Introduction to IMS-IP Multimedia Subsystem

Introduction to IMS-IP Multimedia Subsystem The IMS-Connectivity- One network Experience

Introduction 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. Since services can be accessed from a range of devices, users started demanding uniform access to services regardless of the type of access. 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 their markets as well; the most obvious area being converged business services. Some new types of access have also come up in the recent times. For example WLAN was not a compelling technology few years back but today it is something that operators can't ignore. All this led to need of a common IP centric network core that is access independent. The IP Multimedia Subsystem (IMS) defined by 3GPP provides such an enabling architecture that is access independent, central in the move towards convergence. However since it was first introduced for wireless network, Release 5 (the first 3GPP release containing IMS subsystem) was heavily biased towards 3 rd generation wireless 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 introduced for the first time in release 5. This white paper introduces IMS subsystem and includes details of the functional elements, 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 signaling

access technologies. It is an overlay subsystem on top of existing

network in an all IP network. It surfaced for the first time in

packet 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 flexible

control functions (CSCFs). These are analogous to a Mobile

protocol but that was also a disadvantage since it doesn't have

Switching Centre (MSC) in the Release 4/ Release 99

any standard network topology So IMS was an attempt to build a

architecture and are primarily SIP servers with enhanced call handling functionality. These handle the call request from user equipment for purpose of establishing multimedia session between UEs.

IMS envisages a network core that can work with various kinds of access technologies. It is an overlay subsystem on top of existing packet core.

The softswitch shown in Figure 1 is divided in to three call state control functions (CSCFs). These are analogous to a Mobile Switching Centre (MSC) in the Release 4/ Release 99 architecture and are primarily SIP servers with enhanced call handling functionality. These handle the call request from user equipment for purpose of establishing multimedia session between UEs.

standard network topology using SIP. In other words, IMS is a grand middleware between services and access networks and can be said to be the J2EE of telecom world.

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. network and calls being handled by home network and hence From technology point of view SIP was considered flexible IMS also introduces a node known as HSS which is mainly an protocol but that was also a disadvantage since it doesn't have any standard network topology So IMS was an attempt to build aenhanced HLR. To ensure service availability at all times, calls standard network topology using SIP. In other words, IMS is a are serviced in home Onealways of the main objectives ofnetwork.

IMS was to make switching grand between servicesaccess and access networks was and faster)middleware pace than the core network, independence canthe said to be the J2EE evolve at came aworld.As be handsets typically of distributed and hence various CSCF into picture replacing 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) paceserviced than theincore network, access independence wasline access are always home network. medium intoRelease IMS. 1 of evolve at a different will bring in fixedAs the handsets incorporate one of the cornerstones of the(and IMSsome architecture typically TISPAN NGN which 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 access regardless of the access network used. The IP Multimedia medium IMS.protocols: SIP, H.248 and DIAMETER. While IMS usesinto 3 main future networks, IMS is getting changed so that it is not too subsystem (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 payload independent. This is central in moving towards true As IMS gets adopted by variousneeded in wireless since aircompression on air interface is other organizations for their convergence. Having defined this core network architecture that future networks, IMS is getting changed so that it is not too IMS is able to provide users with same service experience interface traffic has to be optimized to the maximum. Also, IMS is is access independent, efforts are on to 'enable' different types of wireless specific as it is now. For example signaling payload regardless of the access network used. The IP Multimedia all IPV6 but wire air networks have mostly wireless now aircompression on line access to work with the same, be it DSL, WLAN, that is accesssubsystem (IMS) interface is needed inIPV4 right sinceand defines an enabling architecture GPRS or any interface traffic has tohandle presence of maximum. and IPV6 inhence IMS independent. This is as WiMAX.emerging technology, suchcentral in moving needs to be optimized to the both IPV4 Also, IMS is towards true all IPV6 but wire line networks have mostly IPV4 right now and convergence. Having defined this core network architecture that network. Wire line to handle presence of both firewall issues.hence IMS needs is 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.

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So these problems are getting addressed in TISPAN with IMS as the base architecture. To solve all the above mentioned problems in wire line networks, TISPAN has added two main components: RACS and NASS.

Network transformation Now let's look at the network transformation. All the networks are divided mainly into 3 layers: transportation/access, session control & services. So the first convergence has to happen at transportation level. This results in access independence. Next step is that session control has to converge. Different networks handle call in different way so in order to have convergence at session control layer, there has to be a common signaling core architecture which is used by all access mediums. IMS achieves this.

From network topology point of view, IMS creates signaling infrastructure in the form of various SIP proxies which together perform call control like MSC in the old architecture.

