An Article from Microsoft

Joseph Hofstader, an architect at Microsoft who writes periodically on Telecommunications technologies, sent to me a link to an article he published on Communications as a Service (CaaS). This is the first in a series of about 3 or 4 he has planned on the topic, diving deeper into architecture and infrastructure.

http://msdn2.microsoft.com/en-us/library/bb896003.aspx

The article is very interesting, clearly describing the capabilities of SIP as a protocol, and the paradigm shift between the traditional concept of communication services and CaaS.

Concerning the IMS related part of the article, readers of The IMS Lantern will understand that I would tend to elaborate more on the IMS service architecture and the unique benefits it can provide, but I understand that this is not the focus of Joe's article. He actually intends to elaborate more on this aspect in his next articles.

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

What is an IMS Service?

Considering the current fuzziness around IMS, it is a remarkable fact that there is not even a common understanding of what an IMS service is. For anybody who can't provide such a definition or is giving a wrong one, promoting IMS as the next big thing or dismissing it as the next big telco failure are equally blind statements.

In this post I will try to clarify this issue and provide my own definition.

In the following, I will often distinguish between SIP, the Internet protocol, and IMS, a specific architecture making use of SIP.

Often Heard

For many, an IMS service is a service that is developed specifically for IMS, and which uses SIP as its main, if not unique control protocol. Push to Talk over Cellular (PoC) or VoIP based on SIP and IMS are typical such services.

Another common statement is that IMS will not be used to develop new services, but to re-implement existing services with a new architecture and a new control protocol.

From these statements can be derived many interesting questions, including the following.

Aren't web services a better approach and isn't SOA a more adequate architecture to deliver services?

Isn't Jabber/XMPP a better protocol than SIP to support Instant Messaging?

Isn't the fate of SIP (and IMS) already sealed, considering that many of the most important companies delivering communication services to very large communities over the Internet have decided to go for alternative (standard or proprietary) protocols?

Is there any viable future for a network and a protocol that are inherently incapable to foster service innovation?

I will come back on these questions in the following, either directly or indirectly.

There will be be a huge (global) IMS and (global) SIP community, covering both telco networks and the Internet

First, while this is true that major communication applications running on the Internet do not all make use of SIP, it should be acknowledged that this is not the rule. The popular communication application provided by the company founded by the wealthiest man on earth is based on SIP, even if some "optimizations" brought to the protocol do not make it directly interoperable with other SIP-based clients. Th Gizmo project pushes forward the fact that SIP is a standard, and therefore permits interoperability. A famous Internet company uses both Jabber/XMPP and SIP for its communication services.

IMS is an architecture and a standard supported by standardization forums like 3GPP, 3GPP2, ETSI TISPAN, CableLabs, and the WiMax Forum. As a consequence, every operator in the world supporting a single or a combination of access technologies (ex. fixed broadband, cable, licensed or unlicensed radio access) should to at least consider IMS as a candidate in its roadmap. It is very likely that a significant number of operators will opt for IMS, thus creating a huge community of SIP and IMS users across the world.

The intrinsic capabilities of SIP as a control protocol (see Index for several posts on this subject), and the integration of SIP servlets support in all J2EE platforms, even in the versions that are not aimed at the telecommunications market, make that it is possible to eventually see SIP used in other domains that those which directly relate to telecommunications, and for applications which do not focus on person-to-person communication.

All in all, there is a high probability that the SIP community eventually exceeds in dramatic proportions all communities relying on other protocols for communication.

Note that as long as interoperability is ensured, there is no need for all parties in this community to rely on the same architecture or on the exact same SIP profile.

A very valid question is whether a non-IMS SIP community can supersede and eventually kill an IMS SIP community. The temptation of certain telecommunications actors to artificially create an IMS SIP walled garden by adding barriers in IMS standards between IMS SIP and non-IMS SIP clients and applications is an implicit invitation to a war, which may eventually damage the whole SIP ecosystem (however, it is important to note that many telco actors do not want such a fight and may find ways to eliminate these barriers - this may be stating the obvious, but the telco domain is not only standard-driven: business concerns are more fundamental). Otherwise, I guess that this is essentially a question of service offering and business model. If a user can benefit from similar or better services from an alternative service provider, and for a lower cost, it may be tempted to bypass the IMS-based offer.

SIP does not have to be the only communication protocol to be successful

Once again I am stating the obvious, but there is such a thing as protocol gateways, enabling interoperability between communication and control protocols, even if this conversion has to be adapted to semantical differences between protocols and may only address the common functionality between them. In the case where the protocols that need to interoperate are both standard, the conversion may itself be standard, as this is the case between SIP and XMPP (last time I checked the standardization of this particular gateway was work in progress).

An existing service can become an IMS one through integration

The thought that a service has to be specifically implemented in order to run on IMS and to be SIP-centric is wrong.

Access to a service can be preceded by a SIP interaction. The binding between SIP and the service specific protocol(s) usage can be supported by such SIP mechanisms as content indirection or the use of the SIP method REFER. I wrote a post on this subject, giving examples of this type of integration and describing some of the advantages associated to it.

Another way to integrate a non-IMS service with IMS is to encapsulate the delivery of the service within a SIP session: SIP is used to manage a session between the IMS client and the server supporting the non-IMS service (or an IMS application server acting on its behalf), while the service specific protocol(s) is (are) used to deliver the service within the session. I also described this approach here.

Integrating a service with IMS does not necessarily require that the existing implementation of the service be modified to incorporate components addressing the SIP specific logic. This SIP logic can be isolated and deployed on a SIP application server that is remote from the server supporting the legacy non-IMS logic, keeping the overall integration process simple from a service delivery perspective.

Yet another integration approach can be limited to IMS being used for service discovery, subscription and configuration, while service delivery is integrally supported outside of the IMS domain. I wrote a specific post on this subject.

This integration subject is a central topic of the article I wrote for the IEEE Vehicular Technology Magazine.

This means that a lot of future IMS services might already be out there. As an example, an existing Video on Demand (VoD) service might be integrated with IMS using the service discovery, subscription and configuration approach, and/or service delivery through content indirection, REFER, or within a SIP session. The service-specific protocol is RTSP and the binding between the service and IMS for content indirection or REFER is supported through the usage of the RTSP URI within SIP methods.

IMS can be used to combine multiple services

A non-IMS service delivered within a SIP session can be combined with other non-IMS services delivered within the same session, either simultaneously or sequentially. In order to optimize this combination from a user perspective, the IMS-based SIP logic might insert an intermediary on the media plane in order to mix/combine the different services at the media level. For instance, the media intermediary might mix the soundtrack of the video with another audio source, or insert textual messages in the video. See this post for an example.

