IMS Service Anthropomorphism

Anthropomorphism is the attribution of uniquely human characteristics and qualities to nonhuman beings, inanimate objects, or natural or supernatural phenomena (wikipedia).

The pictures taken from various cartoons by Tex Avery that you can see at the top of this post illustrate an anthropomorphic behavior associated to animals.

Service anthropomorphism is therefore the possibility for an IMS service to behave and be treated as a human user of the IMS network. This post describes how IMS supports service anthropomorphism and what an anthropomorphic service can do and benefit from.

Symetric usage of SIP between IMS users and IMS services

The IMS service architecture ensures that every SIP request that is generated or processed by an IMS client can also be generated or processed by an IMS application server.

For instance, if an IMS client can generate a SIP instant message, set up a SIP session, or subscribe to the presence of another user, an IMS application server can do it as well.

Conversely, if an IMS client can receive a SIP instant message, process a SIP session set up request or process a SUBSCRIBE to any SIP event package (e.g. presence), an IMS application server can also do as well.

An interesting consequence of this feature is that the concept of enabler (the possibility for a service to use a remote feature) is simple to implement in an IMS network, and can be extremely powerful.

In a traditional telecommunications architecture, some services offered to an end-user can also be used as an enabler by an application server. This is the case for network-based user location in a mobile network, which can accessed in a location server from both a device and an application server hosting location-based services. However, in most cases, the user to service interface and the service to service interface are different, and both need to be specified and implemented for the application to be used as both an end-user service and an enabler by other services. In IMS, as soon as a service is deployed in the network, for which a SIP based interface has been defined for access by IMS clients, the exact same interface can immediately be used by other IMS application servers to access the service as an enabler for their own services. In theory and when it makes sense from a service perspective, every new IMS service can become an enabler for other services, which themselves can become enablers for other services. There is therefore a high potential for an explosion of service opportunities each time a new IMS service is deployed in the network.

Another difference with a traditional telecommunications network is that, in such a network, a service enabler is always network-based. As devices are more or less stupid, every added-value logic that can serve other services is located on a server in the network. On the other hand, with IMS, every logic processing SIP requests can theoratically be located both in IMS clients and in IMS application servers. While complex logic (e.g. full-featured presence) will tend to be located in powerful IMS ASs, simpler logic can be located in the IMS client and be accessed by an AS through SIP. For instance, IMS clients can support SIP event packages (using SUBSCRIBE, NOTIFY, PUBLISH) that are able to inform and notify ASs about what is located or is happening in the IMS client: session states, information located in device, information or status about a specific device-based application like a game. This implies that, with IMS, the concept of enabler is extended from the network to the endpoints, and this offers potentially huge service opportunities.

Finally, in a traditional network a service and the enabler it accesses are most of the time located in the same operator's domain, as addressing, request routing and security issues are important barriers to overcome for inter-operator interfaces. In an IMS network, SIP routing procedures cross boundaries between networks with secured interfaces, and the addressing of SIP enablers is based on public identities like IMS Public User Identities (IMPUs) and Public Service Identities (PSIs). This implies that, technically, a presence based service from one operator can access the presence of a user subscribed with a different operator (or located in the Internet, but then with security risks).

In an IMS network, all SIP requests originate from a public identity (IMPU, PSI) and are addressed to a public identity (IMPU, PSI). How do IMS application servers fit in this picture?

Service impersonating a user

An IMS AS can process requests addressed to an IMPU of a user it serves, and can generate a SIP request on behalf of a user, by placing an IMPU as the originator of the request it (the AS) actually generates.

For instance, an AS hosting session control logic for a user can receive the INVITEs addressed to an IMPU of this user and serve it on its behalf (e.g. accept or reject the session). In another example, IMS presence requests are always addressed to an IMPU of the user from which the presence is sought. In theory, this request could and should be routed to one or several IMS clients associated to the user, but in practice they are all routed to a network-based AS serving presence requests on its behalf.

