Network Bulls
www.networkbulls.com
Best Institute for CCNA CCNP CCSP CCIP CCIE Training in India
M-44, Old Dlf, Sector-14 Gurgaon, Haryana, India
Call: +91-9654672192
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 and mentioned in the introduction
to this chapter. Wireless LANs (WLANs) use radio frequencies (RFs) 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.
Introducing Wireless Technology
465
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 we
should all be able to get our computers to communicate wirelessly without configuring much.
Various agencies have been around for a very long time to help govern the use of wireless
devices, frequencies, standards, and how the frequency spectrums are used. Table 8.1 shows the
current agencies that help create, maintain, and even enforce wireless standards worldwide.
Because WLANs transmit over radio frequencies, they’re regulated by the same types of laws
used to govern things like AM/FM radios. It’s the FCC that regulates the use of wireless LAN
devices, and the IEEE takes it from there and creates standards based on what frequencies the
FCC releases for public use.
The FCC has released three unlicensed bands for public use: 900MHz, 2.4GHz, and 5GHz.
The 900MHz and 2.4GHz bands are referred to as the Industrial, Scientific, and Medical
(ISM) bands, and the 5GHz band is known as the Unlicensed National Information Infrastructure
(UNII) band. Figure 8.1 shows where the unlicensed bands sit within the RF spectrum.
So it follows that if you opt to deploy wireless in a range outside of the three public bands
shown in Figure 8.1, you need to get a specific license from the FCC to do so. Once the FCC
opened the three frequency ranges for public use, many manufacturers were able to start offering
myriad products that flooded the market, with 802.11b/g being the most widely used wireless
network found today.
The Wi-Fi Alliance grants certification for interoperability among 802.11 products offered
by various vendors. This certification provides a sort of comfort zone for the users purchasing
the many types of products, although in my personal experience, it’s just a whole lot easier if
you buy all your access points from the same manufacturer.
TABLE 8 . 1
Wireless Agencies and Standards
Agency Purpose Website
Institute of Electrical and
Electronics Engineers (IEEE)
Creates and maintains operational standards
www.ieee.org
Federal Communications
Commission (FCC)
Regulates the use of wireless devices in
the U.S.
www.fcc.gov
European Telecommunications
Standards
Institute (ETSI)
Chartered to produce common standards
in Europe
www.etsi.org
Wi-Fi Alliance Promotes and tests for WLAN interoperability
www.wi-fi.com
WLAN Association
(WLANA)
Educates and raises consumer awareness
regarding WLANs
www.wlana.org
466
Chapter 8
Wireless Technologies
FIGURE 8 . 1
Unlicensed frequencies
In the current U.S. wireless LAN market, there are several accepted operational standards
and drafts created and maintained by the IEEE. Let’s take a look at these standards and then
talk about how the most commonly used standards work.
The 802.11 Standards
Taking off from what you learned in Chapter 1, “Internetworking,” wireless networking
has its own 802 standards group—remember, Ethernet’s committee is 802.3. Wireless
starts with 802.11, and there are various other up-and-coming standard groups as well,
like 802.16 and 802.20. And there’s no doubt that cellular networks will become huge
players in our wireless future. But for now, we’re going to concentrate on the 802.11 standards
committee and subcommittees.
IEEE 802.11 was the first, original standardized WLAN at 1Mbps and 2Mbps. It runs in the
2.4GHz radio frequency and was ratified in 1997 even though we didn’t see many products pop
up until around 1999 when 802.11b was introduced. All the committees listed in Table 8.2 are
amendments to the original 802.11 standard except for 802.11F and 802.11T, which are both
stand-alone documents.
TABLE 8 . 2
802.11 Committees and Subcommittees
Committee Purpose
IEEE 802.11a 54Mbps, 5GHz standard
IEEE 802.11b Enhancements to 802.11 to support 5.5Mbps and 11Mbps
AM Broadcase
Cellular
(840 MHz)
Visible
light
Sonar
(extremely low)
X-rays
900 MHz
band
2.4 GHz
band
5 GHz
band
FM Broadcast Infrared
Wirless
LAN
The 802.11 Standards
467
Okay, now let’s discuss some important specifics of the most popular 802.11 WLANs.
