Fixed Mobile Convergence(s)

The term "fixed mobile convergence" is very ambiguous. In this post, I will try to clarify the concept by distinguishing between different types of convergence and by positioning IMS with regards to them.

Device convergence

This type of convergence essentially relies on the ability of a device to connect to a single or to separate networks using either fixed or mobile access (typically WiFi behind xDSL and GSM/UMTS).

UMA (Universal Mobile Access), also called Generic Access Network (GAN) in 3GPP, is a potential solution for this type of convergence, as it permits to add an IP access to the mobile core network while keeping the higher level signaling stacks, thus enabling standard handover between, e.g. WiFi and cellular access. UMA/GAN requires the support of a dedicated "base station" in the access network to add the new access to the mobile core.

The IMS Voice Call Continuity (VCC) specification supports a similar feature, but this time handover is performed through switching between an IMS core network and the mobile core network. An issue with VCC is that voice services are supported across two different core networks (IMS and the pre-IMS mobile core), which may cause trouble with regards to the execution of value added services that should be unified or at least coherent across the two networks. The 3GPP IMS Centralized Services (ICS) initiative could be a way to address this issue, by locating value added voice services for both IMS and the pre-IMS mobile core network in a single IMS Telephony Application Server (TAS)

Overall, device convergence suffers from the significant hurdle to rely on dual mode devices, with the need for associated market penetration. It may also face technical issues related for instance to the quality of user experience or battery consumption. Note that this is the only type of convergence which relies on the ability of a device to connect to the network through different access types.

Service convergence

Service convergence relies on the support of convergence through service implementation.

Typically, fixed and mobile devices each access their own network through their specific access. A service is then made convergent by an ad hoc implementation permitting the coherent delivery of the service across the different access technologies and devices. This generally implies federation between fixed and mobile identities, the usage of a converged service profile and a consolidation of fixed and mobile OSS/BSS functions in order to deliver a coherent provisioning and a converged bill.

The drawback with this approach is that services have to be made convergent one by one, either by totally service-specific implementations or by the integration with a usually proprietary convergence framework dealing for instance with identity federation and shared fix / mobile service profiles. This type of convergence may be costly to support, difficult to evolve, and may sometimes be limited due to technical issues.

Network convergence

Network convergence relies on the ability of a core network to support different types of devices using different access technologies.

A single IMS core network may for instance support a variety of access technologies among GSM, UMTS, LTE, Femtocells, WiFi, WiMax, xDSL, FTTH, circuit-switched fixed access and cable, and consequently the devices making use of them.

IMS based network convergence requires the ability of the device to connect with IMS, either through native or acquired internal support, through a home gateway or through a network gateway. On the other hand, it does not require the device to support various access technologies (like WiFi and GSM/UMTS).

Taken in isolation, network convergence is essentially a cost reduction feature as it permits to avoid deploying access-specific core networks, thus cutting in the CAPEX and OPEX of the operator. Fixed and mobile devices can still make use of different user identities associated to different service profiles, and providing access to discriminated services or service implementations.

However, network convergence can also be seen as a support facilitating the implementation of service convergence, as it implies the support of the same (IMS) protocols by the devices (or through gateways) as well as a unified access to application servers through the unique core network. IMS-based network convergence can therefore be used to support convergent services in a more cost effective and simplified manner.

Network convergence is currently a strong IMS driver for operators deploying new access technologies (e.g. WiMax, LTE) or targeting specific convergent services (e.g. enterprise FMC, converged messaging) across these new or legacy access technologies.

In addition, IMS-based network convergence can also be seen as a step towards the ultimate goal of user convergence.

User convergence

User convergence relies on the support of a unique converged subscription possibly making use of converged user identities (i.e. user identities shared between fixed and mobile devices). It builds and extends upon network convergence.

In this approach, the operator provisions a unique user profile in the HSS, which may include converged identities, separate fixed and mobile identities if needed, and access specific information like authentication methods and credentials (e.g. UMTS AKA for mobile and SIP digest for fixed).

In the most advanced scenario, the user can concurrently register the same user identity (e.g. sip:john.smith@operator.com) from multiple fixed and mobile devices. The service profile stored in the HSS and associated to the shared identity supports converged access to IMS application servers, which associate a converged user profile to this identity. In this approach, every service deployed on the IMS framework and associated to a converged identity is de facto converged because immediately accessible from various devices using vaious accesses.

SIP requests addressed to the converged identity (e.g. incoming calls, incoming instant messages) can be handled intelligently by the IMS core network (and possibly the application layer) with a panel of strategies ranging from alerting the user or delivering the request (e.g. an IM) on several devices to selecting the most appropriate one according to various criteria such as user preferences, device capabilities and more dynamic information such as presence.

Other interesting features include session mobility (currently worked on in the IETF and 3GPP) permitting to transfer an ongoing session from one device to another, as well as the possibility to deliver the different components of a multimedia session over different devices (CPM requirement), like delivering the video component of a voice/video session to the TV and the voice one to the mobile phone.

