Understanding WLANs: Architecture 101 - Page 2

Shifting back

Those very early APs did everything on their own—in some cases, even using peer-to-peer protocols to make control plane decisions. The subsequent evolution from Wireless Gateway to Switch to Controller relocated tasks from all three planes—often making the Switch or Controller an active data plane participant. However, we now see some previously-offloaded tasks shifting back towards the WLAN edge—or onto separate upstream systems.

Hardware advances have made it possible to build more powerful APs at lower cost. Much of that additional CPU and memory has been directed towards internal data plane improvements—notably, MIMO antenna management and advanced signal processing techniques associated with 802.11n. But some products have also used more powerful chipsets to reabsorb control plane functions. Examples include the on-board firewalling implemented by certain “adaptive APs” that can be used with or without a Controller (e.g., Motorola, Aruba) and the “cooperative control” protocols used by mesh APs that can reconfigure themselves in response to changes (e.g., Aerohive, Colubris).

On the flip side, WLAN management plane functions are moving out—and not just away from the data path, but into systems that look beyond 802.11. Management duties that are unique to 802.11 may always fall to WLAN-specific products—for example, channel (re)assignment for interference avoidance or availability. But the writing on the wall is clear: enterprises want to see consolidated network management for common tasks like configuration backup and firmware update. Companies like Proxim and Motorola seem to be focusing on management plane integration across different kinds of wireless, while HP and Cisco are motivated to integrate 802.11 and Ethernet network management.

Contemporary battlegrounds

In this year’s State of the WLAN Market report, Webtorials asked respondents which WLAN architectures they currently used and planned to adopt within the next six months. The results, published in October 2008, show a clear preference for some degree of centralization:

•  47% use thin APs with a central Wireless Controller
•  39% use “intelligent” APs with a central Management Server
•  18% use “intelligent” APs without centralized management
•  17% split intelligence between APs and the Wireless Controller or Switch

However, note the majority interest in “intelligent” APs—a less pejorative term applied to APs that are more fat than thin since they are not wholly-dependent on a Wireless Controller. Given that management plane functions have largely moved elsewhere, today’s “intelligent APs” are primarily performing some combination of control and data plane functions.

For example, consider the growing popularity of AP-to-AP packet forwarding. This may be done based on QoS/security settings to reduce latency and avoid bottlenecks at the data plane—concerns that have increased along with the speed of 802.11n WLANs.

Alternatively, APs that normally forward traffic through a Controller may revert to autonomous operation (that is, adapt) whenever upstream connectivity is lost. Adaptive APs can appeal to smaller remote offices and retail stores where even an entry-level Controller may be considered too expensive to install or maintain locally.

Packaging can also play a role in WLAN architectures. For example, each Xirrus “Wi-Fi array” is a large circular device that combines up to 16 APs with an integrated Controller, firewall, and WIPS, using a sectorized antenna system to divvy airspace.  At first glance, you might be tempted to call this a very fat or intelligent AP – but we consider it to be the extreme in tight coupling between thin APs and a co-resident Wireless Controller.

Different strokes

In fact, the rigid hierarchy that emerged along with Controller-based WLANs also appears to be in jeopardy. Answering that same Webtorials survey question, 30% of respondents reported using Wi-Fi mesh networks, while 7% used single-channel or channel blanket architectures.

Like those old ad hoc WLANs, mesh networks involved peer-to-peer communication—not between clients, but between APs. Today, most new enterprise-class 802.11n APs can participate in mesh networks for high-speed wireless backhaul—especially appealing in hard-to-wire locations. Many of those mesh WLANs must still be configured into a static hierarchy, leading back through a root AP and perhaps a Controller. However, some intelligent APs can now converse with peers to form dynamic meshes and/or optimize traffic flows between themselves, without requiring a separate Wireless Controller.

Within the data plane, Meru and Extricom are challenging the way in which the airwaves have long been divvied between adjacent APs. In most enterprise WLANs, all APs participate in the same Extended Service Set, but each uses its own channel. APs can thus avoid stepping on each other so long non-overlapping channels are assigned. In channel-blanket architectures, APs share a single channel by relying on a Wireless Controller to coordinate airtime use. Because this side-steps co-channel interference altogether, it appeals to high-density WLAN deployments.

The take-away

Clearly three are many different ways to slice and dice WLAN functionality. What will WLANs look like three to five years from now? Further integration seems likely, as hardware grows more powerful, Ethernet dwindles at the edge, high-speed wireless WANs grow ubiquitous, and enterprise focus shifts away from infrastructure deployment to application service delivery.  WLAN vendors will no doubt continue to tweak and twist their offerings to wring the very most out of hardware and spectrum. But don’t become so distracted by individual functions that you overlook how well any platform fits into your own network's data, control, and management planes. After all, architecture is what makes any network more than the sum of its parts.

• For more on understanding WLANs, read "How to: Choose the Right Spectrum for 802.11n Deployments," "Build A Linux-Based Wireless Access Point," and "DD-WRT Tutorial 4: Defining Priorities with QoS." 
• For more by Lisa Phifer, read "Retailers Need to Shore Up Defenses," "Meru Debuts 'Virtual Port'," "RF Barrier Helps Deter Eavesdroppers."
• For more helpful articles on a variety of Wi-Fi topics, visit our extensive Tutorials library.

Lisa Phifer owns Core Competence, a consulting firm focused on business use of emerging network and security technologies. A 27-year industry veteran, Lisa has been involved in Wi-Fi training, product evaluation, network deployment, and security assessment since 2001.