ETH/RZC Zsolt Szendrei

ETH/RZC Zsolt Szendrei +36-1-4377203
IMS IP Multimedia Subsystem Do not forget to update the Slide Master Rev A

IMS is… IP Multimedia Subsystem (IMS) is an architecture for offering multimedia services on the packet domain. IMS is access agnostic, hence it is not just for UMTS or GPRS, but also supports WLAN, fixed line, LTE etc. IMS is not a service, but a service enabler IMS gives Service control by introducing service signalling All IMS services can be done without IMS but without IMS, service creation can be complex, and expensive to the operator and the user IMS gives Convergence of all media communications on to the packet network. Rev A

Service interoperability non-IMS
ETH/RZC Zsolt Szendrei Service interoperability non-IMS Service 2 Service 2 Service 1 Service 1 Operator 3 Operator 1 Service 2 Service 1 Operator 2 Rev A

Service interoperability IMS
ETH/RZC Zsolt Szendrei Service interoperability IMS Service 2 Service 2 Service 1 Service 1 Operator 3 Operator 1 Service 2 Service 1 Operator 2 Rev A

IMS architecture SIP Application Servers SIP Application Servers HSS IMS I-CSCF MRF P-CSCF MGCF CDMA 2000 S-CSCF MGW MSC(Server) RNC Corporate SGSN GGSN IMS is an architecture designed to support the control layer for packet based services, which uses the bearer services of the access network to support the media associated with the service. IMS is access agnostic and as shown. In a multi-access environment it ensures service availability to all access networks (subject to the limitations of the access networks). BSC CN UMTS/GPRS MGW WLAN Rev A

Architecture Elements
ETH/RZC Zsolt Szendrei Architecture Elements Database Elements HSS (Home Subscriber Server) SLF (Subscription Locator Function) IMS Control Elements (Call Session Control Function) S-CSCF P-CSCF I-CSCF Control Plane Interworking Elements MGCF - Media Gateway Control Function BGCF - Breakout Gateway Control Function SGW - Signaling Gateway CSE(SCP) SIP Application Servers OSA Application Server IM-SSF OSA-SCS HSS P-CSCF S-CSCF I-CSCF MRF The new IMS nodes can be split into three main groups of elements: Database Elements HSS (Home Subscriber Server) is the main database element. It is an evolution of the HLR and provides the following functions: HLR functions (subscriber data and authentication data) Location Register Service and Subscription Data Authentication SLF - Subscription Locator Function A supplementary node that support the HSS distributed solution IMS Control Elements Nodes acting on the control (SIP) signalling flows. These nodes are x-CSCFs each with different roles and behaviour: S-CSCF P-CSCF I-CSCF Control Plane Interworking Elements Elements involved in interworking with legacy networks 2G CS, PSTN, Internet: MGCF - Media Gateway Control Function BGCF - Breakout Gateway Control Function SGW - Transport Signalling Gateway MGCF BGFC MGW SGW Rev A

Other Elements CSE(SCP) SIP Application Servers OSA Application Server IMS Service Elements AS (Application Server) External Service and Service Interworking Elements OSA SCS OSA Framework OSA Application Server CAMEL elements - IM-SSF (IP Multimedia Switching Service Function) Resource Elements Media Resources Function (MRF) Media Interworking Elements MGW (Media Gateway) IM-SSF OSA-SCS HSS P-CSCF S-CSCF I-CSCF MRF MGCF BGCF We’ve seen the three main groups of elements, but there are also other application supporting elements: IMS Service Elements These nodes are called Application Server and provide the added value to the SIP signalling in order to: Provide services and applications Enable the network to provide services External Service and Service Interworking Elements OSA and OSA interworking elements OSA SCS OSA Framework OSA Application Server OSA: Open Service Architecture APIs for telephony NW specified by the Parlay group Elements that enable for legacy CAMEL services: CAMEL (Customized Applications for Mobile Enhanced Logic) will provide the GSM operator with the ability to offer operator specific services based on IN service logic to a GSM subscriber even when roaming. IM-SSF – IP Multimedia Switching Service Function Resource Elements These nodes provide additional resources such as announcements Media Resources Function (MRF) Media Interworking Elements Media Gateway (MGW) – supports transcoding and transport interworking MGW SGW Rev A