In yet another example, consider a service that decides on the best way to route messages issued by user A. Possible alternatives include IMS messaging to the same IMPU, IMS messaging to another IMPU, SMS, MMS, email, or an Internet IM service. When user A issues an IMS message to user B, this message is routed to the AS supporting the advanced routing function. This AS can then extract the IMPU of user B from the IM, and generate a presence request originated from user A and addressed to user B. This request will reach the presence server of user B, possibly in another network, which will apply authorization rules associated to user A and generate an appropriate response. This response may provide information to the service about the messaging alternatives available for user B, as well as the associated addresses, preferences, and reachability. It can therefore select the most appropriate approach to deliver the message to user B. In this use case, it is very important for the advanced routing service to endorse the identity of user A, because it acts on its behalf and must benefit from the exact same information as user A would.

This means that an IMS service can act as a network-based agent or proxy of a user (and here I do not mean "SIP User Agent" and "SIP Proxy").

Service acting as itself

A service can also receive requests addressed to a PSI that identifies itself, or generate a request using its own PSI as the originator.

This may permit a service to communicate with a user ("Hey, I am your voicemail") or to benefit from privileges associated to a service (e.g. when the service generates a presence request to a presence server located in the same domain, it has full access rights and top priority toward all users' presence information).

Group Management

Group management permits a user (or an operator) to regroup a set of URIs (SIP or others) into a set which is identified and addressable via a PSI. Such a group PSI can then be used in SIP requests, which then automatically become group requests through appropriate support in IMS application servers. For instance, an IM addressed to a group will be exploded to individual IMs sent to each member of the group, a presence request to a group will be decomposed into individual presence requests, and an INVITE to a group will automatically start a conference. Groups can also be utilized within application servers to define, e.g. black or white lists.

An IMPU and a PSI have the exact same formats (either a SIP URI or a TEL URI).

This means that groups of services identified by a PSI can be defined, as well as groups mixing IMPUs and PSIs. Consequently, everything that can be made towards user groups can also be made towards service groups and service/user groups. For instance, conference session setup can relate to groups including both human beings and automatons like a media server.

Associating services to services

There exist several approaches to route a SIP request addressed to a PSI to an AS serving this PSI. Most approaches rely on a direct mapping between the PSI (SIP or TEL URI) and the address of a server. They differ by where the mapping is defined (DNS, HSS), constraints on how the PSI URI is constructed, and where the routing is performed (originating S-CSCF, I-CSCF).

One approach is different, and while the IMS specifications never describe what it permits, its potential is much more significant from a service perspective. It lies in the association of a service profile to the PSI in the HSS, the same way service profiles are associated to IMPUs, which permit the routing of SIP requests associated to the IMPU to IMS application servers. With this approach a PSI is provisioned in the HSS like an IMPU, except that not all information need to be provisioned (e.g. no need for authentication information).

A first interest of this approach is that not all requests addressed to a PSI have to be routed to the same server. Some requests might be routed to one and others to another, based on initial filter criteria in the PSI service profile.

A very interesting feature of this approach, which is not related to service anthropomorphism, is that if an IMS user decides to make a user group consisting of the members of its family, identified by a PSI like sip:MyFamily@operator.com, an INVITE addressed to the PSI might be routed to a conferencing server, a MESSAGE addressed to the same PSI to an IM exploder, and a SUBSCRIBE to the PSI to a resource list server (in order to explode the request toward each member and compile the results in a single NOTIFY). Compare to the other approaches, which will route requests addressed to sip:MyFamily@operator.com to a single server, actually forcing the user to define a different PSI for each service it would like to apply to its family if it wants different services to apply (e.g. sip:MyFamily@conferencing.operator.com). This need to define a specific SIP URI for a combination group/service is also the way you will have to proceed in a non-IMS SIP network. This is an interesting example of how IMS can add value over non-IMS SIP networks and make the life of a user easier.

To come back to the subject of this post, let us consider a streaming service for a live event like a concert. An IMS user will typically start viewing the event by generating a SIP INVITE to a PSI identifying the live event. With service profile based SIP routing, the operator could define that a SUBSCRIBE to the presence event package would be routed to a presence server instead of the streaming media server (to which INVITEs will be routed). This would permit to associate presence information to the live event, like a description of the live event, information on how to subscribe to it, and status information (delayed, not started, starting, started). A user subscribing to the live event's presence could then be automatically be notified about when it can start viewing it.

