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Transmitting a signal using the typical 802.11 specifications works a lot like it does with a
basic Ethernet hub: They’re both two-way forms of communication, and they both use the
same frequency to both transmit and receive, often referred to as
half-duplex
, as mentioned
earlier in the chapter. Wireless LANs (WLANs) use RF’s that are radiated into the air from an
antenna that creates radio waves. These waves can be absorbed, refracted, or reflected by
walls, water, and metal surfaces, resulting in low signal strength. So, because of this innate vulnerability
to surrounding environmental factors, it’s pretty apparent that wireless will never
offer us the same robustness as a wired network can, but that still doesn’t mean we’re not
going to run wireless. Believe me, we definitely will!
We can increase the transmitting power and gain a greater transmitting distance, but doing
so can create some nasty distortion, so it has to be done carefully. By using higher frequencies,
we can attain higher data rates, but this is, unfortunately, at the cost of decreased transmitting
distances. And if we use lower frequencies, we get to transmit greater distances but at lower
data rates. This should make it pretty clear to you that understanding all the various types
of WLANs you can implement is imperative to creating the LAN solution that best meets the
specific requirements of the unique situation you’re dealing with.
Also important to note is the fact that the 802.11 specifications were developed so that
there would be no licensing required in most countries—to ensure the user the freedom to
install and operate without any licensing or operating fees. This means that any manufacturer
can create products and sell them at a local computer store or wherever. It also means that all
our computers should be able to communicate wirelessly without configuring much, if anything
at all.
2.4GHz (802.11b)
First on the menu is the 802.11b standard. It was the most widely deployed wireless standard,
and it operates in the 2.4GHz unlicensed radio band that delivers a maximum data rate of
11Mbps. The 802.11b standard has been widely adopted by both vendors and customers who
found that its 11Mbps data rate worked pretty well for most applications. But now that
802.11b has a big brother (802.11g), no one goes out and just buys an 802.11b card or access
point anymore because why would you buy a 10Mbps Ethernet card when you can score a
10/100 Ethernet card for the same price?
An interesting thing about all Cisco 802.11 WLAN products is that they have the ability to
data-rate-shift while moving. This allows the person operating at 11Mbps to shift to 5.5Mbps,
2Mbps, and finally still communicate farthest from the access point at 1Mbps. And furthermore,
this rate shifting happens without losing connection and with no interaction from the
5.2 Identify and describe the purpose of the components in a small wireless network
291
user. Rate shifting also occurs on a transmission-by-transmission basis. This is important
because it means that the access point can support multiple clients at varying speeds depending
upon the location of each client.
The problem with 802.11b lies in how the Data Link layer is dealt with. In order to solve problems
in the RF spectrum, a type of Ethernet collision detection was created called
CSMA/CA
, or
Carrier Sense Multiple Access with Collision Avoidance
. Check this out in Figure 5.2.
FIGURE 5 . 2
802.11b CSMA/CA
CSMA/CA is also called a
Request to Send
,
Clear to Send
(RTS/CTS) because of the way
that hosts must communicate to the access point (AP). For every packet sent, an RTS/CTS and
acknowledgment must be received, and because of this rather cumbersome process, it’s kind
of hard to believe that it all actually works!
2.4GHz (802.11g)
The 802.11g standard was ratified in June 2003 and is backward compatible to 802.11b. The
802.11g standard delivers the same 54Mbps maximum data rate as 802.11a but runs in the
2.4GHz range—the same as 802.11b.
Because 802.11b/g operates in the same 2.4GHz unlicensed band, migrating to 802.11g
is an affordable choice for organizations with existing 802.11b wireless infrastructures. Just
keep in mind that 802.11b products can’t be “software upgraded” to 802.11g. This limitation
is because 802.11g radios use a different chipset in order to deliver the higher data rate.
But still, much like Ethernet and Fast Ethernet, 802.11g products can be co-mingled with
802.11b products in the same network. Yet, for example, completely unlike Ethernet, if you
have four users running 802.11g cards and one user starts using an 802.11b card, everyone
connected to the same access point is then forced to run the 802.11b CSMA/CA method—an
ugly fact that really makes throughput suffer. So to optimize performance, it’s recommended
that you disable the 802.11b-only modes on all your access points.
To explain this further, 802.11b uses a modulation technique called
Direct
Sequence
Spread
Spectrum
(DSSS) that’s just not as robust as the Orthogonal Frequency Division Multiplexing
ACK
Source Destination
RTS
CTS
Data
292
Chapter 5
Explain and select the appropriate administrative tasks
(OFDM) modulation used by both 802.11g and 802.11a. 802.11g clients using OFDM enjoy
much better performance at the same ranges as 802.11b clients do, but—and remember this—
when 802.11g clients are operating at the 802.11b rates (11, 5.5, 2, and 1Mbps), they’re actually
using the same modulation 802.11b does.
Figure 5.3 shows the 14 different channels (each 22Mhz wide) that the FCC released in the
2.4GHz range.
FIGURE 5 . 3
ISM 2.4GHz channels
In the U.S., only 11 channels are configurable, with channels 1, 6, and 11 being nonoverlapping.
This allows you to have three access points in the same area without experiencing
interference.
5GHz (802.11a)
The IEEE ratified the 802.11a standard in 1999, but the first 802.11a products didn’t begin
appearing on the market until late 2001—and boy were they pricey! The 802.11a standard
delivers a maximum data rate of 54Mbps with 12 non-overlapping frequency channels.
Figure 5.4 shows the UNII bands.
FIGURE 5 . 4
UNII 5GHz band has 12 non-overlapping channels (U.S.).
Operating in the 5GHz radio band, 802.11a is also immune to interference from devices
that operate in the 2.4GHz band, like microwave ovens, cordless phones, and Bluetooth
devices. 802.11a isn’t backward compatible with 802.11b because they are different frequencies,
so you don’t get to just “upgrade” part of your network and expect everything to work
1 2 3 4 5 6 7 8 9 10 11 12 13 14
2.483GHz
Channels
2.402GHz 22MHz
161
5.15
Lower band
5.15–5.25
indoor
Upper band
5.725–5.825
outdoor
Middle band
5.25–5.35
indoor and outdoor 5.825
Channel center
frequencies
5.180 5.200 5.220 5.240 5.260 5.280 5.300 5.320 5.745 5.765 5.785 5.805
Operating
channels
36 40 44 48 52 56 60 64 149 153 157
293
together in perfect harmony. But no worries—there are plenty of dual-radio devices that will
work in both types of networks. A definite plus for 802.11a is that it can work in the same
physical environment without interference from 802.11b users.
Similarly to the 802.11b radios, all 802.11a products also have the ability to data-rate-shift
while moving. The 802.11a products allow the person operating at 54Mbps to shift to 48Mbps,
36Mbps, 24Mbps, 18Mbps, 12Mbps, 9Mbps, and finally still communicate farthest from the
AP at 6Mbps.
Exam Objectives
Remember the three overlapping channels used with the 2.4Ghz range.
In the U.S., only
11 channels are configurable, with channels 1, 6, and 11 being non-overlapping.
Remember how many channels are non-overlapping in the 5Ghz range.
The 802.11a standard
delivers a maximum data rate of 54Mbps with 12 non-overlapping frequency channels.
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