Proxy-CSCF (P-CSCF) Home IMS Network Entry point to IMS from any access network Local outbound stateful proxy for all SIP requests/responses, ensuring all signalling is sent via the home network Performs compression/decompression for efficient use of the radio interface Performs integrity protection mechanisms using IPsec Includes a Policy Decision Function (PDF) that authorizes bearer resources The P-CSCF and the GGSN are located in the same network, that is, either both in the visited PLMN or both in the home PLMN. CSE(SCP) SIP Application Servers OSA Application Server IM-SSF HSS OSA-SCS P-CSCF S-CSCF I-CSCF MRF S-CSCF MGCF SGW MGW GGSN Home Access Network Visited IMS Network P-CSCF The Proxy-CSCF is the entry point towards the IMS network from any access network. The assignment of a P-CSCF to a user is determined by the access network configuration. In the case of UMTS/GPRS the allocation takes place at PDP context activation, where the UE may use a DHCP query to obtain the list of P-CSCFs or the UE is provided the IP address of the P-CSCF by the GGSN in the PDP activation message. The P-CSCF is located in the same PLMN as the GGSN. The P-CSCF is a stateful SIP proxy and all signalling flows between the user and the IMS system will be routed through the P-CSCF. The P-CSCF will also enforce the routing of signalling messages through the user’s home network. The P-CSCF is responsible for sending the first SIP message (SIP registration query) towards the corresponding I-CSCF, based on the domain name in the registration request. After successful completion of the registration procedure, the P-CSCF maintains the knowledge of the ‘SIP Server’ (the serving S-CSCF, located in the home network) associated to the user, and will forward all requests from the user toward it. The P-CSCF is responsible for establishing a security association with the user, which it maintains for the lifetime of the ‘connection’. Once the security association is established, it is responsible for receiving and validating all session requests. Access network peculiarities are handled by the P-CSCF, such as the compression and decompression of SIP messages. The P-CSCF also includes the Policy Decision Function (PDF) which authorises the use of bearer and QoS resources within the access network for IMS services. The P-CSCF is always located in the same network as the GGSN is located. Therefore, both the GGSN and the P-CSCF are located either in the visited PLMN or the home PLMN. Note that in roaming scenarios the SGSN is always located in the visited PLMN. PSTN PLMN Internet IP networks Visited Access Network GGSN Rev A

PDF Performs policy control and media authorisation based on the policy of the access network (I.e. GPRS) and based on the session parameters carried in the SIP signalling Home IMS Network CSE(SCP) SIP Application Servers OSA Application Server IM-SSF HSS OSA-SCS P-CSCF I-CSCF S-CSCF MRF S-CSCF MGCF SGW MGW GGSN Home Access Network P-CSCF PDF Rev A

Interrogating-CSCF (I-CSCF)
ETH/RZC Zsolt Szendrei Interrogating-CSCF (I-CSCF) Visited IMS Network S-CSCF I-CSCF MRF MGW MGCF HSS SIP Application Servers OSA-SCS SGW Internet IP networks PSTN PLMN Home IMS Visited Access GGSN P-CSCF CSE(SCP) IM-SSF OSA Application Server Home Access First point of contact in the home network from a visited NW Acts as a SIP proxy located at the edge of a network. Queries the HSS to help in selecting a S-CSCF for a user. The Interrogating-CSCF is the first point of contact within the home network from a visited network or external network. It’s main job is to query the HSS and find the location of the S-CSCF. The functionality is similar to that of a Gateway MSC. Rev A

