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.
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