In effect, associating a presence to the live event is associating a service to it, the same way presence can be a service associated to a user.

But there are other ways to associate a service to a service. Service profile -based routing permits to chain different services in the SIP signalling path. Consider an anti-spam application the operator proposes to its IMS customers. Every message addressed to the user can be routed to this application to be analyzed and possibly blocked. Now consider a discussion forum based on IMS messaging. The name of the forum is identified by a PSI (e.g. sip:TheIMSLantern@operator.com) so that each IM addressed to the forum is distributed to all the users currently registered with it. Service-profile based PSI routing would permit the operator to route a SIP request addressed to the forum first to the anti-spam application, then to the forum server. This is as if the discussion forum was treated as an IMS user subscribed to the anti-spam service. Now extend this example with an anti-virus application and an archive server, or think of other examples.

To conclude...

An IMS service can be anthropomorphic because it can act like a user, it can act as a network-based agent of the user, it can interact with other services like a user, it can communicate with "other" users, and it can be associated to other IMS services just like a user.

Note that "IMS service anthropomorphism" is not a standard or a commonly used term. I created it. You might therefore want to be careful and cite your source if if decide to use the term in discussions or in documents.

Christophe

A Bulding Block Approach to Standardization

For decades, the telecommunications industry has standardized solutions from A to Z, with little if any reuse of existing specifications when creating new ones. The progressive migration from circuit switched to IP based services did not initially change this fact much: MMS or OMA IMPS (that I take as an example in this post) are typical examples of creating telco-specific standards based on a loose reuse of IETF ones (SMTP for MMS, HTTP for OMA IMPS).

This has changed with IMS, and more especially its SIP component. 3GPP and the IETF collaborate with each other, and needed extensions to the SIP protocol due to IMS requirements are under the control of the IETF.

By importing IETF specifications into telecom standards, 3GPP implicitly accepted the building block approach to specifications that is common place in the Internet domain. In this post I will try to describe this approach and its benefits.

Building block standardization of SIP

SIP is a textbook example of a building block approach to standardization. The people and groups in charge of specifying SIP constantly try to apply the following rules:
- Do not reinvent the wheel. Reuse and adapt existing specifications if they fulfill your requirements. Only create when needed.

- Make everything as generic as possible. Even if your requirements are very precise, try to make your solution generic enough to be reused for other requirements.

Here follow some examples of how this was applied to SIP standardization:

- SIP sessions make use of the Session Description Protocol (SDP), which was specified prior to SIP. In effect, it is possible to use SDP without SIP.

- SIP SUBSCRIBE and NOTIFY methods were initially created to support a very specific requirement, actually related to the telecom domain (the support of the telephony Automatic Call Back service with SIP). However, it was decided to make the concept a generic and extensible means to distribute event notifications in a SIP network through event packages (see the first draft for SUBSCRIBE/NOTIFY here). When a part of the IETF community decided to support presence through SIP, they simply had to reuse the event package specification and create two presence-specific event packages. While the requirement was initially very specific, it gave birth to a concept that is fundamental for SIP and constantly evolving through the creation of new event packages. It is actually remarkable that this is a telephony -related requirement that led to a SIP concept which opens the door to a large variety of non-telephony related applications of the protocol.

- In the Instant Messaging (IM) area, presence was initially no more than a single state, describing if a recipient could accept an IM. The IETF decision to support presence through the inclusion of an XML document in the body of SIP methods, and allowing extensions to the basic schema, permitted the definition of presence to be gradually extended to become a large set of information about users (or services), their communication means, terminals and applications.

- SIP PUBLISH was initially created specifically for a client to remotely update presence information. The first versions of the draft were tightly linked to the presence event package and made impossible the reuse of PUBLISH in different contexts (see the very first draft here). However, the IETF community rapidly ensured the possibility to reuse PUBLISH for all existing and future event packages. PUBLISH therefore contributed to the enrichment of SIP-based presence, but at the same time a requirement initially scoped to presence contributed to the enrichment of the whole SIP protocol.

