While we acknowledge this as pure fiction, the dream of unfettered communication is becoming more of a requirement in today's business world. Being mobile is more the norm than ever before. Use of mobile handsets to meet enterprise mobility requirements seems to be a partial answer, but only addresses one part of the overall requirements: off-campus connectivity. However, there's no seamless connectivity with the corporate information systems and often there's limited cellular coverage inside office or public buildings.

The advent of enterprise-wide Wi-Fi adoption has brought about the possibility of delivering a technology that bridges wireless carrier connectivity with on-campus WLAN connectivity—Fixed/mobile convergence (FMC). This term was coined by early mobility advocates of providing seamless bridging between Wi-Fi and cellular networks. These technologies, now in pilot testing, extend cellular connectivity to places where cellular coverage is weak or non-existent. Being able to begin a phone call in the wireless network and continue that call when traversing network boundaries will provide true mobility. Such evolving technologies bring the hope of mobility to users, but are not always adequate in addressing the full range of mobility requirements.

FMC alternatives
FMC solutions, in principle, all propose supplying voice and data services based on seamless integration of cellular and wireless packet-switch technologies/services. All of the announced FMC solutions fall into one of two general categories, hosted either by fixed, wireless, or hybrid carriers (carrier-centric), or by enterprises (enterprise-centric). The major difference between these two approaches is where the application control point resides, and results in functionally distinct solutions.

Carrier-centric solutions view Wi-Fi networks as a potential transport mechanism for traffic roaming between fixed and cellular networks, with application control remaining with the carrier/service-provider network (Fig. 1). At its design core, an enterprise-centric solution presumes that the Wi-Fi and wired-network services are a single logical resource under the control of the enterprise, with application control being retained within the enterprise. Such a design allows for a more "native" architecture to be implemented where voice-over-IP (VoIP) can be supported as part of a solution complementing the mobility capabilities of the WAN component.

The enterprise deployment of Wi-Fi, the acceptance of VoIP as a viable technology, and the availability of dual-mode (Wi-Fi/cellular) devices are the key events that have accelerated interest in enterprise mobile-to-mobile market demand. Because these products are required to support business-class telephony, current enterprise-centric solutions are often referred to as premises-based designs in which equipment is configured within the enterprise-managed network behind a PBX or iPBX (Fig. 2).

The benefit of an enterprise-centric mobile-communications product is that it affords important mobility functionality while the enterprise retains application control. Control over resources and data are vital and this fact drives the enterprise buying decision. Therefore, a successful enterprise-centric product must have support for manageability and policy definition and enforcement. In addition, integration with the enterprise PBX/iPBX is a key value-add resulting in a single voicemail design and functional integration of the mobile units with traditional telecomm desk-sets. Developing and deploying an enterprise-centric mobile-communications product is complicated by the need for many key and disparate requirements (e.g., security, mobility, QoS, etc.) to be provided as part of the total solution.

Completely agnostic
A successful enterprise mobility solution must be client agnostic (work with any mobile handset), network agnostic (any enterprise, mobile, or Wi-Fi network), and PBX agnostic (any phone system already deployed by the enterprise). Dual-mode clients are just coming on the market from multiple vendors. Traditional cellular providers have announced and are shipping such devices but without support of mobile-to-mobile roaming. The platform designs are diverse from an operating-system perspective and include products based on Microsoft Windows Mobile, Symbian, and Linux. Any commercially successful solution will be available on multiple platforms.

To meet the mobility goals required by today's enterprise, a successful solution must provide the same feature set for both Wi-Fi and cellular networks. Seamless roaming across the two networks is basic table stakes, which means support of IEEE 802.11a/b/g networks, as well as support of GSM and CDMA cellular network devices.

Seamless roaming across diverse networks affords little value unless it's coupled with a critical enterprise application. Telephony is a life blood of enterprise. The ability to interconnect with a corporate telephone communication system while mobile is a strategic business requirement. Thus, an enterprise mobility solution must also integrate with a wide range of PBXs and iPBXs, as the vast majority of enterprises have chosen to host their own telephony services through these systems.

Mobile convergence appliances
A mobile convergence appliance (MCA) is a new product category designed to satisfy enterprise mobility requirements and remain under enterprise control. An MCA works in conjunction with a mobile handset to provide a seamless user experience. Collectively, the MCA and mobile client collaborate to manage network agility, using logic implemented across OSI layer (Fig. 3).

In the MCA approach, standard Wi-Fi drivers at the media access control (MAC) layer are responsible for managing smooth access point-to-access point roaming and for enforcing the appropriate wireless security policy: WEP, WPA, WPA2, or other. Currently, there are still holes in the IEEE 802.11 service fabric that must be addressed to define fast-roaming (802.11r) and traffic load-balancing (802.11k and 802.11v) within the Wi-Fi space. Without these standards in place, mobile solutions must be optimized through proprietary IP implementations.

Establishing and maintaining a phone call across two dissimilar networks is a balancing act in monitoring the viability and health of the networks to determine which will provide the most reliable connection and best voice quality. For calls established over Wi-Fi, the MCA implements the Session Initiation Protocol (SIP, or IETF RFC 3261), the dominant VoIP standard. SIP affords great flexibility for integration and functionality options.

For calls utilizing a cellular network, the MCA takes advantage of the base functionality, which guarantees the best voice quality yet provides the control elements needed for traversing the networks. It also assures compatibility with both GSM and CDMA networks.

Traditionally, VoIP systems don't couple signaling with the audio streams. But to manage seamless roaming and provide value-add functionality, both signaling and audio must be managed together. The MCA constantly monitors call state and audio quality to make a decision on the best transport network to enable.

Built in to the MCA basic design is the concept of device and application management. Being able to define policies that can be applied to devices even when they're in a cellular coverage area is powerful. Utilizing these features, an enterprise can easily manage its cellular use, optimizing its communications ROI while simultaneously leveraging the WLAN infrastructure for even more efficient utilization.

Implementation of seamless roaming across diverse wireless network while sustaining a phone call requires synchronization and access management of both the originating and target networks, all while maintaining unbroken audio flow to the speaker at each end. Achieving this goal requires mastery of a number of technologies and an overriding architecture that marries these diverse technologies to achieve end-to-end reliability.

About the author
Richard Watson is the director of product management for DiVitas Networks. He contributes regularly to trade journals writing on leading edge topics on wireless VoIP and frequently participates on panels at industry shows. Watson can be reached at Richard.watson@divitas.com.