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Introduction to IMS-IP Multimedia Subsystem
 

Introduction to IMS-IP Multimedia Subsystem

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In the past few years, a number of things lead operators towards convergence. Mobile handsets and mobile application developers have advanced very fast in the last few years and support lot of new ...

In the past few years, a number of things lead operators towards convergence. Mobile handsets and mobile application developers have advanced very fast in the last few years and support lot of new technologies which sometimes network doesn't. Also, all kinds of access devices are allowing people to access an IP centric voice infrastructure

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    Introduction to IMS-IP Multimedia Subsystem Introduction to IMS-IP Multimedia Subsystem Document Transcript

    • Introduction to IMS-IP Multimedia Subsystem The IMS-Connectivity- One network ExperienceIntroductionIn the past few years, a number of things lead operators towards convergence. Mobile handsets and mobile application developershave advanced very fast in the last few years and support lot of new technologies which sometimes network doesnt. Also, all kindsof access devices are allowing people to access an IP centric voice infrastructure. Since services can beaccessed from a range of devices, users started demanding uniform access to services regardless of the type ofaccess. Also, wireline operators needed to consider ways to access wireless services to increase revenues.Wireless operators, in turn, were already seeing handset sales peak and needed new ways to expand theirmarkets as well; the most obvious area being converged business services. Some new types of access have alsocome up in the recent times. For example WLAN was not a compelling technology few years back but today it issomething that operators cant ignore. All this led to need of a common IP centric network core that is accessindependent. The IP Multimedia Subsystem (IMS) defined by 3GPP provides such an enabling architecture that isaccess independent, central in the move towards convergence. However since it was first introduced for wirelessnetwork, Release 5 (the first 3GPP release containing IMS subsystem) was heavily biased towards 3 rd generationwireless networks. Now each access type is being enabled to work with an IMS core, be it DSL,WLAN or GPRS.IMS builds upon the soft-switch based network introduced for first time in release 4 of 3GPP and was introducedfor the first time in release 5. This white paper introduces IMS subsystem and includes details of the functionalelements, interface points, protocols and detailed call flows.
    • IMS envisages a network core that can work with various kinds of IMS is a subsystem in the 3G network and is the core signalingaccess technologies. It is an overlay subsystem on top of existing network in an all IP network. It surfaced for the first time inpacket core. Release 5 of 3GPP and is therefore very much wireless centric.The softswitch shown in Figure 1 is divided in to three call state From technology point of view SIP was considered flexiblecontrol functions (CSCFs). These are analogous to a Mobile protocol but that was also a disadvantage since it doesnt haveSwitching Centre (MSC) in the Release 4/ Release 99 any standard network topology So IMS was an attempt to build a standard network topology using SIP. In other words, IMS is aarchitecture and are primarily SIP servers with enhanced call grand middleware between services and access networks andhandling functionality. These handle the call request from user can be said to be the J2EE of telecom world.equipment for purpose of establishing multimedia sessionbetween UEs. One of the main objectives of IMS was to make switching access to work with the same, be it DSL, WLAN, GPRS or any distributed and hence various CSCF came into picture replacing emerging technology, such as WiMAX. the MSC server (release 4). Wireless networks have always IMS is a subsystem in the 3G network and is the core signaling network in an all IP network. It surfaced for the firstintime infollowed the principle of centralized data repository home Release 5 of 3GPP and is therefore very much wireless centric.IMS envisages a network core that can work with various kinds of network and calls being handled by home network and henceaccess technologies. It is an overlay subsystem on top of existing From technology point of view SIP was considered flexiblepacket core. IMS also introduces a node known as HSS which is mainly an protocol but that was also a disadvantage since it doesnt haveThe softswitch shown in Figure 1 is divided in to three call state any standard network topology So IMS was an attempt to build aenhancedcontrol functions (CSCFs). These are analogous to a Mobile HLR. To ensure service availability at all times, callsSwitching Centre (MSC) in the Release 4/ Release 99 standard network topology using SIP. In other words, IMS is aarchitecture and are primarily SIP servers with enhanced call are always main objectives ofnetwork. to make switching One of the serviced in home IMS washandling functionality. These handle the call request from user