Through an adequate conferencing support, the service can be shared between several users, and combined with person-to person communication components. For instance, the video of a VoD service can be viewed by different users in remote locations, who can exchange their comments through a messaging, a video or a voice communication component.

If there is the need to find a single reason to integrate a non-IMS service with IMS, then the possibility to combine it with person-to-person communication is this one, considering that communication is the core business of telecom operators. However, I hope I have illustrated in past posts that there could be other important motivations for such a combination.

Existing services can be enriched through SIP and IMS

IMS can support a large number of enablers implemented through application residing in SIP application servers or directly supported by terminals or endpoint servers connected to the IMS.

This permits existing services to be enriched with logic making use of IMS-based application like presence (providing information about the user, its communication means, its terminals, its applications) and group management (defining a standard way to access and manage groups of users, to which the service can be delivered simultaneously or which define who is authorized to access the service).

Enablers supported directly by terminals may enable real time communication between the service and the user, provide real time and accurate information about the user, its terminal(s) and its applications, or use SIP as a transport protocol to exchange service control semantic with a remote application server. SIP event packages may typically be used for this. For instance, existing event packages may permit the service to know if a user is currently involved in a SIP session or typing on its keyboard. New event packages can be defined permitting the service to access information or being notified about events related to the user, a terminal, or a specific application. A straightforward example is one where the service would retrieve the location of a GPS-enabled terminal.

Enrichment of an existing service through the usage of IMS-supported enabler(s) requires a modification of the existing service logic to make use of these enablers, even if the SIP/IMS specific part of it can be encapsulated through the usage of appropriate APIs.

Services can be delivered through complementary usage of SIP and other protocols and architectures

It would be a fundamental misunderstanding to believe that an IMS service has to uniquely or even essentially use SIP to be delivered.

The IETF applies a building block approach to specification, with a continuous concern to make every concept as generic as possible, and to always look at what has already been specified, even in adjacent application areas, prior to reinventing the wheel. SIP is a remarkable example of this, and as a result. The IETF community systematically tries to ensure that every SIP extension avoids to betray the spirit that governed the initial creation of the protocol, that every extension is made generic enough to be used beyond its initial purpose, and that mechanisms exist to combine SIP with other protocols that are optimized for fulfilling specific needs. SIP sessions, content indirection and SIP REFER are examples of mechanisms created to enable such a combination see here or in the Index). The building block approach to SIP specification is an interesting topic in itself, that I will address in the next post.

Another fundamental misunderstanding would be to believe that the IMS service architecture is incompatible with other service architectures and their associated protocols. It was specified from a SIP-centric perspective, focusing on the relationship between the IMS application layer and the IMS core network. However, 3GPP did not want to address the details of how the the IMS application layer would be implemented, leaving room for integration with other appropriate architectures and protocols.

As an example, SOA (Service oriented Architecture) and web services are not alternatives to IMS and its service architecture, but powerful complements to them and the usage of SIP as a service control protocol. I already wrote several times on this particular topic (just look at the IMS Lantern Index).

An important question is how the integration of IMS and SOA should be performed. At the moment, the prevailing understanding is that it should take place uniquely through the definition of web services, permitting to integrate a SIP-centric IMS with a web services and SOA -centric application layer. I believe that this view is incomplete and that the optimal integration between IMS and SOA should be more intimate, considering that IMS service logic should combine the direct usage of SIP and web services for service delivery, instead of only developing SIP-centric logic and web services -centric logic connected through a loose web services adaptation layer, similar to the one that connects the pre-IMS networks with SOA. I gave a name to this more intimate integration architecture, which extends both OSA and the standardized IMS service architecture: the User Oriented Architecture (UOA).

This distinction between a future service architecture integrating IMS and SOA solely through a web services adaptation layer (which makes that IMS is simply integrated in a SOA-centric service architecture) or through both a web services layer and direct usage of SIP by applications, might look artificial, but it provides alternatives considerations to the following issues:

- How to share a service context between IMS-centric logic and web services -centric logic? In the SOA-centric architecture, contextual information needs to be exchanged through web services defined between the IMS domain and the SOA domain, and may be limited to what is feasible or what has been done there. In a UOA architecture, this context can be naturally internal to the logic making use of both SIP and web services.

- Which IMS service enablers are provided to services in the application layer? In a SOA-centric application layer, this set of services enablers is limited to the set of web services exposing IMS capabilities to applications, while in a UOA architecture it can be extended to whathever the SIP protocol permits at any time. In a SOA-centric architecture, if no web service has been defined to expose a particular capability to applications, then no application will try to use this capability and the potential for innovative services will be limited. Typically, web services defined by people and organizations viewing IMS as a network and a set of standardized enablers like presence and group management are likely to miss most of the capabilities that are terminal and endpoint -related, as terminals and IMS endpoints tend to be considered more as consumers of network-based services than intrinsic parts of the service architecture. Will the set of web services exposing IMS capabilities to applications ever permit applications to access information generated by another application residing in an IMS terminal, like a game, if the service architecture assumes that IMS capabilities are only those that are accessible through web services defined by network people?

- How can IMS service enablers be used by applications? The synchronous nature of web service is likely to limit these enablers to those that can easily be rendered in a synchronous manner. In some cases, mapping the asynchronous capabilities of SIP to a synchronous interface might limit the usability of these capabilities or make them complex and unattractive to use. Directly using an asynchronous protocol like SIP to access asynchronous capabilities is likely to be simpler.

- Can all services be efficiently delivered to users? A complex architecture clearly discriminating between IMS/SIP -centric entities on the one hand, SOA/web services on the other hand, and forcing a web services -centric mapping between the two is likely to induce delays in the end-to-end delivery of services, as service delivery needs to cross several technical domains, while these delays could be largely reduced in a more integrated architecture.

- What is the cost for IMS services? The previous point hints at a more complex architecture induced by the definition of IMS and SOA -centric servers, and a gateway functionality between them. To this you can add the systematic duplication between web services and SIP interfaces induced by the SOA-centric architecture.

- How fast can new services be created and deployed? If you need to specify and implement a web service for each and every IMS capability, service creation is likely to be slower than if direct usage of SIP by applications is possible.

