802.11b Tips, Tricks, and Factsby 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 wireless possibilities.
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" ThinkPads.
- 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 beneath it!
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."
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