I had the opportunity to describe user oriented convergence in a previous post, but I will use this one to demystify prejudices that sometimes exist about it.

First, while IMS can eventually support user convergence, it will not mandate it for all users. An operator implementing a coherent IMS strategy will be able to offer convergent subscriptions while keeping the possibility to offer mobile and fixed specific subscriptions as well (using IMS network convergence in this case). Moreover, even for converged subscriptions, the operator will be able to keep separate fixed and mobile identities if the user wishes it or for access to specific services. This is only a matter of user profile provisioning in network databases.

Second, user convergence does not necessarily mean the impossibility to distinguish between fixed and mobile access for:
- Service discrimination: some services may remain fixed or mobile specific.
- Service implementation discrimination: variants may still exist between the implementations of a service depending on the access used to deliver it.
- Charging discrimination: differentiated fixed and mobile charging may apply as long as users are informed about it, possibly in an interactive manner before service delivery.

The key element permitting to support these differentiations is the fact that all SIP signalling originating from a device includes an identification of the access technology it uses (e.g. UMTS, xDSL). This information can be used for service routing in the network (i.e. definition of initial filter criteria) or within service logic prior to or while delivering the service. A device switching from one access to another during service delivery would inform the network about it, for instance by initiating session re-negotiation, thus permitting the network and services to dynamically adapt to the change.

User convergence will not happen overnight, as it implies, besides the addressing of pending technical issues, major changes related to organizations, processes, business models, OSS and BSS, as well as regulation. However, I cannot imagine that it will never happen, whether this is under the control of operators using IMS or through disruptive players using alternative solutions enabled by all-IP networks.

Conclusion

IMS can support all types of convergence: device convergence, service convergence, network convergence, and user convergence.

Personally, I consider that device convergence and service convergence are of a limited and tactical appeal, due to the limitations and costs they imply. Moreover, to support them, IMS is only a candidate framework among others, which alone may not justify an investment.

The real interest of IMS for fixed mobile convergence appears when network and user convergence are part of the roadmap for the operator. These targets justify the usage of IMS for device and service convergence, as an initial step towards a more ambitious strategy combining both network and user convergence. Network convergence in itself can be used as an intermediate step towards user convergence, which will be a true revolutionary aspect of IMS, together with its ability to support multimedia communication, to combine services and to merge the Internet and telecommunications domains.

Defining an IMS Strategy

As a set of specifications or as a set of products provided by telecom suppliers, IMS is not the answer about how to ensure a bright future for telecom operators.

Basically, IMS should be perceived as a toolkit, which can be used alternatively or successively in various manners leading to drastically different results. As a consequence, an operator needs to define a short, mid and long term strategy based on IMS that suits its business strategy, and it has to take the adequate measures to apply this strategy. Here, we are touching the biggest IMS issue. It is not technical but rather cultural, practical and organisational.

In this post, I will try to outline some potential elements of an IMS strategy.

Deploying IMS

While some operators may simply decide to deploy IMS as part of their network evolution roadmap, many require an initial motivation to take the decision to go for it.

At the time being, the main triggers for deciding to deploy an IMS network are certainly the following:

- Replacement of the legacy circuit-switched network for fixed telephony services (IMS used as PSTN emulation subsystem).

- Deployment of a carrier grade VoIP solution for fixed residential customers, possibly with mobile/fixed convergence services including or not call contuinity between fixed and cellular access (knowned as voice call continuity).

- Deployment of carrier grade enterprise services including for instance business trunking, IP centrex or fixed mobile convergence.

- Deployment of a compelling set of mobile services through the Rich Communication Suite (RCS) or something else.

- Deployment or migration of an IPTV solution making use of IMS.

- Deployment of a selected set of IMS-enabled fixed mobile services.

However, such initial motivations are not always strong enough to validate investments in IMS.

For instance, a fixed operator may wonder why it should invest in IMS to reimplement a telephony service whose revenues are rapidly decreasing and which is likely to eventually be offered for free. Likewise, the usage of IMS for a specific service like IPTV or RCS or for a specific business segment like enterprise may not be economically worth for the operator.

It is therefore necessary for the operator to have a strategy for extending the usage of IMS beyond its initial deployment scope in order to have an optimal return on investment. Such a strategy is also fundamental for the operator to decide on future investments.

More especially, any decision to eventually replace a pre-IMS service implementation (e.g. telephony, messaging, IPTV) with an IMS one may impact the investment and evolution plan for the pre-IMS implementation.

The Future of Telephony

The future of existing telephony services and their implementation is a tricky issue that needs to be addressed carefully.

Basically, IMS permits to:

- Re-implement classical telephony services for legacy terminals and narrowband access (IMS as a PSTN emulation subsystem in ETSI TISPAN)

- Implement a new generation of voice-centric telephony services for new terminals (e.g. SIP phones) and broadband access

- Implement a new multimedia communication experience, in which voice is only a media component among others, whether they are related to person to person communication (e.g. messaging, video), content (e.g. streaming video, files) or data (e.g. events, application sharing, shared browsing).

