Explain WLAN architecture.

Subject Computer Network
NU Year Set: 3.(b) Marks: 4 Year: 2017

In one of our last segments, we discussed the various WLAN topologies in common use. This segment will discuss the concept of WLAN architectures. While a topology essentially describes the physical configuration of a WLAN, or any LAN for that matter, architecture describes a design concept within which a topology can exist. Network architecture describes the logical relationship between network entities, while a topology describes the actual physical connection required to achieve the logical design. WLANs are described by three broad categories of architectures: autonomous, centralized (controller-based), and cooperative (controller-less).

In an autonomous architecture, access points (APs) are stand-alone (sometimes called "fat") APs that contain all the necessary features and capabilities to operate without any reliance on another device. An autonomous AP operates on all three network planes: management, control, and data. Autonomous architecture allows for several APs to connect to the wired infrastructure and provide a portal for its basic service set (BSA). Multiple autonomous APs can be interfaced to the same infrastructure and form an extended service set (ESS). If all autonomous APs in an ESS adhere to the 802.11 standard, then the APs will cooperate and communicate over the wired backhaul and provide seamless roaming of a client device without any active management. This architecture is suitable for small and small office, home office (SOHO) implementations properly configured to avoid adjacent and co-channel interference. Loss of any AP, however, creates a dead spot in the ESS that would not readily be recovered by nearby APs. Losing connectivity will affect adjacent APs and result in disassociated clients re-associating to an available AP. This will cause a rise in the client/AP ratio and a sudden increase in traffic.

The second type of architecture relies on a centralized controller to regulate the operation of the WLAN. The controller usually takes the form of a hardware device that either is wired to the APs at the network edge, or uses a wireless system to provide local connection to clients on one frequency, while performing control on another. Controller-based APs are referred to as "lightweight" APs and usually operate completely on the data plane. Data forwarding and routing, along with network configuration and management, are done by the controller, which operates on the control and management planes. The AP has diminished functions and is essentially just a radio transceiver in the production area. A lightweight AP communicates with the client devices or other APs, but under the strict authority of the controller. Controller-based WLAN architecture requires a full-time, dedicated controller for the network to function.

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