In addition to combining SIP with other IP and Internet service control and delivery protocols, like HTTP or RTSP, and integrating SOA and IMS, web services and SIP in a new service architecture called UOA, another important aspect that is not described in IMS specifications is to permit IMS applications to access capabilities available in pre-IMS telco networks, like user location (access to location servers), user registration status in the circuit-switched network, call control in the circuit-switched network, SMS, MMS or email as messaging enablers. Some of these enablers that are IP-based can be used in a straightforward manner. Others may require gateways between the IP and circuit-switched worlds, which may be based, on the usage of OSA/Parlay gateways but also on more specific protocol converters.

Therefore, the IMS service architecture can go well beyond the support of SIP and access to access to IMS-based enablers. Here is an overview of this scope.

Conclusions

An IMS service is a service that makes use of SIP and the IMS either centrally or marginally.

SIP itself and even more the combination of SIP with other protocols can give birth to a flurry of new services, some of them implemented on IMS.

The ability of SIP to combine various existing services of different types (communication, data, content, applications) can give birth to a new user experience, which is by itself a new service. This is an important matter to consider when comparing SIP with more purpose-centric protocols.

These new services can reach a huge community covering all the continents, all types of access technologies and spreading between telco domains, other business domains, and the Internet, possibly redefining the definitions of these domains.

IMS and SOA are not alternative architectures to deliver new services. They should rather be seen as building blocks permitting to create a new and more powerful service architecture called UOA.

This draws a potential future world, in which there might be a little bit of SIP everywhere, and consequently a a good potential for IMS to fit as a particular SIP service architecture deployed by telco operators.

However, history shows that the best technologies do not always prevail. In a possible future, the potential of SIP as a service control protocol used in different architectures including IMS, and/or IMS as a service architecture augmenting the intrinsic capabilities of SIP, might eventually fail. Conversely, would SIP and/or IMS be only used at a fraction of their potential (e.g. for VoIP and a limited set of additional services), they could still be a success.

Christophe

Use Case: Multimedia Service Delivery

One of the three axes that would permit IMS to revolutionize the telecom world, together with user oriented convergence and the definition of a new service architecture combining the power of SOA with a new User Oriented service Architecture (UOA), would be the full exploitation of the multimedia capabilities supported by the SIP protocol.

This post presents an example which makes use of these multimedia capabilities.

IMS Service Features Illustrated with the Example

The example I present today shows, among other things...

How IMS can be used by an operator to deliver a multimedia service which includes content and application components, but not a single person-to-person communication one.

How IMS can be used to share a multimedia content and application experience between several users (not shown on the figure).

How IMS can be used to mix person-to-person communication with content and application sharing.

How non IMS and even non SIP-aware services can be integrated with IMS (i.e. a service delivered over IMS does not have to be specifically developed for it and does not necessarily require SIP-related application components).

How IMS can be used for an operator to offer to its subscribers services actually delivered by 3rd party service providers located in other domains, possibly including the Internet, without requiring a prohibitively strong technical coupling with them.

How IMS can permit an operator to add-value to multimedia content delivered to its subscribers by third party service providers.

That IMS Services might be highly distributed, with application logic running in devices and in a variety of application and content servers. The SIP logic itself might be confined to only a subset of these service entities.

Why the existing conferencing solutions available on the market are too limited, and why a more generic multimedia conferencing architecture is needed.

Description of the Service

In order to experience the multimedia service, the user starts a session addressed to a Public Service Identity (PSI) identifying the service. The PSI may be specific to the user and routed to the SIP AS according to the user's service profile (originating trigger). Alternatively, the PSI may be a shared, public one. In this case, the routing to the SIP AS may also be based on the user's service profile (i.e. users need to be authorized to access the service so that their service profile allows routing to the SIP AS), unless it is based on the normal resolution of the PSI to the SIP AS (i.e. all users can access the SIP AS when they issue a request addressed to it). I already described these routing alternatives here and also with another service use case.

The user negotiates (and possibly re-negotiates during the session) the content of the service through any appropriate interface. For instance by accessing a web page supported by the SIP AS. Alternatively, this could be through a client downloaded on the terminal and communicating with the SIP AS via an application to application interface, like the exchange of XML documents describing the desired content of the multimedia session.

The SIP AS then performs the required actions to deliver the content (e.g. files, streaming media, web pages, applications) within the session. Depending on the type of content, the desired control of the operator over the delivery, and the technical means available for each component, the SIP AS uses the appropriate mechanism for each of the components (which do not have to be co-located with the service control logic hosted by the SIP AS). This may include some of the following:

- Establishing a SIP session with the component endpoint and bridging this new session with the user to SIP AS one (typical B2BUA behavior).

- Controlling the delivery of the component via an appropriate non-SIP interface (e.g. web services, H.248) towards the component source and negotiating/re-negotiating the SIP session accordingly towards the user.

- Providing the user's terminal and/or the component source with appropriate information to establish an end-to-end connection. In an example, the SIP AS would provide within the session a URI (e.g. HTTP URI, FTP URI, RTSP URI) via SIP content indirection or a referral, permitting the user's terminal to directly connect with the component source. In another instance that I used when I was the architect of an IMS demo for an equipment supplier, the SIP AS retrieves from the source the information required to connect to a whiteboard server, and transmits it to the user's terminal, so that a whiteboard client connects to the server and interfaces with it through the relevant whiteboard protocol. In yet another instance, the SIP AS provides the component source with the information relevant to push the content to the user's terminal. In any of these instances, the SIP AS may keep a control interface towards the component source in order to terminate the delivery of the component when the service session is completed (the idea is to ensure that the component is not delivered anymore after the service session is ended).

During session establishment or session renegotiation for a specific component, the SIP AS may decide to insert a number of media-level intermediaries (typically media servers) between the user's terminal and the component source(s). In such a case there is no peer to peer connection between the user's terminal and the component source, as both connect to the intermediary which is under the control of the SIP AS. The potential control interface between the SIP AS and the intermediary in the network might be an alternative way for the SIP AS to synchronize the delivery of the service component with the service session (start/stop delivery). However, the usage of a media intermediary may serve other more added-value objectives, like combining/mixing different components together (e.g. inserting text information in a video stream), caching media for better delivery quality, transcoding media to fit the capabilities of the user's terminal, or inserting localized advertizement in the media stream (possibly to decrease the service fee to be paid by the user).

It should be noted that the component sources may be provided by the operator or by 3rd party service providers located in other domains, and possibly in the Internet. In this case, the operator acts as a service broker, adding value to individual components by integrating them in a single multimedia session, acting on the media plane, and providing the level of access (no need for the user to authenticate to each individual service component provider), the QoS and the security that can be expected by the user from its telecommunications operator.

