A key selling point of dual-band cards is their ability to roam seamlessly between the 11a and 11b networks. To test roaming, I configured a Proxim Harmony 8570 802.11a AP and the Cisco AP1200 802.11b with identical SSIDs (service set IDs) and set the card to work in auto-wireless mode. I collocated the two APs above the dropped ceiling, and used the continuous ping test to check for the packets dropped and the time required to roam between the systems as I walked further and further from the APs.
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Vendor Info
NetGear WAB501 Dual Band PC Card, $180. Netgear. (408) 907-8000. www.netgear.com
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As expected, the combo card associated with the Proxim AP provided the best available data rate. As I walked away from the APs, the data rate fell from 54 Mbps to 6 Mbps until maximum 11a range was reached. At this point the card lost association with the Proxim AP and started to scan for other available networks and dropped many packets. After a delay of 12 to 15 seconds, the card switched to the Cisco 802.11b AP. The card maintained its association with the 11b AP even as I walked back into range of the 11a AP. In fact, I had to force the card to associate to an 802.11a network and then force it to automode to reinitiate roaming. Netgear and Atheros told me this behavior is normal. Rather than incur the session disruption that would occur if it roamed back to the 11a AP, the system is designed to maintain connectivity with the 11b system.
Roaming will pose an issue for time-sensitive applications, but the basic functionality of this dual-mode NIC is impressive. For organizations taking a long-term approach to wireless, it's worth a little extra money to get dual-mode capabilities in a single card--and you don't have to sacrifice performance.
Alireza Dehghanpur is a research associate at the Center for Emerging Network Technologies at Syracuse University. Write to him at [email protected].
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