The Dual-Band Option

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By designing a network around a dual-band solution, you can increase the WLAN's flexibility and provide additional user bandwidth by simultaneously supporting both 802.11b and 802.11a wireless networks. Going the dual-band route means that a network manager can deploy additional wireless PC cards and access points (dual-mode, of course) into the WLAN infrastructure without forcing a complete and immediate turnover of technology. Thus, a dual-band solution can protect any investment in existing 802.11b technology, while providing a gradual upgrade path to the higher-throughput 802.11a (or 802.11g) standard when the organization is ready.

Most 802.11b radio cards come in a PC card form factor, so they can be easily swapped out of client devices or access points. This can be an important feature if and when the WLAN is upgraded in the future to higher speed technologies. However, upgrading single-mode access points requires an upgrade of the entire wireless system at the same time. This can be costly; it also requires a potentially lengthy shut down period. Using a dual-band system to deploy a new WLAN obviates these problems.

The benefits of a dual-band network include 802.11b's range and sustainable 11 Mbps data rate being complemented by 802.11a's space-concentrated, 54 Mbps data rate and relief for network executives grappling with long-term migration issues. But 802.11b and 802.11a are not a one-for-one trade off. Mixing these technologies engenders a complexity that requires strict attention to the network's design. This is mainly due to the following factors:

• The propagation characteristics among the radios are very different-802.11b's coverage is far greater than that of 802.1 la (although there are a few exceptions).

• In a dual-AP environment, a network that's designed to accentuate the properties of 802.11b cells could leave large dead zones between 802.11a cells. You might be able to address the problem by lowering the output power of the 11b radio (if the access point allows) so that the coverage areas are concentric, or by supplementing the patchy 11a coverage with single-mode 11a access points.

• For the end-user, there is still some question as to how smoothly different computing devices can transition from one band to the other, especially smaller devices such as PDAs and handheld computers.

• Seamless roaming also might be quite a technical challenge when a network is designed around dual-band equipment. However, this is a problem primarily for applications such as voice over IP, where retransmission is not an acceptable solution to data loss.

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