- Instant Messaging through SIP was initially supported only through the creation of a new SIP method: MESSAGE. However, it rapidly emerged that this approach was far from optimal to support all potential requirements associated to instant messaging: the concept of chat, which embeds IMs in a specific dialog context, the need to potentially exchange large documents via IM (e.g. a video file) while SIP is a control protocol and not a transport one like HTTP, or the need to support potentially high IM traffic while a SIP infrastructure might not have been implemented with this purpose in mind. It took time and several tries for the IETF community to address these requirements, and the final decision was to reuse the concept of SIP session as well as another protocol to transport an IM within the session. As a protocol like HTTP was not optimal to support the requirements for this IM transport protocol, it was decided to specify a new one called MSRP. This decision makes the comparison between Jabber/XMPP and SIP to support IM very biased. Maybe Jabber/XMPP is a better protocol than SIP for IM. However, Jabber/XMPP was initially specified and optimized for it, making its extension for, say VoIP, far from straightforward. On the other hand, in a SIP context, IM can be perceived as one communication component among others in a multimedia session.

OMA IMPS vs. IETF Presence and IM

The vertical standardization mindset that still prevailed a few years ago in the telecom community can be illustrated with OMA IMPS (initially called Wireless Village), a mobile specification to support instant messaging, chat rooms and presence.

Instead of reusing IM and presence related protocols available in the Internet, the Wireless Village group decided to specify a client to server protocol and a server to server protocol that would be specific to the mobile telecom domain, just reusing HTTP as a semantic-less transport protocol for OMA IMPS commands.

The group also decided to define IM, chat rooms and presence as tightly coupled together from a protocol and an architecture perspective, and to tightly link presence information to the mobile context.

In order to support its requirements, the Wireless Village group had to define various kinds of user lists (or groups) serving different purposes. Instead of creating a generic user group concept, they decided that each group fulfilling a specific purpose was a distinct object. Consequently, each group object led to a set of specific commands in the protocol, for creating/deleting the group, adding/removing elements to it, etc. With such an approach, if you define, say 4 types of user groups and 6 management commands, you end up with 24 distinct commands in the protocol.

In comparison, to address similar objectives, the IETF decided to decouple various concerns.

While presence is a concept originated in an IM context, the IETF decoupled one from the other, permitting each to evolve independently, thus permitting presence to apply to a much broader scope than simply IM.

By reusing the SIP session concept for session-based IM, the IETF permitted both the implementation of IM-specific systems, and multimedia systems using IM as one component among others in a SIP session.

The approach to address user groups and associated management, specified in RFCs related to XCAP, followed this approach:

- A user group is a user group, no matter what it is used for. The same user group can serve different purposes, and the set of applications for user groups is not arbitrarily bounded.

- A user group is user data, and there might be other user data that require similar access and management. No need to specialize access and management methods to user groups.

Consequently, XCAP is an HTTP-based protocol defining a few data management methods. The data itself is specified in XML, and there exist specifications for these data being user groups. As one of the requirements associated to data management was to be able to notify a user about changes made to data, the IETF decided to use a SIP event package. In effect, the IETF specifications for user data management include the joint usage of XCAP and SIP.

Building block standardization approach in IMS

The building block mindset to specifications has spread to IMS and non IMS standardization into 3GPP.

For instance, despite a terminology which is heavily related to SIP sessions (e.g. CSCF - Call Session Control Function), the IMS core network can be seen as a SIP connectivity network able to route SIP signaling, whether it is session-related or not, within an IMS domain, across IMS domains, and between IMS and non-IMS SIP domains.

In this context, IMS Presence, Messaging, and Chat Rooms are implemented as independent applications on top of the IMS core network and that make use of it. Once again, the comparison with OMA IMPS is quite interesting:

- OMA IMPS specifications lead to an implementation based on a network of IMPS servers over the mobile IP network. An IMS implementation relies on deploying application servers on top of an IMS core network. The IMS core network directly supports some of the requirements that are supported vertically in the OMA IMPS specifications (and implementations), like user authentication or routing and interfacing between various operators' OMA IMPS networks.

- OMA IMPS specifications tightly link the concepts of IM, presence and user groups. On the other hand, IMS specifications treat each of them as independent enablers which can be used together or in different contexts.