IEEE 802.11c Bridge operation procedures; included in the IEEE 802.1D standard
IEEE 802.11d International roaming extensions
IEEE 802.11e Quality of service
IEEE 802.11f Inter-Access Point Protocol
IEEE 802.11g 54Mbps, 2.4GHz standard (backward compatible with 802.11b)
IEEE 802.11h Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) at
5Ghz; adds more non-overlapping channels for 802.11a
IEEE 802.11i Enhanced security
IEEE 802.11j Extensions for Japan and U.S. public safety
IEEE 802.11k Radio resource measurement enhancements
IEEE 802.11m Maintenance of the standard; odds and ends
IEEE 802.11n Higher throughput improvements using MIMO (multiple input, multiple
output antennas)
IEEE 802.11p Wireless Access for the Vehicular Environment (WAVE)
IEEE 802.11r Fast roaming
IEEE 802.11s ESS Extended Service Set Mesh Networking
IEEE 802.11t Wireless Performance Prediction (WPP)
IEEE 802.11u Internetworking with non-802 networks (cellular, for example)
IEEE 802.11v Wireless network management
IEEE 802.11w Protected management frames
IEEE 802.11y 3650–3700 operation in the U.S.
TABLE 8 . 2
802.11 Committees and Subcommittees
(continued)
Committee Purpose
468
Chapter 8
Wireless Technologies
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, for example, 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
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 8.2.
FIGURE 8 . 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). With 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 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
Source Destination
RTS
CTS
Data
ACK
The 802.11 Standards
469
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 commingled 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
(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 (11Mbps, 5.5Mbps, 2Mbps, and
1Mbps), they’re actually using the same modulation 802.11b does.
Figure 8.3 shows the 14 different channels (each 22MHz wide) that the FCC released in the
2.4 GHz range.
FIGURE 8 . 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 8.4 shows
the UNII bands.
FIGURE 8 . 4
UNII 5GHz band has 12 non-overlapping channels (U.S.).
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Channels
2.402 GHz 22 MHz 2.483 GHz
5.15 5.825
Channel Center
Frequencies
Operating
Channels
Lower Band
5.15-5.25
Indoor
Middle Band
5.25-5.35
Indoor & Outdoor
Upper Band
5.725-5.825
Outdoor
5.180 5.200 5.200 5.240 5.260 5.280 5.300 5.320 5.745 5.765 5.785 5.805
470
Chapter 8
Wireless Technologies
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, metal file cabinets,
and Bluetooth devices because they operate in the same frequency range. 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 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.
Similar 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.
Comparing 802.11
Before I move on to Cisco-specific products, take at look at Table 8.3, which lists the pros and
cons of 802.11a, b, and g.
Now let’s take a look at Figure 8.5, which delimits the range comparisons of each 802.11
standard and shows us the different ranges using an indoor open-office environment as a factor.
We’ll be using default power settings.
TABLE 8 . 3
802.11 Comparison
802.11b 802.11g 802.11a (h)
2.4GHz 2.4GHz 5GHz
Most common Higher throughput Highest throughput
Up to 11Mpbs Up to 54Mbps* Up to 54Mbps
DSSS DSSS/OFDM OFDM
3 non-overlapping channels 3 non-overlapping channels Up to 23 non-overlapping
channels
**About 25 users per cell About 20 users per cell About 15 users per cell
Distance limited by Multipath Throughput degraded by
802.11b clients
Lower market penetrations
*Runs Direct Sequence Spread Spectrum when also running the 802.11b at speeds of 11Mbps and below.
**This happens to be Cisco’s rule of thumb. Know that the actual amount of users per cell varies based on many factors.
Basic Service Sets
471
FIGURE 8 . 5
Range comparisons of 802.11 standards
You can see that to get the full 54Mbps benefit of both 802.11a and 802.11g, you need to
be between 50 feet and 100 feet (at the farthest) away and maybe even closer if there are any
obstructions between the client and the access point.
All good, but there’s one more IEEE 802.11 standard I want to cover that we’ll use to get
even higher speeds at greater distances.
2.4GHz/5GHz (802.11n)
802.11n builds upon previous 802.11 standards by adding Multiple-Input Multiple-Output
(MIMO), which employs multiple transmitters and receiver antennas to increase data throughput.
