and select the destination network.
• Within the Home network the I-CSCF receives the call signaling from the
Visited network, chooses the appropriate S-CSCF to process the call and the call is completed.
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IMS UE Registration
HSSHSS SS--CSCFCSCF PP--CSCFCSCF
UA/UE
RegisterRegister
•The UA/UE Registers with the S-CSCF
MAR/MAA
•The S-CSCF consults HSS for Authentication
Unauth Unauth
•The S-CSCF Challenges the UA/UE
RegisterRegister
•The UA/UE Registers with Credentials to the S-CSCF
MAR/MAA
•S-CSCF Authenticates with HSS and Downloads User Profile
IMS registration is the procedure where the IMS user requests authorization to use
the IMS services in the IMS network. The IMS network authenticates and authorizes the user to access the IMS network.
• The UA/UE initiates the registration process when the terminal is connected or
otherwise introduced into the network. The SIP registration is passed to the S-CSCF. If the user happens to be roaming in another network then the P-CSCF in
the Visited network would pass the registration to the S-CSCF in the Home network through a I-CSCF. Users are always registered in the Home network.
• The S-CSCF forwards the request to the HSS via the Multimedia Auth Request (MAR) message to 1) download authentication data via the Multimedia Auth Answer
(MAA) message and 2) inform the HSS that this S-CSCF is in control and any other queries to the HSS should be returned to this S-CSCF.
• The S-CSCF creates a SIP 401 Unauthorized response that includes a challenge that the IMS terminal should answer.
• The IMS terminal sends a new Register that contains the response to the
challenge.
• The S-CSCF validates the user and sends a Session Auth Request (SAR)
message to the HSS informing it that the user is now registered and requesting the user profile, including services, that come in a Session Auth Answer message
(SAA).
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IMS Subscription to UE State Changes
HSSHSS SS--CSCFCSCF PP--CSCFCSCF
UA/UE
Subscribe
• The P-CSCF Subscribes to the UA/UE Registration State
Notify
• S-CSCF Notifies the P-CSCF of Registration State
SubscribeSubscribe
• The UA/UE Subscribes to its Registration State
Notify Notify
• S-CSCF Notifies the UA/UE of Registration State
Now the Elements can Inform Each Other of Registration State Changes
Now that a user is registered with the network there is a need for notification of state changes. For example, a user registration may be valid for a fixed period of time and then the network requires the user to register. Or the user or network element may go out of service and need to inform the other of some state change. This is done by having the UA/UE subscribe to the registration state. Not only does the UA/UE subscribe, but the P-CSCF serving the UA/UE subscribes so it can be informed.
• When the IMS terminal has completed registration the P-CSCF sends a Subscribe request for the registration event. The request is directed at the S-CSCF (which is in the Home network).
•The S-CSCF receives the request and installs that subscription, i.e. the S-CSCF takes the role of a notifier. The S-CSCF sends a Notify request to the P-CSCF. This request includes Public User Identities and the registration state.
• When the IMS terminal has completed registration it sends a Subscribe request for the registration event. The request is directed at the S-CSCF (which is in the Home network).
•The S-CSCF receives the request and installs that subscription, i.e. the S-CSCF takes the role of a notifier. The S-CSCF sends a Notify request to the user. This request includes Public User Identities and the registration state.
In case the S-CSCF has to shutdown or there is some other stimulus the S-CSCF will inform the user (and the P-CSCF) of the event.
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A Typical Example of an IMS Call
User B
DSL/Cable ModemDSL/Cable Modem
DSLAM/CMTS
RNC
GGSN
Network Z (UMTS/GPRS)Network Z (UMTS/GPRS)
Network XNetwork X
User A
SGSN
Network YNetwork Y
GRXGRXP-CSCF
P-CSCF
I-CSCF
I-CSCF
S-CSCF
S-CSCF
HSS
HSS
ASAS
This example illustrates how IMS multimedia calls may be coordinated between
parties. The first is a voice call originated by User A. The second is a video call originated by User B and the third is a data call originated by User A.
1. The voice call originates from user A and enters the IMS network X at the P-
CSCF
2. The P-CSCF passes the call to the S-CSCF
3. The S-CSCF interrogates the Application Server for originating services
4. The S-CSCF forwards the call to the I-CSCF of network Y.
5. The I-CSCF interrogates the HSS to determine the S-CSCF and passes the call
to it.
6. The S-CSCF interrogates the Application Server for terminating services.
7. The S-CSCF passes the call to the P-CSCF assigned for the user and the voice
call is completed.
Now a video call is set up from User B to User A and the signaling path is reversed.
Finally, User A sets up a data call to User B using the same signaling path.
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23
IMS (3G) Architecture
This slide comes directly from one of the 3GPP specifications and defines protocol
interfaces required to deliver a call to the legacy Emergency Services Network or an IP-capable PSAP. Note that PSAP selection is left to implementation. Also the IP
PSAPs are treated a peer network since the interface is from a S-CSCF to the PSAP and a P-CSCF is not included to manage the PSAP interface.