- OMA IMPS specifications were totally under the control of the Wireless Village group, and then OMA. On the other hand, by reusing IETF specifications, IMS specifications directly benefit from the evolutions performed in the IETF community, including some originating from people and companies which do not belong in the telecom or IMS domains.

A quite similar comparison can be applied to MMS and the equivalent support through IMS messaging.

Another interesting example is the 3GPP Generic User Profile specification, which permits to provide a centralized and homogeneous access to user data actually residing in various locations (e.g. HLR, HSS, AuC, application servers) and normally accessed through a variety of protocols (e.g. MAP, Diameter, LDAP). At the beginning of the erratic standardization process for GUP, 3GPP intended to standrdize a specific GUP protocol as well as a specific GUP schema to describe user data. Later on, it was decided to align on the specifications for Liberty Alliance, which define web services permitting 3rd party service providers to access user data owned by the network operator. As a consequence, GUP can be used directly as the means to access user data in network databases to support the Liberty Alliance web services exposed to 3rd parties.

The GUP specifications were also made generic enough to clearly distinguish between the methods used to access and manage user data and the data itself, that needs to be specified by instantiating and extending the generic GUP schema. On the other hand, the GUP specifications also include a SOAP-based user data modification notification mechanism, which duplicates what SIP event packages can and do support for XCAP. However, one can argue that the usage scope of GUP is broader than IMS and cannot rely on a protocol that 3GPP only uses in the context of IMS.

Some advantages associated to building block standardization

Reusing existing specifications instead of defining them from scratch permits to speed up the standardization process.

A protocol component or an application performing a generic task can be implemented once and reused several times, leading to faster development and validation.

In some cases, building blocks can be re-arranged with others to create new solutions. I gave the example of session-based messaging which, by applying the concept of SIP session to instant messaging, permits to integrate IM as one component among others in a multimedia SIP session.

Christophe

User Application Data: Opinions

After describing a panorama of the standards and technologies that can be used for user application data management and storage, here is a more subjective post.

User Data Distribution Can Be Managed!

Our industry has been struggling for years between two extremes:
- Distributing user data in application servers in order to optimize service delivery.
- Centralizing user data in order to simplify its management and ensure its integrity.

I think that with both the IMS service architecture and the Generic User Profile (GUP), we now have the tools to permit user data to reside where it is the most useful for service delivery, i.e. in the application servers hosting services.

As I tried to illustrate with the Automatic Service Discovery & Configuration example, and this is the same for presence, discovering the XCAP location of user data can be as easy as generating a SIP SUBSCRIBE addressed either to the identity of the user the data is associated to, or the identity of the service it applies to. The SIP/XCAP combo also permits to synchronize duplicates of user data with the master copy.

GUP is the exact equivalent in the web services sphere. GUP could be used to support centralized user data / subscription management, by hiding distribution details to the BSS system. See more below on GUP.

Exploiting Presence

Presence should be considered as one of the main sources of information related to the user, its devices and its applications.

This repository of user data will store both static and dynamic user-related data. This data will be populated/modified by the user, but also automatically by client and network based applications acting on behalf of the user, as well as network entities (e.g. IMS core, location server).

Presence data may be used by many applications, as an alternative to user application data specifically managed for them.

SIP and XCAP will be the natural protocols to access presence, according to 3GPP and IETF specifications.

Centralization for Common Application Data

User data that need to be shared between multiple applications can be stored in a centralized repository. Which one?

I do not like the idea of using the HSS as a user application data repository. Besides the potential impacts on HSS characteristics, another issue with this approach is that Diameter is not an optimal protocol for application data management. Moreover, the Sh reference point is very weak in terms of data management semantic, more especially compared to available alternatives. You have to realize that 3GPP specified the possibility to store application data in the HSS before GUP, XCAP, and CPS were defined.

I would like a centralized user repository to support SIP and XCAP in order to permit a smooth integration in the IMS service architecture. This means that the repository would be a SIP application server in the IMS architecture, and not a standalone XDMS as the OMA architecture seems to suggest.

This user repository could be based on the CPS architecture, which means that LDAP could also be an interface to it, and it would support GUP as well.

