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Outlining the basics of WLAN communications

By Craig Mathias, Network World, 10/1/07

Wired LANs are made out of a reasonably small set of common components – switches, routers, and gateways. It's easy to assemble the right set of these components for a given solution, and deploy that solution with a reasonable assurance that performance objectives will be met. Wireless LANs follow a similar philosophy, but with a somewhat greater degree of variance primarily relating to where in the network particular functions are located, and how traffic moves through the wireless LAN itself.


At the edge of a wireless LAN are access points (APs), which function as bridges between a wired network and (typically mobile) client devices. The functionality inherent in a specific AP can range from little more than a radio and an antenna to the ability to route traffic across subnets. APs are typically interconnected via Ethernet cabling, but can also relay data between them over the air using wireless mesh techniques. This capability can be useful in difficult-to-wire or temporary installations, and is also now being used as a reliability mechanism as well.


The key differentiator in the design of APs is how much local intelligence they possess. Before the introduction of the wireless LAN switch in 2001, APs were atomic network elements, separately provisioned and managed. The WLAN switch centralized common functions, like security and management, into a single location which resembled an Ethernet switch. The switch is less common today and has largely evolved into the wireless LAN controller, using intermediary switches for interconnection and PoE, but again centralizing common traffic-flow functions. Controllers are usually designed to work in together to allow scalability across large geographies and for fault tolerance.  Some can also take on management functions.


With so much architectural variability, it's often useful when analyzing WLAN architectures to think in terms of three planes describing key functions, as follows:


  • The data plane describes how traffic flows from the AP to other nodes in the network. Some APs must be connected directly to a wireless switch, while others can communicate with a wireless switch or controller over an IP connection – Layer-2 vs. Layer-3. An interesting approach today is the ability of some APs to directly forward traffic to a destination without going through a centralized controller, which some vendors claim will yield meaningfully-higher performance.


  • The control plane is responsible for the real-time control of APs, which can include when a particular AP transmits or receives and which client node will receive attention next. This function can be distributed, whereby each AP makes its own decisions, or centralized, where a controller handles this task. Security can also be centralized or distributed in each AP. The decision of how much control to locate where will be the key architectural differentiator going forward.


  • The management plane handles configuration, monitoring, reporting, exception handling, and other functions common to network operations. All enterprise-class WLAN systems are designed around centralized management, as having to independently manage each AP would become unwieldy beyond a small number of nodes. This function is often resident in a server, or, increasingly, in an appliance capable of managing even a very large distributed WLAN installation.


Farpoint Group believes that the future of enterprise-class WLAN systems lies in distributed data, centralized management, and both centralized and distributed control. It's now clear that purely "thin" or "thick" AP cannot be a universal solution, and the ability of an AP to adapt to changing conditions, perhaps even switching to mesh mode when a wired backhaul link fails, is the key to flexible, reliable, mission-critical enterprise WLAN installations. But the degree of architectural variability is in fact likely to increase before any convergence takes place; it's still fairly early, after all, in the history of this technology.


There is, thus, today no single enterprise Wi-Fi architecture today that can claim the title of best. Part of the reason for this is that it's very difficult (and often very expensive) to characterize performance and do comparative benchmarking analysis, especially considering the impact of radio artifacts (fading, interference, etc.) and the high degree of variability in traffic loads, duty cycles, volumes, and mixes. This situation is slowly improving, however, with the development of new performance-analysis tools, and we'll gradually see over the next few years, enough success stories to at least generalize on the best approach for a good variety of applications.



This story, "Guide to Enterprise Wireless LAN Systems" was originally published by Network World.

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