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24
3GPP IMS R7 Emergency Sessions
HSSHSSDNSDNS
ENUMENUM
SS--CSCFCSCF
IMS Core Network
ASASASASASAS
PP--CSCFCSCF
MGCFMGCF
MGWMGW
SelectiveSelective
RouterRouter
H.248
ISUPBGCFBGCF
SIP
SS7SS7
SIP
SIP
SIP
SIP
SIP
Diameter
TDM
MSMS MSMS
MRFCMRFC
SIP
CAMAE-MF
IP PSAP
Connectivity to NetworkCapable of DeliveringCall to IP PSAP
Connectivity to NetworkCapable of DeliveringCall to IP PSAP
Connectivity to
Legacy Emergency
Services Network
Connectivity to
Legacy Emergency
Services Network
• 3GPP View of Emergency Sessions
RTP
IP Access NetworkInitiates EmergencyCall
IP Access NetworkInitiates EmergencyCall
PSAP SelectionLeft to Implementation
PSAP SelectionLeft to Implementation
IP Connectivity
Access Network (ICAN)
IP-PBX
IP-phones
IP-phones
(IP-Centrex,DSL, Cable)
Legacy PSAP
(PSAP=Public Safety Answering Point)
Serious work on Emergency Services has not begun within IMS standards.
The initial assumption is that Emergency Services would follow legacy methods. This slide illustrates the conceptual model currently defined in
3GPP.
The IP Connectivity Access Network (which represents the wireless access, cable access, etc.) forwards the call to the P-CSCF in the IMS Core
Network and the call is routed to the S-CSCF.
The S-CSCF performs PSAP selection. However, 3GPP currently defines
this as left to the implementation. There is current work within IETF to define this.
The emergency call may be delivered to the legacy Emergency Services Network through Media Gateways.
The emergency call may be delivered to a PSAP capable of directly handling
SIP calls. Note that a significant amount of work is required to define the interactions between the IMS network and a IP-capable PSAP. (Note that
the IMS Core network treats the PSAP as a foreign network since the interface is from a S-CSCF to the PSAP and a P-CSCF is not included.)
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25
Potential Long TermIMS Emergency Services Network
HSSHSSDNSDNS
ENUMENUM
II--CSCFCSCF SS--CSCFCSCF
Customer
Home
Network
IMS Emergency
Services Network
SIP
ASASASAS
PSAP PSAP
SelectionSelection
ASAS
P/SP/S--
CSCFCSCF
BackboneBackbone
PacketPacket
NetworkNetwork
AccessAccess
MGCFMGCF
MGWMGW
BGCFBGCF
SIP
SIP
SIP SIP
SIP
SIP
SIP
SIPSIP
Diameter
RTP
RTP
RTP
MSMS MSMS
MRFCMRFC
PP--CSCFCSCF SIP
SIP
IP PSAP
Call Delivered toIP PSAP withLocation
Call Delivered toIP PSAP withLocation
Call Enters IMS ESNet
With Location
Call Enters IMS ESNet
With Location
Location is Used to Determine PSAP
Location is Used to Determine PSAP
H.248
Selective RouterSelective Router
ISUP
SS7SS7
TDM
CAMA
E-MF
Legacy PSAP
CAMA
E-MF
OR
Call Delivered to Legacy Network Without Location
(Query for Location Needed)
ORCall Delivered to Legacy Network Without Location
(Query for Location Needed)
This slide illustrates a potential architecture where the Emergency Services
Network is IMS enabled.
• The emergency call comes into the IMS ESNet from another IMS network with its location object.
• The S-CSCF interrogates the PSAP Selection Application Server for routing instructions. The AS must convert the location object to a PSAP
URI or other designation that maybe used by the S-CSCF to route the call.
• The call is delivered to the IP-capable PSAP with location.
• It is possible that the call could be delivered directly to the Legacy PSAP via CAMA trunks in the Media Gateway. The issue with this is that now
the location is lost and the Legacy PSAP would have to have a key to query for location information (e.g. ALI).
Note that the concept of managing the interactions between an IMS Emergency Services Network and the Legacy Emergency Service
Network requires much further thought. For example, legacy station sets will need to terminate to IP PSAPs and IP terminals may need to
terminate to an IP PSAP.
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26
Emergency Call DeliveryVoIP to IP PSAP
P-CSCF
I-CSCF MRF
MGWMGCF
IMS ES NetworkIMS ES Network
S-CSCF
SIP ApplicationServersSIP Application
Servers
HSS
IP PSAP
Legacy ESNet/PSAPPSTNPSTN
LegacyEO
STP
VSP VoIP NetworkVSP VoIP Network
PIDF-LO
PIDF-LO
ANI
ANI
• Call Originates with Location
• Location used for PSAP Selection
• Call Delivered with Location
In recognizing that a legacy ESNets may have to exist in parallel with IMS ESNet,
questions regarding how the two interwork for various call flows arise. This slide, and the following 3 slides, illustrate topics for consideration in call delivery.
