17C H A P T E R S E V E N T E E N

Understanding WirelessNetworking

ObjectiveThis chapter covers the following Cisco-specific objective for the “Explain and select the appropriate administrative tasks required for a WLAN” section of the 640-802 CCNA exam:

. Describe standards associated with wireless media (including: IEEE WI-FIAlliance, ITU/FCC)

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OutlineIntroduction 560

The Business Case 560

The Players in the 802.11 Market 561

Entering the Radio Frequency World 562

Understanding Wireless Transmission 562

Unlicensed RF Bands 563

The Key to Successful Wireless: Channel Surfing 564

The 802.11 Standards Family 567

Party Like It’s 1999: 802.11a and 802.11b 567

2003 Delivers: 802.11g 567

The Next Generation: 802.11n 568

Chapter Summary 569

Key Terms 569

Apply Your Knowledge 570

Exercise 570

Review Questions 571

Exam Questions 571

Answers to Review Questions 573

Answers to Exam Questions 574

Suggested Readings and Resources 574

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Study Strategies. Read the information presented in this chapter, paying special attention to tables, Notes,

and Exam Alerts.

. Spend some time soaking up how Wireless Radio Frequency operates. This understand-ing is key to an optimized wireless network deployment.

. The ICND1 and CCNA exams focus only on the concepts behind wireless technologyrather than the configuration. The Exam Alerts in this chapter and Chapter 18, “WirelessSecurity and Implementation Considerations,” will guide you to specific areas of study.

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IntroductionIt was one of those moments I’ll never forget. I (Jeremy) had just installed my brand-new D-Link 802.11b wireless access point in my home and slid the new PCMCIA wireless cardinto my laptop. I sat on the couch in another room, held my breath, and opened InternetExplorer. I was so excited to see my home page appear, I jumped up off the couch and yelledto my wife, “Sue! I’m on the Internet! Look!” I held my laptop high in the air to show thatthere were no wires attached. “Wow…” came the not-as-excited reply (she had become usedto my ultra-geekdom by this point). I then proceeded to run around the house, laptop in hand,yelling, “I’m still on the Internet! I’m still on the Internet!” as I went in and out of each room.Amazing. Network connectivity without wires!

Fast-forward two years from that monumental moment. I am standing in an office talking tothe VP of Sales for the organization. “What do you mean the wireless is down?” he says to me.“The wireless access point went bad. We have one on order. It should be here early next week,”I reply. “So what am I supposed to do until then?” he counters. He follows my gaze to theEthernet wall jack, about three feet from the desk. “You’re going to have to plug in,” I mutteras I pull an Ethernet cable from my bag. From the look of utter annoyance on his face, I mightas well have told him that he’ll have to walk five miles, uphill, in the snow before he will gethis Internet access back.

What happened to the excited people running through companies screaming, “I haveInternet!” with their laptops held high above their heads? Convenience happened—and theultimate convenience at that: solid network connectivity without wires. Yes, 802.11 wirelessnetworking has taken over the world faster than nearly any other network technology ever cre-ated. It is now rare to find even a small business without a wireless access point sitting on ashelf somewhere. It doesn’t matter if users are sitting at home, at work, or at Starbucks; they’veall come to expect that they will always be connected to the great network power of the sky.

The Business CaseThe funny thing about wireless LANs (WLANs) is that the technology was just so cool thatnetwork administrators started deploying it and making up the business case as they wentalong. Wireless networks were a new way of looking at infrastructure, and within a few yearsof mainstream deployment, the applications that used this infrastructure began to emerge.One of the first changes businesses began to realize was the increased use of laptops within theorganization rather than their stationary desktop counterparts. In 2005, laptops outsold desk-tops for the first time, accounting for 53% of all PC sales. That figure continued to increasein the following years as vendors began including embedded wireless network cards in thedevices. Companies are now purchasing laptops for their employees for two reasons.

