Ten years ago, designing a wireless LAN was more art than
science. Most of the early wireless LAN deployments were
“hand-crafted” in the sense that RF engineers designed
the layout and placement of access points (APs) with simple tools
and their good instincts.
Back then, when 802.11 networks operated at 1 and 2 Mbps and APs
were “fat,” the rule of thumb for coverage planning in
WLANs was 20% overlap of adjacent cells. That would result in a
continuous coverage area, but the access points were far enough
apart that they would not directly interfere with each other.
Client roaming behavior was smooth and predictable. As wireless
clients moved away from their access point and into these
overlapping areas, they were able to detect that they were in the
fringe coverage and would scan for better access points. In a
well-designed network, a roaming 802.11 client would find a
neighboring AP with better RF signal and automatically associate to
the new AP.
Over the last few years, WLAN systems for the enterprise have
become much more sophisticated. APs are now “thin,” and
a central WLAN controller consolidates management, security and
deployment functions. Modern APs are capable of automatically
setting their channel, and have detailed control of their transmit
power levels, even at deployment. Tools are available for
802.11a/b/g to plan networks, predict wireless coverage and
recommend AP placement. The central controller can take a
system-wide view and make adjustments as necessary to assure
adequate coverage throughout the network. There is still some
debate about whether this automated approach results in a better
wireless network design than the good old handcrafted approach.
However, the WLAN controller architecture has clearly driven the
success of WLANs in the enterprise by reducing complexity and
lowering deployment and operating costs.
So – what happens when 802.11n comes to the enterprise?
Will we deploy WLANs the same way? Not exactly.
802.11n for the enterprise changes everything. There are many
different facets of this change. New 802.11n APs capable of
delivering ten times the capacity of the current generation will
create tremendous load on enterprise networks and force redesigns
for both wired and wireless networks. The debate about WLAN
architectures – controllers and thin APs versus standalone fat APs
– has resurfaced as WLAN infrastructure vendors migrate to 802.11n
and optimize their systems to take advantage of the new
Let’s focus on the range and coverage capabilities of
802.11n to illustrate how a new 802.11n deployment will be
different from current 802.11a/b/g network designs. We previously
covered the outstanding range of draft 802.11n products coming to
market today for consumers and small businesses. They really do
deliver at least twice the range of 802.11g products, and
enterprise APs and clients based on 802.11n could do even better.
But combining tens or hundreds of new 802.11n APs in an office may
have unpredictable results in terms of the coverage achieved and
the behavior of wireless applications.
There will be a wide variance of capabilities in the client
devices using 802.11n. Certainly, legacy 802.11a/b/g clients will
have very different range capabilities than 802.11n clients when
communicating with new 11n APs. Most enterprise WLAN deployments
will plan to support both legacy and new 11n clients for the next
few years. However, even a greenfield 11n deployment could have a
variety of client devices with differing range. There will be a
significant range difference between an 11n dual-mode
Wi-Fi/cellular handset and an enterprise 11n Wi-Fi client in a
notebook PC. The notebook client will have multiple antennas built
into the case and multiple radios operating at higher power. The
dual-mode handset will likely only have one radio dedicated to
Wi-Fi, and is not large enough to support the many antennas that
are physically separated. The range of these products will be very
When designing an enterprise WLAN, how do you do coverage
planning? Do you design your network for the least capable client?
When 802.11n systems move into the enterprise, the disparity
between the best-performing client and the worst-performing client
will be greater than ever before, and new network designs must
Environmental factors are more influential in coverage with
802.11n networks than with legacy 802.11 systems. The new physical
layer for 11n employs MIMO (multiple in, multiple out) techniques
to improve the link margin, and actually takes advantage of
multi-path to improve the signal. Multi-path is essentially
multiple reflections of the RF signal arriving at the receiver at
slightly different times. Indoors, multi-path is caused by the
radio signal bouncing off walls and other obstructions. 11n
benefits from multi-path.
However, multi-path is a signal impairment for earlier versions
of 802.11 – it’s essentially interference. A difficult
location that was a “dead spot” or “coverage
hole” in an 11g network may be a great location with
excellent signal strength in an 11n network. Our intuition about
designing network coverage for 11a/b/g networks is all wrong when
it comes to designing 11n networks. The current set of automated
network planning and site survey tools for 11a/b/g will likely
produce strange results. The size and shape of the coverage areas
with this new infrastructure will be very different. The coverage
achieved by 802.11n will be more sensitive to building design, and
will vary more from one installation to another. We will need to
develop new tools and best practices for installing large
enterprise WLANs based on 11n.
The method for achieving extreme range in 802.11n changes the
behavior at the system level. With all of the optional capabilities
enabled, an 11n client may be communicating to its AP with multiple
spatial streams using multiple antennas and beam forming. Beam
forming concentrates the RF energy in a particular direction,
toward the intended recipient. The beam is formed dynamically and
can be pointed automatically, creating the effect of a temporary
high gain antenna between the two communicating stations. Range
extension in this manner will change the shape of the effective
coverage area even more. Coverage will literally follow a mobile
client with smart antennas beyond the normal coverage area.
AP-to-AP roaming behavior will be different for new 11n clients
with smart antenna systems. In current WLANs, the mobile client is
usually associated to the closest access point. That may not be the
norm for 11n networks. As a result, applications may perform
differently. Overall, interference from adjacent APs operating in
this manner may occur less frequently than with current systems,
but when an enhanced mobile client is nearby and active, it will
create interference for adjacent APs at a higher power level. Even
interference in the enterprise will be different with 11n.
These are not huge problems. They just make the system behavior
of WLANs in the enterprise very different than it has been for the
last ten years. It is possible to deploy an 802.11n system in the
enterprise without understanding or addressing any of these issues
— existing 802.11 networks could upgrade their APs from 11g to
11n, placing all of the new APs in exactly the same locations.
Legacy Wi-Fi devices would continue to work, though there may be
funny behavior with some applications. Old trouble areas for
coverage may be improved, and new coverage holes may emerge. New
11n Wi-Fi clients would be able to use the same infrastructure and
operate at higher data rates.
Since new 11n APs will be more expensive than current thin APs,
the simple AP swap-out could be an expensive system upgrade for an
incremental performance improvement. It may be possible to achieve
the same performance with fewer APs, or to have much better
performance with the same investment in a system tuned to address
the 802.11n system behavior.
To fully exploit the promise of 802.11n in the enterprise, we
need to develop new tools, new intuition about range and coverage,
and new best practices for WLAN design and deployment.
The technical part of the IEEE 802.11n standard is complete,
though the official standard won’t be published for more than
a year. Wireless chip vendors are already shipping their second
generation 11n chipsets. Now, further innovation will come at the
system level and there will be many new and different 11n
enterprise WLAN systems introduced in the next few months.
It will be interesting to see how WLAN infrastructure vendors
deal with the complexity, challenges and new capabilities of 11n.
Fat APs may come back in style. WLAN architectures and deployment
strategies that were once considered strange may become the new
best practices for building WLANs.
Article courtesy of Wi-Fi Planet