grand middlewarethe core network, access independence was faster) pace than between services and access networks andequipment for purpose of establishing multimedia session canthe said to be the J2EE evolve at came into picture replacing distributed and hence various CSCF aworld.As be handsets typically ofbetween UEs. telecom different (and some times one of the cornerstones of the IMS architecture and due to thisthe MSC server (release 4). Wireless networks have always followed it has been adopted by otherdata repository in homeattribute the principle of centralized access technologies as network and calls being handled by home network and hence well also introduces a nodeIMS for their future networks. known as HSS which is mainly an enhanced HLR. To ensure service availability at all times, calls Release 7 of IMS will address non-IMS end points as well and will faster) pace than the core network, access independence wasline access are always serviced in home network. medium into IMS. incorporate Release 1 of evolve at a different will bring in fixedAs the handsets one of the cornerstones of the IMS architecture typically TISPAN NGN which (and some times and due to this IMS uses has been adopted by other and DIAMETER. Whileattribute it 3 main protocols: SIP, H.248 access technologies as Figure 1: IMS access independence well for their future networks. Release 7 of IMS will address non-IMS end points as well and will SIP & DIAMETER came from IETF, H.248 was contributed by ITU.IMS is able to provide users with same service experience incorporate Release 1 of TISPAN NGN which will bring in fixed line IMS gets adopted by various other organizations for theirAs accessregardless of the access network used. The IP Multimedia medium intomain protocols: SIP, H.248 and DIAMETER. While IMS uses 3 IMS. future networks, IMS is getting changed so that it is not toosubsystem (IMS) defines an enabling architecture that is access SIP & DIAMETER came from IETF, H.248 was contributed wirelessby ITU. specific as it is now. For example signaling payloadindependent. This is central in moving towards true As IMS gets adopted by variousneeded in wireless since aircompression on airconvergence. Having defined this core network architecture that interface is other organizations for their future networks, IMS is getting changed so that it is not tooIMS is able to provide users with same service experience interface traffic has to be optimized to the maximum. Also, IMS isis access independent, efforts are on to enable different types of wireless specific as it is now. For example signaling payloadregardless of the access network used. The IP Multimedia all IPV6 but wire air networks have mostly wireless now aircompression on lineaccess to work with the same, be it DSL, WLAN, that is accesssubsystem (IMS) interface is needed inIPV4 right sinceanddefines an enabling architecture GPRS or any interface traffic has tohandle presence of maximum. and IPV6 inhence IMSindependent. This is as WiMAX.emerging technology, suchcentral in moving needs to be optimized to the both IPV4 Also, IMS istowards true all IPV6 but wire line networks have mostly IPV4 right now andconvergence. Having defined this core network architecture that network. Wire line to handle presence of both firewall issues.hence IMS needsis access independent, efforts are on to enable different types of IP networks also have NAT &IPV4 and IPV6 in network. Wire line IP networks also have NAT & firewall issues. 1
    • So these problems are getting addressed in TISPAN with IMS as From network topology point of view, IMS creates signalingthe base architecture. To solve all the above mentioned problems infrastructure in the form of various SIP proxies which togetherin wire line networks, TISPAN has added two main components: perform call control like MSC in the old architecture.RACS and NASS. CSCFs perform a number of functions. They perform multimedia session control function which is similar but far more enhancedNetwork transformation than the call control functionality done in MSC. They also perform address translation / DNS look up / ENUM support etc.Now lets look at the network transformation. All the networks are to translate the called user identity into a directly reachabledivided mainly into 3 layers: transportation/access, session address on the IP network. This function is similar to the digitcontrol & services. So the first convergence has to happen at translation function in the MSC. CSCF also transfers control totransportation level. This results in access independence. Next an APP server, potentially via. a service broker, similar to the INstep is that session control has to converge. Different networks concept used in MSC and SCP. For most of these functionalitieshandle call in different way so in order to have convergence SIP protocol is used. Role of the VLR and HLR is replaced byat session control layer, there has to be a common signaling centralized subscriber profile storage space i.e. HSS. HSS iscore architecture which is used by all access mediums. IMS used by CSCF whenever subscriber profile information isachieves this. needed by the CSCF. Figure 3: IMS Architecture, various Nodes Figure 2: Network transformation Why IMS is needed It is