This week, we look at the hurdles to enterprise WLANs. On Wednesday we covered Management, on Tuesday we covered Quality of Service. Today, we look at roaming and interoperability.

802.11f is not enough
Although the 802.11f standard - already part of most 802.11-based hardware - enables roaming among APs on the same network segment, 802.11 roaming often breaks down as users move across network segments, especially for voice traffic, says Abhijit Choudhury, director of ASIC architecture at chipmaker SiNett.

For data connections, the client can usually get away with using DHCP to get a new IP address with no noticeable downtime, notes Ben Guderian, director of industry relations at SpectraLink. Also, the reauthentication effort during roaming can interrupt connections that are streamed, especially VoIP.

The reauthentication issue becomes especially problematic with the new 802.11i security protocol or the use of RADIUS server authentication, both of which tend to take several hundred milliseconds. "Handoff for voice needs to be no more than 20 milliseconds," HP's Congdon says. So the 802.11r task group is studying faster algorithms and preauthentication to keep authentication time low. "If you have to go back to a RADIUS server every time you need to reauthenticate, there's no hope," Congdon says.

In the meantime, enterprises can use NAT and mobile IP, in which the "home" IP address is static and is redirected to the changing IP address as the device moves from one AP to another, notes Shrikant Sathe, SiNett's vice president of marketing.

But 802.11r is not complete yet
Until 802.11r is complete, however, enterprises will need to use proprietary hardware from vendors such as SpectraLink to get fast roaming for applications such as VoIP, Sathe adds. IT managers should note that wireless VoIP systems use the insecure WEP (Wired Equivalent Privacy) encryption to keep authentication time under the 20-millisecond threshold, Airespace's O'Hara warns.

Mobile interoperability can wait
The least pressing issue facing IT managers is interoperability with other wireless technologies, namely cellular data and 802.16 wide-area wireless. "Interoperability among the three is not an issue right now," says Jeff Orr, product manager for broadband wireless at equipment maker Proxim. "We don't even have the hardware yet."

WiMax is not there yet...
The absence of hardware has not stopped some vendors and analysts from promoting 802.16 as just around the corner. Even Intel, the most aggressive 802.16 chipmaker, doesn't expect to have 802.16 chip sets ready for sale to laptop makers until mid-2006, notes Phil Solis, a wireless analyst at ABI Research. For fixed-wireless 802.16 deployments, Intel doesn't expect to see carrier trials until fall 2005, says Jim Johnson, vice president of the wireless networking group at Intel.

Interoperability with 802.16 should be the simplest to achieve, Orr notes, because it is Ethernet- and SNMP-based, as is 802.11, and will thus support the same security mechanisms and policies. "Policies need to be handled as a superset of what they do for the wireless LAN," he says.

Initial 802.16 deployments will be in fixed-wireless environments, based on the recently completed 802.16d standard popularly known as WiMax, for which the WiMax Alliance industry group plans to certify interoperability. Such 802.16d hardware will be used mainly as a substitute for cable modems and DSL service to connect desktops to the Internet, Wi-Fi Alliance's Hanzlik notes. By 2008, the mobile version of WiMax, based on the still-evolving 802.16e specification, should allow mobile deployments.

...and cellular uses a different model
As WiMax gets off the ground, cellular data technology is already being deployed, with the GPRS and EDGE (Enhanced Data rates for Global Evolution) variations of the GSM technology used by carriers. Cellular and 802.11b are very different technologies at all levels, notes BelAir's Belanger, which makes handoff and billing difficult.

For example, authentication on cellular networks is typically handled by the use of embedded hardware IDs, whereas they are software-based under 802.11. Even more important are the business rules because cellular carriers would need to address roaming from their pay-as-you-go cell networks to largely free 802.11 networks as well as billing for use of private 802.11 hotspot networks such as those offered by Boingo Wireless, SBC, Sprint, and T-Mobile.

The Wi-Fi Alliance has a task group to explore 802.11b/cellular convergence, including billing and infrastructure issues, Hanzlik notes. Standards in this area could take years, Airespace's O'Hara says, adding, "I'm not sure there's been enough time looking at the problem to know what the standards might need to do."