The service session, which takes place between the user's terminal and the SIP AS (usage of SIP for the service may be confined to these two entities), may terminate when either the user or the SIP AS decides that it is time to. As for individual components in the session, their delivery may be terminated through either the user's terminal, the component source, or the SIP AS if it has the control means to do it.

Some of the Benefits of Using IMS to Deliver the Service

As I already described in an earlier post, using SIP has a prerequisite to access a service has numerous advantages for both the operator and the user, and using a SIP session to deliver the service even adds on top of this:

- The SIP signalling generated by the user's terminal and reaching the SIP AS transports meaningful service information, such as the authenticated identity of the user, information relevant to charging like the address of the charging nodes and correlation identifiers which will permit the billing system to correlate charging information generated at the media plane level (e.g. type and volume of media), at the IMS core network level (duration of the session), and at the SIP AS level (any additional application-level event), information about the location of the terminal (e.g. cell ID), and information about the access technology used by the terminal, which can be exploited by the SIP AS to optimize the delivery of the components.

- Routing of the SIP signalling between the user's terminal and the SIP AS may be directly linked to the authorization of the user to access the service (see above).

- The establishment and re-negotiation of the session permits the user's terminal and the SIP AS to re-use core network support to set the relevant QoS and security associations just like for a person-to-person voice or multimedia session.

- The SIP session determines a well defined context for the delivery of the service, with a clear begining and end.

- The session permits the coherent combination and aggregation of individual components within a multimedia service.

- The session offers the possibility for the operator to insert media-level intermediaries for both control and added-value purposes. This example thus illustrates how an operator can both use multimedia sessions to deliver its own services, and add value to peer-to-peer multimedia exchanges.

Some Possible Extensions to the Use Case

Though not supported by the standards today, the service could be extended with session continuity, permitting the terminal to switch from one access to another (e.g. WiFi to UMTS) without stopping the service session and the delivery of its components.

Currently under work in the IETF, session mobility would permit the user to transfer the ongoing session from a terminal to another (e.g. mobile phone to TV set) without stopping it. Such a transfer could be necessary from a convenience perspective (e.g. the user started the service on the run and is now at home, benefiting from terminal alternatives), or depending on the renegotiation of the service session (e.g. the user would like to add a component like an application, which is not available on the terminal he or she is using). It would also be possible for the user to receive the content or run applications on several terminals, each optimized for a subset of the components or applications.

While the example concentrated on the delivery of a mutimedia service to a single user, it would be possible to share the experience between multiple ones. This could be done by providing a conferencing entity to the architecture. As this conferencing support would not be limited to person-to-person communication (e.g. voice, messaging), this would require a more generic conferencing architecture than those proposed today by suppliers. I tried to describe a potential architecture in a past post. Sharing the same experience could imply the synchronization of applications on each of the users' terminals, permitting for instance shared browsing, a shared whiteboard, or multi-player gaming.

As soon as multiple users are involved in the service session, person-to-person communication would be an appreciated plus and would be enabled by the conferencing support permitting to add bi-directional communication components between the participants. Such a possibility would make the use case come back to the core concern of the operator: delivering person-to-person telecommunication.

A service supporting both the delivery of content/application and person-to-person communication may experience different modes. In addition to this example in which a service session is extended to communication, an alternative use case would see a communication session between two or more users extended to shared multimedia service delivery.

Everything is Possible, Nothing is Given

Obviously, such a service would require an adequate support in terminals (application architecture, application components and an intuitive user interface), a relevant architecture in the IMS application layer, the right agreements and technical settings between the operator, its SIP AS, and the 3rd parties and their servers, and an attractive business model for all the parties (the operator would need to find the right charging policy for its subscribers).

It would require that ongoing standardization efforts in 3GPP do not prevent, through artificial barriers, the delivery of such a service or similarly out-of-the-mainstream others. I will come back on this topic in a future post.

Christophe

Conflicting Views on the IMS Service Architecture

The two figures above both depict the IMS Service Architecture.

The first one is taken from the 3GPP specifications and is therefore the primary basis for anyone to understand the IMS Service Architecture.

The second one is my own representation of the IMS service architecture, based on the exact same specifications, and is therefore only accessible by the readers of this blog. As I have not seen so far a similar representation anywhere else, you can assume that this is not at the moment the mainstream perception of the IMS service architecture in the telco community.

IMS as a New Intelligent Network (IN)

The 3GPP representation of the IMS service architecture is network centric. It represents the IMS application layer solely according to its interface to the IMS core network, placed at the center of the figure with the S-CSCF.

This representation is potentially misleading as technicians with a classical telecom background can perceive the IMS service architecture as an IP replica of the Intelligent Networks that can be found in all existing fixed and mobile circuit-switched networks. Actually, I have seen this view supported numerous times, both by these people who know everything about telecommunications and by alleged IMS experts.

This IN perception of the IMS service architecture is re-inforced by the fact that two of the IMS application server types represented in the figure are tightly linked to IN: the IM SSF is a gateway between IMS and an IN application server, and the OSA gateway is usually (and rightly) perceived as being essentially an API on top of IN. Adding to the confusion, the terminology used for the IMS service architecture (e.g. trigger points) heavily borrows from IN (to be frank, some companies participating to IMS standardization did see this architecture as an IN one).

In an IN network, the (fixed or mobile) switch interfaces with the application server through a dedicated control interface (e.g. INAP, CAP, an IS.41 subset), which permits the application server to control the switch by issuing instructions to it. These instructions essentially serve the purpose to control voice calls, as the circuit-switched network is voice centric.

In such an architecture, the basic voice services are supported by the core network, which is complemented by application servers for the delivery of "supplementary" or "value-added" services.

With this kind of background, it is easy to perceive ISC as the IMS equivalent to INAP/CAP and the IMS application servers as an extension to the core network. Some might even argue that the IMS application layer is not needed, if you can implement all the required supplementary services within the S-CSCF.

IMS as a Totally New Service Architecture

However, the IN-oriented interpretation of the IMS service architecture does not resist a thorough analysis of the specifications that I tried to describe in past posts (here, there, there and there).

First, with the exception of the user registration part of the ISC reference point (which permits the S-CSCF to notify application servers about registration events, but does not permit the AS to block registrations), ISC cannot be seen as a logical interface between the S-CSCF and the application server.