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The Players in the 802.11 Market561

Employees love laptops, because they get a “new computer” they can use at home, on vaca-tion, and at the office. The companies love the laptop because the employee can now workwhile they are at home, on vacation, and at the office. Along with laptops, personal digitalassistants (PDAs) began to support 802.11 wireless standards. These devices now allowemployees to check email, voice mail, and the company intranet over a virtual private network(VPN), all while walking to a flight.

While a host of new wireless networking devices continue to emerge (including wireless VoIPphones), the most tangible benefit of wireless is found simply in not having wires. The IT costof moving an employee from one cubicle to another ranges from $175 to $375, depending onthe equipment present. When you realize that an average of 10 to 15% of staff moves everyyear, you can begin adding up the numbers for your organization. Likewise, eliminating cableruns in new or existing buildings can always provide traceable cost savings provided by wire-less technology.

Finally, realize that wireless technology has moved to the point where even someone whoknows nothing about networking can drive to a local electronics store and pick up a wirelessaccess point requiring zero setup and configuration. Some users are so desperate to have wire-less access at work that they bring in their own wireless network devices and build “mini-WLANs” in their cubicles, knowingly or unknowingly violating company policy. This can sub-ject your organization to an enormous security risk that could have been minimized by usinga centrally managed WLAN system.

The Players in the 802.11 MarketObjective:

. Describe standards associated with wireless media (including: IEEE WI-FI Alliance, ITU/FCC)

802.11 holds a host of standards that every vendor is scrambling to implement and every net-work admin is trying to keep straight. Three organizations play key roles in managing thiswireless world:

. International Telecommunication Union-Radiocommunication Sector (ITU-R):Regulates the radio frequencies (RF) used for wireless transmission

. Institute of Electrical and Electronic Engineers (IEEE): Maintains the 802.11wireless transmission standards

. Wi-Fi Alliance: Ensures certified interoperability between 802.11 wireless vendors

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Entering the Radio Frequency WorldObjective:

. Describe standards associated with wireless media (including: IEEE WI-FI Alliance, ITU/FCC)

Because of the simplicity associated with setting up 802.11 wireless networks in smalloffice/home office (SOHO) environments, it can be easy to underestimate the task of deploy-ing a wireless network on a corporate scale. Wireless access points (WAPs) use radio frequen-cies that are subject to interference and distortion. Likewise, when we move into the wirelessnetwork world, we take a step backward in our switch-based mind-set to the days of networkhubs. Only one device attached to a wireless access point can send or receive at a time (halfduplex). Understanding these facts is key to a successful WLAN deployment.

Understanding Wireless TransmissionBack in the day, the two competing LAN technologies were Ethernet and Token Ring. Manyargued that Token Ring was the better technology because the users would never experience acollision of data on the network. This is because only one user could transmit data at a timedue to the token-passing mechanism. Ethernet ended up winning the competition because itsmethod of collision detection (CSMA/CD) allowed for higher transmission speeds.

When we move into the WLAN environment, we move our transmission methods to a colli-sion avoidance (CSMA/CA) strategy because wireless devices have no collision detectionmechanism. Likewise, because a wireless device that is sending data cannot receive data at thesame time, WLAN devices run at half duplex as a rule. When you combine this limitation withsignal degradation due to wireless range and interference, you’ll find that wireless devicesrarely run at the advertised speed (11Mbps for 802.11b, 54Mbps for 802.11b and 802.11a).Most of the time, the actual speed you are receiving is less than half of the maximum data rate!

Much of the wireless signal degradation is due to the nature of RF. Most of us are used to theland of Ethernet, where data signals travel cozily through a padded copper wire, free frommost interference. Wireless RF signals are sent through the air using an antenna, and unfor-tunately, the air is an ugly place. The first challenge that the wireless signal faces is physicalobjects. Every object the signal must pass through can degrade the signal in some way.Reflective surfaces, such as metal or glass, cause RF waves to bounce off, shooting in a differ-ent direction. Uneven surfaces, such as a gravel road, piles of merchandise in a warehouse, adesk, or a cubicle can cause the signal to reflect and scatter in many directions. Finally, as wire-less signals pass through physical objects, they are absorbed. This absorption rate differsdepending on the type of material the signal passes through.

