ObjectivesHighlight Current Standards ActivityPresent an Overview of IMS Architecture Define IMS Core Network ElementsIllustrate Registration and Call FlowPresent an Overview of IMS in the context of Emergency Services

Where does IMS come from?In Wireline there has been a long term move towards a Voice over Packet (VoIP) replacement for the PSTNDrivers have always been to reduce OPEX in fixed line networks by optimising bandwidth usage, and replacing outdated TDM switching fabricCompressed speech into less than 64kbps channels (G Standards)Further optimisation using VAD, Silence Suppression, ATM AAL2 or IP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 relatedIMS is born out of a need for feature rich servicesBandwidth 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-Related Standards Initiatives

IMS Releases

What is IMS?An Industry Standard Service Architecture (SA) and Core Network (CN) architectureAn IP Multimedia Services ArchitectureDefined with Open Standards from 3GPP and ETSIBased 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 TransparencyCapable of Interworking with PSTN (i.e. legacy IN-based services)CSCF Call Session Control FunctionHSS Home Subscriber ServerAccess & Transport PlaneCore NetworkSession Control PlaneService ArchitectureApplications/Services PlaneHSSCSCFAccessNetworkOtherNetworksWeb PortalApplicationServersSession ControlCentralizedDatabasesMedia Control &GatewaysMediaServer

IP Multimedia Subsystem (IMS) - Key AttributesAccessNetworkOtherNetworksWeb PortalApplicationServersSession ControlCentralizedDatabasesMedia Control &GatewaysMediaServerAccessLayerSession ControlLayerApplication LayerOpen Industry Standard Support for a Variety of Applications: Speed Applications to marketCommon Session Control Element to Provide Service InterworkingPredictable interactions between multiple servicesCommon Subscriber Database with Open InterfacesService Consistency Across Wireless, Wireline and VoIP Endpoints:Retain ownership of the subscriber and their servicesAbility to provide differentiated servicesDistributed Session Control IMS flexibility and scalability reduce OPEX Support mobility/portability

Common support for CoS, QoS, security, scalability, reliability, and performanceCommon OAM&P Environment Ease integration into OSS/BSS/NMS Common OAM&P,Billing, etc.Capable of Interworking with the PSTN (i.e. legacy IN-based services)

IMS Access Network IndependenceRNCMSC(Server)SGSNGGSNCNMGWBSCUMTS/GPRSWLANCDMA 2000

ClientsDevicesAccessTransportSessionControlApplicationsIMS Converged Communications Services VisionPresenceServersTelephonyServersUnifiedMessaging

IMS Functional ElementsSession Management (SIP)RoutingDatabasesNetwork Interoperability ElementsServices and Support ComponentsCharging Components

IMS Functional ElementsApplication 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)

VisitedNetworkIMS Home Network - Functional ElementsHSSDNSENUMI-CSCFS-CSCFHome NetworkHome Subscriber Server Centralized DB HLR successor User profile Filter criteria (sent to S-CSCF) Which applications Which conditionsApplication Servers Push-to-talk Instant messaging Telephony AS 3rd party or IMS VendorDomain Name ServerP-CSCFMGCFMGWPSTNH.248ISUPBGCFSIPSS7SIPSIPSIPSIPSIPSIPSIPDiameterRTPTDMCall SessionControl Function SIP registration SIP session setupMSMSMRFCMedia Gateway Control Function Interfaces to PSTN/PLMN by Converting SIP ISUP Interworking RTP to circuit H.248 control of MGWBreakout Gateway Control Function Selects network (MGCF or other BGCF) in which PSTN/ PLMN breakout is to occur Media Resource Function Controller Pooling of Media servers (e.g. conference)

Proxy CSCF 1st contact point for UA QoS Routes to S-CSCFInterrogating CSCF Entry point for incoming calls Determines S-CSCF for Subscribers Hides network topologyServing CSCF Registrar Session control Application InterfaceP-CSCFSIPSIPUA/UEUA/UE

