Build wireless LANs that last: access points can be designed to work in adjacent groups without co-channel interference.

Wireless LAN (WLAN)adoption in enterprises began in earliest with the emergence of low-cost 802.11 b access points (APs). By 2003, the aggregate demand for 802.1 l b access led industry leaders to begin including 802.11b wireless clients in business laptops and PDAs. Today, there are more than 50 million 802.11b client devices in the world, and the prevalence of so many clients has spurred more large organizations to deploy 802.11b WLANs.

With maximum throughput of 11 Mbps and a foundation in shared media (wireless frequency), however, 802.11b lacks the bandwidth and quality of service to support large groups of users, particularly when organizations introduce delay-sensitive applications like voice over IP (VoIP). As a result, early-adopter companies are now planning to overlay initial 802.11b networks with 54-Mbps 802.11a and 802.11g networks.

If each new set of applications or increase in user density requires companies to overlay a new WLAN infrastructure on top of existing ones, managing wireless LANs may soon become a costly and difficult problem.

The 802.11 standard was designed to support a wireless LAN composed of a single AP linked to a wired Ethernet network-essentially, the AP would look like any other Ethernet MAC device (such as an Ethernet card in a desktop system), with its own IP address. The AP continually broadcasts its signal, any wireless client within range can choose to lock onto that signal in order to make a connection (or "associate with") the AP, and the AP treats all client requests equally.

Based on preconfigured access-control mechanisms, such as wireless equivalency protocol (WEP), Wi-Fi protected access (WPA) or 802.11x, a given client may be allowed or denied access when it tries to associate. If several clients are trying to gain access to the same AP at the same time, standard collision avoidance algorithms cause all requesting clients to back off and retry their access requests. Collision avoidance is why most vendors recommend no more than five or six clients per AP, and why throughput drops off" significantly when more clients are present.

CLOSE DEPLOYMENT A PROBLEM

In a corporate network composed of multiple APs based on commodity AP technology, each AP still functions as a separate entity, and the APs on a network are not aware of one another. This is not a problem if a corporation is deploying widely separated APs to create isolated Wi-Fi hotspots, but it becomes a problem when APs are placed side by side in a pervasive deployment.

In such a deployment, the idea is to create an unbroken blanket of wireless coverage. When companies arrange APs close enough to provide continuous coverage, however, each AP generates signals that interfere with neighboring APs, causing what is known as co-channel interference. In this scenario, two or more APs transmit packets at the same time on the same channel, corrupting packets and causing clients to experience transmission delays and lower performance.

In addition, clients located equidistant between two APs may flip-flop between accessing one and the other. Finally, since APs have no ability to prioritize incoming client requests, a high-priority VoIP client has the same chance of access as a low-priority e-mail client.

To minimize channel interference, most WLAN vendors recommend that users operate adjacent APs on alternating channels (e.g., 1, 6 and 11 in 802.11b). Sophisticated site-survey and AP-management tools help users plan such alternating AP deployments. These systems may also allow users in regulate the power output of individual APs to make their coverage areas larger or smaller, and so fit better into an overall coverage map.

These measures do not eliminate the co-channel interference problem, and do not increase the capacity or quality of service offered by the WLAN. Lowering an AP's power output can exacerbate the problem by increasing signal-to-noise ratios that can also cause packet corruption.

One current solution is to overlay 54-Mbps 802.11g or 802.11a networks, and then assign certain groups of employees or applications, like Vole to these new WLANs. Doing this may require forklift upgrades of equipment and may make the job of managing the overall WLAN infrastructure more difficult.

A DESIGN ALTERNATIVE

Rather than using APs that create co-channel interference and cannot prioritize client requests, and then giving customers tools for arranging them so as to mask the problem, standards-compliant APs can be designed to work in adjacent groups without co-channel interference. This approach offers several benefits for performance, deployment simplicity and migration.

Performance. In a future-proof WLAN architecture, all of the APs are aware of one another, as well as all of the clients. As in a cellular phone network, the APs and the network infrastructure collaborate to decide which client is best served by which AP (rather than passively waiting for random client requests to connect), and the APs are coordinated and balanced to maximize quality of service.

For example, when one AP looks like it will be overloaded by application or user density demands, the WLAN system can simply route some of the clients to a neighboring AP. In addition, the coordinated infrastructure can recognize and prioritize client access based on application or user level, so as to ensure guaranteed quality of service for VoIP or other important traffic.

Deployment simplicity. Rather than having to alternate AP channels, all of the APs in such a system can operate on the same channel, because the WLAN switch completely manages every AP's interaction with every client. As a result, users do not have to pay for RF analyses or site surveys, or worry about coverage gaps--they can simply deploy APs as needed to create blanket coverage.

Migration. With co-channel interference eliminated, upgrading APs is also possible, migrating from 802.11b to 802.11g or 802.11a without the need to overlay a new WLAN infrastructure. Instead, the entire WLAN can operate on a single set of dual-radio APs and support 802.11a, b or g without worrying about the distance between APs or the RF channel and power settings of every AP and client-and without affecting performance of higher-speed clients in the presence of lower-speed clients.

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Uses of Wireless LANs
(Percent of respondents rating 'Very Important' or 'Critical')

        Creation of specific coverage zones within a site  45%
                              LAN extension/user mobility  42%
                          Building-to-building connection  23%
                                Replacement for wired LAN  23%
                                    LAN-to-WAN connection  21%
Reduction of number of wireless LAN "rogue" access points  20%
                                     Backup for wired LAN  11%
                          Enabling wireless voice over IP  11%

Source: Infonetics Research 2004 survey

Note: Table made by bar graph.