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802.11b Tips, Tricks, and Facts
by Rob Flickenger
There's much more to 802.11b spec than that teeny little "b" indicates. 802.11b is
not just the downstairs apartment of 802.11; it's a whole new world of
Before we examine what makes that little "b" so special, let's take a look at the original 802.11:
- Approved in 1997 by the IEEE 802 committee, 802.11 details the framework
necessary for a standard method of wireless networked communications.
It uses the 2.4-GHz microwave band designated for low-power unlicensed
use by the FCC in the USA in 1985.
- It allows for two different (and incompatible) methods of encoding,
FHSS and DSSS.
- FHSS (Frequency Hopping Spread Spectrum) spreads the conversation
across 75 one-MHz subchannels, continually skipping between them.
- DSSS (Direct Sequence Spread Spectrum) breaks the band into 14
overlapping 22-MHz channels and uses one at a time.
Two operating modes are defined: infrastructure and ad hoc. Most
dedicated hardware (the "access point") provides a basic or
extended service set that builds the wireless "infrastructure." It goes
a bit beyond basic bridging, allowing clients to roam from access point to access point
(provided they all exist on the same physical Ethernet segment; roaming across routers isn't allowed -- at least, not yet). The ad hoc (IBSS, or Independent Basic Service Set) mode allows
individual nodes to participate in a peer-to-peer network
without an access point.
- The 802.11 spec also allows for Wired Equivalent Privacy encryption at the MAC (Media Access Control) layer.
But 802.11 isn't perfect. For example, how do you detect
collisions with a device that can transmit or receive at any given moment, but can't do both at the same time?
What happens when packets you've sent
bounce off of a distant wall and come right back at you microseconds later?
Another major problem with 802.11 equipment was its relatively low speed
compared to wired networking -- only up to 2 Mbps -- and the fundamental
incompatibility (and confusion) between FHSS and DSSS equipment. But because they were incompatible, a choice had to be made. And that choice led to the 802.11b spec.
The move to DSSS and 802.11b
The FHSS frequency-hopping cards were the first to hit the marketplace, as they
were cheaper to produce and easier to implement than DSSS. As time marched
on (and with Moore's Law in effect), the processing power needed to
cheaply implement DSSS soon became available. As it turned out, given the
FCC's broadcasting constraints and some terribly clever engineering, DSSS
began to prove itself as the more reliable solution.
In September of 1999, the 802 committee extended the specification, deciding
to standardize on DSSS. This extension, 802.11b, allowed for new, more
exotic encoding techniques. This pushed up the throughput to a much more
respectable 5.5 Mbps (up to 11 Mbps). While breaking compatibility with FHSS
schemes, the new extensions made it possible for new equipment to continue
to interoperate with older 802.11 DSSS hardware.
With the ever-present need for speed temporarily quenched, everyone who is
anyone started jumping on the wireless roller coaster. While Lucent and Cisco
are the major producers, Apple, Xircom, Linksys, IBM, and others have all
come out with OEM equipment.
Now that the spec is set (for the moment), let's review a few tips to make your wireless networking experience as solid as possible.
Wireless networking tips
Here are some practical 802.11 facts that any self-respecting wireless hacker should
be aware of:
- There's a constant trade-off between range and throughput. Your
hardware should auto-sense signal strength (unless you tell it
otherwise), and back off the transmission rate if your signal gets
weak. It will automatically back it down from 11 Mbps to 5.5, 2, and
even 1 Mbps. Do keep in mind that, although 1 Mbps may sound low, many
businesses have a T1 as their main pipe to the Internet. As
a T1 only moves data at 1.544 Mbps, this should not be a problem. (I
hardly ever notice the difference myself.)
- The top speed is 11 Mbps, but that's only over the air. Access
points typically have 10baseT Ethernet connections, so your
theoretical maximum to the wire is still only 10 Mbps. You might be able
to get around this by bridging a wireless card to your network with a
do-it-yourself Linux router and a PCI bridge, but it gets worse ...
- The client cards currently on the market only have one radio in them.
That means half-duplex communications only (you can talk or listen with
a radio, but not both.)
So in short, while they're billed for 11 Mbps, your mileage will most
certainly vary. The most I've been able to squeeze through a card doing WEP
was about 6.5 Mbps sustained (roughly 8 Mbps without WEP), and even that was
downhill with a good, stiff tailwind.
Rob using a parabolic dish in search of the strongest signal
Antenna tips and tricks
Antenna selection has a tremendous impact on the range of your wireless network. Here a few things I've learned:
- The design of every external card puts the antenna in the worst possible
orientation: sideways, and very close to the laptop (or desktop). The
radiation pattern is almost straight up and down! You can watch the
wonders of RF (radio frequency) by opening up your strength meter
(wmwave for Linux is
very good for this) and tilting your laptop sideways. Watch that signal
bar grow. Go for the green! Learn to type sideways!
- The one notable exception to this is the Apple built-in AirPort card.
They've thought enough to include an internal antenna connector that
runs up the LCD panel. This is an excellent design with much better range. It looks
like IBM is the first to play copycat (as usual) with their "i Series"
- You will see tremendous differences in signal strength by attaching a
small omnidirectional external antenna, and orienting it properly.
Which way is properly? That depends on your environment. Try every
possible position (with your signal strength meter open). I've put mine
on top of my monitor, below the desk, sideways, on the table behind me,
slung over my shoulder, etc.
- Make sure your card can take an external antenna. Many
low-priced cards don't include external connectors anymore. You
will have trouble finding a connector to fit the ones that do. Word has
it that part of the FCC rules require "proprietary" external antenna
connectors on all 2.4-GHz equipment. Check out your friendly local radio
supplier for proprietary-to-standard adapters.
- Higher is not always better for large external antennae. Most people
immediately think of putting an antenna on their roof, without
considering where their traffic is coming from. Look at the radiation
projection of various antennae, and try to get the best parts pointed in
the direction you are most likely to be coming from. Usually, the worst
place to be in relation to an omnidirectional antenna is directly
Finally, here's a real gem: Lay your antennas in the spring. Trust me.
Why, you ask?
Well, your worst natural enemy is water. Low-power microwaves will
bounce off leaves like a mirror. If you set up a well-placed antenna in
the winter, you will be horribly disappointed in April when the trees
are blooming and your signal is dropping.
To sum up: 802.11b has brought speed and cheap, reliable hardware to the
networking world. For the first time, people can bring up high-speed,
encrypted communication lines for only the cost of hardware. As disruptive
technologies go, this one has its disrupter set on "obliterate."
is a long time supporter of FreeNetworks and DIY networking. Rob is the author of three O'Reilly books: Building Wireless Community Networks, Linux Server Hacks, and Wireless Hacks.
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