Therefore, overall, the user repository would be a SIP AS, using CPS as a backend data store, and supporting the GUP interface.

Generic User Profile & Liberty

I believe that Liberty and GUP would permit a strong integration of the future telecommunications network into a Service Oriented Architecture and into Web 2.0.

GUP is the optimal architecture and set of protocols to support Liberty in the operator's network.

I also think it should be used for subscription and user data management by the operator. Do you know that 3GPP specified the whole HSS data model using a GUP-compliant XML schema? This means that GUP could be imposed on suppliers right now as an open interface for HSS provisioning.

On the other hand, SIP/XCAP is a more lightweight approach to support access and management of user application data by IMS clients and IMS application servers.

It should be noted that at the moment the plans of most Network Equipment Providers to support GUP seem to be quite fuzzy, to say the least. There is therefore a risk that GUP remains a good idea on paper. At least as long as operators do not exert more pressure on their favorite vendors.

Common Profile Store to Break Silo Databases and for Coherent Data Modeling

In terms of actual database implementation, I think that the CPS approach is rolling and nothing will stop it. Some major suppliers have already embraced the concept and are offering HLRs and HSS's implemented on it. Others still resist, but will have to surrender sooner or later.

In the mid-term, all network databases (e.g. HLR, HSS, AAA) as well as some of the application servers will be implemented according to the CPS approach. All open service platforms (whether based on J2EE or JAIN SLEE) should integrate with a CPS.

Moreover, the CPS approach provides a unique opportunity to define a coherent model for user profile data across multiple network databases and application servers, while permitting application-specific extensions as required for a dynamic and differentiated service network.

Christophe

User Application Data: a Panorama

I tend to think that IMS and all-IP will make the telecommunications industry move from an era where the typical technical challenge was to design a specific solution for every problem to a new one where the main difficulty will be to select one solution among many candidates.

An illustration of this is user profile management and storage in the future application layer.

In this post I will try to make a panorama of standards and industry trends that may impact this topic in the years to come. In another one, I will express some personal opinions about the way to go.

User Profile Data in the HSS (Diameter, unspecified data format)

The Home Subscriber Server (called UPSF for fixed networks) is the database to store all the user data required by the IMS core network to fulfill its duties.

Additionally, the IMS service architecture has an interface between the HSS and the SIP Application Server called Sh.

This interface is based on Diameter and can be used by the application server for two purposes:

- Accessing user data normally stored in the HSS (or in the HLR), such as the user registration status, the S-CSCF that serves the user, or the service profile associated to the user.

- Storing application data in the HSS. This data is transparent to the HSS, i.e. it does not understand its format and semantic.

Sh supports a Diameter-based notification mechanism, permitting to alert the application server when data has been modified.

Most suppliers support the data repository feature of the HSS. It should be noted that the HSS is a mission critical database, which tends to constitute a major portion of the cost of an IMS core network. In this context, the application data repository feature of the HSS and associated questions (e.g. how much data will be stored in it? How much traffic will it generate?) raises questions about the required characteristics for the HSS and its eventual cost for an operator.

User Profile Data in SIP Application Servers (SIP/XCAP, XML)

In the IMS service architecture, there is a reference point called Ut between IMS clients and Application Servers that is used for service customization. This interface is based on XCAP, a simple HTTP based protocol specified in the IETF, which permits to access and manage data defined as XML documents.

The IMS service architecture therefore implies that a SIP application server can own the user data corresponding to the services it supports. This is a quite straightforward and optimized approach to co-locate user service data with the services that make use of them.

In the Automatic Service Discovery & Configuration example, I showed how a SIP event package (i.e. SUBSCRIBE, NOTIFY, PULBLISH) can be used by an IMS client to easily retrieve the XCAP location of user data associated to the user (SUBSCRIBE is addressed to one of the user's Public Identities) or to a specific service associated to the user (SUBSCRIBE is addressed to Public Service Identity). The XCAP URI can then be provided to the client in a NOTIFY. The event package also permits the client to monitor changes to the data (as in the IETF, a SIP event package is used to monitor changes in data manipulated using XCAP). These mechanisms can also be used within the network for an application server to retrieve user data in another application server.