possible to have an IMS installation and use the roamingAs IMS is fundamentally data centric and follows the “home principle built into access network e.g. GPRS roaming to accesscontrol model”, mobile networks are going to be the first adapters the same IMS network from anywhere in the world. This isto IMS and can easily build a bridge to IMS network from non-IMS depicted in the following figure;networks of today e.g. pure circuit switched network like GSM.Also, there are already more than 40 Million 3G networksubscribers on UMTS Release 4 based 3G network. So movingthese customers onto IMS based network would not bea big task.Reference ArchitectureFrom service description point of view, IMS is primarily an overlayon top of existing and future packet networks using SIP,MEGACO & DIAMETER as the main protocols. It reuses SIPprotocol as defined by IETF and has added some extensions tofulfill mobility and other requirements. As far as interoperabilitywith existing CS domain networks is concerned, IMS providesreference points from where IMS network can connect to circuitswitched network using gateways. Figure 4: Using transport plane roaming 2
    • This approach suffers from the disadvantage that user plane Short description of Nodestraffic (media) will not be routed efficiently since GPRSinfrastructure will not be application signaling aware and also Following is short description of various nodes in IMS:since user plane traffic has to come to the home network in order 1. P-CSCF (Proxy-CSCF): It is the first point of contact forto get routed elsewhere. mobile equipment in the visited network. It accepts the registration request from user equipment and forwards theSo ideally IMS should be deployed as shown below, where user same to the home network. It enforces quality of service in thegets connected to the IMS domain of the local service provider visited network under the control of the home networks S-and then that IMS domain knows the home domain to contact CSCF and may also provide local control for emergencywhich will eventually provide the service. services in the visited network. Its role is similar to the outbound proxy (OBP) in SIP networks. It is also responsible for generating billing information for S-CSCF for subscribers usage of visited network resources. 2. I-CSCF (Interrogating-CSCF): It is the first point of contact in a home network whenever a call is to be terminated on an IMS user. It is optional element in an IMS network and P-CSCF may directly also contact the S-CSCF. It may be used to support load balancing between various S-CSCFs as configured in HSS. It may also provide topology hiding and firewall functionality. 3. S-CSCF (Serving-CSCF): It performs multimedia session control for calls originated by or terminated to users in the home network that is served by S-CSCF. In a network there can be more than one S-CSCF. S-CSCFs can also be added to flexibly increase capacity of the home network. It may also transfer the control of call to the P-CSCF in case user has asked for a service that can be better served locally in the visited network e.g. finding list of restaurants in the vicinity of user. S-CSCF is mainly responsible for billing the subscriber. Figure 5: Using IMS RoamingFollowing requirements are achieved by IMS: One of the key points here is that although P-CSCF is the first point of contact for a UE, it is the S-CSCF that handles user1. Ensuring QoS: On the public internet quality of service is request and hence enables user to use his subscribed feature unpredictable. Together with the underlying transport always irrespective of the capabilities of the visited network. network, IMS attempts to provide predictable quality of This is also called Virtual Home Environment (VHE). service by regulating and controlling usage of bearer resources. UEs use SIP signaling to negotiate the media attributes that are needed for call and then request bearer 4. HSS (Home Subscriber Sub System): It is a replacement of network to provide the same. IMS ensures that only HLR and AUC in the traditional wireless network. It stores authorized media streams are carried as per the negotiated subscriber profile and also some authentication information media attributes and hence has view of currently available that is used by S-CSCF for authenticating the subscriber at network resources all the time. the time of registration. 5. App Server: App server hosts services that are provided to subscriber when they desire. Application Servers are invoked2. Secure communication: Security is necessary in any when S-CSCF notices that criteria related to a call are met network and IMS also ensures this. It provides data integrity and hence app server needs to take control of the call. protection by using IPSec. Users are authenticated to prevent unauthorized users from entering the network. Similarly when a call attempt is made, PDF generates authentication token 6. Breakout Gateway Control Function (BGCF): When that is used by UE when requesting resources from the S-CSCF determines that the call needs to breakout of the IMS network thus restricting resource theft. domain and go into other networks like PSTN, it passes call to BGCF. BGCF then determines whether call needs to breakout in the