The operator needs to clearly identify these services, decide which ones it wants to deploy, how, and in which timeframe.

Once this diversity has been understood and associated decisions have been taken, the operator can start to think about the implementation of related value-added services. For instance, are or should the value added services be the same or be different for each variant? Should they be implemented on the same or different platforms?

The operator can also think, if relevant with its plans, about how these different types of voice-related services should co-exist and possibly interwork.

The relationship between IMS and classical telephony services is of particular interest. It goes beyond the question of replacing the circuit-switched core network with an IMS based IP network as the two networks will co-exist for some time, and during this transition phase the support of value-added telephony services for both may be a question in itself.

The operator may have the choice between several alternatives and decide to opt for one or to evolve from one to the other over time:

- Using different service implementations in both networks (e.g. Intelligent Network in CS and a Telephony Application Server (TAS) in IMS)

- Reusing legacy Intelligent Network solution(s) in IMS

- Re-implementing services on a new platform able to support both IN and IMS ISC interfaces

- Reusing the IMS TAS for delivering value-added services to circuit-switched voice, in replacement to Intelligent Network servers (ongoing standardisation in 3GPP as IMS Centralized Services)

The future of messaging

In mobile networks, various messaging services can exist such as SMS, MMS and IMPS (OMA standard for mobile Instant Messaging).

IMS messaging has the potential to emulate all these messaging services and to add more to the user experience. The operator may therefore have to decide if and when it wants to eventually replace all of the existing messaging services and how it wants to manage the transition phase during which IMS messaging takes off and co-exists with them.

Moreover, as for voice, the multimedia nature of IMS makes that messaging may eventually cease to exist as a standalone service and be part as a multimedia component, of a more global and powerful multimedia communication paradigm. Should the operator skip the silo IMS messaging step and go directly to the multimedia communication service or should it implement one after the other? If so, how should the migration be performed?

IMS as a multi-service platform

An important feature of the IMS service architecture is its ability to support the harmonious co-existence of multiple services related to communication, content and data. This is a discriminating feature compared to a non-IMS SIP network, which lacks a standard service architecture and requires ad-hoc and often cumbersome support for the co-existence of several services, with limited multivendorship possibilities, and this until the architecture explodes after the hack too much (as this happens with all silo architectures whose limits are pushed too much).

An advantage of using an IMS core network as a multi-service platform is its ability to factorize and make coherent such functions as service discovery, user identification, user authentication, QoS and policy control, security, detection of the access technology used by the subscriber for service adaptation purpose (e.g. UMTS vs. LTE, xDSL vs FTTH) or interworking among operators, among others.

There are important cost savings to be achieved through deploying multiple services on IMS, both in terms of simplifying service implementation and harmonizing the support of functions common to multiple services.

There can also be important benefits related to the centralized knowledged in the IMS of service usage by subscribers, like the ability to apply QoS policies that take into account the multiple services used in parallel from the same home or device.

The operator needs to prepare and plan for the gradual support of new services and the porting of existing ones on IMS. This implies, among other things, not to take architectural decisions related to IMS that are specific to one service and may prevent the deployment of new ones in the future. To take a simplistic example, an Initial Filter Criteria routing all SIP INVITEs to a TAS may work as long IMS is used to support a single telephony service. On the other hand, it can be a source of nightmare as soon as an IPTV or IMS Messaging service (both making use of INVITEs as well) is introduced.

Using IMS as a multi-service platform is not always straightforward for operators which are used to deploy silo services and whose decision processes and organizations are defined according to this paradigm. An IMS strategy may therefore include organizational and decision process changes as well.

IMS for new services

I have often read that IMS cannot support new services. This is a wrong assessment.

The versatile nature of the SIP protocol, illustrated by the invaluable concept of event packages, the ability to integrate the IMS application layer in a more global SOA (Service Oriented Architecture) and UOA (User Oriented Architecture) framework, and the possibility to strongly integrate IMS with the Internet and its services create thousands of opportunities for new services, many of which are not connected to person to person communication.

The operator needs to define a strategy related to this opportunity for innovation: whether it wants to use this opportunity, how, what changes need to be done in organizations, skills and practices.

IMS for a new user experience

Possibly more important than new services, IMS permits to create a brand new user experience which makes that the frontier between services as we know them can totally disappear.

As already mentioned above, multimedia sessions permit to deliver communication, content and data services in a combined context.

Moreover, IMS-based fixed mobile convergence has the potential of eventually making the device and access technology used by the subscriber a mere detail of service delivery. Using a converged subscription, a user could access all its services from all its device, from all access technologies (whether fixed or mobile) and with the same identity.

Such a full convergence cannot happen overnight due to various reasons related to organizations, existing business processes, cultures, regulation, and to a much lesser extent, technical issues.

