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.