same domain or it needs to pass the call to another BGCF in some other IMS domain. If former is the3. Roaming: IMS supports roaming to ensure that user plane case it passes control to the MGCF which then uses media traffic is routed more efficiently instead of relying on user place and signaling gateways (MGW and SGW respectively) to roaming. pass call to other network.4. Faster service development and easier access to service: IMS achieves the latter by way of access independence so that user can access same service independent of the access 7. Media Gateway (MGW): Role of Media Gateway (MGW) is to medium used. Also, since IMS cleanly separates, transport, convert media from other networks into RTP as it enters the signaling / call control and service layer, it helps easier and IMS domain and vice versa. It is controlled by MGCF. faster development of services. 3
    • 8. Media Gateway Control Function (MGCF): MGCF gets call S-CSCF (by using the domain look up mechanisms for UEs from BGCF and passes it on to the PSTN. It receives / sends home domain) via I-CSCF of the home domain. signaling from signaling gateway and performs interworking between the SIP signaling and signaling used in As the REGISTER messages traverses the network, various other networks. nodes in the message path e.g. p-cscf and i-cscf and other proxies, add themselves to the “Path” header in SIP REGISTER message. Hence when this message reaches S-CSCF, home9. Signaling Gateway (SGW): Signaling gateway receives SS7 network is aware of the path that needs to be taken to reach the or other forms of signaling from the network that it interfaces UE. In other words, home network is aware of where the user has with and sends it in similar format over IP to MGCF e.g. M3UA roamed. over SCTP. SGW doesnt need to convert the signaling to SIP since this kind of interworking is performed by MGCF. Similarly in the 200 OK for REGISTER message, S-CSCF sendsHow it works a path that call origination requests must take from UE wheneverLets take an example call flow involving P, I & S CSCF. it makes a call. This information is present in the “Service-Route” header and is stored at the UE.Following figure shows two IMS domains having various CSCF.UE-B is a domain B subscriber currently roamed into network Aand similarly UE-A is a subscriber from domain A, which has Assuming that users A and B that belong to IMS domains A & Broamed into domain B. respectively, have roamed into IMS domains B & A respectively and have registered with their home networks, following is the path that would be taken by signaling and media when user A calls user B: Signaling Media IP Connectivity Step 1-c Step 1-b Step 2-a Step 3-a Step 2-b Figure 6: IMS Networks with roaming subscriber Step 1-aFollowing figure shows registration of roaming users A with theirhome networks: Figure 8: IMS Call Flow High Level Following summarizes the steps taken in this call flow: Step 1: UE-A prepares an INVITE message and sends it to the P- CSCF-B since it is the default outbound proxy for UE-A. P-CSCF sends the same to S-CSCF, in accordance with the “service-route” header present in the “200 OK” message received for REGISTER from S-CSCF-A. Step 2: S-CSCF sends INVITE to domain B S-CSCF. Step 3: S-CSCF-B remembers the Path” header received in REGISTER message from UE-B and uses the same to route the INVITE to UE-B. Message is routed through I-CSCF-A and P-CSCF-A. Figure 7: REGISTER Message Flow Subsequent messages are exchanges in similar way and call is established. However media flows between UE directlyAs shown in the call flow, UE sends the REGISTER message to through their P-CSCFs.its local P-CSCF which then forwards the same to home domain 4
    • Now let us look at each IMS node in detail:IMS Architecture Details P-CSCFThis section explains the IMS nodes and reference points P-CSCF is the first point of contact for a UE in the IMS network. Itin detail. is similar to the out bound proxy. It acts as proxy when it passes messages sent to and received from the UE and sometimes also acts as UA when it has to take some action on its own e.g.IP Connectivity releasing the call in case of media or signaling inactivity or when bearer network cant provide enough resources as are neededIn order to be connected to an IMS domain, user has to be for call. Following tasks are performed by P-CSCF:connected to the IP world first. Release 5 of IMS assumes GPRSfor the IP connectivity but in release 6 when WLAN wasintroduced, IMS adopted the concept of IP Connectivity accessnetwork (IP-CAN) as any access network that can connect the To forward REGISTER message received from UE to the I-user equipment (UE) to the P-CSCF. Future releases will test CSCF of domain to which UE belongs.more variants of the IP-CAN. To forward the requests / responses to and from the UE for terminating and originating call. Also similar routing ofSo here is how multi access IMS network will finally look like: messages needs to be done for independent transactions that are not related to a call. To perform signaling compression and decompression for messages destined to and received from the UE. To handle emergency calls. In release 5 emergency calls are P-CSCF not handled by P-CSCF and UE is redirected to the