CSCFs perform a number of functions. They perform multimedia session control function which is similar but far more enhanced than the call control functionality done in MSC. They also perform address translation / DNS look up / ENUM support etc. to translate the called user identity into a directly reachable address on the IP network. This function is similar to the digit translation function in the MSC. CSCF also transfers control to an APP server, potentially via. a service broker, similar to the IN concept used in MSC and SCP. For most of these functionalities SIP protocol is used. Role of the VLR and HLR is replaced by centralized subscriber profile storage space i.e. HSS. HSS is used by CSCF whenever subscriber profile information is needed by the CSCF.

Figure 3: IMS Architecture, various Nodes Figure 2: Network transformation As IMS is fundamentally data centric and follows the “home control model�, mobile networks are going to be the first adapters to IMS and can easily build a bridge to IMS network from non-IMS networks of today e.g. pure circuit switched network like GSM. Also, there are already more than 40 Million 3G network subscribers on UMTS Release 4 based 3G network. So moving these customers onto IMS based network would not be a big task.

Why IMS is needed It is possible to have an IMS installation and use the roaming principle built into access network e.g. GPRS roaming to access the same IMS network from anywhere in the world. This is depicted in the following figure;

Reference Architecture From service description point of view, IMS is primarily an overlay on top of existing and future packet networks using SIP, MEGACO & DIAMETER as the main protocols. It reuses SIP protocol as defined by IETF and has added some extensions to fulfill mobility and other requirements. As far as interoperability with existing CS domain networks is concerned, IMS provides reference points from where IMS network can connect to circuit switched network using gateways.

Figure 4: Using transport plane roaming

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This approach suffers from the disadvantage that user plane traffic (media) will not be routed efficiently since GPRS infrastructure will not be application signaling aware and also since user plane traffic has to come to the home network in order to get routed elsewhere.

So ideally IMS should be deployed as shown below, where user gets connected to the IMS domain of the local service provider and then that IMS domain knows the home domain to contact which will eventually provide the service.

Short description of Nodes Following is short description of various nodes in IMS: 1. P-CSCF (Proxy-CSCF): It is the first point of contact for mobile equipment in the visited network. It accepts the registration request from user equipment and forwards the same to the home network. It enforces quality of service in the visited network under the control of the home network's SCSCF and may also provide local control for emergency services in the visited network. It's role is similar to the outbound proxy (OBP) in SIP networks. It is also responsible for generating billing information for S-CSCF for subscriber's 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 Roaming Following requirements are achieved by IMS: 1. Ensuring QoS: On the public internet quality of service is unpredictable. Together with the underlying transport network, IMS attempts to provide predictable quality of 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 network to provide the same. IMS ensures that only authorized media streams are carried as per the negotiated media attributes and hence has view of currently available network resources all the time.

2. Secure communication: Security is necessary in any network and IMS also ensures this. It provides data integrity 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 that is used by UE when requesting resources from the network thus restricting resource theft.

3. Roaming: IMS supports roaming to ensure that user plane traffic is routed more efficiently instead of relying on user place roaming.

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 medium used. Also, since IMS cleanly separates, transport, signaling / call control and service layer, it helps easier and faster development of services.

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 user request and hence enables user to use his subscribed feature always irrespective of the capabilities of the visited network.

This is also called Virtual Home Environment (VHE).

4. HSS (Home Subscriber Sub System): It is a replacement of HLR and AUC in the traditional wireless network. It stores subscriber profile and also some authentication information that is used by S-CSCF for authenticating the subscriber at the time of registration.

5. App Server: App server hosts services that are provided to subscriber when they desire. Application Servers are invoked when S-CSCF notices that criteria related to a call are met and hence app server needs to take control of the call.

6. Breakout Gateway Control Function (BGCF): When S-CSCF determines that the call needs to breakout of the IMS 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 the case it passes control to the MGCF which then uses media and signaling gateways (MGW and SGW respectively) to pass call to other network.

7. Media Gateway (MGW): Role of Media Gateway (MGW) is to convert media from other networks into RTP as it enters the IMS domain and vice versa. It is controlled by MGCF.

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8. Media Gateway Control Function (MGCF): MGCF gets call from BGCF and passes it on to the PSTN. It receives / sends signaling from signaling gateway and performs interworking between the SIP signaling and signaling used in other networks.

9. Signaling Gateway (SGW): Signaling gateway receives SS7 or other forms of signaling from the network that it interfaces with and sends it in similar format over IP to MGCF e.g. M3UA over SCTP. SGW doesn't need to convert the signaling

S-CSCF (by using the domain look up mechanisms for UE's home domain) via I-CSCF of the home domain.

As the REGISTER messages traverses the network, various 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, home network is aware of the path that needs to be taken to reach the UE. In other words, home network is aware of where the user has roamed.

to SIP since this kind of interworking is performed by MGCF.

How it works Let's take an example call flow involving P, I & S CSCF. Following figure shows two IMS domains having various CSCF. UE-B is a domain B subscriber currently roamed into network A and similarly UE-A is a subscriber from domain A, which has roamed into domain B.

