It's all about the handoff between 802.11b LAN technology and wireless WAN standards.You're in a hotel lobby, using its 11M bit\/sec 802.11b\u00a0wireless LAN\u00a0to download e-mail to your laptop, when your cab arrives. You get in, and your laptop continues downloading, though at a slower rate, all the way to the airport.At the airport, you proceed to a passenger lounge where the transfer rate picks up again thanks to another\u00a0802.11b \u00a0network. All this happens seamlessly, with no indication, other than the change in data rate, that you are moving from a LAN to a WAN to another LAN.You can't do this today because wireless LANs based on IEEE 802.11 protocols can't talk with wireless WANs that use data-over-cellular standards such as\u00a0GSM, General Packet Radio Service (GPRS) and\u00a0Code Division Multiple Access. However, the major wireless carriers, including\u00a0AT&T Wireless,\u00a0Nextel,\u00a0Sprint PCS,\u00a0T-Mobile\u00a0and\u00a0Verizon , are expected to begin supporting such LAN\/WAN handoffs, with the first implementations possibly later this year.Let's take a step back to describe how we got to this point. Soon after the Wi-Fi or 802.11b wireless LAN standard was ratified in 1999, the technology began taking over public access hot spots such as airports and hotel lobbies.Initially, cellular operators saw the new wireless LAN standard as competition. They had hoped to charge by the byte for data services such as text messaging, Web surfing and downloading or sending e-mail on\u00a03G \u00a0networks in which they had invested billions of dollars. They didn't want high-speed public LANs "stealing" that traffic.During the next few years, however, a couple of things became clear: First, 3G rollout and market penetration would take longer than operators had hoped. Second, 802.11 became the overwhelming technology of choice for companies installing hot spots. Operators decided if they couldn't beat 'em, they'd better join 'em.\u2022\u00a0 In December 2001, Sprint PCS invested $15 million in\u00a0Boingo Wireless , which now operates about 1,000 hot spots. Sprint PCS has talked about developing a dual-mode WAN\/wireless LAN card and a service to take advantage of it.\u2022\u00a0 In January 2002, VoiceStream Wireless, now T-Mobile, purchased bankrupt MobileStar Networks, which operated hot spots at Starbucks locations. Today, the rebranded T-Mobile HotSpot service lets customers connect in about 2,000 locations such as Starbucks, Borders bookstores, airports and airline clubs. Customers also can use the GSM\/GPRS WAN when no LAN connection is available.T-Mobile also has announced plans for an integrated GPRS\/Enhance Data Rates for Global Evolution\/802.11b service offering, expected to arrive sometime in 2003, that would let customers switch manually between Wi-Fi and cellular.\u2022\u00a0 In April 2002, Nextel Communications announced that it was testing technology from\u00a0RadioFrame Networks\u00a0that would let enterprise customers receive cell phone calls indoors via a software-defined radio (SDR) system designed to combine multiple cell phone networks with 802.11 networks.And technology is emerging to support the merged WAN\/LAN vision. Nokia has announced a PC card that incorporates GSM\/GPRS and 802.11b. In addition, you can create a dual-mode platform by adding a GPRS card to a laptop with built-in 802.11b.However, there are still problems. For one thing, even though wireless carriers have bowed to the inevitability of public access 802.11b, it's still not clear how well the wireless LAN fits into carriers' business models.Wireless LANs are less expensive than WANs, if only because they run on unlicensed ("free") spectrum. Accordingly, hot spot operators charge much less per megabyte than wireless carriers. The result is lower revenue, even for carriers that get into the hot spot business. And there's a limit to how much carriers can lower data prices on WANs, because too much data could reduce their ability to carry voice traffic - their bread and butter.In addition, it's going to take a lot more than the right hardware in a laptop to enable seamless wireless LAN\/WAN roaming. There are difficult issues involved in switching the TCP\/IP sessions between networks operated by different companies, says Christian Gunning, director of product management for Boingo."With built-in 802.11b, you'll only really need a WAN card," says Fran Rabuck, president of Rabuck Associates, a mobile consultancy. "How or if the handoff will work between the services is a really big question."That's one area where T-Mobile has the advantage of both being the largest hot spot operator and owning a nationwide GSM\/GPRS network, Rabuck says.T-Mobile likely will phase in wireless LAN\/WAN integration, says spokeswoman Kim Thompson, with some level of integration taking place this year, although probably not transparent handoffs.T-Mobile might find a worthy competitor in the recently formed\u00a0Cometa Networks, a joint venture of AT&T, Intel and IBM (along with Apax Partners and 3i), established to provide wholesale nationwide broadband wireless Internet access, Rabuck says.Two technologies to watch as likely enablers of transparent handoffs are Mobile IP and SDR. The Mobile IP protocol is designed to handle handoffs at the network layer (Layer 3), while leaving the transport layer (Layer 4) and above unaffected, so that the existing routing infrastructure, nonmobile hosts and applications don't have to change.SDR operates at the data link layer (Layer 2), which is responsible for link establishment and maintenance. Because wireless LAN and wireless WAN technologies are radically different at Layer 2, a device is required to tie them together. SDR is a good option.