The operator must therefore plan and prepare for these evolutions, defining clear steps towards a the target. For instance, the operator may initially decide to use the cost savings aspect of IMS based fixed mobile convergence, by sharing the same IMS core network between fixed and mobile subscriptions. It also must decide if and how it eventually wants to support full convergence. For instance, will the operator eventually offer only converged subscriptions or will it permit subscribers to chose between converged and non-converged subscriptions? In this latter case, what would be the implications?

Managing the complexity of the IMS application layer

This blog has often tried to describe the fantastic power of the SIP protocol and the IMS service architecture to deliver rich services.

One of the outstanding features of the service architecture is its ability to combine services or service logic components residing in the devices, in the operator's network and across different operators' networks, the Internet or Enterprise networks, in a very flexible manner.

There are basically two composition mechanisms:

- Implicit composition: device and network based service logic or services are chained in the SIP signalling path.

- Explicit composition: device and network based service logic or services make use of each other through an appropriate interface like SIP, web services, HTTP, RTSP, H.248 or another protocol.

While very powerful from a functional perspective, this potential leads to new complexity related to distribution, compared to the traditionally centralized implementation of telecommunications services.

The operator must therefore find ways to deal with this complexity by addressing issues like the definition of relevant compositio handling functions like the SCIM, the management of service composition data like Initial Filter Criterias and SCIM information, the addressing of potentially redundant or conflicting functions across different servers, or the determination of approaches to measure quality of experience or determine root causes for potential problems.

Optimizing the IMS application layer

These distribution and composition mechanisms also lead to potentially critical optimization issues.

These optimization issues may for instance relate to:

- End-to-end performance if too many physical servers are involved in service delivery. In an extreme case, a spectacular service on paper may simply be irrealistic in practice due to disqualifying delays in service delivery to the end user.

- The load of IMS entities, more especially the S-CSCF and application servers, which are essential components in service delivery and composition.

- The cost of deploying and operating services if the application layer is too heterogeneous and too distributed or if features are duplicated across multiple servers.

- The impact of the failure of a server, for instance an application server, on subscribers. For instance, how many users will be impacted by the failure of an AS and for which services?

The operator must therefore define an optimal strategy to deploy services, which optimizes the user experience while minimizing costs and risks. This issue has to deal with service platforms and service topology. For instance, IMS offers alternatives to the dedication of an application server to a certain service for a large portion of subscribers by permitting on the contrary an application server to support a variety of interdependent services for a smaller subset of subscribers.

Service Platforms

The choice of service platforms is a key component of an IMS strategy, though only one component among others.

An operator has basically the choice between two alternatives for a service platform:

- A black box delivered by a service supplier.

- An open service platform delivered by an IT vendor, usually called Service Development Platforms (SDPs) in a telecom context, on which services are applications delivered by application providers or implemented in-house by the operator.

Concerning SDPs, the operator has further choices between alternatives like IT-centric platforms (e.g. J2EE, .Net) or telecom centric ones (e.g. JAIN SLEE, OSA application servers).

Each approach has its advantages and drawbacks, and these may differ depending on the considered timeframe. The operator should therefore define criteria permitting to select the most appropriate platform for a given service in the short, mid and long term, and prepare for the potential migration from one to the other, when appropriate.

IMS clients and devices

The best package of IMS services is useless if it is not adequately supported by clients and devices.

The operator must address such key issues like:

- The distribution of service logic between devices and the network. Note that depending on the characteristics of a device (e.g. high end vs low end, mobile vs. fixed), a given service may require different implementations or implementation variants for access by different devices.

- The distribution of service logic and IMS support between different devices on the customer premises, e.g. a home gateway and end-devices.

- The management of IMS client updates to support new IMS services.

- Optimizing device management by using IMS-intrinsic benefits.

- Spreading IMS support in the wider range of devices.

- Presenting IMS services in an optimal way to end-users in order to make them attractive and make their access and usage intuitive.

Conclusion

IMS by itself is not an answer to the challenges an operator will face in the future. An operator should define a coherent strategy comprising which components of the IMS potential it wants to use, in which timeframe and how.

While many operators have already deployed IMS or are planning to do so, usually with a well defined short term objective, I have the feeling that only a few are actively working on a mid and long term IMS strategy.

This is a pity, because defining such a strategy would permit to:

- Help IMS acceptance in the company by outlining the short, mid and long term benefits expected from it.

- Anticipate on changes required in organizations, practices, and skills to apply the strategy. By doing so, speed up the implementation of the strategy.

- Have a better control of short and mid term investments, for instance by avoiding excessive investments in implementations or services whose lifetime will be shortened by the implementation of the IMS strategy.

As you may infer from this post, I have put a lot of thoughts in these topics. Do not hesitate to contact me on this.

Christophe

Course on the IMS Application Layer

Through my company Arismore, I am offering a 2-day course on the IMS application layer. It can be delivered either in French or in English (all the material is in English). There are sessions regularly organized in my company's premises in Saint-Cloud, Paris, France, which are open to individuals. The course can also be delivered on site, within or outside Europe.