As if physical objects weren’t enough, the wireless signal is also subject to interference. All802.11 technology uses unlicensed wireless bands (discussed further in a moment). This means

Chapter 17: Understanding Wireless Networking

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Entering the Radio Frequency World563

that they are not sanctioned by the United States Federal Communications Commission(FCC), and any wireless device can use them. In many ways, this is fantastic. Without unli-censed wireless bands, every new wireless network in the world would need approval from theFCC, along with all the licensing fees associated with the process! However, unlicensed RFbands also present the challenge of conflict. Cordless phones, Bluetooth, microwave ovens,and WLAN technology all share the same unlicensed RF band. Never before have networkadministrators faced the challenge of a Stouffer’s Microwavable Macaroni Dish being respon-sible for a 1-minute network outage. It’s a good thing this is reduced to 30 seconds in high-powered microwaves!

The U.S. designates the FCC to manage the radio frequency bands. Other countries each have their ownagency that manages radio frequencies. Often, these agencies mirror many of the rules that the FCC hasspecified. However, you may find differences as you move from country to country.

NOTE

Unlicensed RF BandsThe FCC maintains three unlicensed RF bands called the Industrial, Scientific, and Medical(ISM) bands. Figure 17.1 shows where these RF bands fit in the entire wireless spectrum.

FIGURE 17.1 FCC unlicensed bands in view of the entire wireless spectrum.

The three specific bands are as follows:

. 900MHz band: 902MHz to 928MHz

. 2.4GHz band: 2.400GHz to 2.483GHz

. 5GHz band: 5.150GHz to 5.3GHz and 5.725GHz to 5.825GHz

902-928 MHz26 MHz

2.4-2.4835 GHz83.5 MHz802.11b

and 802.11g

5 GHz802.11a

Frequencies varywith countries.

ExtremelyLow

VeryLow

Low

AudioAM Broadcast

Shortwave Radio FM Broadcast

TelevisionCellular (840 MHz) Infrared Wireless LAN

NPCS (1.9 GHz)

Medium HighVeryHigh

UltraHigh

SuperHigh

InfraredVisibleLight

Ultra-violet

X-Ray

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Keep in mind that these ranges of frequency are for the U.S. Other countries may supportmore frequencies, and others support fewer. Some countries may not allow unlicensed wire-less transmission at all!

Every company that manufactures equipment that transmits wirelessly and that does not wantto license frequencies from the FCC is forced to share these unlicensed bands. Combining dif-ferent devices in the same area that are sharing the same frequencies results in interference.For our WLAN, that means impaired performance or potentially a completely unusable net-work, depending on the severity of the interference.

By far, more equipment uses the 2.4GHz band, making it more difficult to keep the airwavesfree from interference. The more popular 802.11b, 802.11g, and 802.11n standards fit into thisRF band. The 5GHz band is home to the 802.11a standard. On that note, let’s also discusssome key facts about RF:

. Fact #1: The higher frequencies allow for higher data rates.

. Fact #2: The higher frequencies have shorter transmission distances (range).

. Fact #3: Shorter distances can be compensated for by using high-powered antennas.

. Fact #4: Every country has its own restrictions on how powerful your radio transmis-sion can be for the unlicensed bands.

So, building a 500-foot-high tower on top of your company building powered by a smallnuclear generator most likely is not an option for achieving campus-wide network coverage.The alternative solution requires you to purchase many small transmitters and strategicallyplace them throughout your network.

The Key to Successful Wireless: Channel SurfingBecause government restrictions keep us from saturating our campus network with an ultra-powerful wireless signal, we are forced to deploy many transmitters (WAPs) that have a muchsmaller coverage radius. The problem with this solution is that these transmitters all use thesame RF band. Because a good wireless design requires overlapping signals, we are stuck witha problem: the adjacent transmitters will interfere with each other.