IMS Network-to-Network ConnectivityHSSDNSENUMI-CSCFS-CSCFVisitedNetworkHome NetworkSIPP/S-CSCFBackbonePacketNetworkAccessMGCFMGWPSTNH.248ISUPBGCFSIPSIPSIPSS7SIPSIPSIPSIPSIPSIPDiameterRTPTDMRTPRTPMSMSMRFCP-CSCFSIPSIPProxy/Serving CSCFManages call originationSelects destination network Routes to I-CSCF

Interrogating CSCF Entry point for incoming calls Determines S-CSCF for Subscribers Hides network topologyUA/UE

IMS UE RegistrationHSSS-CSCFP-CSCFUA/UE

IMS Subscription to UE State ChangesHSSS-CSCFP-CSCFUA/UENow the Elements can Inform Each Other of Registration State Changes

A Typical Example of an IMS CallUser BDSL/Cable ModemDSLAM/CMTSRNCGGSNNetwork Z (UMTS/GPRS)Network XUser ASGSNNetwork YGRXP-CSCFP-CSCFI-CSCFI-CSCFS-CSCFS-CSCFHSSHSSASAS

IMS (3G) Emergency Services Architecture

3GPP IMS R7 Emergency SessionsHSSDNSENUMS-CSCFIMS Core NetworkP-CSCFMGCFMGWSelectiveRouterH.248ISUPBGCFSIPSS7SIPSIPSIPSIPSIPDiameterTDMMSMSMRFCSIPCAMAE-MFIP PSAPConnectivity to NetworkCapable of DeliveringCall to IP PSAPConnectivity toLegacy EmergencyServices Network 3GPP View of Emergency SessionsRTPIP Access NetworkInitiates EmergencyCallPSAP SelectionLeft to ImplementationIP ConnectivityAccess Network (ICAN)IP-PBXIP-phones(IP-Centrex,DSL, Cable)Legacy PSAP

Server

Potential Long TermIMS Emergency Services NetworkHSSDNSENUMI-CSCFS-CSCFCustomerHomeNetworkIMS EmergencyServices NetworkSIPP/S-CSCFBackbonePacketNetworkAccessMGCFMGWBGCFSIPSIPSIPSIPSIPSIPSIPSIPSIPDiameterRTPRTPRTPMSMSMRFCP-CSCFSIPSIPIP PSAPCall Delivered toIP PSAP withLocationCall Enters IMS ESNetWith LocationLocation is Used to Determine PSAPH.248

Emergency Call DeliveryVoIP to IP PSAPP-CSCFI-CSCFMRFMGWMGCFIMS ES NetworkS-CSCFSIP ApplicationServersSIP ApplicationServersHSSIP PSAPLegacy ESNet/PSAPPSTNLegacyEOSTPPIDF-LOPIDF-LOANIANI

Emergency Call DeliveryVoIP to Legacy PSAPP-CSCFI-CSCFMRFMGWMGCFIMS ES NetworkS-CSCFSIP ApplicationServersSIP ApplicationServersHSSIP PSAPPSTNLegacyEOSTPPIDF-LOPIDF-LOANIANILegacy ESNet/PSAP

Emergency Call DeliveryLegacy PSTN to IP PSAPP-CSCFI-CSCFMRFMGWMGCFIMS ES NetworkS-CSCFSIP ApplicationServersSIP ApplicationServersHSSIP PSAPPSTNLegacyEOSTPPIDF-LOPIDF-LOANIANILegacy ESNet/PSAP

Emergency Call DeliveryLegacy PSTN to Legacy PSAPP-CSCFI-CSCFMRFMGWMGCFIMS ES NetworkS-CSCFSIP ApplicationServersSIP ApplicationServersHSSIP PSAPPSTNLegacyEOSTPPIDF-LOPIDF-LOANIANILegacy ESNet/PSAP

Emergency Services - Additional TopicsBridge/TransferIncluding Location, if FeasibleAlternate RoutingDefault RoutingWireless Phase 2 UpdatesTTY/TDD3rd Party Call Centers

