Three Axes for IMS: #1 Fixed Mobile Convergence

I personally see three areas where IMS and SIP can dramatically modify the telco and Internet landscape. When I write "IMS and SIP" this is because SIP has intrinsic capabilities that are independent from IMS, and can be exploited within the Internet, while on the other hand the IMS specifications define a specific architecture for a SIP network, an architecture that has its own merits on top of those from SIP. I will certainly come back on this.

In this entry I will describe the area for which IMS is mostly mentioned: Fixed Mobile Convergence.

At the moment, in the industry FMC is often equalled to the ability of a mobile handset to connect to both the mobile and fixed network, through the support of dual cellular / WiFi (or Bluetooth) access. The term is also tightly linked to voice services, and the industry tends to compare the merits of UMA and IMS/VCC as alternative implementations of FMC. The skeptics then argue that FMC will not reach the mass in the next few years, due to the need to have hybrid devices to support it.

For me, the role of IMS into Fixed Mobile Convergence can be much more important than the support of dual mode devices and Voice Call Continuity (VCC), which permits to smoothly switch from WiFi to cellular access, and vice versa.

A first level of IMS-based convergence is technological. Basically, IMS is the common standard for next generation fixed (TISPAN), cable (PacketCable) and mobile (3GPP, 3GPP2) networks. Such an understanding of IMS as an FMC technology is very limited, as it may lead to the conclusion that an operator with fixed and mobile networks can converge through the deployment and interworking of separate fixed and mobile core networks. This technical convergence would lead to a certain homogeneity between fixed and mobile networks, the ability for both to support very similar services, and the possibility to find synergies between the fixed and mobile organizations of the operator. Fine, but is this enough?

The next level goes one step further: if IMS is the common standard for next generation fixed and mobile networks, it is possible to achieve FMC through the deployment of a unique IMS core network for fixed and mobile access. Moving from two separate fixed and mobile networks to a single shared one clearly has cost-related advantages, both from a CAPEX and an OPEX perspective. You can also assume that the user experience will benefit from this homogeneous core network support. Fine, but is this enough? Limited to core network unification, FMC still considers that mobile and fixed subscribers are different users.

The last level brings FMC at the user level, and for this relies on the intrinsic features of the SIP protocol. Through IMS, it is possible to finally unify the mobile and fixed dimensions of a human being into one. IMS permits to associate to users access independent public user identities (e.g. sip:user@operator.com, tel:+33123456) that can be concurrently registered from multiple devices using different access technologies. For instance, a user can concurrently register the same identity(ies) from its mobile device, PC, Television set, and fixed phone.

When a SIP request is addressed to this identity, the network implements (standard SIP) mechanisms to alert the user on the different devices or to look for the best device to reach the user. The basic mechanism for this is called "forking", and the ability to more intelligently select devices is based on IETF RFCs relating to callee capabilities (the ability to register the services and features supported by a device) and caller preferences (the ability to instruct how forking should be performed according to callee capabilities).

In IMS, public user identities are associated to so-called "service profiles" stored in the network user database, the HSS. This service profile determines how SIP requests generated from or addressed to the identity will be routed to IMS applications. It therefore constitutes the key for the door to IMS services. The fact that this key is associated to a user identity and that this user identity can be shared across devices and access technologies makes that all services deployed on IMS are inherently converged, i.e. uniformly accessible from all devices and access technologies employed by the user. Service implementation can then adapt the delivery of the service to the specific characteristics of a device or the access technology it uses.

It is important to note that such a user-centric fixed mobile convergence is totally network-based and does not require that devices be able to connect to the network through several access technologies. This means that IMS-based convergence does not put any other requirement on devices than their ability to register with and access IMS over IP. Once the IMS core network is deployed, all IMS-enabled devices making use of an IMS-compatible access (e.g. WiFi, WiMax, xDSL, GSM, UMTS, Cable) can be converged.

Obviously, these services are not limited to voice. The other two axes for the exploitation of IMS capabilities will provide a certain view on what IMS services can be about.

Christophe