XCAP Data Management Servers (XCAP, XML)

Based on XCAP, the Open Mobile Alliance (OMA) defined an architecture for user data management which consists of XCAP Data Management Servers (XDMS). You can see figures in this page from Tech-invite.

OMA decided to define a Shared XDMS storing common data between different services/enablers, as well as a specific XDMS for each of them (e.g. push to talk, presence, resource lists). Each of these XDMS is logically separated from the SIP application server supporting the corresponding service/enabler.

In practice, this ambiguous architecture is likely to lead to two implementation options:

1) The supplier decides to implement a network database called XDMS to store all XCAP data, i.e. OMA Shared XDMS and all the service/enabler -specific XDMS. This database is accessed through the XCAP protocol. A priori, it does not support any SIP event package, which may lead to synchronization problems with clients.

2) The supplier decides to combine each service/enabler specific XDMS in the SIP application server supporting the corresponding application. This approach is more in line with the 3GPP service architecture for which Ut (XCAP) and ISC (SIP) terminate to the same entity. It also corresponds to the user-profile-data-in-SIP-AS option above. It permits to support the SIP event package for monitoring of changes in the user data. The supplier may also decide to implement the Shared XDMS as a SIP AS.

Presence (SIP/XCAP, XML)

It might seem strange to list presence, a specific IMS service/enabler, as a potential data repository.

Actually, presence has considerably evolved since the early days when it only supported instant messaging and the IETF suggested to implement it as part of the basic SIP registrar function.

Through multiple extensions to its baseline XML schema, presence can now be considered as an extensive set of static and dynamic information about the user, its devices and its applications. In this context, presence can be seen as an essential repository of user data, that can be used as an enabler by many IMS applications.

In a network in which presence is a basic enabler associated to every user, some data currently considered as user application data, that the user manages service per service, may simply be part of the user's presence. To take a trivial example, presence may eventually make the explicit definition of forwarded-to-numbers for call management services totally irrelevant.

Presence is a specific example of the user-data-in-SIP-AS approach described above.

Generic User Profile (SOAP, XML)

GUP is a 3GPP standard, which is independent from IMS but that can optimally be used with it.

To make it short, GUP permits to define a virtual centralized user database, by enabling an homogeneous and centralized access to distributed user profile components, stored in network databases (e.g. HLR, HSS) and application servers.

The GUP location server is called GUP Server. A client application asks to access user data by providing the identity of the user (e.g. an IMS public identity) and the name of user profile components it wants to access. The GUP server either provides in return the address of the GUP data repository (e.g. an application server) or accesses the data on behalf of the application).

GUP interfaces are based on SOAP and the user data is defined according to a hierarchical XML schema (GUP does not specify the data itself). As GUP aims at supporting Liberty Alliance (set of web services towards 3rd party service providers) within the 3GPP network, GUP interfaces totally align with the Liberty Alliance Project specifications.

The GUP architecture can be compared to the user-data-in-SIP-AS approach in which:

- SOAP replaces SIP/XCAP to access and monitor changes in user data

- The GUP server and its location data plays the role the S-CSCF making use of service profiles endorses with SIP

More especially, the two approaches are able to dispatch a request based on the identity of a target user.

Common Profile Store (LDAP)

Every supplier or operator has its own name for this concept, but CPS is the term under which 3GPP has been addressing it.

CPS originates from a recent telecom industry trend which aims at changing the monolithic nature of network databases (e.g. HLR, HSS, AAA, application servers) into a multi-tiered IT architecture, in which the business logic of the database (e.g. Diameter features of HSS, MAP features from HLR) is physically separated from data storage. An LDAP server supporting telco-grade requirements (e.g. availability, capacity, performance) is shared between stateless business logic frontends implementing a variety of network entities.

CPS permits to optimize and rationalize the architecture of network databases. It also permits to reduce vendor locking achieved through monolithic network databases.

While it does not impose it, CPS also favors a more coherent modeling of user data across the various network databases and application servers, which permits to decrease operating costs and to foster new synergies.

In the context of IMS, CPS would permit to remove the need for a direct interface between the HSS and the HLR (required for Sh) by permitting user data sharing through access to the common backend.