What makes this course unique is that, while others usually provide a litteral description of IMS specifications and essentially concentrate on the IMS core network, largely overlooking the IMS service architecture, sometimes even delivering misleading information, this one totally concentrates on this domain in which resides the true potential of IMS for differentiation and revenue generation (both for operators and suppliers). You can take this course after a regular IMS one, but my experience shows that even IMS novices can follow it without missing any core network background.

The course does not only describe IMS application-related specifications that span multiple standardization organizations (3GPP, the IETF, OMA, ETSI TISPAN). It also helps understanding what lies behind them, by providing revealing insights on how they were produced (e.g. historical timeline, assumptions, mistakes, conflicts between companies, need to address specific issues), shows in details how they effectively work, explains how they can optimally be exploited to deliver innovative services or innovatively deliver existing ones, and finally addresses the main opportunities and challenges that IMS brings to the industry. After this course, the student can understand what IMS can deliver, why there exist so many conflicting perceptions of IMS, and why IMS is as much a challenge as a promise.

Several on site sessions have been given or are planned in the near future, and the response has been so positive that these sessions usually lead to discussions about further collaboration between the customer company and Arismore.

Contact me at cgourraud@yahoo.ca if you are interested in either an on site session or in a session organized in our offices in Paris (planned dates can be found in the side bar of the blog).

Here follows a description of the course. You can also request from me a slightly more detailed powerpoint content description.

Part 1: The IMS Service Architecture

This is the most comprehensive part of the course. It addresses several objectives: understanding how the IMS service architecture works (dynamic view), getting an overview of the specifications (static view), understanding the drivers behind standardization decisions (motivations, design by accident, pending issues), understanding how the architecture can be used (application layer architecture, service patterns).

Standardized topics include:
- Overview of the architecture
- IMPUs & PSIs
- ISC usage scenarios & examples
- Details of ISC
- Service profiles and initial filter criterias (iFCs)
- OSA SCS / IM SSF / SIP AS
- MRFC/MRFP
- SCIM & Service Broker
- IMS Communication Services
- User Data Management (Sh, OMA XDM, GUP)

The service features of SIP are treated as a specific topic. It highlights the potential of the protocol when used in IMS or in other networks:
- Multimedia sessions
- Event packages
- Routing alternatives (SIP forking, callee capabilities/caller preferences), GRUUs)
- Support of Group Services
- Combination with other protocols (content indirection, SIP REFER, SIP session, body in SIP message)
- Interoperability aspects (between SIP profiles, between SIP and other protocols)

Service oriented features specific to the IMS service architecture, and complementing the intrinsic SIP ones are also detailed. This part permits to understand the value IMS adds on other SIP-based networks:
- Service composition
- Logic mutualization
- Personalization / Differentiation of user experience
- Simple access to services
- Service flexibility / agility
- Authorization to services
- Unlimited service scalability
- Multi-vendorship / Differentiation for each service
- Service anthropomorphism
- Usage of a single identity for multiple services

Part 2: IMS standardized services and enablers

This part covers IMS services and enablers standardized or under standardization in 3GPP, OMA and ETSI TISPAN. As for Part 1, background information about the standardization process is given.

User data services & enablers:
- Presence (IETF, 3GPP, OMA specifications)
- Group Management / Group Services

Voice oriented services & enablers:
OMA Push To Talk Over Cellular (Poc) V1 and V2
3GPP Combining circuit-switched and IMS services (CSI)
3GPP Voice Call Continuity (VCC)
3GPP/TISPAN Multimedia Telephony (MMTel)
3GPP IMS Centralized Services (ICS)
3GPP Conferencing

Messaging services & enablers:
OMA SIP Push
Messaging (IETF IM, 3GPP IMS Messaging, OMA SIMPLE IM)
3GPP SMS over generic IP access
OMA Converged IP Messaging (CPM) - Messaging part

Multimedia services & enablers:
OMA Converged IP Messaging (CPM) - Multimedia part
TISPAN IPTV

This part ends with a description of the Rich Communication Suite (RCS) initiative

Part 3: Opportunities & Challenges

This final part addresses essential topics which go beyond IMS specifications.

It is structured around the following topics:
- Fixed Mobile Convergence (from technical to user oriented convergence: opportunities and challenges)
- Which role(s) for the operators (e.g. bitpipe provider, intelligent pipe/channel provider, service provider with more or less control)
- Exploiting IMS Capabilities (e.g. User Oriented Architecture, Distributed Service Architecture, need for optimizing the IMS service architecture)
- Service Platforms (e.g. black boxes vs. open platforms, JAIN SLEE, J2EE)
- IMS as a service integration framework (different types of IMS services, how to integrate non-IMS services into IMS, relationship to 3rd party service providers)

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

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: ISC for IMS Application Server

After a long summer break, here is a fourth post on the details of the IMS service architecture related to the handling of SIP signalling. While the previous post described ISC from the perspective of the IMS core network (S-CSCF), this one concentrates on the other side of the interface: the IMS Application Server.