Similarly in the 200 OK for REGISTER message, S-CSCF sends a path that call origination requests must take from UE whenever it makes a call. This information is present in the “Service-Route” header and is stored at the UE. Assuming that users A and B that belong to IMS domains A & 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-a

Following figure shows registration of roaming users A with their home 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 As shown in the call flow, UE sends the REGISTER message to its local P-CSCF which then forwards the same to home domain

Subsequent messages are exchanges in similar way and call

is established. However media flows between UE directly through their P-CSCFs.

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Now let us look at each IMS node in detail:

IMS Architecture Details

P-CSCF

This section explains the IMS nodes and reference points in detail.

P-CSCF is the first point of contact for a UE in the IMS network. It 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. releasing the call in case of media or signaling inactivity or when bearer network can't provide enough resources as are needed for call. Following tasks are performed by P-CSCF:

IP Connectivity In order to be connected to an IMS domain, user has to be connected to the IP world first. Release 5 of IMS assumes GPRS for the IP connectivity but in release 6 when WLAN was introduced, IMS adopted the concept of IP Connectivity access network (IP-CAN) as any access network that can connect the user equipment (UE) to the P-CSCF. Future releases will test more variants of the IP-CAN.

So here is how multi access IMS network will finally look like:

To forward REGISTER message received from UE to the I-

CSCF of domain to which UE belongs. To forward the requests / responses to and from the UE for

terminating and originating call. Also similar routing of 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

P-CSCF

not handled by P-CSCF and UE is redirected to the CS-CN. 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

Figure 9: Multi access IMS Detailed Description of Functional Elements Following is a diagram that shows all IMS components from the point of view of transport, signaling core and service plane separation.

using the same media types / formats as those which were negotiated when call was established. It also inspects the SDP present in INVITE message being sent and can reject this if UE is trying to use media types / formats that it is not allowed to. To maintain session timers. P-CSCF maintains session timers

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

Figure 10: Various planes in IMS Architecture In this diagram HSS is shown overlapping in both the signaling as well as services plane since HSS is used by both.

Protocol or PDP context). Informs the P-CSCF when the bearer (e.g., PDP context) is

lost or modified. A modification indication is only given when the bearer is upgraded or downgraded from or to 0 kbit/s.

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Passes an IMS-charging identifier to the GGSN and to pass a

GPRS-charging identifier to the P-CSCF. I-CSCF I-CSCF is the first point of contact for an operator's network. For example when a call has to be terminated to a subscriber, ICSCF of that network has to be contacted first. It performs following functions: Contacts HSS to obtain the name of S-CSCF that is serving a

user.

HSS HSS is the data storage for an IMS domain. It stores data related to user identities, registration information, access parameters and service-triggering information. For authentication, it stores a secret key for each mobile subscriber, which is used to generate dynamic security data for each mobile subscriber. Data are used for mutual authentication of the International Mobile Subscriber Identity (IMSI) and the network.

There may be more than one HSS in the network in which case 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. Forwards SIP requests or responses to the S-CSCF. Sends accounting-related information to the CCF.

SLF It is used to resolve the HSS applicable for a particular subscriber when there are multiple HSS in the network.

Provides topology hiding functionality.

S-CSCF S-CSCF is the CSCF that finally serves call termination to / call origination from a UE. It also handles REGISTER request sent by UE when it is registering from visited network. There may be multiple S-CSCF each performing similar / different roles.

MRFC / MRFP These come into picture when specialized media processing is required for a call. For example if announcements, conferencing etc is needed then media resource function (MRF) is needed.

MRFC is contacted by S-CSCF or AS via Sip signaling and it in turn communicates with via H.248 interface to invoke the media services.

Functions performed by S-CSCF are: To authenticate users by means of the IMS Authentication and

Key Agreement (AKA) scheme. To download user information and service-related data from

the HSS during registration. To route mobile-terminating traffic to the P-CSCF and to route

mobile originated traffic to the I-CSCF, the Breakout Gateway Control Function (BGCF) or the application server (AS). To perform session control.

BGCF It comes in picture when call has to leave IMS network and enter some other network e.g. PSTN. The Breakout Gateway Control Function (BGCF) is responsible for choosing where a breakout to the CS domain occurs. The outcome of a selection process can be either a breakout in the same network or another network.

If the breakout happens in the same network, then the BGCF selects a Media Gateway Control Function (MGCF) to handle a session further. If the breakout takes place in another network, then the BGCF forwards a session to another BGCF in a 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. To translate an E.I64 number to a SIP universal resource

identifier (URI) using ENUM. To select an emergency centre when the operator supports

IMS emergency sessions. This is a Release 6 feature. To execute media policing. The S-CSCF is able to check the

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 operator's policy or user's subscription, the S-CSCF rejects the request. To maintain session timers. It allows the S-CSCF to detect

and free resources used up by hanging sessions. To send accounting-related information to the CCF for offline

MGCF MGCF provides the call control functionality for calls that are terminated into / originated from non-IMS networks. It uses gateways to convert media and signaling functionality from other network formats to IP. it gets the IP converted common channel signaling information and performs interworking between this and SIP protocol since CSCFs in IMS use SIP for call control.