Serving-CSCF (S-CSCF)
ETH/RZC Zsolt Szendrei Serving-CSCF (S-CSCF) Visited IMS Network S-CSCF I-CSCF MRF MGW MGCF HSS SIP Application Servers OSA-SCS SGW Internet IP networks PSTN PLMN Home IMS Visited Access GGSN P-CSCF CSE(SCP) IM-SSF OSA Application Server Home Access Stateful SIP server providing session control. Acts as a SIP registrar Always located in the home network. Central point for control of operator provided services. Performs authentication. The Serving-CSCF is the node that performs the session management within the IMS network for the UE. The S-CSCF operates in a stateful manner. The S-CSCF also ensures end-to-end reachability for users and services by interacting with other S-CSCFs, SIP servers and application servers. The S-CSCF also authenticates the user. The S-CSCF is the main control point for services. The S-CSCF enforces the rule set for services based on the general policy of the operator and the users subscription parameters. The S-CSCF may reject a service request according to these above factors. The S-CSCF decides on the handling of service requests from the user based on the user’s profile, provided by the HSS during registration. Where the services of an application server are required to complete the requested service, the S-CSCF will forward the request to the appropriate application server either based on the user’s profile or based on the operator’s local policy. The S-CSCF is always located in the home network. There can be several S-CSCFs in the network. They can be added as needed based on the capabilities of the nodes or the capacity requirements of the network and if required can be assigned dedicated functions. The management of S-CSCFs in the IMS network is dynamic and the I-CSCF can allocate the S-CSCF for a user at registration time. The S-CSCF may be chosen based on the services requested or the capabilities of the mobile. One key advantage of this architecture is that the home network provides the services and service features. This means that the user’s configurations are always the same and are always provided by the home network operator. The user is not restricted to the capabilities of the visited IMS network as is seen in the current wireless network (i.e. if an MSC does not support a feature that you have subscribed to, you will not be able to use that feature). However the user is still limited by the visited access network capabilities. This also has distinct charging advantages. Rev A

HSS and SLF Visited IMS Network S-CSCF I-CSCF MRF MGW MGCF HSS SIP Application Servers OSA-SCS SGW Internet IP networks PSTN PLMN Home IMS Visited Access GGSN P-CSCF CSE(SCP) IM-SSF OSA Application Server Home Access Home Subscriber Server (HSS) An evolution of the HLR HLR (subscriber and authentication data for CS and PS domains) + the new IMS functionality Location Register IMS subscriber records IMS service profile processing IMS authentication data Home Subscriber Server (HSS) The main database element is the HSS (Home Subscriber Server). This element is an evolution of the HLR element. The HSS contains the features of the HLR (subscriber data and authentication data) and other functions like Location Register and IMS Service Profile Processing and IMS Subscription and Authentication Data. The HSS will be accessed by the I-CSCF, the S-CSCF and external platforms. The HSS uses the Diameter protocol with the Diameter Multimedia Application. Subscription Locator Function (SLF) Another database element is the SLF. This database is accessed by the I-CSCF and the S-CSCF in order to obtain the HSS which stores some user data when more than one HSS is present in the network. The query will contain the identification of the user and the response will contain the HSS (the HSS address) to ask for the user data. Subscription Locator Function (SLF) A look-up function that can be used in networks where several HSS exist. Rev A

Breakout Gateway Control Function (BGCF)
ETH/RZC Zsolt Szendrei Breakout Gateway Control Function (BGCF) Selects the appropriate PSTN/CS domain break out point for the session. Selects the network in which the interworking with the PSTN/CS Domain is to occur. If the interworking is in another network, then the BGCF will forward the SIP signalling to the BGCF of that network. If the interworking is in another network and network hiding is required by the operator, the BGCF will forward the SIP signalling via an I-CSCF toward the BGCF of the other network. Selects the MGCF in the network in which the interworking with PSTN/CS domain is to occur and forwards the SIP signalling to that MGCF. BGCF External Network S-CSCF Home GSM/UMTS CS Network GGSN MGW MGCF Home Network External Network The BGCF selects the network in which PSTN/CS Domain breakout is to occur for IMS calls terminating in a legacy network. If the BGCF determines that the breakout is to occur in the same network in which the BGCF is located, then the BGCF selects an MGCF which will be responsible for the interworking with the PSTN/CS domain. If the break out is in another network, the BGCF will forward this session signalling to another BGCF in the selected network. The BGCF may make use of information received from other protocols, or may make use of administrative information, when making the choice of which network the interworking shall occur. BGCF Functionality Receives request from S-CSCF to select appropriate PSTN/CS domain break out point for the session. Selects the network in which the interworking with the PSTN/CS Domain is to occur. If the interworking is in another network, then the BGCF will forward the SIP signalling to the BGCF of that network. If the interworking is in another network and network hiding is required by the operator, the BGCF will forward the SIP signalling via an I-CSCF(THIG) toward the BGCF of the other network. Selects the MGCF in the network in which the interworking with PSTN/CS domain is to occur and forwards the SIP signalling to that MGCF. This may not apply if the interworking is a different network. Rev A