On the contrary, ISC is only one branch in an end-to-end SIP interaction between the application server and other application-related entities. In such an interaction, the S-CSCF and the whole IMS core network only act as intermediaries and service-related routing functions between application-related entities. This is why I tend to represent the whole IMS core network as a SIP-oriented service bus supporting the IMS application layer (I will come back on this in a future post). This bus is essential for the IMS application layer, but its significance is close to zero from a service perspective. Suffice to say that it duly supports its service bus role, and that the best additional value it can offer to applications is to be totally transparent to them.

The second essential characteristic of IMS that dismisses the claims of an IN replica is that SIP is not the equivalent of SS7 over IP:
- SIP sessions are not voice calls. They are much more generic service sessions, which may even not include any voice or person-to-person communication component.
- SIP is not only about SIP sessions. Other SIP methods make SIP a very generic service control protocol.

With such a knowledge in mind, it is irrelevant to represent the IMS service architecture without representing what is on the other side of ISC (and other SIP-relate reference points supported in the IMS core network). My figure tries to represent all the possibilities.

However, this figure is still incomplete, as it focuses on the SIP dimension of the IMS service architecture. For a more complete view, look at this previous post.

The End-To-End IMS Service Architecture (SIP Dimension)

Many possibilities exist, that I already described in the past (here, there and there), but I will summarize below (note that the examples are very basic).

An IMS endpoint (terminal, endpoint application server) in the operator's domain initiates a SIP service interaction with the IMS application server. ISC is the last branch used in this interaction. Example: user accesses presence.

An IMS endpoint (terminal, endpoint application server) in another operator's domain initiates a SIP service interaction with the IMS application server. ISC is the last branch used in this interaction. Example: user accesses presence.

An endpoint (terminal, endpoint application server) in a non-IMS domain (e.g. enterprise network, the Internet) initiates a service interaction with the IMS application server. ISC is the last branch used in this interaction. Example: user accesses presence. Note that the non-IMS endpoint may either use SIP or another protocol to initiate the interaction. In the latter case, a protocol converter (e.g. Jabber to SIP) will translate the initial request into SIP.
Specific case: the non-IMS endpoint is an application server (e.g. Internet application requiring user's presence).

The application server initiates a SIP service interaction with an IMS endpoint (terminal, endpoint application server) in the same operator's domain. ISC is the first branch used in this interaction. Example: application server sends instant message to user.

The application server initiates a SIP service interaction with an IMS endpoint (terminal, endpoint application server) in another operator's domain. ISC is the first branch used in this interaction. Example: application server sends instant message to user of another operator.

The application server initiates a SIP service interaction with an endpoint (terminal, endpoint application server) in a non-IMS domain (e.g. in an enterprise network or in the Internet). ISC is the first branch used in this interaction. Example: application server sends instant message to user in the Internet. Note that the non-IMS endpoint may either accept SIP requests or not. In the latter case, a protocol converter (e.g. SIP to Jabber) will translate the initial request to the required protocol.
Specific case: the non-IMS endpoint is an application server (e.g. presence server).

The application server initiates a SIP service interaction with another IMS application server in the same operator's domain. ISC is both the first and last branch used in this interaction. Example: presence-based application accesses user's presence.

The application server initiates a SIP service interaction with another IMS application server in another operator's IMS domain. ISC is both the first and last branch used in this interaction. Example: presence-based application accesses user's presence.

An IMS application server acts as an intermediary in any of the interactions listed above, plus the essential case where none of the service interaction endpoint is an IMS application server (e.g. two IMS devices, an IMS device and a non-IMS device, an IMS device and a non-IMS endpoint application server). The idea is for the intermediary AS to either control or add-value to the end-to-end interaction. ISC is a branch somewhere in the middle of the SIP signalling path. Note that several IMS ASs can be inserted as intermediaries in the signalling path, forming a chain of combined services. Examples: IMS AS authorizes and/or charges for access to service, IMS application server inserts media plane intermediary to improve user experience (e.g. media mixing, media transcoding).

User Oriented Service Routing

The SIP dimension of the IMS service architecture relies on two SIP routing mechanims:
1) Standard SIP routing (labelled in the figure as IETF SIP routing) based on the resolution of the target SIP address towards an endpoint.
2) IMS User Oriented and Service Profile based routing (labelled in the figure as user profile based routing), which permits to dynamically alter SIP routing on a user-per-user basis towards IMS application servers. The service profiles stored in the HSS can be seen as service routing routing under the control of the operator and modifiable over time. This permits to incrementally modify SIP routing as applications are added or removed from the network, and this without impacting endpoints.

This draws a service architecture which is very flexible in terms of service logic location (in service entities like application servers in the network, endpoint application servers, devices), very flexible in terms of location of service entities in different domains (the operator's domain, another operator's domain, an enterprise domain, the Internet) and very agile in its possibility to dynamically modify service logic at work in the network.

Christophe

IMS Service Routing: Big Picture

This post is the first in a series that will describe how the routing of SIP service-related requests is performed in the IMS.

Before going into the details of service profiles, initial filter criteria and the ISC (IMS Service Control) interface, I will start with an attempt at placing IMS service routing in the larger context of end-to-end SIP routing within IMS.

I think this is something missing in most representations of the IMS service architecture, which tend to fragment the overall perception of this architecture, and make it more difficult to comprehend for the novice.

The figure you can see at the top (click to enlarge) shows the end-to-end interaction between two IMS users. However, there exist variants related to which entity initiates the SIP interaction (IMS client or IMS AS), which entity terminates the SIP interaction (IMS client or IMS AS), and whether the end-to-end SIP interaction fully takes place between IMS entities or involves entities in an external network like the circuit-switched network or the Internet. I will address these variants below using specific colors.

Before describing what IMS SIP routing permits to achieve, let's start with one of its apparent limitations compared with how SIP can be used in a non-IMS network.

Restriction to IETF SIP Routing

The routing of a SIP request to its destination is based on the resolution of the request-uri address (IMPU or PSI in IMS) to one or several targets. However, before reaching its destination, the SIP request may traverse several servers, and the originator of the request may itself define in a specific Route header (part of) the path to be followed .

This may permit the client to decide the routing of the SIP request through a set of servers that will deliver added-value services.

This is not possible in the IMS. More precisely, whatever SIP route an IMS client may define in a SIP request it originates will be discarded by the IMS core network (the P-CSCF for that matter) and replaced by another route which ensures the phase 1 of IMS SIP routing described below.

Is this a limitation to the power of IMS, compared to what is possible in, say, the Internet? I don't think so.

