Improving wireless handoffs

Is there any way to improve the effectiveness of a roaming handoff when the signal is dropping on the associated access point while there is an adjacent access point with a stronger signal?

When the 802.11 protocol was first developed, the prevailing opinion was that only the client could know which access point is the “best” option to deliver wireless services. For most clients, this means 802.11 operates as a “bug light” protocol, where the client will “fly” to the brightest light (the strongest signal) it can see. Most clients won’t look for a new light until they can’t see the old light any more. This results in the behavior implied by your question, in which a client will hang on to an access point far too long, causing lower throughput, greater bandwidth consumption, greater interference and generally poorer performance in the WLAN.

This problem has been tough to solve in traditional WLAN implementations because access points work as stand-alone devices. Without a holistic view of the wireless LAN-scape, it is difficult to make intelligent handoff decisions between individual access points.

Some vendors address this by using proprietary extensions to the 802.11 specification, letting wireless clients make more intelligent decisions based upon known traffic load, signal quality and other access point performance metrics. In this environment, the access point is still a passive device, with the client making all its own decisions. Because there is more information available to the clients, the decisions can be more informed. The drawback, however, is that proprietary software is required on both the access point and client. And because decisions are in the hands of the client, this may not be in tune with the goals of the network (or the enterprise).

Newer products aim to solve this by placing RF intelligence on a centralized WLAN switch or appliance. These devices supervise all the access points in a wireless LAN, providing a systemwide view of the entire network. This way, the network can play an active role in handling roaming clients. For example, a switch or appliance can monitor signal strength and quality across several access points, the number of clients associated at each access point, and the load applied to each access point. Based on this information, individual access points can be told whether to accept or deny new client associations, as well as force clients to re-associate with a better device. This makes roaming more effective, without requiring proprietary modifications to the 802.11standard.

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Copyright © 2003 IDG Communications, Inc.