Multimedia Resource Function (MRF)
ETH/RZC Zsolt Szendrei Multimedia Resource Function (MRF) Home Access Network GGSN S-CSCF I-CSCF MRF MGW MGCF HSS SIP Application Servers OSA-SCS SGW Home IMS CSE(SCP) IM-SSF OSA Application Server P-CSCF The media resource function provides the additional media resources complementary or fundamental for services. The functions to be applied will be: Conference management such as, a floor control mechanism to allow some members of a conference to control the conference. To provide additional media streams to the session, such as media fade-in for rich calls. Play announcements and tones The MRF is split in to two functional parts : Multimedia Resource Function Controller (MRFC) Multimedia Resource Function Processor (MRFP) The media resource function provides the additional media resources complementary or fundamental for services. The functions to be applied will be: - Conference management such as, a floor control mechanism to allow some members of a conference to control the conference. - To provide an additional media streams to the session, such as media fade-in for rich calls. - Play announcements and tones The MRF is split in to two functional parts : - Multimedia Resource Function Controller (MRFC) - Multimedia Resource Function Processor (MRFP) MRF MRFC MRFP Mp Rev A

IMS service environment
ETH/RZC Zsolt Szendrei IMS service environment Home IMS Network There are three different types of application servers, depending on the application hosted: SIP AS (Application Server): IMS native applications IM-SSF (IP Multimedia Switching Service Function) Legacy applications OSA-SCS (Service Capability Server): Service frameworks (e.g. OSA) CSE(SCP) SIP Application Servers OSA Application Server IM-SSF HSS OSA-SCS P-CSCF S-CSCF I-CSCF MRF MGCF SGW MGW GGSN Home Access Network Visited IMS Network P-CSCF The IMS service environment is designed to support native IMS applications, legacy applications and service frameworks such as OSA. The drivers behind the service environment are: Provide enhanced services taking advantage of the network features. Reduce the development cost and time of services by using a standard developing platforms, languages, interfaces and environments. Reduce the hardware cost of services by using standard hardware platforms. SIP Application Servers: These are application servers designed to support native IMS applications. They host/enable and execute services and applications. The SIP Application Server influence and impact the SIP session on behalf of the Services. SIP application servers may act as SIP proxies or SIP User Agents, depending on the service provided. IM-SSF (IP Multimedia Switching Service Function): The IM-SSF provides the interworking between the IMS and legacy service environment of CAMEL and IN. The functionality required to support IN based services are found in the IM-SSF, such as the trigger detection points, CAMEL Service Switching Finite State Machine, etc. This also ensures that the IMS nodes do not need to support legacy protocols such as CAP and CAP/INAP towards the legacy environment is terminated at the IM-SSF. OSA-SCS (Service Capability Server): Interfaces to the OSA framework Application Server and provides one standardized way for third party secure access to the IMS. PSTN PLMN Internet IP networks GGSN Visited Access Network Rev A

SIP Application Server
ETH/RZC Zsolt Szendrei SIP Application Server Home IMS Network Accept requests and responses. It will be able to control, finish or initiate a new SIP transaction Route the session towards another user or network Interact with other service platforms for the support of services Communicate with the HSS in order to obtain information about subscriptions and services CSE(SCP) SIP Application Servers OSA Application Server IM-SSF HSS OSA-SCS P-CSCF S-CSCF I-CSCF MRF MGCF SGW MGW GGSN Home Access Network SIP Application Servers The SIP Application Server is a node which hosts the logic of a native IMS application, able to control the session and apply a defined service. This node is reachable via the S-CSCF and is able to: Accept requests or services. It will be able to control, finish or initiate a new SIP transaction (new session, messaging sending,…). Route the session towards another user or network. Interact with other service platforms for the support of services. The Application Server will be able to communicate with the HSS in order to obtain information about subscriptions and services. This interface is known as Sh and is based on Diameter protocol. In order to apply the established service logic, the Application Server might be able to communicate with another nodes like external databases or another network node, all these solutions are not standardized. Rev A