I do not think that relying on a SIP client to determine a set of application servers supposed to add value to the end-to-end SIP interaction is a very pragmatic and agile approach to service delivery. The mechanism is limited by the knowledge and sophistication available in the SIP client, which may be put under pressure as soon as more than one application server needs to be inserted in the signalling route (which application servers? In which order?).

Then, you have to compare what is lost (the possibility for an IMS client to define a service route) with what is gained with the IMS architecture: service profile based routing.

The IMS routing of SIP requests based on service profiles (phases 2 and 5 below), is sophisticated, very agile and permits to keep IMS clients very simple, as they do not have to care about service-related routing. It also accurately reflects the business relationship between an IMS user paying an operator, and the operator providing value for money in return.

While some may look at the IMS architecture as a way to deprive users from their freedom to access the services they want to access (note anyway that the limitation is only in the route to access a destination, not the destination itself), others can see the same architecture as a way to simplify the life of users (and their devices) and to concentrate service-related complexity in the hands of those who are asking money for value.

Service Routing for Each Party (See Figure)

An important feature of the IMS service architecture, which is not apparent in figures taken from IMS specifications, is the fact that each party in a two-party or multiparty SIP interaction benefits from its own service routing, and therefore its own services.

As shown in the figure above, an end-to-end SIP interaction between John and Mary will first lead to the invocation of services associated to John (phase 2), and then to services associated to Mary (phase 5).

This feature is very natural for a telecommunications service architecture (it is similar to IN), but it is not to most of the non-IMS SIP implementations, more especially those supporting enterprise VoIP services. Typically, these non-IMS implementations will combine the execution of features that apply to all parties of a call. This difference is usually the most important one to overcome when porting a non-IMS application server on the IMS.

In the IMS architecture, a specific party in an end-to-end SIP interaction is either identified by an IMPU or a PSI. While the figure shows an example of IMPU to IMPU interaction, the following alternatives could also exist:

- IMPU to PSI: an IMS client or an AS-based application acting on behalf of a user interacts with an AS-based application identified by a PSI.

- PSI to IMPU: an AS-based application interacts with an IMS client.

- PSI to PSI: an AS-based application interacts with another AS-based application.

Here is a practical advice for when you define or analyze an IMS message flow: S-CSCFs do not hang just like that in the architecture; they always serve someone or something, and it certainly helps to make it explicit (see for instance the figures in this or the previous post).

Preliminary to the Description of End-To-End IMS Signalling

Here follows the decomposition of an end-to-end SIP interaction into 6 distinct phases.

But first a couple of things...

There is of course a pre-requisite: John is registered with IMS. This implies that his client discovered the P-CSCF it will be associated to (according to 3GPP or TISPAN procedures), and performed the IMS registration procedure leading to John's authentication, the association of an S-CSCF to him, and the setting up of a security association between its client and the P-CSCF.

The figure shows direct interfaces between different operators' networks. However, the GSM Association (GSMA) defined the possibility for 3rd party transit networks to be used in-between, in order to reduce the need for bilateral operator agreements.

Phase 1: routing of the SIP request to the S-CSCF serving the originating party

As described above, the P-CSCF removes any potential route defined by the IMS client and replaces it with a route to the S-CSCF serving the user in its home network. This route was defined during the IMS registration of the user, and may traverse an I-CSCF in case the home network wants to hide its network topology to the potential visited network (note that in practice a border gateway might be used instead).

Phase 2: service profile -based routing for originating user

The S-CSCF serving the originating party applies the service profile for the originating IMPU. This service profile consists of a list of initial filter criteria. It corresponds to the set of services the originating party is associated to. In practice, this means that the service route for the SIP request originated by John is defined by the operator and implemented by the S-CSCF.

In this phase, the S-CSCF may route the SIP request to 0, 1 or several application servers (one after the other in a signalling chain or pipe).

It is possible that one of the application servers terminates the SIP request (i.e. acts as a SIP User Agent). This may happen in three cases:

1) The request was addressed to a PSI hosted by the application server (e.g. user tries to access content)

2) The originating party issued a request addressed to itself, which was actually targeted at one of its services in the network. For instance, SIP requests that update the presence information of a user (PUBLISH for presence event package) are addressed to the user's own IMPU. Another example can be seen in the automatic service discovery and configuration example I described in an earlier post.

3) The application server terminates the request on behalf of the B-party. This may happen, for instance, with services that block traffic to unauthorized destinations. Another example could be the case where a service, potentially after having accessed the presence of the B-party, decides to transform an IMS instant message (SIP MESSAGE) into an email, an SMS, or an Internet IM. In this case, IMS SIP routing is terminated to be superseded by an alternative protocol outside of IMS.

Signalling path termination: the SIP interaction may terminate at phase 2 if one of the application servers decides to act as an endpoint for the SIP request.

IMS exit point: the SIP interaction may exit IMS at phase 2 if an application server serves as a gateway towards a different protocol/domain (e.g. email, SMS, MMS, Internet IM protocol). Note that phase 2 takes place before the IMS core network tries to route the SIP request based on the request-uri. This means that the request-uri might not be an IMS routable URI: it could be a URI specific to the protocol and/or domain to which the SIP request is actually targeted.

In case no application server terminates the SIP request, the S-CSCF proceeds with the next phase.

Phase 3: routing of the SIP request to the destination network
The S-CSCF routes the SIP request according to its request-uri.

In case the request-uri is a TEL URI, the S-CSCF may route the request to the circuit-switched network (see previous post - the request will be routed to the circuit-switched network when the TEL URI does not resolve into a SIP URI through ENUM).

Through DNS, the S-CSCF may also determine that the domain of the request-uri is resolved into a non-IMS network like the Internet.

IMS exit point: the SIP request is routed outside of IMS by the IMS core network if it is a TEL URI that does not resolve into a SIP URI, or if it is a SIP URI whose domain is outside of the IMS (e.g. in the Internet).

Side comment: to come back to the possibility for an IETF SIP client to define by itself a route towards application servers... If it is not possible for an IMS client to define such a route, it is possible for an IMS application server to do it on behalf of the client. This route could binclude application servers in the Internet and could be provided by the IMS client to the IMS AS through signalling or self-provisioning.

In case the B-party is an IMS user (same or different operator), DNS resolves the request-uri to an I-CSCF of the target operator, that acts as an entry point to its network. The request may traverse an I-CSCF of the originating network, in order to hide the network topology to the destination network (note that in practice a border gateway might be used instead).

Phase 4: routing of the SIP request to the S-CSCF serving the destination user

Either the B-party is registered with IMS or not.