CS-CN. P-CSCF But in release 6 onwards it will be possible for P-CSCF to handle the call since P-CSCF is the one who has best idea about location of UE and hence can handle emergency call in best possible way. To send charging related information to the charging collection function (CCF). To provide integrity protection of SIP signaling by maintaining IP SEC association with UE so that data is transferred with UE in a secure fashion. To perform media policing. This entails ensuring that UE is using the same media types / formats as those which were negotiated when call was established. It also inspects the Figure 9: Multi access IMS SDP present in INVITE message being sent and can reject this if UE is trying to use media types / formats that it is notDetailed Description of Functional Elements allowed to.Following is a diagram that shows all IMS components from thepoint of view of transport, signaling core and service plane To maintain session timers. P-CSCF maintains session timersseparation. so that there are no hanging sessions and if UE loses bearer network connectivity, call can be cleared. It also maintains RTCP timers for the same purpose. PDF PDF applies policy logic to the session and media related information received from P-CSCF. It performs following tasks: Generates an authorization token that identifies the session for which P-CSCF makes authorization request. Provides an authorization decision according to the stored session and media related information on receiving a bearer authorization request from the GGSN / IP-CAN. Updates the authorization decision at session modifications, which changes session and media-related information. Enables the usage of an authorized bearer (e.g., Packet Data Protocol or PDP context). Figure 10: Various planes in IMS Architecture Informs the P-CSCF when the bearer (e.g., PDP context) isIn this diagram HSS is shown overlapping in both the signaling as lost or modified. A modification indication is only given whenwell as services plane since HSS is used by both. the bearer is upgraded or downgraded from or to 0 kbit/s. 5
    • Passes an IMS-charging identifier to the GGSN and to pass a HSS GPRS-charging identifier to the P-CSCF. HSS is the data storage for an IMS domain. It stores data related to user identities, registration information, access parametersI-CSCF and service-triggering information. For authentication, it stores a secret key for each mobile subscriber, which is used to generateI-CSCF is the first point of contact for an operators network. For dynamic security data for each mobile subscriber. Data are used example when a call has to be terminated to a subscriber, I- for mutual authentication of the International Mobile Subscriber CSCF of that network has to be contacted first. It performs Identity (IMSI) and the network. following functions:Contacts HSS to obtain the name of S-CSCF that is serving a There may be more than one HSS in the network in which case user. SLF is used to locate the relevant HSS.Assigns an S-CSCF based on received capabilities from the HSS. An S-CSCF is assigned if there is no S-CSCF allocated. SLFForwards SIP requests or responses to the S-CSCF. It is used to resolve the HSS applicable for a particular subscriberSends accounting-related information to the CCF. when there are multiple HSS in the network.Provides topology hiding functionality. MRFC / MRFPS-CSCF These come into picture when specialized media processing is S-CSCF is the CSCF that finally serves call termination to / required for a call. For example if announcements, conferencing call origination from a UE. It also handles REGISTER request etc is needed then media resource function (MRF) is needed. sent by UE when it is registering from visited network. There MRFC is contacted by S-CSCF or AS via Sip signaling and it in may be multiple S-CSCF each performing similar / different turn communicates with via H.248 interface to invoke the roles. media services.Functions performed by S-CSCF are:To authenticate users by means of the IMS Authentication and BGCF Key Agreement (AKA) scheme. It comes in picture when call has to leave IMS network and enter some other network e.g. PSTN. The Breakout Gateway ControlTo download user information and service-related data from Function (BGCF) is responsible for choosing where a breakout the HSS during registration. to the CS domain occurs. The outcome of a selection process can be either a breakout in the same network or another network.To route mobile-terminating traffic to the P-CSCF and to route If the breakout happens in the same network, then the BGCF mobile originated traffic to the I-CSCF, the Breakout Gateway selects a Media Gateway Control Function (MGCF) to handle a Control Function (BGCF) or the application server (AS). session further. If the breakout takes place in another network, then the BGCF forwards a session to another BGCF in aTo perform session control. selected network.To interact with service platforms. Interaction means the capability to decide when a request or response needs to be routed to a specific AS for further processing. MGCFTo translate an E.I64 number to a SIP universal resource identifier (URI) using ENUM. MGCF provides the call control functionality for calls that are