Registration to the network
ETH/RZC Zsolt Szendrei Registration to the network UE GSN HLR IMS 1. Bearer Level Registration: GPRS 2. PDP Context Activation 3. CSCF Discovery This slide shows the high-level sequence of events that take place prior to the IMS registration The UE attaches to the GPRS level The UE establishes a PDP context. This may be a general PDP context or a dedicated PDP context for IMS signalling. As a result, the UE gets an IPv6 /64 prefix. This allows the UE to create an IPv6 address. The UE performs the CSCF discovery procedure. As a result, the UE gets the IP address of the P-CSCF allocated to the UE The UE performs the IMS registration procedures 4. Application Level Registration Rev A

Session establishment model
ETH/RZC Zsolt Szendrei Session establishment model 12 13 Steps: 1) Route signalling to A’s home network 2) Route signalling to B’s home network 3) Route signalling to B terminal Steps: 1) Route signalling to A’s home network 2) Route signalling to B’s home network Steps: 1) Route signalling to A’s home network Home B Home A AS HSS AS 11 10 8 7 5 4 9 6 3 S-CSCF I-CSCF S-CSCF I-CSCF 16 17 18 15 19 2 Visited B Visited A P-CSCF P-CSCF 14 20 1 Session establishment model In summary, the IMS architecture provides the capability for the home network operator to control all sessions for its users whether they are roaming or not and by having such control, additional services can be invoked and various charging models can be applied. The diagram shows an overview of how the different IMS nodes come together to ensure that the IMS signalling (even when user A and user B are both subscribers of different home networks and are both roaming to different visited networks) is always routed via the user’s home network. This applies to both the calling and the caller user. GGSN GGSN SGSN SGSN Radio Access Network Radio Access Network User plane (RTP) Rev A

Application level Registration (I)
ETH/RZC Zsolt Szendrei Application level Registration (I) Home A The S-CSCF challenges the subscriber S-CSCF The S-CSCF downloads the authentication vector for this subscriber from the HSS S-CSCF The I-CSCF contacts the HSS to find the S-CSCF serving this subscriber. I-CSCF The P-CSCF finds the I-CSCF with the help of DNS P-CSCF A UE initiates a SIP Registration including his public user ID and his private user ID Subscriber The I-CSCF finds the S-CSCF allocated to this user or selects a new S-CSCF for this user. I-CSCF SIP-AS HSS 7 3 4 6 5. REGISTER I-CSCF S-CSCF 8. Unauthorized 2. REGISTER 9. Unauthorized Visited A P-CSCF 10. Unauthorized 1. REGISTER 1. The UE sends a SIP REGISTER message. This message includes a Public User ID, the Private User ID, and the home network SIP URI. 2. The P-CSCF finds, with the help of DNS, the home network entry point (I-CSCF). The P-CSCF forwards the REGISTER request to that I-CSCF. 3. The I-CSCF queries the HSS to find out if there is an already allocated S-CSCF to this user. 4. The HSS returns either: The address of the S-CSCF allocated to this user. A set of capabilities that will help the I-CSCF to choose an appropriate S-CSCF for this user. The I-CSCF may choose a new S-CSCF or use an already allocated one. In both cases, it forwards the REGISTER request to that S-CSCF. The S-CSCF informs the HSS that this S-CSCF is taking care of the user. The HSS returns one or more authentication vectors. The S-CSCF challenges the UE. The I-CSCF forwards the SIP response. The P-CSCF forwards the SIP response to the UE. GGSN SGSN Radio Access Network A Rev A

Application level Registration (II)
ETH/RZC Zsolt Szendrei Application level Registration (II) Home A The S-CSCF authorizes the registration and informs the subscriber S-CSCF The I-CSCF contacts the HSS to find the S-CSCF serving this subscriber. I-CSCF The I-CSCF finds the S-CSCF allocated to this user I-CSCF The S-CSCF informs the HSS that this S-CSCF is serving the subscriber. The HSS sends the S-CSCF the service profile S-CSCF The P-CSCF finds the I-CSCF with the help of DNS P-CSCF The UE calculates the credentials and re- initiates the SIP Registration Subscriber The S-CSCF informs the other Application Servers about the user's registration S-CSCF SIP-AS HSS 17 13 14 18. REGISTER 16 19. OK 15. REGISTER I-CSCF S-CSCF 20. OK 12. REGISTER 21. OK Visited A P-CSCF 22. OK 11. REGISTER The registration procedure continues: The UE calculates the credentials, includes them into the REGISTER request, and sends it to the P-CSCF. The P-CSCF finds, with the help of DNS, the home network entry point (I-CSCF). The P-CSCF forwards the REGISTER request to that I-CSCF. The I-CSCF queries the HSS to find out if there is an already allocated S-CSCF to this user. The HSS returns the address of the S-CSCF allocated to this user. The I-CSCF forwards the REGISTER request to that S-CSCF. The S-CSCF informs the HSS that this S-CSCF is taking care of the user. The S-CSCF returns the user profile containing the filter criteria The S-CSCF evaluates the filter criteria, and may contact, if needed, one ore more application server. In this example, the filter criteria indicates that an application server has to be informed about the user's registration. The S-CSCF creates a new REGISTER (3rd party REGISTER)requests and sends to an application server. The AS acknowledges the reception of the REGISTER request The S-CSCF informs the UE about the successful registration. The I-CSCF forwards the response. The P-CSCF forwards the response GGSN SGSN Radio Access Network A Rev A