Gateway (MGW + SGW) Signaling and media gateway convert the signaling and media from non-IMS networks to IMS networks respectively. Signaling gateway converts the transport of SS7 from MTP3 to IP and transports the ISUP protocol PDU on to IP network towards the MGCF, as shown in the diagram:

charging purposes and to the Online Charging System (OCS) for online charging purposes.

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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 UE's 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 PCSCF 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 / Services Application servers are used when S-CSCF is serving a call and it finds that call criteria meets the service criteria. In this case call control is transferred to the application server. Application server hosts enhanced service like prepaid service and executes the same on received call and handles it accordingly.

Mw reference point This is the SIP based signaling interface between various SIP servers viz. S-CSCF, I-CSCF & P-CSCF. Following actions / procedures apply to this interface / reference point: During UE registration process this interface is used to send

Reference Points Following diagram shows reference points and interfaces between all IMS nodes, it is taken from 3GPP specification 3GPP TS 23.002:

message from P-CSCF to I-CSCF and further to S-CSCF. In the reverse direction when responses flow, same interface is used to send responses as well. The same is used by SCSCF to perform network-initiated de-registration and also 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).

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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

Figure 12: IMS reference points

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. 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.

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Cx reference point Cx interface is between CSCFs and HSS. It is based on DIAMETER protocol and is used by I-CSCF and S-CSCF whenever they require access to subscriber profile data. I-CSCF needs this to get the S-CSCF assigned to the subscriber. SCSCF also needs this interface for a variety of reasons e.g. for getting authentication information when it needs to authenticate a user that is registering through the P-CSCF.

which decides whether it should use a MGCF or BGCF in another IMS domain for breakout to occur. Protocol used is SIP.

Mj reference point When BGCF receives a call from CSCF, and it selects MGCF for the breakout, it uses this interface to communicate with MGCF. Protocol used is SIP. Evidently, it is used in conjunction with Mi reference point.

Dx reference point When multiple HSSs are present in the system, CSCFs need to contact SLF for knowing which HSS to use for handling a given subscriber. This interface is based on DIAMETER protocol and is always used in conjunction with Cx interface. It uses the redirection logic of DIAMETER protocol.

Mk reference point In case BGCF needs to communicate with another BGCF instead of MGCF for the breakout to occur, it uses Mk reference point to communicate. Protocol used by this interface is SIP.

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 Sh reference point This based on the DIAMETER protocol and is between CSCF and HSS for fetching user data. It has commands like user-datarequest (UDR) to request data and gets the same in User-dataanswer (UDA). This interface can also be used by AS to subscribe for updates in user data in the HSS.

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 requires communication between the IMS (control plane) and the GPRS network (user plane). The Go reference point is used for this purpose. In addition, the charging correlation was added as an additional functionality.

Mm reference point

The protocol used is the Common Open Policy Service (COPS) protocol. Go procedures can be divided into two main categories:

This reference point is used by CSCF to send the call to other IP multimedia networks which are not IMS. Protocol used is SIP.

Media authorizationPolicy Enforcement Point (PEP) (e.g.,

Mg reference point It is the interface between MGCF and CSCF and is used for session control procedure for calls that are destined to / received from circuit switched networks. It uses SIP signaling. It is job of MGCF to convert CS domain signaling into SIP signaling and send the same to CSCF using this interface.

GGSN) uses the Go reference point to ask whether a requested bearer activation request (by UE) can be accepted from the PDF that acts as a policy decision point. The PEP also uses the Go reference point to notify the policy decision point about necessary bearer modification and bearer releases (e.g., PDP context).

Charging correlation via the Go reference point in IMS is

Mi reference point When call needs to breakout of IMS domain and go into other network then CSCF uses this interface to send call to BGCF

achieved by passing an IMS Charging correlation via the Go reference point in IMS is achieved by passing an IMS charging identifier to the IMS. With this procedure it is possible to later merge GPRS charging and IMS charging information in a billing system.

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Gq reference point

Globallogic expertise in IMS Technology

When a stand-alone PDF is deployed the Gq reference point is used to transport policy set-up information between the application function and the PDF. The term "application function" is used because it is intended that a PDF could authorize other traffic than IMS traffic. In the IMS case the P-CSCF plays the role of an application function.

Globallogic is developing various IMS nodes for leading telecom equipment vendor and a telecom solution provider . It also has substantial expertise in performing quality assurance and testing of software being written for IMS architecture.

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.

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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.

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