802.11n can have up to eight antennas, but most of today’s access points use four. These
are sometimes referred to as smart antennas, and if you do have four of them, two would be used
for transmitting simultaneously with the other two receiving simultaneously. This setup would
allow for much higher data rates than 802.11a/b/g. In fact, the marketing people claim it will
provide about 250Mbps, but personally, I’m not buying it. I just don’t believe that’s what our
actual throughput levels can be, and even if what they’re saying is true, exactly how would that
help if all you’ve got is a 1Mbps or 2Mbps cable or DSL connection to the Internet?
With all this in mind, let’s move on and take a look at how to use these various 802.11
specifications.
Basic Service Sets
With a range of products that support IEEE 802.11a/b/g and soon “n” technologies, Cisco
really does offer a pretty complete and impressive line of in-building and outdoor wireless
LAN solutions. These products include access points, wireless controllers, wireless LAN client
Indoor open-office environment
802.11 b
802.11 g
802.11 a
50 ft. 100 ft. 150 ft. 200 ft. 250 ft. 300 ft. 350 ft.
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
54 Mbps
48 Mbps
36 Mbps
24 Mbps
18 Mbps
12 Mbps
9 Mbps
11 Mbps
54 Mbps
48 Mbps
36 Mbps
24 Mbps
18 Mbps
12 Mbps
9 Mbps
6 Mbps
472
Chapter 8
Wireless Technologies
adapters, security and management servers, wireless management devices, wireless integrated
switches and routers—even antennas and accessories. Did I say impressive or what?
Since about the year 2000, a lot of corporations have relied upon basic access points as their
main wireless networks and connected them into an infrastructure (wired network), which
allowed users to roam within their complete network.
Let’s discuss service sets in a little more detail.
There are typically three types of wireless networks that you can create with wireless
networks:
Ad-Hoc
Basic Service Set (BSS)
Extended Service Set (ESS)
Ad-Hoc
is just a term for connecting two or three wireless hosts together without an AP. This
is helpful for home, or very, very, small office transfers of data. Ad-Hoc would not typically be
used in today’s corporate networks.
BSS and ESS networks define what we call a Service Set ID (SSID) that’s used to advertise your
wireless network so hosts can connect to the access point (AP). An example of a BSS is one access
point at home with one SSID. You can have multiple SSIDs configured on an access point for
security reasons; typically this will be found in a corporate environment. For example, you can
designate that one SSID is open access for a public hot spot, while another SSID can use WEP
or WPA2 for the employees that work at this public hot spot. The SSID name is broadcasted out
the AP by default so the clients can find the AP and connect to the wireless network, and of
course you can turn this feature off for security reasons.
BSS/IBSS
A BSS only involves a single access point. You create a BSS by bringing up an AP and creating
a name for the service set ID (SSID). Users can then connect to and use this SSID to access the
wireless network, which provides connectivity to the wired resources. When hosts communicate
with each other, they must go through the AP.
When the AP connects to a wired network, it then becomes known as an Infrastructure
Basic Service Set, or IBSS. Keep in mind that if you have a BSS/IBSS, users won’t be able to
maintain network connectivity when roaming from AP to AP because each AP is configured
with a different SSID name.
BSS wireless networks are also really helpful if you happen to have a couple of hosts that
need to establish wireless communication directly between just them, for example, a home network.
You can also make this happen through the ad-hoc network I already mentioned, but
if you have an AP between the hosts, it’s just called a BSS.
Figure 8.6 shows a basic service set using one SSID and not connecting to an infrastructure.
ESS
Mobile wireless clients can roam within the same network if you set all your access
points to the same Service Set ID (SSID). Doing this creates an Extended Service Set (ESS).
Figure 8.7 shows four APs configured with the same SSID in an office, thereby creating the
ESS network.
Basic Service Sets
473
For users to be able to roam throughout the wireless network—from AP to AP without losing
their connection to the network—all APs must overlap by at least 10% or more, and the channels
on each AP shouldn’t be set the same either. And remember, in an 802.11b/g network, there are
only three non-overlapping channels (1, 6, 11), so design is super important here!
FIGURE 8 . 6
Basic Service Set (BSS)
FIGURE 8 . 7
Extended Service Set (ESS)
If the AP connected to a wired network, this would now be
called an Infrastructure BSS (IBSS).
802.11 Client
802.11 Client 802.11 Client
Access Point
Extended Service Set: All AP’s are set to the same SSID and connect
to an Infrastructure.
No comments:
Post a Comment