The details of the procedures to be supported by an IMS application server for ISC can be found in chapter 5.7 of TS 24.229. This post does not intend to describe all the details of the IMS AS procedures (for instance I will not address such specific issues as the handling of GRUUs, local numbering or carrier selection - the reader is invited to read the specification for this). I will only provide my own description of the basics, sometimes summarizing the specification, sometimes going beyond them or placing them in a larger context.

In the following, I decided to distinguish between two cases.

In the first one, the IMS application server receives a SIP request that was originated by another SIP entity. This entity could be an IMS client (e.g. phone, TV set, home PC) or an IMS application server (e.g. a messaging server, a multimedia content server). It could be located in the same operator domain as the IMS application server, or in a different one (e.g. an IMS client or an IMS application server of operator X issues a SIP request received by an IMS application server of operator Y).

This entity could also be a client or application server located in a non-IMS network, like an enterprise network or the Internet. A SIP request originated from outside the IMS and routed to it can be discriminated from an IMS-issued request, and the IMS application server can adapt its behavior to this origination. For instance, a request originated from the Internet was not authenticated by the IMS, and has to be processed accordingly.

In the second case, the IMS application server issues a SIP request to another SIP entity. This SIP request may be the consequence of the initial reception of a SIP request (first case), but it is not directly related to it. For instance, upon the reception of a call set up request for user A, the IMS application server issues an instant message to user A (or B). It may also be generated out of the blue, or through an initial interaction based on, e.g. web services or the access of a user to a web page. The SIP request may be addressed to an IMS client or IMS application server located in the same or a different operator domain (e.g. a service from operator X accesses the presence of an IMS user located in operator Y's domain). Alternatively, it may be addressed to a client or a server located in a non-IMS network, like an enterprise network or the Internet.

Note that, through the usage of appropriate gateways, the IMS application server may use SIP to interact with entities which do not reside in the IMS and do not support the SIP protocol.

IMS Application Server Receiving SIP Requests from the Network

SIP Roles

When an IMS application server receives a SIP request from the core network - request which may have been initiated by an IMS client, a non-IMS client or an entity in a non-IMS network - it may support one of four different roles.

Acting as a SIP User Agent, the IMS application behaves as an endpoint for the SIP request. The request may have been addressed to a service or service feature supported by the IMS application Server (typically when the request-uri is a Public Service Identity or PSI), or it may have been addressed to an IMS user (the request-uri is an IMS Public User Identity, also known as IMPU). In this latter case, the service logic hosted by the IMS application server decides to terminate the request on behalf of the user it is addressed to. This is typically the case for presence requests terminated at a presence server (the presence request is addressed to the user whose presence is sought). Other examples include a call termination service or an IMS messaging store that decide that the destination user cannot accept the call or the message right now.

Acting as a SIP redirect, the IMS application server will redirect the initiating party to another destination. I do not expect an IMS application server to extensively support this (feature-poor) role.

The IMS application may also decide to act as an intermediary in the end-to-end SIP interaction that takes place between two SIP entities (e.g. client to client, client to other AS, other AS to other AS, other AS to client).

Acting as a SIP proxy server, the IMS application server has very little opportunity to impact the end-to-end interaction. This behavior is adequate in cases where the AS hosts service logic that has to apply only prior to enabling the end-to-end interaction (e.g. authorization, target selection): the AS performs its logic, then lets end-to-end SIP signalling go on without any interference. It can also be used when the AS hosts service logic that monitors end-to-end SIP signalling without any intention to interfere.

When the AS hosts service logic that requires greater control on the end-to-end SIP interaction, it has to support the so-called routeing back-to-back user agent (routeing B2BUA) behavior. As a B2BUA, the AS acts as an endpoint to both parties in the SIP interaction. Doing so, it may decide to explicitly appear as an endpoint to them (by inserting a PSI as recipient / originator of the SIP interaction) or to remain transparent to the endpoints. Here are examples of situations where service logic needs to rely on a Routeing B2BUA role: need to modify the body of a SIP request (e.g. change a codec in session description), potential need to transfer a session during its course (e.g. to another party, to an announcement), need to insert a media plane intermediary in a multimedia session (e.g. to mix/adapt/control content, to insert advertizement).

I expect the latter example of the insertion of a media plane intermediary in an end-to-end multimedia session to be a fundamental IMS service use case in the future.

Direction of Requests

This is possibly the most important feature of the IMS service architecture when you want to integrate a non-IMS SIP application server with an IMS network, more especially in the context of VoIP services: the ISC interface (more especially the initial filter criteria that govern the forwarding of SIP requests over ISC) distinguishes between requests that originate from a certain IMS identity (IMPU or PSI) and those that terminate to this IMS identity. Moreover, it permits to distinguish between the case where the IMS identity is currently registered with IMS or not (only from 3GPP R7 for requests originating from a non-registered identity, i.e. requests initiated on behalf of a non-registered user by an IMS application server).

