Fixed Mobile Convergence: Determining the Right Model for Your Enterprise

By  Isabelle Guis, Senior Manager, Mobility Solutions, Cisco — September 03, 2008

Work is no longer a place, but an activity that happens in multiple locations, using many different devices and applications. Fixed mobile convergence (FMC) promises to keep users connected to business applications such as inventory management, supply chain management and accounting, and to do so independent of networks, devices, location, or time.

Today's FMC implementations are largely limited to mobile roaming between fixed, cellular, and Wi-Fi networks, with unified communication features, such as single- number reach, single voice mailbox, and corporate directory.

There are two primary options for implementation: carrier-led and enterprise-managed. In carrier-led implementations, the hand-off between networks is managed by the carrier network, whereas in the enterprise managed model, management is based on the enterprise wireless LAN (WLAN). This article evaluates the different carrier- and enterprise-managed models and discusses the criteria enterprises should use when selecting an FMC implementation.

Carrier-Led Implementations
There are two main types of carrier-led implementations: Unlicensed Mobile Access and IP Multimedia Subsystem.

Unlicensed Mobile Access
Unlicensed Mobile Access (UMA) is a standard that enables handoff between the carrier (GSM and General Packet Radio Service, or GPRS, networks only) and an enterprise WLAN. On the cellular network, the mobile handset communicates through a base station controller with a cellular base station to servers in the core network of the carrier (see Figure 1).

When the handset detects a WLAN, it establishes a secure IP connection through a gateway to a server called a UMA network controller (UNC) on the carrier's network. The UNC translates the signals coming from the handset and emulates a base station controller (BSC). Thus, when a mobile handset moves from a cellular network to a WLAN, it appears to the core network as if it is simply on a different base station. This helps to ensure that it has the same access rights and subscription to applications.

An UMA-based architecture always anchors the voice call to the cellular network and thus it is not possible to tightly integrate enterprise IP PBX features. When a call moves from the WLAN within the enterprise to the cellular network, any extended business communication features (such as four-digit-dialing and corporate directory) are lost. An UMA-based architecture that is hosted by the carrier may be attractive to smaller businesses that don't own their own IP-PBX infrastructure or have the technical expertise to maintain such network services. Enterprises, however, should be aware that UMA is only supported on GSM and GPRS networks.

Figure 1: Highly Simplified UMA Architecture

IP Multimedia Subsytem

IP Multimedia Subsystem (IMS) is a service delivery framework for multimedia services based on pre-Fast Serial Interface Processor (FSIP) interfaces. IMS supports multiple access technologies, including Wi-Fi, WiMAX, DSL, broadband cable, and enterprise-level T1. It is a layered architecture that separates the device layer, transport layer, and control layer (see Figure 2) for more flexibility and interoperability than other implementation types as it can run on a broader variety of networks.

The layers are as follows:

  • Device layer: Users can connect with a variety of devices such as computers, mobile phones, PDAs, and digital phones. Traditional analog phones can establish connectivity via a public switched telephone network (PSTN) gateway.
  • Transport layer: The transport layer is responsible for initiating and terminating SIP sessions via a variety of transmission media, including Wi-Fi, DSL, cable, SIP, GPRS, and 3G. In addition, it allows IMS devices to make and receive calls to and from the PSTN network.
  • Control layer: The Call Session Control Function (CSCF), a SIP server, handles SIP registration of the endpoints, and processes SIP signal messaging to the application servers. The Home Subscriber Server (HSS) database stores a unique service profile for each end user.
  • Service layer: The service layer offers a variety of multimedia services such as telephony, instant messaging, video, and presence.
  • This IMS approach holds much promise for FMC, with truly converged multimedia services for end users across a variety of networks. Currently, IMS is at varying stages of adoption.  Mobile users can look forward to taking advantage of its range of services in a few years.
Figure 2:  Highly Simplified IMS System

Enterprise-Controlled FMC Implementations
In addition to the carrier-led FMC technologies just described, there are several technologies that are enterprise-controlled. In enterprise-managed FMC implementations, the PBX functions as the anchor point for calls and unified communication features.

Implementations differ in the mechanism used to trigger the handoff. WLAN infrastructure-assisted connection management (Figure 3) combines the intelligence of the WLAN and the capabilities of a software agent on dual-mode devices to work together in determining the handoff. The advantage of this implementation is that the handoff between networks is based on the intelligence of the WLAN, and not just on signal strength. Indeed, signal strength alone can lead to unnecessary, short-term switching between networks. Furthermore, this enterprise-driven implementation allows a business to manage the security of the network and secure access of numerous mobile devices.

Figure 3 WLAN Infrastructure-Assisted Handoff

Deployment Options: Enterprise-Owned or Service-Provider-Hosted?

Enterprises today have varied deployment options for FMC solutions for their businesses. Solutions can be owned and managed by the enterprise, or alternatively, businesses can turn to a service provider to provide and manage the solution for them. Some considerations when making this decision include:

Which deployment options fits best with I.T. priorities, and which one will contribute more to profitability?
Does the business have in-house I.T. support staff with expertise in telephony and business communications?
What type of solution is desired--single-number reach or mobile unified communications--and is there a need or requirement to anchor to business infrastructure for regulatory compliance?
What scale of solution is needed: single country, multiple countries, or truly global?
Which type of solution is open for the integration of third-party applications and will accommodate new technology as it becomes available in the future?


FMC solutions are available today to help businesses maintain and increase productivity of their most important assets--people--as they move within and outside the main sites of the business. Overall, FMC solutions can enable enterprises to reduce cell phone costs and improve the productivity and responsiveness of their workforce. What's more, using dual-mode phones and making Wi-Fi and cellular networks available to mobile users can result in more consistent in-building coverage.

Enterprises that already have a telephony system in place can upgrade to FMC technology with an enterprise-controlled implementation. On the other hand, smaller businesses that are not planning to invest in an enterprise voice and unified communications infrastructure might look to carrier-led FMC solutions.

The IMS standard promises to expand today's current implementations to include multimedia applications, which will offer a true FMC experience to users. Customers should implement a FMC solution that is designed to integrate IMS features once they become available.

In the future, context-aware information for business assets and people will also become part of FMC. Open solutions that enable third parties to integrate devices and applications tightly will lead the market. In adopting FMC, enterprises should consider first and foremost the usage patterns and needs of their wireless users.  It is likely that multiple types of FMC deployments will coexist in the future, depending on specific business needs.

Isabelle Guis is Senior Manager, Mobility Solutions, at Cisco.


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