Mobile Observer
June 28, 2006
Wireless Propagator: A Wireless Repeater to Switch Transition? -- Part 1
By Frank Bulk (mailto:fbulk@nwc.com)
Blurb: As wireless networks continue to spread and become more pervasive in
organizations today, the wireless network manager needs to understand the
caveats of a micro-cell infrastructure and some other options.
Perhaps it hasn't quite happened at your workplace yet, but some organizations
are shifting the balance of their network traffic from the wired to the wireless
one. Telecommuters or road warriors who visit the office once or twice a week
(or month) can now work out of a temporary cubicle in the same way they do in
their home offices, on the road or at a hotel: Turn on their wireless card and
associate to the network.
In other locations, Wi-Fi is enabling applications and services that weren't
available or affordable before. Hospitals are able to take advantage of VoWLAN
phones from the likes of SpectraLink and Vocera to enable instant and directed
communication to provide better bedside assistance. Universities are making
Wi-Fi available in the classroom and public areas so students can have quick and
convenient access to information or engage in class assignments.
Basically, as usage increases, the wireless network needs to morph from one
designed around coverage to one designed around capacity, from spare to dense.
It's been almost two years since Aruba began heavily marketing the "wireless
grid," a term for its micro-cell concept that emphasizes smaller and overlapping
cells of wireless coverage. The other leading enterprise WLAN vendors recommend
roughly the same design for dense installations. Rather than leave APs (access
points) on their default output power of 100 mW, the APs (automatically) can
turn down their power so the wireless signals don't propagate as far. At the
same time, the vendors tout their AP placement planning software, auto-RF tuning
and load balancing, all while selling a few more APs and supporting wireless
controllers along the way.
The micro-cell concept is conceptually intuitive, but the devil is in the
details. If a three-channel design (1, 6 and 11) was complex the first time
around, designing a dense wireless network requires many more access points,
which significantly increases the complexity of AP placement. More importantly,
the potential for co-channel interference rises, for at least two reasons.
First, even if the access points are configured to link at 36 Mbps, to establish
a minimum bound for performance reasons, the APs' basic rates for management
frames can be as low as 1 or 6 Mbps, which transmit decipherable information
much farther than the higher rates. Compounding the problem, client cards are
usually able to pick up RF energy up to twice as far as the limits of the 1 Mbps
range. This is enough to fail the client card's clear channel check, which in
turn delays transmission.
Second, unless clients support automatic client output power control -- as is
the case with CCX (Cisco Compatible Extensions)-enabled cards and Cisco APs --
they will not automatically adjust their output power, which leads to the
problem of power asymmetry. The client's Wi-Fi card, operating at full power,
essentially broadcasts far beyond the range of the lower power access point to
which it is associated. This means a neighboring AP on the same channel will
hear it, too, as will many of the other clients on that channel associated to
the neighboring AP(s). The IEEE is working on standards that will enable
AP-client link management communications, but we're stuck with status quo for at
least the next two years.
These co-channel problems are not new. I had personally hoped that enterprises
would require 802.11a radios in their second-generation wireless networks and
take advantage of the dual-radio client cards embedded in many laptops. However,
although the majority of enterprise APs are dual-band, many are not dual-radio,
which means it's an either/or instead of a both. Intel will very likely pump out
only dual-band Centrino chipsets moving forward, but it seems that most of the
time the drivers are not configured by default to prefer 802.11a.
Unfortunately, our most recent surveys confirm that not many organizations, not
to mention end users, are taking advantage of 802.11a. It's a pity, because most
implementations of 802.11a support at least eight nonoverlapping channels. This
significantly simplifies the channel selection process and reduces co-channel
interference, and the higher attenuation characteristics of that frequency are
conducive to a micro-cell design. Hopefully, 802.11n, which can take advantage
of both the 2.4- and 5-GHz bands, will obfuscate the spectrum issue and make
channel choices a moot point. Think of it as you would the three bands our GSM
cellular phones use, which we don't worry about.
Another possible solution is to use a four-channel design, an option that Aruba
recently re-introduced. In a lecture hall with 250 students each using
school-provided laptops, it's understandable for an enterprise wireless
infrastructure vendor to want to reduce the per-AP load below 80 clients.
According to Vice President of Product Marketing Keerti Malkoote, Aruba has
tweaked its product to minimize the adjacent-channel defects generated by the
slight channel overlap found in the 1, 4, 8 and 11 line-up. It sounds to me like
a semi-desperate attempt to deal with density issues, and it's not to be
recommended. (For some primary reading on this topic, check out:
http://dades.joor.net/article_fitxers/1253/FourPoint.pdf and
http://www.cisco.com/univercd/cc/td/doc/product/wireless/airo1200/accsspts/techr
ef/channel.pdf )
Two vendors have followed the coordinated single-channel route: Meru and
Extricom. Both use coordinated access of the channel between APs to scale the
number of clients and provide predictable levels of service, essentially
eliminating the penalties of roaming between access points. And each scales in
capacity by overlaying the network with another channel. Competing vendors have
thrown some fierce barbs over the years because of the companies' admittedly
unique and unconventional technology, but customer installations thus far have
substantiated Meru's claims and Extricom's installed base is decidedly smaller.
Adjusting a network from casual and light use to mission-critical and pervasive
use will mean some growing pains. In three weeks we'll talk about how you might
possibly convert those wireless APs from hubs to switches and how new antenna
technologies will help reduce the problems of general and co-channel
interference.
Frank Bulk is a contributing editor to Network Computing Magazine covering
wireless and mobile technologies and works for a telecommunications company
based in the Midwest.
For more analysis and opinion from Frank Bulk.
http://www.networkcomputing.com/channels/wireless/
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