Integration with open service platforms (e.g. J2EE, JAIN SLEE) would permit to use CPS for all applications implemented on these platforms.

So many possibilities... In the next post, I will provide personal opinions on them.

Christophe

IMS for Mobile Operators

I will try to dedicate a series of posts to analyze how I think IMS is perceived by different types of operators.

I will first focus on mobile operators, as this is a world I have known for years. I will then speak about fixed operators, and finally about operators that have both fixed and mobile organizations. Unfortunately I have too little knowledge about cable operators to dare saying anything about them.

IMS was an invention of the mobile industry, as its standadization started as early as 2000 in 3GPP (with initial work started the year before in the AT&T Wireless -led 3G.IP initiative).

The positioning of IMS in the cellular world cannot be voice-centric, for the simple reason that 3GPP specified a softswitch architecture as part of 3GPP R4, which permits to support VoIP within the mobile core network while keeping circuit-switched channels on the cellular access. Many mobile operators have deployed or are currently deploying this new architecture, and this is not to replace it with IMS in the next few years.

This is why, from the begining, 3GPP defined IMS as a network initially for "new multimedia services" and then, years later, to eventually replace the circuit-switched core network and therefore support legacy services as well (at least those that still make sense at the time).

An issue is that 3GPP never came up with a clear definition or list of such "multimedia services" (though it could be rightly argued that this was not the role of 3GPP to do it), and little progress has been made to this respect in the last years.

There are other important handicaps for using IMS in a mobile context:
- Current cellular access cannot efficiently support real time media over IP (e.g. conversational voice or video)
- Mobile handsets are still closed devices, and introducing openness and innovation into them is a slow and difficult process.

A consequence is that mobile operators will try to think of IMS in terms of "innovative services". However, the telco industry has shown so far little proficiency in creating innovative IMS services, and even when it does it faces the challenge to implement the relevant support for these services in mobile handsets and in application servers.

Push To Talk Over Cellular (PoC) was the first mobile IMS service to appear. The idea was to adapt a successful enterprise group walkie talkie service implemented on a circuit-switched network to the residential mass market and IMS. PoC has the (technical) advantage of relying on talk bursts instead of real time bidirectional voice. From a business perspective, the results are mixed, to say the least.

Then came so-called combinational or rich voice services, which try to combine a voice call established using the circuit-switched network with a data session over IMS. The approach permits to address the cellular voice over IMS problem, by integrating circuit-switched voice with non real time IMS services. It requires mobile handsets able to concurrently use circuit-switched and packet-switched sessions, with a client that transparently combines them for the end user. This approach led to "Push To X" services, permitting for instance to send pictures or to stream a unidirectional video during a voice call. Whether they are successful or not, such services are certainly not enough to support a convincing IMS business case.

IMS Messaging might deliver very interesting services for mobile operators, but this part of IMS/SIP standardization is not totally mature (at least from an implementation perspective), and in the meantime operators tend to directly support Internet IM integration (e.g. Google, Yahoo). The positioning of IMS Messaging with legacy SMS and MMS (with risks of cannibalization) is also an issue to consider. On the other hand, and unless I am mistaken, the positioning of IMS messaging with the pre-IMS mobile IM standard called OMA IMPS (formerly Wireless Village) is currently being addressed by this latter finally not taking off.

IMS Presence might also be very interesting, but presence is an enabler more than a service, and interesting applications of this enabler have not spread too much in the public sphere. A related issue associated to presence is that few know what the real potential of this enabler is, as many still associate it only to Instant Messaging or the basic indication of the ability/willingness of a user to communicate.

One of the latest trends for mobile operators is to link IMS to...VoIP, but not through cellular access. Voice over IMS is possible for dual mode mobile handsets making use of WiFi. IMS may therefore be used to provide VoIP services on WLANs, more especially for enterprises or for residential customers at home or in hotspots. In this area, IMS can be seen as the long-term solution while UMA is more a tactical alternative.

There is no question that IMS is part of the normal mobile evolution roadmap for mobile operators. However, their main problem is to decide when it makes sense to start deploying IMS, and with which services they should start. This is a reason why most mobile operators are still in an exploration phase.

Christophe