terminated into / originated from non-IMS networks. It usesTo select an emergency centre when the operator supports gateways to convert media and signaling functionality from other IMS emergency sessions. This is a Release 6 feature. network formats to IP. it gets the IP converted common channel signaling information and performs interworking between thisTo execute media policing. The S-CSCF is able to check the and SIP protocol since CSCFs in IMS use SIP for call control. content of the SDP payload and check whether it contains media types or codecs, which are not allowed for a user. When the proposed SDP does not fit the operators policy or users subscription, the S-CSCF rejects the request. Gateway (MGW + SGW) Signaling and media gateway convert the signaling and mediaTo maintain session timers. It allows the S-CSCF to detect from non-IMS networks to IMS networks respectively. Signaling gateway converts the transport of SS7 from MTP3 to IP and and free resources used up by hanging sessions. transports the ISUP protocol PDU on to IP network towards theTo send accounting-related information to the CCF for offline MGCF, as shown in the diagram: charging purposes and to the Online Charging System (OCS) for online charging purposes. 6
    • Let us look at the each reference point in detail: Gm reference point It is the reference point between UE and the P-CSCF, which is UEs first contact in the IMS network. UE uses this reference point for the following actions / procedures: During registration UE sends REGISTER and receives the response to REGISTER message. It also establishes Path and service route header to be used later when receiving / making calls. UE also establishes security association with P- CSCF and uses this interface to exchange authentication data with P-CSCF during registration procedure. During session initiation / termination same interface is used to send / receive SIP signaling messages. P-CSCF may inspect messages received on this interface to verify the same with PDF to ensure disciplined usage of resources.Figure 11: Signaling Gateway & MGCF Protocol Layers Transaction procedures allow UE to create independent transactions on this interface. These can be used to exchange capabilities (OPTIONS) or to send messages in an internet chat (MESSAGE).Application Server / ServicesApplication servers are used when S-CSCF is serving a call and Mw reference pointit finds that call criteria meets the service criteria. In this case call This is the SIP based signaling interface between various SIPcontrol is transferred to the application server. Application server servers viz. S-CSCF, I-CSCF & P-CSCF. Following actions /hosts enhanced service like prepaid service and executes the procedures apply to this interface / reference point:same on received call and handles it accordingly. During UE registration process this interface is used to send message from P-CSCF to I-CSCF and further to S-CSCF. InReference Points the reverse direction when responses flow, same interface is used to send responses as well. The same is used by S-Following diagram shows reference points and interfaces CSCF to perform network-initiated de-registration and alsobetween all IMS nodes, it is taken from 3GPP specification 3GPPTS 23.002: when user has to be re-authenticated by network. In case of re-authentication, P-CSCF needs to delete the user related state information that it might have accumulated in last registration. For mobile originated as well mobile terminated calls this reference point is used between S-CSCFs. The same is also used by P-CSCF to release a call if P-CSCF is notified by PDF about degradation / loss of bearer from underlying transport. It is also used for transaction procedures that allow UE to create independent transactions on this interface. These can be used to exchange capabilities (OPTIONS) or to send messages in an internet chat (MESSAGE). W ISC reference point ISC stands for IMS service and is based on SIP protocol. It is between S-CSCF and AS and is used for following: When S-CSCF receives a call that matches service criteria, it forwards call to AS by sending INVITE. AS may modify the SIP message and may send the call back to S-CSCF using the same interface. Figure 12: IMS reference points AS may initiate a call towards S-CSCF as per service logic. AS may also initiate a transaction e.g. for sending messages using MESSAGE. 7
    • Cx reference point which decides whether it should use a MGCF or BGCF inCx interface is between CSCFs and HSS. It is based on another IMS domain for breakout to occur. Protocol used is SIP.DIAMETER protocol and is used by I-CSCF and S-CSCFwhenever they require access to subscriber profile data. I-CSCFneeds this to get the S-CSCF assigned to the subscriber. S- Mj reference pointCSCF also needs this interface for a variety of reasons e.g. for When BGCF receives a call from CSCF, and it selects MGCF forgetting authentication information when it needs to authenticate the breakout, it uses this interface to communicate with MGCF.a user that is registering through the P-CSCF. Protocol used is SIP. Evidently, it is used in conjunction with Mi reference point.Dx reference pointWhen multiple HSSs are present in the system, CSCFs need to Mk reference