This architectural feature (shared with IN) permits to execute different service logic depending on the direction of a request. For instance, taking classical call control services, it permits to execute an outgoing call screening service only for originating call requests, and call forwarding services only for terminating call requests. It also permits to forward an IMS instant message to a store and forward messaging AS only in the case where the recipient of the message is currently not registered with IMS, and therefore unreachable through IMS connectivity.

A clear distinction between originating and terminating parties (and their respective services) is mandatory in a multi-operator environment, in which the parties might be subscribed to different operators. However, most of VoIP application servers that exist on the market today were implemented for a closed, single service provider, environment like the enterprise. They therefore tend to execute both originating and terminating services at once. Such a behavior is incompatible with the IMS service architecture, and must be corrected when the AS is ported on IMS. On the other hand, the direction of SIP requests is not a issue when porting a presence server on IMS (the presence logic depends on the SIP requests more than their direction).

Note that, while the direction of a request (originating, originating unregistered, terminating, terminating unregistered) is an information used to determine how SIP requests should be forwarded to IMS application servers, it is not specified how to convey this information in the SIP request that is to be forwarded. A typical approach to make the AS aware of the direction is to define a distinct AS SIP URI for each direction in the initial filter criterias (IFCs), or to add a specific parameter in the AS address. In both case, the information about the direction is made explicit when provisioning AS addresses for iFCs in the HSS.

Authentication

Authentication aspects are clearly described in section 5.7.1.4 of TS 24.229.

In short, a SIP request may originate from an authenticated identity that required privacy (it will be considered as "anonymous"), from an authenticated identity whose identity can be found in a 3GPP-specific header called P-Asserted-Identity, or from a non-authenticated identity (typically originating from a non-IMS network) that either set itself as "anonymous" or to a certain value. The IMS application server is then supposed to act according to the case it handles. For instance, if a non-authenticated identity was provided in the request, it may challenge it for authentication, typically in an Internet manner (e.g. using SIP digest). In another example, the AS may host service logic that applies to anonymous users or not.

Is is an important IMS feature, that SIP requests initiated from IMS entities are authenticated by the IMS core network, and that the authenticated identity is transported across IMS entities and IMS networks sharing a security trust relationship. This implies that a SIP request issued by a subscriber of operator X can reach an IMS application server from operator Y, without the need for the AS to authenticate the user, as it was already authenticated by operator X and the fact that it was authenticated as well as the authenticated identity are transported in the SIP request. This can be seen as a cross-network single sign on feature of the IMS network, that benefits all IMS services reached through SIP.

P-Headers

As already described in a previous post, a SIP request reaching an IMS application server includes a set of IMS-specific headers ("P-" headers) which are either important for the proper behavior of the IMS application server (e.g. the already mentioned P-Asserted-Identity header, the two headers related to charging) or which can benefit the logic it supports (e.g. information about the access technology currently used by the IMS client).

With the direction of the request, this is the other part of ISC which may require an evolution of a SIP application server to be ported on IMS.

What is Possible, What is Not

In the case where a SIP request initiates a dialogue (e.g. a session, subscription to events), the initial filter criterias and associated procedures at the S-CSCF may make that this request is forwarded to an IMS application server (or several). The IMS application server may either decide that it will only process this initial request (including the responses to it) or that it will process the whole dialogue initiated by this request until the end.

This means that the following cases are not possible:

- An AS cannot get involved in the middle of an ongoing dialogue: it has to be involved from the start.

- An AS cannot cherry pick the SIP signalling it wants. Either it handles the initial request and its responses, or it handles the whole signalling in the dialogue.

- Once it has decided to be involved in a dialogue past the initial request, an AS cannot drop an ongoing dialogue it is involved in before the end.

Any deviation from this approach would be a betrayal of core SIP routing procedures. This implies that what would be gained (an alleged flexibility in the relationship between the IMS core network and the IMS application layer) would be at the expense of the integrity of the SIP protocol, and everything it can bring to the delivery of next generation services.

This feature of the IMS service architecture is sometimes regarded as one of its "limitations". My belief it that those who think this is a limitation of the IMS service architecture are only projecting their own thinking limitations upon the IMS. They would like to engineer an IMS application layer exactly the same way they would engineer a circuit-switched application layer, instead of creating an application layer optimized around and making use of the unique features of IMS and the SIP protocol.

This feature also implies that the concept of "subsequent filter criteria", initially introduced in IMS specifications and never defined afterwards will never come to a reality, as they would violate the basic SIP routing procedures used over ISC. Subsequent filter criteria were introduced to mimic dynamic triggers in the Intelligent Network, which permit an application server to dynamically inform the switch about the events it is interested in.

Registration Case

The IMS application server may reeive 3rd party registration requests from the S-CSCF, indicating that an IMPU has registered/re-registered/de-registered with the IMS core network. this implies that in that case the AS acts as a SIP registrar.