BACKUP SLIDES

FUNCTIONAL ELEMENTS DESCRIPTIONSHome Subscriber Server (HSS)Application Server (AS)Call Session Control Function (CSCF)Breakout Gateway Control Function (BGCF)Media Gateway Function (MGW)Media Gateway Control Function (MGCF)Multimedia Resource Function Controller (MRFC)Multimedia Resource Function Processor (MRFP)

Home Subscriber Server (HSS)Presence, Location and ProfileEnd-User IdentityPrivate and Public End-User InformationRegistration InformationService Initiation InformationSubscriber Service Profile (SSP)Downloaded to CSCF at RegistrationHSSDiameter

Application Server (AS)Contains Call Related Application LogicFacilitates a Service Creation EnvironmentQueried by S-CSCF in Real Time to Execute LogicGenerally Specialized for Each Service May Provide Gateway to Legacy Applications (e.g. AIN)SIPDiameter

Call/Session Control Function (CSCF)CSCF Processes SIP SignalingP-CSCF First Point of User ContactAuthenticates userMay Include Policy FunctionsC-CSCFCentral Node of Control PlaneActs as Registar for User (Downloads SSP from HSS)Invokes Application ServersPerforms Primary Routing FunctionI-CSCFLocated at Edge of Administrative DomainIs the Ingress Network Point Defined in DNSShields Network Topology from External NetworksI-CSCFS-CSCFP-CSCFSIPSIPSIPSIPSIPDiameter

PSTN (Circuit Switched) GatewayBGCF Routes to Gateway Based Upon Telephone NumberMGCF Controlling Function for SGW and MGWSGW Provides Signaling Conversion Between SIP and ISUPMGW Provides Conversion between RTP and TDM

MGCFMGWH.248ISUPBGCFSIPSIPTDMSGWSIP

Multimedia Resource Function (MRF)Offers Services Such as ConferencingMRFC SIP User Interface toward S-CSCFMRFP Controls the Media Server (MS)SIPMSMSMRFC

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.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.

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.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. 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. 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. IMSs 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. 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.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 IMSs 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. 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 LECs deploy 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. 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.

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.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).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.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).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.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.The voice call originates from user A and enters the IMS network X at the P-CSCFThe P-CSCF passes the call to the S-CSCFThe S-CSCF interrogates the Application Server for originating servicesThe S-CSCF forwards the call to the I-CSCF of network Y.The I-CSCF interrogates the HSS to determine the S-CSCF and passes the call to it.The S-CSCF interrogates the Application Server for terminating services.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.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.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.)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.

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.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).A Legacy PSTN that may consist of Class 5 end offices and only pass the callers number (ANI) in the signaling.An IP capable PSAP that is the next generation PSAP that is able to receive a SIP call that includes the callers location (PIDF-LO).The Legacy ESNet or a Legacy PSAP which interconnects using CAMA or E-MF and is only able to receive the callers 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 callers location. 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 callers 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 callers location is lost. Therefore, a query mechanism is required to obtain the callers location.Calls from the PSTN or legacy end offices can only pass the callers 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 callers 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 callers cell site information.

Also, wireless Phase 2 exacerbates the problem since the PSAP may need to obtain updated location information. 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 callers 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 is required to obtain the callers location.

Call delivery is only one of the things that must be resolved in order to implement a VoIP ESNet. Many of the Emergency Services functions must be replicated. A call may need to be bridged from one PSAP to another PSAP. It would be desirable to have the original callers location information transferred as well. Transferring between IP capable and Legacy PSAPs must be addressed. Specifications are required for routing to an alternate PSAP if the primary is not available. If PSAP selection fails (corrupted data, data not yet provisioned) a method must be incorporated to choose a default PSAP that can manage calls. Wireless Phase 2 information often is not available at the time the call is set up. In addition the PSAP may want to request a location update. These mechanism must be considered. NRIC VII and others have noted considerations for TTY/TDD over IP. This topic needs careful consideration with regard to PSAPs. The ability for 3rd party call centers and Relay Centers to provide emergency calls on behalf callers needs to be specified.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. 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.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.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. 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.