Session Establishment
ETH/RZC Zsolt Szendrei Session Establishment Home A Home B The I-CSCF fetches from the HSS the address of the S-CSCF that is serving subscriber B I-CSCF The S-CSCF in Home B performs service invocation for subscriber B. It forwards the INVITE to the P-CSCF and UE S-CSCF The I-CSCF routes the message to the originating S-CSCF I-CSCF The S-CSCF in Home A performs service invocation for subscriber A. The S-CSCF finds the entry point in Home B S-CSCF The P-CSCF is the first point of contact in the IMS for the subscriber. It forwards the INVITE request to the I-CSCF P-CSCF The HSS returns the address of the S-CSCF serving subscriber B HSS Subscriber A initiates a SIP invitation including the SIP URI of subscriber B. Subscriber SIP-AS SIP-AS HSS 5 6 7. INVITE 3. INVITE 4. INVITE I-CSCF S-CSCF I-CSCF S-CSCF 14. OK 13. OK 12. OK 2. INVITE 15. OK 11. OK 8. INVITE Visited A Visited B P-CSCF P-CSCF 1. INVITE 16. OK 10. OK 9. INVITE The Session Establishment procedures: The UE sends an INVITE request through the originating leg (P-CSCF, optional I-CSCF and S-CSCF). The INVITE request includes a Request-URI with the SIP URI of destination subscriber. The INVITE request contains a Route header that helps the P-CSCF to route the INVITE request to the I-CSCF. The I-CSCF, with the help of the Route header, forwards the INVITE request to the S-CSCF. The S-CSCF evaluates the filter criteria for subscriber A. In this example, subscriber A does not have any originating services, so no AS is contacted. Then the S-CSCF inspects the Request-URI to find the destination network. With the help of DNS, the S-CSCF finds the entry point (I-CSCF) of Home B. The S-CSCF forwards the INVITE request to that I-CSCF. The I-CSCF queries the HSS to find out which S-CSCF is serving the destination subscriber B. The HSS returns the address of the S-CSCF serving subscriber B The I-CSCF forwards the INVITE request to that S-CSCF The S-CSCF evaluates the service criteria for subscriber B. In this example, there are no AS involved. The S-CSCF, inspecting the Contact information (at registration), forwards the INVITE request to the P-CSCF. The P-CSCF forwards the INVITE request to the UA UE B accepts the session. The 200 (OK) response is forwarded backwards to UE A. GGSN GGSN SGSN SGSN Radio Access Network Radio Access Network A Calls B A B Rev A