If it is, the I-CSCF retrieves the location from the HSS and routes the request to the S-CSCF serving it.

If it is not, there is a procedure to dynamically allocate an S-CSCF to the B-party and to route the request to this S-CSCF. The rationale is that the B-party's IMS services in the network might need to be reachable even when the B-party is not available (e.g. presence, a voice call continuity server that will route the call to the circuit-switched network, a call forwarding service).

Alternative entry points to phase 4: instead of originating from the IMS and having followed phases 1 to 3, the request received by the I-CSCF may originate from the circuit-switched network (request is received from MGCF, i.e. signalling gateway with CS), or from a non-IMS network like the Internet.

IMS entry point: instead of an S-CSCF in the originating network, the request may come from the circuit-switched network (through an MGCF), or from a non-IMS SIP network (e.g. the Internet).

Phase 5: service profile -based routing for the terminating user

The S-CSCF serving the terminating party applies the service profile associated to the IMPU (or PSI) corresponding to the request-uri. This service profile consists of a list of initial filter criteria. It corresponds to a set of services associated to the terminating party.

In this phase, the S-CSCF may route the SIP request to 0, 1 or several application servers (one after the other).

It is possible that one of the application servers terminates the SIP request (i.e. acts as a SIP User Agent). This may happen in two cases:

1) The request was addressed to a PSI hosted by the application server (e.g. user tries to access content, user posts message in chat room). This is the case where an operator offers services to the world. Note that there exist alternative mechanisms to route a request to a PSI.

2) The application server terminates the request on behalf of the B-party. This might be the case for presence (the request was addressed to the B-party but was actually aimed at its presence server) and any other service acting on behalf of the terminating party (e.g. voice call continuity, call or message blocking or forwarding services).

Signalling path termination: the SIP interaction may terminate at phase 5 if one of the application servers decides to act as an endpoint for the SIP request.

IMS exit point: the SIP interaction may exit IMS at phase 5 if an application server serves as a gateway towards a different protocol/domain (e.g. email, SMS, MMS, Internet IM protocol).

Phase 6: delivery to the terminating party

In case there is still a SIP request to be delivered to a B-party, the S-CSCF routes the request to the P-CSCF(s) serving the B-party (there might be several in case several endpoints are registered with the same IMPU), which in turn contacts the device of the B-party.

SIP requests issued by application servers

This post essentially addressed the situation where the SIP request is initiated by an IMS client corresponding to an IMPU. However, to be complete, you also have to consider that IMS application servers are able to generate SIP requests, using either an IMPU or PSI as the originating party, and an IMPU or PSI as the request-uri.

When an AS issues a request with an IMPU as originating address, it is supposed to route it to an S-CSCF serving the IMPU. Starting with 3GPP R7, it is possible to allocate an S-CSCF to the IMPU even if it is not registered with IMS (i.e. the IMS AS issues a request on behalf of an IMPU which is not registered with IMS). IMS routing then proceeds with phase 2.

When an AS issues a request using a PSI as the originating party, it may either reach a S-CSCF serving the PSI, or route the request directly to an I-CSCF for the terminating party. Depending on the case, IMS routing then proceeds with phase 2 (the service profile is associated to the PSI) or directly with phase 4.

A specific case for having an IMS AS issuing a SIP request to the IMS is when it acts as a gateway towards another protocol and/or domain. This is the symetric case to what was described in purple for phases 2 and 5 above.

Christophe

There is no direct relationship to this post, but I just added the initial architecture proposed in 3GPP for the SCIM in the first post I wrote about it in May.

An IMS Application Server in Context

The figure above (click to enlarge) shows how an IMS application server can fit in a service delivery context that includes a legacy telecommunications environment, the Internet, the IT environment of the users it provides services to, and (of course) the IMS.

This picture might not be perfect (though I like the shades of blue), but let me know if you can find much better in the current IMS literature.

A specific instance of an IMS application server may only implement a subset of the features shown in this figure, depending on the capabilities of the platform supporting it, and its potential specialization based on the logic it supports.

It is not the purpose of this post to discuss the technologies that can be used to support such an IMS application server. However, if you look for platforms able to support most of what is implied here, J2EE and JAIN SLEE are certainly the best candidates, and a combination of both might be the winning ticket right now.

Pre-IMS Context (white boxes, black arrows)

The white boxes and black arrows represent the typical pre-IMS context of an application server, that applies to most current Service Delivery Platforms (SDPs).

Applications implemented on the application server can make use of a set of network capabilities such as call control, location, SMS, MMS.

Each set of capabilities is supported by a dedicated network server (e.g. switch, location server) and through a specific network to network interface (e.g. INAP, MLP).

On top of these protocols, the typical SDP provides for the service logic it hosts such features as horizontalization (e.g. hiding of the underlying stove pipes), homogenization (e.g. via a set of similar APIs), simplification (e.g. hiding details of ugly telecom protocols), and abstraction (e.g. permitting to manipulate a call at a high level).

It also exposes the capabilities to 3rd party service providers, through CORBA APIs (e.g. Parlay Classic) or web services (e.g. Parlay X).

The set of capabilities available to service logic in the SDP and to 3rd party service providers is limited and static. It followed a very slow standardization and implementation process, starting with the standardization of the network enabler (e.g. IN), and finishing with the implementation of (standardized) web services (e.g. call control). This means several years.

IMS, and more generally the move to all-IP can change all this and make the picture more colorful.

Where is the IMS Core Network? (lower part)

You might have noticed that the figure does not show any IMS core network entity (e.g. CSCFs, HSS, gateways).

This is not because they are not there or they are not important to the IMS service architecture. Actually, without them, the IMS service architecture would not exist at all!

The point is that the IMS core network itself is of limited relevance from an application perspective, compared to the sophisticated SIP routing mechanisms it provides. This is an aspect that is usually overlooked, as the IMS core network is often the tree that hides the forest to those who try to understand the IMS service architecture.

For instance, I believe that ISC (IMS Service Control) should not be considered as an interface between the S-CSCF and the IMS application server, like INAP is an interface between a switch and an Intelligent Network server. ISC is just the SIP road that permits to connect the IMS application server to the IMS/SIP motorway that connects all the blue entities in the lower half of the picture.

One of my core beliefs is that, among all the benefits the IMS core network can provide to its application layer, one of the most important should be its transparency. An application should only care about what the IMS architecture can provide to it, not about the sophisticated details of how this is done. The application should only know that by issuing this SIP message to this user identity, it will be able to magically access this enabler (e.g. the presence information of this user), and that is all. This is unfair for the hundreds of pages that specify the IMS core network, and the thousands of workers that implement or will operate it, but life is unfair.

