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wireles network hacks mods - briere D.

briere D. wireles network hacks mods - Wiley publishing, 2005. - 387 p.
ISBN: 0-7645-9583-0
Download (direct link): wirelesnetworkhacks.pdf
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802.11g is nothing more or less, when you get down to the essence of it, than a new, improved, fancier, souped-up version of 802.11b. 802.11g uses the same frequencies — the 2.4 GHz band — and the same channel assignments (with the same overlaps and same three non-overlapping channels). 802.11g even includes the DSSS and CCK modulation from 802.11b — which means that any 802.11g system is backwards-compatible with 802.11b and fits perfectly into any 802.11b network.
The reverse is also true — 802.11b systems can be used in an 802.11g network, but in almost all cases, this slows down the 802.11g network to 802.11b speeds. Typically, the 802.11g clients and access points slow down to a lower speed (like 22 Mbps) while still allowing the 802.11b clients to operate on the network — with some APs you can configure the AP to not allow the 802.11b clients on the network in order to maintain the highest possible speeds.
The real breakthrough for 802.11g is a new modulation scheme known as OFDM, or orthogonal frequency division multiplexing. Orthogonal is a mathematics term that relates to things being at right angles — or, more generally, refers to things that are independent and well-separated. In the wireless world, this means that the data is sent across the airwaves in a series of well-separated frequency modulations that, in essence, can be jam-packed into the airwaves but still distinct enough to be demodulated at the far end.
The net result is a much greater amount of data being sent across the wireless network — five times as much data can be sent simultaneously across an 802.11g network as can travel across an 802.11b one. That’s a big deal if you’re trying to send audio signals — or the home movie you just created in iMovie — across the network in real time.
Of course, as with 802.11b — and all wireless networks — there’s a difference between the theoretical maximum (in this case, 54 Mbps) and the actual throughput you get in the real world. It’s not unreasonable to expect upwards of 20 Mbps throughput in an 802.11g network — which gives you enough bandwidth to handle even an HDTV channel’s worth of data.
The combination of backwards compatibility and significantly higher throughput have made 802.11g the top dog of the wireless world today.
Most of the wireless LAN gear you can buy today (or that you might get “included” with devices such as digital media adapters) uses 802.11g.
30
Part I: Making Your World Wireless
802.11a: Still hanging in there
The limited number of non-interfering channels (just three of them!), when combined with the large amount of gear (like cordless phones) can cause 802.11b and g networks to face interference that causes decreased range (meaning that users have to stay close to the base station or access point) and poor performance.
One way of getting around this issue is to use a different, less crowded set of frequencies. That’s exactly what systems based on the 802.11a standard do — they move from the crowded city streets of the 2.4 GHz band on out to the relatively unexplored frontier of the U-NII (Unlicensed National Information Infrastructure) band. U-NII is in the 5 GHz range of frequency spectrum, where relatively few other devices operate (a few cordless phones are the only devices operating at this frequency that we’ve seen on the marketplace).
Besides having few other devices contending for scarce spectrum, the U-NII band has the big advantage of just being a bigger chunk of frequencies than the 2.4 GHz band used for 802.11b and g. This provides some big benefits:
^ More room for data on each channel — the channels assigned in 802.11a are “bigger” (they use a wider swath of frequencies) than those in 802.11b or g.
^ There are more channels — 14 versus 11 (although 4 are designated for outdoor use).
^ Most importantly, there are more non-overlapping channels than 802.11b and g — 8 versus 3.
This leaves you with a much lower chance of interference and, typically, greater throughput. So 802.11a wireless LANs — which, by the way, use the same OFDM modulation scheme as do 802.11g LANs — can reach maximum speeds of 54 Mbps (the same as 802.11g), and in the real world, often reach speeds in the vicinity of 30 Mbps. This makes 802.11a networks the fastest for many folks.
Figure 2-2 shows the channels in 802.11a.
So what’s the downside? Why aren’t we all using 802.11a? The biggest reason is inertia — 802.11a is not compatible with 802.11b, and therefore won’t work with a lot of the legacy wireless network equipment found in homes, offices, and public locations throughout the world. 802.11g does provide this compatibility, which makes it a safer choice.
As we discuss in Chapter 3, where we talk about choosing equipment, many wireless networking devices these days are dual mode and can support both 802.11g and 802.11a. (Many vendors advertise these as 802.11a/b/g access points.)
Chapter 2: Wireless Network Basics
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