As the 3rd party registration provides little details about the registration (for instance the capabilities of the IMS client are not provided), the IMS AS may need to automatically SUBSCRIBE to the registration event package asociated to the IMPU as soon as it has received the 3rd party REGISTER indicating it has registered.

IMS Application Server Initiating SIP Requests to the Network

An IMS application server can issue SIP requests on its own, without this request being tightly related to a request the AS previously received. The generated request might be a side effect of one or several SIP requests previously received by the AS, of interactions with an end-user performed through a non-SIP interface (e.g. web page, SMS), of interactions with a 3rd party service (e.g. web services), but these are only examples: anything can do, and the more intelligent the service is, the more spontaneous the generation of SIP requests can be.

SIP Roles

The IMS application server can act as a SIP User Agent. It is the initiator of the request and it will support the end-to-end SIP interaction with its SIP peer, whether this is an IMS client, an IMS application server, or another SIP entity outside of the IMS.

The IMS application server can also act as the 3rd party initiator of a dialogue between two other SIP endpoints, like two IMS clients or an IMS client and an IMS application server. In this case, the AS acts as an initiating back-to-back user agent (initiating B2BUA). This role can be used for example by a service to automatically set up a call between two users or implement a click-to-dial-back feature supported by a web page (the user clicks on a link to get called back by an operator. the service logic selects the operator and establishes the call between the two).

Note that an alternative way to implement 3rd party control is for the application server to use SIP REFER towards one of the SIP entities it wants to involve in the dialogue (e.g. the AS asks user A to set up a session with user B). This approach is simpler to implement from an application server perspective, as it does not require the usage of an initiating B2BUA, but it also requires the support of the REFER method by the SIP client, which is not a given in current SIP networks. The approach may also have an impact on how charging will be performed.

Originating IMS Identity

When it initiates a SIP request towards another IMS or non-IMS entity, the IMS application server (more precisely the service logic it hosts) has the choice between two possibilities.

The AS may act on behalf of an IMS user, and use an IMPU of this user as the identity initiating the request. Doing so, the AS can impersonate a user it serves, and for instance send an instant message or subscribe to the presence of a 3rd party just like the user would.

Note that this ability of an AS to issue a SIP request on behalf of a user (which may not be registered with the network at the time the request is initiated) is the reason why 3GPP had to introduce the initiating unregistered session case in the R7 specifications of initial filter criterias.

The AS may alternatively populate the P-Asserted-Identity header with a PSI, thus endorsing an identity associated with the service logic that initiates the request. For instance, an application server may initiate a request as a specific conference, voice mail account, or chat room.

Because it has a secure connection with the IMS core network and it is part of the IMS trust domain, the AS can directly set up the P-Asserted-Identity header with the IMPU of the user whose behalf it acts on or a PSI, ensuring that the IMS request was duly authenticated by the IMS network.

Routing of the Request

3GPP initially tended to be very restrictive about how the routing towards the IMS core network of a SIP request initiated by an AS could be performed. Fortunately, the latest specifications permit room for variants.

When the request is sent on behalf of a user, the AS may have to route the request to an S-CSCF serving the IMPU of the user, in order for originating services associated to the user to be executed (in this case the AS has to insert the "orig" parameter to indicate this is an originating request). The routing of the request to this S-CSCF might be direct, which implies that the AS knows the address of an S-CSCF serving the IMPU (possibly through a previously received request, or by retrieving it from the HSS via Sh), or through an entry point to the network serving the IMPU (which is less efficient but requires less knowledge from the AS).

Alternatively, and if the operator policy allows it, the AS may directly route the request to the network serving the destination address, thus bypassing potential originating services and charging procedures associated to the IMPU. This flexibility was not part of initial 3GPP ideas, but I always supported it as a simpler approach placing fewer constraints on the AS, and which is certainly adequate for some services.

The procedure for the case where the request is initiated by a PSI is similar, except that when the request is routed to an S-CSCF serving the PSI in order to apply originating procedures and execute originating services, the address of this S-CSCF is a priori static and can be configured in the AS (but it can also be retrieved from the HSS as well).

Note that the case where the AS has to route the request to a S-CSCF serving the IMPU or PSI requires an IMS-specific behavior that will impact non-IMS SIP application servers when they are ported on IMS.

P-Headers

This is another constraint associated to an IMS application server, and which needs to be considered when porting a non-IMS AS on IMS: the IMS AS is responsible for generating and inserting 3GPP-specific headers in the SIP request prior to forwarding it to the IMS core network.

Beside the already mentioned P-Asserted-Identity header, the AS has to insert a P-Charging-Vector header including a unique id for the transaction/dialogue (called icid) as well as an identifier for the network the request originates from (called orig-ioi). It will also have to process 3GPP-specific information coming from responses, like the identity of the terminating network (term-ioi).

Non-SIP Interfaces

Just a reminder: SIP is only one of the protocols that may be used by an IMS client to access an IMS application server. Therefore, this post is focusing on just one part of the inclusion of the IMS application server in its environment.

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.