Having a 10 to 15% overlap in signals, as shown in Figure 17.2, is a good design. With theright configuration, this overlap can allow clients to roam seamlessly between transmitters.However, the client will experience heavy interference and performance degradation withinthe overlapping coverage areas.

Chapter 17: Understanding Wireless Networking

EXAM ALERTYou will want to know the two primary unlicensed RF bands used for wireless networking: 2.4GHz and5GHz.

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Entering the Radio Frequency World565

The solution? Wireless channels. Just because a wireless access point uses the 2.4GHz rangeof RF doesn’t mean it consumes the entire range. The 2.4GHz RF band is divided into differ-ent channels, as shown in Figure 17.3.

InterfaceInterfaceInterference

FIGURE 17.2 The problem withoverlapping wireless signals.

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2401Mhz 2483MhzFrequency

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The 2.4GHz band supports up to 14 different channels (the FCC allows only channels 1 to 11in the U.S.). Each channel consumes 22MHz of frequency bandwidth. When you walkthrough the setup wizard of a wireless access point, one of your options is a channel selector,typically populated with channels 1 to 11. Your goal is to put adjacent access points on sepa-rate channels. Be careful when doing this—it’s not as simple as it looks! First, most of the wire-less channels the FCC has sanctioned are overlapping. Figure 17.3 shows that Channel 1 (2401to 2423MHz) overlaps with Channel 2 (2406 to 2428MHz), Channel 3 (2411 to 2433MHz),Channel 4 (2416 to 2438MHz), and Channel 5 (2421 to 2443MHz). Placing one access pointon Channel 1 and an adjacent access point on Channel 3 would cause interference problems.

FIGURE 17.3 2.4GHz band wirelesschannels.

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For this reason, it is commonly said that the 2.4GHz band supports only three channels: 1, 6,and 11. These three channels do not overlap with each other. With this in mind, we need toarrange our wireless access deployment into logical, nonoverlapping “cells,” as shown inFigure 17.4.

Chapter 17: Understanding Wireless Networking

Channel 6

Channel 1

Channel 11

Channel 1

Channel 6

Channel 1

Looking at Figure 17.4 should make you realize that, indeed, deploying an organization-widewireless network does take planning.

One of the big advantages that the 5GHz band has over the 2.4GHz band is the number ofchannels supported. Depending on your locale and the channels you choose, you could haveup to 23 nonoverlapping channels in the 5GHz range! This makes it much more viable to finda clear channel without interference. Remember our Stouffer’s Macaroni Dish microwaveissue? Using different channels can be one solution to help alleviate some of the microwaveinterference.

FIGURE 17.4 Arranging 2.4GHzchannels into logical, adjacent cells.

EXAM ALERTUnderstanding the concept of wireless RF channels is key—not only for proper network design, butalso for the ICND1 and CCNA exams.

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The 802.11 Standards Family567

The 802.11 Standards FamilyNow that you have a foundational understanding of RF signaling, we can move to the 802.11standard. When many network administrators hear of 802.11 technology, their mind immedi-ately equates this with the three or four popular wireless networking standards: 802.11a,802.11b, 802.11g, and the prestandard (at the time of this writing) 802.11n. However, 802.11describes much more than this when dealing with wireless networks. For example, 802.11edescribes quality of service (QoS) standards for 802.11, and 802.11i is an enhanced wirelesssecurity standard, to name just a couple. In a way, 802.11 is similar to TCP/IP in that it depictsa suite of protocols and standards.

Of course, in this suite of standards, the most popular are the common 802.11a, 802.11b,802.11g, and 802.11n. Let’s look at the evolution of these standards.

