Comparing Centralized and Distributed Management

Historically, WLANs have been approached in a manner similar to wired networks. Because the access points were deemed access layer devices, or edge devices, they were considered no differently from a typical Layer 2 switch. Both provided connectivity to the network, after all.

Most enterprise networks rely upon intelligent devices. This includes access layer switches and (historically, once again) access points. Control and management were provided "at the edge," so the devices required a degree of intelligencethat is, configurability and manageability. The access points were considered "intelligent" or "autonomous" because they were effectively standalone devices. Each access point stored all the features, settings, and configuration and acted as a unique, intelligent host on the network. In other words, if you had a WLAN with 25 access points, you had to configure and manage each of the 25 devices. The rest of the network infrastructure (the switches to which the APs were connected) did not require wireless-specific awareness. This is the so-called distributed model, also known as the autonomous AP architecture, and many products and manufacturers still use this framework.

Recently the centralized model, commonly known as the thin AP or centralized AP architecture, has gained popularity. This pardigm approaches WLAN intelligence differently. Instead of embedding the intelligence into the access points themselves, it is centralized into the device to which the AP is physically or logically attached. The attachment points are typically dedicated appliances (so-called "wireless LAN controllers") that act as a centralized management system for the access points. The APs are configured and managed not individually but centrally from these controllers. Conversely, this model requires dedicated wireless controllers (or wireless switches).

Often, the WLAN controller automatically configures the access point settings entirely, obviating the need for IT staff to configure each device with specific or distinct radio settings, for example. On the other hand, this system sometimes results in reduced configurability, removing your IT staff's ability to fine-tune or mold the wireless network to your particular needs. In these circumstances, you can disable the automatic "self configuring" features while retaining the other benefits of the centralized model.

Both the centralized and distributed models have their advantages and disadvantages. The traditional, distributed architecture provides a robust system that does not rely upon dedicated wireless appliances. Access points can be installed anywhere on the WLAN, and each site (small offices, remote branches, individual buildings) does not require additional equipment in the form of a dedicated controller. The access points contain all the configuration and settings required and use common operating systems, such as those already familiar to your IT support staff. A distributed model is very scalable because new APs can be installed wherever you have a free Ethernet port.

The centralized "wireless switch" or "wireless controller" architecture simplifies deployment. Almost all the wireless settings are configured centrally, and there is no (or reduced) need to configure each access point. Many products that adopt this approach also include added features, such as VPN tunnel termination, guest networking capabilities, and wireless intrusion detection systems. The wireless management is provided by either the controller or, more often, another separate WLAN management appliance. The centralized model can often make small or medium installations very easy to deploy. On the other hand, the centralized model does not scale as well because it requires dedicated controllers for each WLAN site and may even require several controllers for larger buildings or deployments. This can rapidly become costly and a deployment challenge in its own right.

The manufacturer of the product that you select will most likely dictate whether you use a centralized or distributed architecture. Some manufacturers, such as Cisco Systems, offer both. In either case, some fundamental WLAN management strategies are necessary for both models, and neither obviates the need for a carefully considered and robust management framework. Despite what any marketing or sales people tell you, there will always be a need for a holistic approach that takes into account more than just the simple "intelligence" or configurability of the access point.