Inter-network Roaming

By Gerry Blackwell

June 28, 2002

Roaming between cellular networks and wireless LANs may be some time off for the every day user, but police departments are already finding ways to make it work.

Seamless roaming between 802.11 WLAN hotspots and mobile wireless network cells is one of those ideas that has a very high cool factor.

For example, imagine a situations in which you are downloading your e-mail to a laptop or handheld at a Wi-Fi hotspot, for example, and are suddenly called away. You wouldn't want to interrupt the transmission and start again later. With wireless LAN-WAN roaming, you could walk away and keep downloading. The connection would automatically be handed off from the hotspot to your mobile network.

GSM service provider VoiceStream Wireless has already said it intends to exploit this kind of nice-to-have application. It will develop wireless LAN-WAN roaming services over the next six to 18 months in partnership with sister company T-Mobile International, which owns the assets of defunct hotspot network provider MobileStar.

While we wait for compelling horizontal applications of WLAN-WAN roaming to drive the marke, there are already strong cases for it in some vertical markets. It is already being used, for example, in the public safety arena.

Padcom, a Bethlehem, PA-based wireless data systems integrator, has developed hardware and software that it deployed first last year for the Baltimore Police Dept.

Officers in 140 of the department's 320 cruisers now carry ruggedized laptops connected to a Padcom device in the car that switches them seamlessly between Verizon's CDPD (Cellular Digital Packet Data) network and any of nine Wi-Fi hotspots at station houses around the city.

As soon as a Baltimore P.D. cruiser pulls into a station house, the Padcom system switches it to higher-speed Wi-Fi access via the local hotspot. When it moves out of range of the hotspot, it switches back to CDPD.

"The primary complaints about wireless data networks are the lack of bandwidth or the lack of coverage," says Padcom director of marketing Mark Ferguson. "To get bandwidth, you're going to have to give up coverage, to get coverage, you have to give up bandwidth. It's always a trade-off."

With the Padcom solution, users still don't have high-speed access everywhere, but they do get ubiquitous coverage, and they can also have high-speed access wherever possible. An organization designing a wireless network doesn't have to make an either-or choice.

The department implemented the system to replace a proprietary RF network from Motorola. Officers in the cruisers used the old system mainly to communicate by voice with dispatchers. The dispatchers accessed data -- checking license plate numbers on the National Crime Information Center (NCIC) database when an officer stopped a car, for example -- and relayed it to the cars by voice.

There were a number of problems with the status quo. Dispatchers were having to take too many calls. They were under constant stress, jeopardizing effectiveness and threatening burn-out.

The department realized it would be faster and more efficient for officers in the cars to directly access the information themselves -- if they had the bandwidth. The Motorola system offered data throughput of less than 9.6Kbps, a fifth of most dial-up modems.

The Motorola system also provided coverage only within the Baltimore P.D.'s jurisdictional area. In an age of increased inter-agency co-operation, when officers have more occasion to move outside the city's boundaries, it was essential for their safety that cruisers be able to stay connected, says department MIS manager Lt. Craig Meier.

So the first decision was to go with CDPD for wide area data communications. It provides the coverage needed, but CDPD is still slow -- 19.2Kbps or less.

The department wanted the cruisers to be like offices on wheels. In the past, Meier says, patrol car officers would spend as much as a couple of hours of each shift doing work at the station that could not be done in the cars or could not be done easily because of communications limitations -- writing and uploading reports, for example, downloading mug shots.

"The new system enables them to do more work in the car and stay on the street longer rather than having to come in to the station all the time," Meier says.

Plus, if the officers were going to start using laptops, the IT department wanted to be able to update the software on them over a network so they didn't have to bring them in from the field each time. They would need higher-bandwidth network access for that as well.

The combination of high-speed Wi-Fi hotspots and CDPD coverage offered by the Padcom system fulfilled all of the department's objectives.

The hardware device, mounted in the trunk and connected to the laptop via Ethernet cabling, houses switching equipment and software, as well as CDPD and 802.11b network cards.

The client software detects when anything changes in coverage status and automatically switches to whichever network is available and best suited. It also ensures that the in-car device has the same IP address no matter which network it's using.

Padcom also provides software for a host system at the department's communications center. It does the same thing for traffic originating from the host side.

The Baltimore P.D. was naturally concerned about security at the Wi-Fi hotspots. Padcom used 802.11b gear from Cisco Systems. It implemented Cisco's advanced secure access system and also uses end-to-end encryption.

The cost of the Padcom switching software works out to about $500 per client unit, Ferguson says. That includes both host and client software, but does not include the proprietary in-trunk devices Padcom sold the Baltimore P.D., nor the network access cards.

The Padcom hardware is not essential, Ferguson points out. The software can run on the laptop and the access cards can go in the laptop's PC card slots. The Baltimore police were investing in ruggedized laptops and did not want to reduce the ruggedness by having easy-to-break wireless transceivers sticking out of the card slots.

The Baltimore P.D. went live last September using the Padcom solution. It's still early days, but Meier is convinced it is already having exactly the impact the department wanted. It will be expanded in the near term to include over 200 squad cars, and eventually the whole fleet.

Padcom, meanwhile, is implementing the technology at other police departments. As in most things wireless, California is the hot market. The company has installed systems in Oakland, Malpitas, Santa Ana and San Leandro, as well as in Tucson, AZ.

"At this point," says Ferguson, "it's mainly a public safety market, but enterprises are becoming aware of the advantages."

The fact that Nokia is scheduled to introduce a dual-mode (GPRS/Wi-Fi) wireless network card later this year could fuel interest in the concept, Padcom believes.

Ferguson admits he sees no real "killer apps" as yet, but colleague Chris Bogdon, the company's director of product management, believes voice over IP could eventually become a major driver for wireless LAN-WAN roaming.

As VoiceStream has hinted, connecting via a Wi-FI hotspot will likely be cheaper than using a mobile network. So there would be a benefit for business travellers in being able to roam into Wi-Fi hotspots while talking on their cell phones and thereby reduce their costs.

Bogdon speculates on the possibility of a new breed of tri-mode IP telephone. It could plug into the wired office system, work unplugged on the Wi-Fi WLAN allowing employees to stay in touch while roaming around the building or campus. Then it could also seamlessly hand off calls to a wide area network when they moved out of range of the WLAN.

It still sounds nice-to-have, but is fairly far off in the future as well. In the meantime, though, there may very well be other vertical market applications outside public safety.

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