pointcontact SLF for knowing which HSS to use for handling a given In case BGCF needs to communicate with another BGCFsubscriber. This interface is based on DIAMETER protocol and is instead of MGCF for the breakout to occur, it uses Mk referencealways used in conjunction with Cx interface. It uses the point to communicate. Protocol used by this interface is SIP.redirection logic of DIAMETER protocol. Mn reference point This is the interface between CSCF and MGW for creating context and setting up bearer for IMS to CS call and vice versa. Protocol used is H.248 Mp reference point This is the interface between MRFC and MRFP and is used to setup media channels for specialized media handling when CSCF require so. Protocol used on this interface is H.248 Mr reference point This is the interface between CSCF and MRFC and is used when specialized media handling is required for the call. Protocol used is SIP. Go reference point Figure 13: Cx and Dx Reference Points Operators want to ensure that the QoS parameters along with source & destination addresses of the intended IMS media traffic matches the negotiated values at the IMS level. This requiresSh reference point communication between the IMS (control plane) and the GPRSThis based on the DIAMETER protocol and is between CSCF network (user plane). The Go reference point is used for thisand HSS for fetching user data. It has commands like user-data- purpose. In addition, the charging correlation was added as anrequest (UDR) to request data and gets the same in User-data- additional functionality.answer (UDA). This interface can also be used by AS tosubscribe for updates in user data in the HSS. The protocol used is the Common Open Policy Service (COPS) protocol. Go procedures can be divided into two mainMm reference point categories:This reference point is used by CSCF to send the call to other IPmultimedia networks which are not IMS. Protocol used is SIP. Media authorizationPolicy Enforcement Point (PEP) (e.g., GGSN) uses the Go reference point to ask whether a requested bearer activation request (by UE) can be acceptedMg reference point from the PDF that acts as a policy decision point. The PEPIt is the interface between MGCF and CSCF and is used for also uses the Go reference point to notify the policy decisionsession control procedure for calls that are destined to / received point about necessary bearer modification and bearerfrom circuit switched networks. It uses SIP signaling. It is job of releases (e.g., PDP context).MGCF to convert CS domain signaling into SIP signaling andsend the same to CSCF using this interface. Charging correlation via the Go reference point in IMS is achieved by passing an IMS Charging correlation via the GoMi reference point reference point in IMS is achieved by passing an IMS charging identifier to the IMS. With this procedure it isWhen call needs to breakout of IMS domain and go into other possible to later merge GPRS charging and IMS chargingnetwork then CSCF uses this interface to send call to BGCF information in a billing system. 8
    • Gq reference point Globallogic expertise in IMS Technology When a stand-alone PDF is deployed the Gq reference point is Globallogic is developing various IMS nodes for leading telecom used to transport policy set-up information between the equipment vendor and a telecom solution provider . It also has substantial application function and the PDF. The term "application function" expertise in performing quality assurance and testing of software being written is used because it is intended that a PDF could authorize other for IMS architecture. traffic than IMS traffic. In the IMS case the P-CSCF plays the role of an application function. About the Author Himanshoo Kumar Saxena has over a decade of Software product development experience in convergence domain. He has been involved in development of various convergence products including some IMS nodes from concept to finish. He currently plays role of Consultant Architect at Globallogic. Related Articles: product lifecycle management The Impact of Social Commerce on Ecommerce Figure 14: Go and Gq Reference Points Media authorization and correlation of Go and Gq interface is also shown in the figure 14. As shown in the above diagram, during media characteristics negotiation procedures, PCSCF queries PDF to authorize the media properties that UE is willing to use for call. Later when UE tries to get the bearer for actually sending media (e.g. PDP activation through a GGSN), bearer controller (GGSN), acting as the PEP (policy enforcement point), queries the PDP to ensure that this usage is properly authorized.Corporate Headquarters Global Delivery Global Delivery Global Delivery Global DeliveryVirginia, USA Center Noida, India Nagpur, India Kiev, Ukraine California, USA8605 Westwood Center Drive, B-34/1, Sector 59, Noida Harihar Nagar, Besa, Bozhenko 86D, 03150 7950 Silverton Avenue,Suite 401 Vienna, VA 22182 201301 (U.P) Nagpur - 441108 Phone: (+38) (044) 492.9693 Suite 205 San Diego, CA 92126Phone: (+1) (703) 847.5900 Phone: (+91) (120) 406.2000 Phone: (+91) (710) 328.1162 Fax: (+38) (044) 492.9694 Phone : (+1) (760) 672.4818Fax: (+1) (703) 847.5901 Fax: (+91) (120) 258.5721 Fax: (+91) (710) 328.1161 Fax : (+1) (858) 693.4907 9