IMS-CS interworking The home BGCF selects another BGCF, perhaps in another network BGCF Subscriber A initiates a SIP invitation including the E.164 number of subscriber B. Subscriber The MGCF performs the signalling interworking between SIP and the BICC/ISUP in PSTN MGCF The S-CSCF detects: No SIP URL assigned to the E.164 number. The call is routed to a home BGCF. S-CSCF. The S-CSCF queries the DNS for the tel URL. DNS returns a negative match. S-CSCF/DNS The BGCF selects the MGCF in the breakout network BGCF The MG performs the bearer interworking between RTP/IP and the bearer used in the PSTN network MG Home A IMS Gateway DNS BGCF 5 7. INVITE 4 6. INVITE 15. OK 14. OK 3. INVITE 8. INVITE 11. IAM I-CSCF S-CSCF BGCF MGCF 16. OK 13. OK 12. ANM 2. INVITE 17. OK 9. ADD 10. RESP Visited A P-CSCF MG 1. INVITE 18. OK Session initiation involving PSTN interworking: The UE sends an INVITE request. The Request-URI (destination) contains a TEL URL. The INVITE request contains a Route header that helps the P-CSCF to route the INVITE request to the I-CSCF. The I-CSCF, with the help of the Route header, forwards the INVITE request to the S-CSCF. The S-CSCF evaluates the filter criteria for subscriber A. In this example, subscriber A does not have any originating services, so no AS is contacted. Then the S-CSCF inspects the Request-URI to find the destination network. As the Request-URI contains a TEL URL, the S-CSCF queries DNS to translate the TEL URL into a SIP URI. The DNS returns a "no match" for this TEL URL. The S-CSCF, as it has no options, routes the call to the BGCF in the home network. The BGCF in the home network selects a suitable gateway operator (another BGCF) The BGCF selects an appropriate MGCF. The MGCF reserves resources (TDM slot, DSP processors, etc.) in the Media Gateway. The MG confirms the successful reservation of resources. The MGCF initiates BICC/ISUP signalling towards the PSTN. Eventually, the subscriber in the PSTN answers the call. 12-16 The call is completed. GGSN SGSN Radio Access Network A Calls B (CS Subscriber) A Rev A

Interfaces of IMS CSE(SCP) SIP Application Servers OSA Application Server Home IMS Network IM-SSF ISC Sh OSA-SCS SIP Diameter IP TDM SS7 Sh ISC ISC BGCF Cx HSS Cx Mi P-CSCF S-CSCF Mw I-CSCF Mw Mr MRF Mg Gi MGCF T-SGW Mw MGW Gi ISUP/BICC GGSN Home Access Network Visited IMS Network P-CSCF PSTN PLMN Internet IP networks Visited Access Network GGSN Rev A

End-to-End QoS and Policy Enforcement Architecture
ETH/RZC Zsolt Szendrei End-to-End QoS and Policy Enforcement Architecture Control Plane Terminal P-CSCF SIP UA S-CSCF PDF The terminal also provides the link between the two planes. The Go interface provides the link between the control plane and the bearer plane in the network. Bearer Plane Ensuring QoS is enabled and supported for IMS services is fundamental. The key differentiator for IMS services will be QoS, especially as fundamental services such as Voice will be offered on IMS. Furthermore the QoS architecture must be end-to-end. IMS relies on the existing comprehensive QoS architecture already defined in UMTS. In the case of GPRS access, the QoS mechanisms exist but are not as extensive as UMTS and are not able to support conversation class of services. The QoS architecture defined for IMS enables the IMS system to use the existing UMTS/GPRS QoS mechanisms. The IMS deals only with the control plane for services (i.e. signalling). The bearer plane is provided by the access network. The link between the bearer plane and control plane takes place at two points: the first in the network between the P-CSCF and GGSN: the second in the terminal. If the aim of the architecture was only to provide QoS, then a single point (in the terminal) of interaction will be adequate. However the architecture also supports policy enforcement, i.e. a means for the operator to police the bearer connection to the mobile. Hence the network needs to be aware of the appropriate QoS for the service in order to police the bearer. Furthermore, in case the terminal is not able to request the appropriate resources the network can allocate the required QoS. The interface between the P-CSCF and the GGSN is known as the Go interface and the protocol is the Common Open Policy Server (COPS). Over the Go interface, the IMS domain informs the UMTS/GPRS system of the level of QoS required for the service. In the terminal the QoS required for the service is determined and the QoS is requested from the network Bearer GGSN Rev A

Policy Control UMTS Control Plane IMS Terminal SIP UA P-CSCF S-CSCF GGSN Session Manager PDF (0) P-CSCF receives ‘final ‘SDP (1) Generate QoS parameters (2) Generate Authorisation token (3) Forward token to terminal (4) Generate QoS parameters (5) Request PDP activation with token (6) Determine PDF. (7) Req authorisation from PDF. (8) Authorise resources. (9) Authorise PDP activation (10) Commence policing of traffic NOTE: When the service is renegotiated, the approval process as described above is repeated in its entirety with new resource allocations. Rev A

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