Party Like It’s 1999: 802.11a and 802.11bIn 1999, two competing standards were released: 802.11a and 802.11b. As with most brand-new standards, vendors did not really begin implementing and releasing equipment that used802.11a or 802.11b until about 2002. The two standards offered the following features:

802.11a 802.11b

RF Band 5GHz 2.4GHz

Bandwidth 54Mbps 11Mbps

Channels Up to 23 3 (nonoverlapping)

Outdoor Range Approximately 75 meters Approximately 100 meters

Indoor Range Approximately 25 meters Approximately 45 meters

It’s an interesting story how these two standards competed. 802.11a was clearly the better stan-dard. It offered faster speeds (54Mbps versus 11Mbps), more channels (23 versus 3), and anoverall cleaner RF band (nowhere near as many devices use the 5GHz band as the 2.4GHzband). Alas, the 802.11b standard won the competition (for now) just because it was moreavailable. The silicon used to make the 802.11a chips was in short supply, and the industry washungry for wireless, so it gobbled up 802.11b instead.

2003 Delivers: 802.11gAfter the newness of the 802.11b standard wore off, users and administrators alike demandedmore speed. The IEEE answered with the 802.11g standard. This standard borrowed much ofthe 802.11a technology and implemented it in the 2.4GHz RF band. One of the major hurdles

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that was overcome was to achieve backward compatibility with 802.11b, thus allowing 802.11gaccess points to also support 802.11b clients. For this reason, most access points are labeled as802.11b/g. Let’s pull the same stats on 802.11g as we did for 802.11a and 802.11b:

802.11g

RF Band 2.4GHz

Bandwidth 54Mbps

Channels 3 (nonoverlapping)

Outdoor Range Approximately 95 meters

Indoor Range Approximately 40 meters

The Next Generation: 802.11nAt the time of this writing, 802.11n is currently in prestandard state, set to become a standardin September 2008. Therefore, I will write about the basics of what we currently think the802.11n standard will become. 802.11n adds multiple input, multiple output (MIMO) tech-nology to wireless cards and access points. Simply put, you now have multiple antennas thatcan send or receive between devices. This can bring about increased range and throughput.The irony of the situation is that even though the 802.11n standard is not yet complete, ven-dors are releasing prestandard devices, placing their bets on what they believe the standard willbe. This is quite a risk, because it has yet to be decided which RF band 802.11n will use; itcould use the 2.4GHz and/or 5GHz bands. Perhaps by the time you read this the standard willhave been decided. Here is what we currently know about 802.11n:

802.11n

RF Band 2.4GHz and/or 5GHz

Bandwidth 248Mbps (with two receiving and two transmitting antennas, called “2×2”)

Channels Unknown

Outdoor Range Approximately 160 meters

Indoor Range Approximately 70 meters

Chapter 17: Understanding Wireless Networking

Many vendors are now supplying wireless network cards and wireless access points that support all three(802.11a/b/g) wireless standards. I would expect that the 802.11n standard will obtain similar supportbefore long.

NOTE

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Chapter Summary569

Chapter SummaryIn less than a decade, 802.11 wireless technology has changed everything we thought a LANenvironment to be. Many people believe that someday networks will become “a world withoutwires” as wireless technology becomes more developed and deployed. To successfully deploy awireless network, it is essential to understand the basics of radio frequency (RF). The 802.11wireless networking standards are designed to operate in the unlicensed bands of 2.4GHz(802.11b/g) and 5GHz (802.11a). These bands are subject to interference from a variety ofdevices, so care must be taken to choose the correct location, number of access points, andchannels when deploying wireless networking devices. Many wireless spectrum analysis toolsare available from the general market that can help with this process.

Because of interference and half-duplex operation, the actual throughput of a wireless deviceis typically half of the maximum specified by the standard. As new standards develop, thethroughput and range of wireless technology will continue to increase.

Key Terms. 802.11a, b, g, n

. Radio frequency (RF)

. ITU-R

. IEEE

. Wi-Fi Alliance

. 2.4GHz band

. 5GHz band

. Wireless channels

. CSMA/CA

. Multiple input, multiple output (MIMO)

. Wireless cells

. Wireless roaming

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