Wouldn't it be nice if you had a dual-mode phone that let you talk via voice over Wi-Fi when you're in the office, warehouse or hospital, and then seamlessly switched over to cellular when you were outside the coverage area of your wireless LAN.

That's the promise of new converged or hybrid services that will become available sooner than you think. The benefits are increased mobility, productivity and convenience, not to mention some cost savings.

Of course, there are trade-offs. This hybrid service means replacing a public switched telephone network (PSTN) service with a VoIP service running over a broadband Internet connection.

Compared with VoIP, reliability and call quality are both superior on the PSTN, while the calls are fairly inexpensive. Moreover, indoor coverage can be spotty, making many users wary of abandoning that fixed-line phone altogether.

And without a reliable system for LAN to WAN handoffs, many calls initiated on the cellular network will remain on that network for the call's duration, regardless of whether there is a better WLAN signal available.

The question for the IT executive is whether it's in the company's best interest to trade away some degree of reliability and call quality for cost savings and mobility. For now, it's a tough call, but as VoIP becomes more reliable and as the bridging technology between these networks becomes more sophisticated, the scales will tip in favor of converged services.

The future isn't that far away

Although most carriers have shown interest in fixed-mobile convergence, one carrier actually has begun deployments.

T-Mobile offers the dual-mode iPaq H6315, which lets users switch between GSM/General Packet Radio Service (GPRS ) and Wi-Fi networks as they travel. "The device automatically notifies you as you enter a Wi-Fi hot spot and switches to the fastest network available, allowing you to maintain your Internet session as you travel from your home, to Starbucks, to the airport, to a business meeting and to your hotel," says Todd Achilles, director of handset product management at T-Mobile.

According to Achilles, the current GSM/GPRS network provides wide-area coverage for applications to which customers want constant access, such as e-mail and calendar, while users can turn to broadband hot spots when they need to access larger data files.

On an even more ambitious scale, Avaya recently teamed with Motorola and Proxim to develop a fixed-mobile offering targeted at corporations. All you need is a dual-mode phone from Motorola, Session Initiation Protocol (SIP )-enabled IP telephony software from Avaya, and a voice-enabled WLAN infrastructure from Proxim. Trials are underway, with general availability slated for this spring.

Within the enterprise, Motorola's dual-mode phone connects with a Proxim WLAN access point and functions as a VoIP phone. As a user moves out of the office, the phone acts as a GSM cell phone. A wireless gateway jointly developed by Proxim and Avaya manages the handoff between the two networks, while Avaya's SIP-based IP telephony software pushes features commonly associated with desk phones, such as conferencing, out to the mobile handset. The handset also can access data applications on both networks.

According to Frank Lovasco, mobility solutions practice leader at Avaya, cost savings will exist, but cost won't be the biggest draw initially. "In terms of what will motivate an enterprise to make this switch, you have to think in terms of business continuity," he says.

Early adopters will be high-value users such as executives, doctors, lawyers and salespeople. Saving airtime minutes isn't a big deal to them - they already buy big buckets of minutes - but having a single point of contact is a big deal. Added to that is a converged service that pushes important features such as conference calling from the desk to the mobile phone, meaning that these users are more productive when on the move.

The new service "allows you to access all of your important applications, be they voice or data, from a single device," he says, noting that Avaya's SIP-based software adds features such as presence while simplifying a user's life beyond just the subtraction of devices. "You now have one point of contact, one phone number where you can be reached all of the time, as well as a single voice mail box," he says.

The seams are still visible

These two early offerings still have some wrinkles to be worked out.

As of now, billing is not centralized. Users still would pay a carrier for the cellular plan, while the VoIP calls within the company would be rolled in with the corporation's telephony plan.

Also, these services don't shield users from the underlying networks. In other words, while T-Mobile's service informs you when a new network is available, you must finish your session and reconnect with the new network. With the Avaya/Proxim/Motorola offerings, the corporate network is the only network where you get guaranteed Wi-Fi connectivity. Currently, the solution does not integrate hot-spot access.

For fixed-mobile convergence to really fly, it needs is the seamless handoff between various Wi-Fi networks and cellular networks, with users maintaining their sessions and with the underlying networks essentially invisible to them.

"There is still a lot of work to be done to improve the handoff capabilities," says Phil Solis, a senior analyst at ABI Research. Unlicensed Mobile Access (UMA) "could help, but in the end what networks you can roam to might be determined by your carrier and the bundle you sign up for."

UMA technology is a set of specifications for linking cellular networks and unlicensed spectrums such as 802.11 and Bluetooth .

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