Four salient components are shown interacting with the IMS ESNet.
1. A VSP VoIP network (or potentially the Intenet) that is capable of delivering the emergency call to the IMS ESNet with the location of the caller (Presence
Identification Format – Location Object (PIDF-LO).
2. A Legacy PSTN that may consist of Class 5 end offices and only pass the
caller’s number (ANI) in the signaling.
3. An IP capable PSAP that is the next generation PSAP that is able to receive a
SIP call that includes the caller’s location (PIDF-LO).
4. The Legacy ESNet or a Legacy PSAP which interconnects using CAMA or E-MF and is only able to receive the caller’s number (ANI).
This slide represents the long term vision of emergency services. That is, a call originates with its location (PIDF-LO) and enters the ESNet via a SIP INVITE.
The location is used to select the PSAP and the call is delivered to the IP capable PSAP with the caller’s location.
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27
Emergency Call DeliveryVoIP to Legacy PSAP
P-CSCF
I-CSCF MRF
MGWMGCF
IMS ES NetworkIMS ES Network
S-CSCF
SIP ApplicationServersSIP Application
Servers
HSS
IP PSAP
PSTNPSTN
LegacyEO
STP
VSP VoIP NetworkVSP VoIP Network
PIDF-LO
PIDF-LO
ANI
ANI
• Call Originates with Location
• Location Could be used for PSAP Selection
• Call Delivered only w/Callback (ANI)
• Query Mechanism Needed for Location
Legacy ESNet/PSAP
If a call originates from a VoIP network capable providing the location of the caller
(PIDF-LO) and must be delivered to the Legacy ESNet or a Legacy PSAP then there are issues that need to be resolved related to the Legacy PSAP obtaining the
location of the caller.
• The call originates in a VoIP network where the caller’s location is included in the SIP INVITE. When the call enters the ESNet the PIDF-LO may be used to select
the Legacy Emergency Services Network or the Legacy PSAP. The call must then traverse a Media Gateway to get to the Legacy ESNet or PSAP. In doing so the
caller’s location is lost. Therefore, a query mechanism is required to obtain the caller’s location.
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28
Emergency Call DeliveryLegacy PSTN to IP PSAP
P-CSCF
I-CSCF MRF
MGWMGCF
IMS ES NetworkIMS ES Network
S-CSCF
SIP ApplicationServersSIP Application
Servers
HSS
IP PSAP
PSTNPSTN
LegacyEO
STP
VSP VoIP NetworkVSP VoIP Network
PIDF-LO
PIDF-LO
ANI
ANI
• Call Originates with only Callback (ANI)
• Mechanism to Provide Location and
PSAP Selection Required • Call Delivered with Location
Legacy ESNet/PSAP
Calls from the PSTN or legacy end offices can only pass the caller’s number.
Therefore if a call were to come from a Legacy EO to the ESNet it would only have ANI. If the call is destined to a IP PSAP, the IMS ESNet would require a mechanism
to acquire the caller’s location, use it to select the PSAP and forward the call to the IP PSAP with location (PIDF-LO)
This flow only illustrates a wireline call. However a wireless call can be extrapolated. For the Wireline Compatibly Mode, an ESRK would be passed to the IMS ESNet
and the flow would have the same attributes as wireline. For Hybrid CAS calls, 20 digits would be delivered to the IMS ESNet and the ESNet would have to use the
ESRD to obtain the caller’s cell site information.
Also, wireless Phase 2 exacerbates the problem since the PSAP may need to
obtain updated location information.
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29
Emergency Call DeliveryLegacy PSTN to Legacy PSAP
P-CSCF
I-CSCF MRF
MGWMGCF
IMS ES NetworkIMS ES Network
S-CSCF
SIP ApplicationServersSIP Application
Servers
HSS
IP PSAP
PSTNPSTN
LegacyEO
STP
VSP VoIP NetworkVSP VoIP Network
PIDF-LO
PIDF-LO
ANI
ANI
• Call Originates with only Callback (ANI)
• Mechanism to Provide Location and
PSAP Selection Required
• Call Delivered only w/Callback (ANI)
• Query Mechanism Needed for Location
Legacy ESNet/PSAP
If a call originates from a Legacy EO and must be terminated to a Legacy ESNet or
PSAP then the issue of obtaining the location for routing and acquiring the location at the PSAP must be addressed. The IMS ESNet would require a mechanism to
acquire the caller’s location, use it to select the PSAP and forward the call to the Legacy PSAP. When the call is received at the Legacy PSAP, a query mechanism