SOA and UOA (upper vs. lower parts)

As I wrote in an earlier post, the IMS service architecture as it is depicted in 3GPP specifications is like a picture of a person which would only show its lower half. The reason for this is that 3GPP IMS specifications are core network centric, and only address the relationship between the IMS application server and the core network.

Whereas the pre-IMS application server only exposes a few web services to (hypothetical) 3rd party applications, the future application server should be an integral part of a service oriented architecture, in which applications deployed in the operator's domain could as much be consumers of 3rd party services as they expose some, and could as well integrate with the user's IT environment (e.g. calendar, applications running at home or in the office).

This Internet and IT -centric dimension of the application layer is complemented with its IMS and SIP counterpart (SIP is not the only service protocol that will be used in the IMS, and IMS is not the only network making use of SIP), which adds user orientation to the global service architecture.

To over-simplify, the service oriented architecture routes service requests according to the identity of services, while the user oriented architecture enabled by IMS and SIP can route service requests according to user identities, service identities, or a combination of both. As a consequence, the identity of a user placed in a SIP message (as originator or recipient) may impact service routing and direct the SIP service request to network-based applications serving the user, client or endpoint applications serving the user, or the user itself.

In terms of semantic, it is time for the large part of the telecom industry that still ignores it to understand that the concept of SIP session is much broader than its legacy "call" counterpart, and that SIP is not limited to session control. There are other SIP methods and mechanisms which could be of great interest in the future application layer (e.g. SIP event packages), and which sometimes overlap with the semantic usually associated to web services (e.g. will you use a web service or a SIP SUBSCRIBE to access user information?).

IMS Capabilities: Numerous, Dynamic and Everywhere (lower part left)

The semantic power of SIP, as well as its extreme extensibility and versatility compared to telecom protocols (just look at the rate new SIP drafts pop up in the IETF) make that capabilities usable by applications can be varied, dynamic and multiple.

These capabilities will seldomly be located in standardized network servers like today.

Some will be located in IMS application servers. Every new IMS application exhibiting a SIP interface to end users might be usable as an enabler by other applications using the same SIP interface. Some of these applications could themselves be used as enablers, and so on,

Other capabilities will be located in endpoint servers and devices connected to the IMS, including those associated to end users in a fixed mobile converged environment (e.g. mobile phone, PC at home, home gateway, set top box, fixed phone). Every new application deployed in these endpoints and devices may give birth to new enabling capabilities usable by the IMS application layer. Just to take an example, there exists today a SIP event package that can be used to monitor the activity of a user on a keyboard.

Access to these enablers located in network application servers, IMS devices and enpoint servers, can cross network boundaries. This is, an application located in operator X's network can access an enabler located in or registered with operator Y's network or in the Internet. For instance, an application associated to John, subscriber of operator X, can access the presence of Alice, subscriber of operator Y, and the presence of Bob, located in the Internet.

Web Services, SOA: Much More Possibilities (upper part center)

The multiplicity and dynamicity of IMS service capabilities permits the operator to offer a much richer and differentiated set of IMS services to 3rd party service providers.

Besides the usual communication-related suspects, a large portion of these services are likely to be informational: information about the user, its preferences, its data, its applications, its devices, its activity, etc. It will be the responsibility of the operator to preserve the intimacy and privacy of the users.

The dynamic approach implies that operators will have to rely less on standards and more on differentiation in order to attract third party service providers. This is one example, among others, about the need for the telecom industry to find a new balance between standardization and differentiation.

Web services are not the only way to integrate 3rd party services in the operator's offer. This integration can also be performed on a more horizontal axis.

3rd Party Services Integrated Through End-to-End SIP (bottom part right)

3rd party applications located in the IMS (e.g. in a different operator's network) or in a non-IMS network (e.g. the Internet) can have an end-to-end SIP interaction with the IMS clients of the operator's subscribers. These rd party applications may either be network-based (e.g. implemented in an IMS application server) or device/endpoint server based.

This end-to-end service interaction can be placed under the control of the IMS application server, as soon as the service profile of the user is provisioned with an initial filter criteria that identifies end-to-end SIP signaling between the user and the 3rd party application.

For instance, routing of SIP signaling to the IMS application server may be determined by the fact that the SIP request is addressed to a specific SIP URI (e.g. sip:this_service@3P_SP.com), a specific SIP URI domain (e.g. 3P_SP.com), or because the SIP request explicitly identifies a 3rd party application in a SIP header or in an SDP (session description protocol) body. For instance, the name of a game could appear in the session description of a SIP INVITE aiming at starting a gaming session.

Service logic inserted in the signaling path between the user and the 3rd party service can serve various purposes: control, charging, monitoring, adding value. Do not overlook the latest item: telecommunications should not be only about control and charging.

3rd Party Services Through SIP Indirection or Inclusion in SIP Session (same)

This is just a variant of the previous one.

This case differs from the one above because SIP interaction only takes place between the user and the service logic hosted in the IMS application server.

The 3rd party logic or content is accessed/controlled through other protocol(s) than SIP (e.g. HTTP, RTSP, web services).

I already gave an example of such an integration through SIP indirection. In the future, I will post another one showing inclusion of 3rd party content in a SIP session controlled by the operator.

The Ut Reference Point (lower part left)

The picture shows the support of the Ut interface which, in the IMS service architecture, permits an IMS client to interface with the IMS Application Server using XCAP (HTTP-based protocol that manipulates XML documents) for service data management.

I extended this interface to HTTP in general, as it is very important to permit that IMS clients interact with service logic through advanced web-based user interfaces. Other relevant protocols may be added as needed.

Conversion Between SIP and Non-IMS Protocols (upper part right)

It can be rightfully argued that this should not be part of the SOA layer, but please bear with it.

The IMS service architecture makes it very convenient to detect and divert SIP signaling which needs to be converted into another protocol (e.g. Jabber/XMPP, OMA IMPS CSP) to an IMS application server serving as gateway towards this other protocol. Conversely, the IMS Application Server can receive non-IMS messages and convert them towards SIP for interaction with IMS clients.

This approach is currently used in 3GPP for the support of SMS over generic IP access (which encapsulates legacy SMS's into SIP IMS Messaging).

Note that the IMS core network only supports protocol conversion from/to legacy circuit-switched networks for voice calls. The rest is up to the application layer.

Christophe